zener diode experiment precautions

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You are here, lab 3 - diodes.

University of California at Berkeley

Donald A. Glaser Physics 111A

Instrumentation Laboratory

Semiconductor Diodes

© 2015 by the Regents of the University of California. All rights reserved.

References:

Physics 111-Lab Library Reference Site

Reprints and other information can be found on the  Physics 111 Library Site.

Lab 3 Appendices : Data sheets and  Curve Tracer operation.

  NOTE: You can check out and keep the portable breadboards, VB-106 or VB-108,  from the 111-Lab for yourself  ( Only one each please)

This is the first of three labs on basic semiconductor components. You will study semiconductor characteristics and some of their applications, leading up to the design and construction of a differential amplifier.  Note: Keep all your parts with you, as they are for you permanently.

DO NOT RETURN THEM TO THE CABINETS .

This lab studies diodes. You will find the relationship between the voltage and current in a diode, and study temperature effects, rectification, nonlinear phenomena, and frequency doubling.

This lab uses liquid nitrogen (LN $_\mathrm{2}$ ), which is very cold: approximately  $77\,\mathrm{K}$ . L N $_\mathrm{2}$  can cause severe skin and eye "burns".   Use Safety Goggles while working with  LN $_\mathrm{2}$ ; goggles are provided free in the  111-Lab and are yours to keep.  Open-toe shoes are not permitted in the lab while LN $_\mathrm{2}$   is in use.  

Read the Cryogenic safety materials from EH&S .     Take the cryogenic training: Go to  https://uc.sumtotal.host/Core/search  and enter 109 in the search window.  You may need to select UCB and then login.  ~~~  (Alternatively, you may be able to get into the training via   http://blu.berkeley.edu  , Login, then click  UC Learning Center  in the left hand column, and then when the UC Learning Center opens click Find A Course, enter 109 in the search window that opens up and click Search.  You should now see  EHS 109 Cryogens Safety  which you can click to enter the training.) ( Yet another way in is  https://jwas.ehs.berkeley.edu/lmsi  which links from  https://ehs.berkeley.edu/training/uc-learning-center-access and then click Find A Course and then search 109 )

If you cannot log into blu nor sumtotal, please show your issue to your instructor or GSI.  If they see no obvoius fix please capture a screenshot or snip of the issue and upload this along with your name and email via the form:

  https://app.smartsheet.com/b/form/ecc351889e484beb99f77dc0dc0d5d59  (Please check the box that says "Send me a copy of my responses" and then forward the email your receive to [email protected] ) 

Obtain the LH $_\mathrm{2}$ from a GSI, who will provide it for you in a styrofoam cup.  Be careful not to spill the cup.  You are not allowed to obtain the LN $_\mathrm{2}$ form the storage dewar yourself.

The semiconductor components used henceforth in this course can get quite hot...especially if they are hooked up incorrectly.  They can easily burn you.   If you must touch components while the power is on, touch them gingerly first to asses their temperature before grasping them with force.  Better yet, turn the power off, and wait a few moments for the components to cool down.

Before coming to class complete this list of tasks:

·         Completely read the Lab Write-up

·         Answer the pre-lab questions utilizing the references and the write-up

·         Perform any circuit calculations use Matlab or anything that can be done outside of lab use RStudio (freeware). [NOTE added: R is now a dead language. Learn Python instead.]

·         Plan out how to perform Lab tasks.

All part spec sheets are located here the Physics111 Library site

1.     In a few sentences, explain what diodes are and how they are useful.

2.     Show that the second term in Eq. (1) (the $-1$  in the brackets) may be neglected for typical operating parameters :

$V>0.1\;V,\;kT\approx\frac{1}{40}eV,\;n\approx 2$ .

3.     Why is there a ripple on top of the DC voltage output by the circuit in 3.11?

4.     What is a load line used for?  What are the relative advantages of graphical and iterative analysis?

This lab is one of the most analysis and plotting intensive labs done this semester. Do not leave your analysis section to the last minute. Using a computer to do the analysis is much easier & quicker than doing it by hand.  Bring a thumb drive to class or alternatively e-mail yourself the data files from the diode characteristic traces found by the  Curve Tracer so you will have access to data both at your lab station and home. Excel is a powerful timesaver when graphing similar data sets multiple times.

Light Emitting Diodes (LEDs) will burn out if you hook them directly to the 5V power supply without a current limiting resistor.

Diodes and pn Junctions

Diodes and transistors are made from semiconducting materials: typically crystalline silicon. Pure silicon has few free electrons and is quite resistive. To increase its conductivity, the silicon is normally "doped", i.e. d eliberately contaminated,   with other elements. Some dopants, like phosphor, arsenic and antimony, easily give up one of their electrons to the now impure silicon crystal.  These donated electrons are free to move about the crystal, and its conductivity increases dramatically.   Only a few dopant atoms will significantly increase the crystal's conductivity.  For example, one dopant atom per 100 million silicon atoms will increase pure silicon’s conductivity by approximately 10 5 .  Of course, dopants atoms that give up an electron become positively charged. The net charge remains zero.

Other dopants, like boron, indium and aluminum, grab electrons from the surrounding silicon atoms, leaving positively charged silicon ions behind. In turn, these now positive silicon ions try to regain their neutrality by grabbing electrons from their neighbors…the net result is that there are regions of “positiveness” floating around the crystal lattice. Such “absences of electrons” are called holes . Amazingly, holes behave almost exactly like positively charged electrons; they move, respond to electric fields, and appear to have a mass close to the electron mass.

A doped semiconductor with more mobile electrons than holes is called an “n‑type” semiconductor; conversely, a doped semiconductor with more holes than mobile electrons is called a “p‑type” semiconductor.

If doping’s only effect was to increase semiconductor conductivity, semiconductors would be obscure, little-used materials. The utility of semiconductors comes from the remarkable effects of placing p and n‑type materials next to each other. Such juxtapositions are called “pn” junctions. An isolated pn junction makes a semiconductor diode. Other semiconductor components are made from more complicated arrangements; bipolar npn transistors, for example, are made by sandwiching a p layer in between two n layers, hence the name npn.

The current through an ideal pn junction is given by the diode equation.

$\displaystyle I(V)=I_\mathrm{sat}\bigg[\exp\bigg({\frac{eV}{nkT}\bigg)}-1\bigg],$              (1)

where $V$  is the voltage drop across the junction, $I_\mathrm{sat}$  is a constant called the saturation current and depends on the temperature, on the particular geometry of the junction, and on the material of the junction,  $e = 1.6\times10^{-19}\,\mathrm{C}$ is the charge of an electron, $k = 1.38\times10^{-23}\,\mathrm{J/K}$  is Boltzmann’s constant, and $T$  is the temperature in Kelvin. (A good approximation to memorize is that a t room temperature,  $kT\approx 1/40\,\mathrm{eV}$ .)  The constant $n$  varies between 1 and 2 depending on the particular diode, but is typically equal to 2 for discrete diodes.

Notice from Eq. (1) that the diode’s response is directional and highly nonlinear. When forward biased, ( $V$ positive) enormous currents can flow through the diode because of the exponential dependence of $I$  on $V$ .  When the diode is reverse biased, ( $V$ negative), the current then approaches $-I_\mathrm{sat}$ . Since $I_\mathrm{sat}$  is typically very small (picoamps are not uncommon), very little current flows.  Thus the diode acts like a one-way valve; current can only flow in one direction.

Be aware that while Eq. (1) properly describes the diode voltage dependence, the implicit temperature dependences in  $I_\mathrm{sat}$  dominate over the explicit dependence in Eq. (1).  For example, the current typically goes down, not up, as the temperature decreases.

Nonlinear Circuit Equilibrium

Unlike purely linear circuits, circuits containing nonlinear elements like diodes cannot be reduced to systems of linear equations. Consequently, the equilibrium voltages and currents in nonlinear circuits are much more difficult to determine. Although these equilibrium quantities can be found using complicated computer programs like MultiSim , quick, approximate analysis methods are often useful, particularly for simple circuits. Two quick methods will be used in this course: (I) Graphical Analysis and (II) Iterative Analysis

(I) Graphical Analysis – Load Lines

The possible values fall on a curve given by the parametric equations $I=V_0/(R+Z)$   and $V=ZV_0/(R+Z)$ , where $Z$  varies between zero and infinity. Eliminating $Z$  demonstrates that the curve is actually a straight line given by the equation

$\displaystyle I(V)=(V_0-V)/R$ .    (2)

This equation could have been derived directly from its end points, $I=V_0/R$  at $V=0$  and $I=0$  at $V=V_0$ . The line determined by Eq. (2) is called the load line because it is determined solely by the load (and the power source), not by the nonlinear component.

The nonlinear component obeys its own equation, or “characteristic” curve  $I_Z(V)$ . In equilibrium, both the load line and the characteristic curve must be satisfied simultaneously.  Consequently the equilibriu m current and voltage for the c ircuit are given by the intersection of the load line (Eq. (2)) and the characteristic equation  $I_Z(V)$ .

For example, assume that the nonlinear component is a diode ( $I_\mathrm{sat}=4\times10^{-10}\,\mathrm{A}$ ) driven by a  $1\,\mathrm{k}\Omega$  1k resistor from a  $2\,\mathrm{V}$  battery, as shown below left.    The load line and diode characteristic (Eq. 1) for this circuit intersect, as shown below right, at equilibrium voltage  $V=0.75\,\mathrm{V}$  and current  $I= 1.25\,\mathrm{mA}$ .

The equilibrium voltage and current across the diode is sometimes called the  operating point   of the circuit.

(II) Iterative Analysis

The apparent precision of the iterative method is deceptive as it relies on precise knowledge of  $I_\mathrm{sat}$  and  $n$ .  Don’t worry; answers with accuracy's better than 10% are rarely required in electronics.

[Be careful: iterative methods do not always converge. In fact, running the described sequence backwards (guess the diode voltage, calculate the current, find the resistor voltage drop, and subtract from the battery voltage to refine the diode voltage guess) does not converge. Try it yourself! The study of the convergence of these methods is called Iterated Map Theory, and, surprisingly, is the basis for Chaos Theory.]

Both the load line analysis and the iterative analysis yield the same values for the equilibrium voltage and current. Note that the diode equation [Eq. (1)] was used in both methods. With some loss in precision, experimental data taken from an actual diode can be used instead.  

Perturbation Analysis

Determining a circuit’s response to small changes in its parameters is as important as determining its initial equilibrium. The general subject of the response of a system to small parameter changes (perturbations) is called perturbation analysis. In the circuit of Fig. 1, for example, perturbation analysis can be used to determine the change in the diode voltage when small changes are made to the source voltage.

(I) Graphical Perturbation Analysis

(II) Small Signal Impedance Perturbation Analysis

The above graphical procedure is completely equivalent to the following method: First calculate the small signal impedance $z$  of the nonlinear diode at its operating point. The small signal impedance is the reciprocal of the slope of the diode characteristic curve  at the operating point. 

$z=\dfrac{\partial V}{\partial I}\bigg|_{operating\;point}$ .

Then use $z$  in a “linear” circuit analysis. Because the characteristic curve is not linear, $z$  must be recalculated if the voltage across the diode changes (i.e. if the operating point changes).

For more information, see  http://en.wikipedia.org/wiki/Small_signal_model

Problem 3.1 - Forward and Reverse Diode Behavior

With the Double Banana plug ground hooked up to the COM (Fluke) or LO (Keithley) Banana input plug, the red minigrabber lead will be positively biased referenced to the black lead; thus, for diode conduction, the red lead should be attached to the diode anode, and the black lead should be attached to the cathode. On the diode itself, the cathode is marked by a black band; in the 1N4448 image above, the band is at the bottom.

The DMM diode settings attempt to impress  $1\,\mathrm{mA}$  through the diode, while simultaneously measuring the voltage across the diode.  If the diode is forward biased, the voltage measurement will be the "forward voltage drop" of the diode at  $1\,\mathrm{mA}$ .  If it is reversed biased, the maximum output voltage of the DMM (about  $7\,\mathrm{V}$  for the Fluke, and  $2.5\,\mathrm{V}$  for the Keithley) will be insufficient to drive  $1\,\mathrm{mA}$ , and the meter will read an error code (OL for the Fluke, and Open for the Keithley).

Confirm  that the diode conducts unidirectionally by measuring the forward voltage drop when the diode is forward biased, and an error code when the diode is reversed biased.   You should get a forward voltage drop on the order of   $0.6\,\mathrm{V}$ .

Obtain  a plastic-stick-mounted 1N4448 diode from the laboratory staff.  Repeat  your measurement of the forward voltage drop using the DMM. Does this diode have exactly the same forward voltage drop as the diode you used in part 3.1? Forward voltage drops vary between types of diodes and even between diodes of the same type.

Squeeze the diode between you fingers.  The forward voltage drop should change as the diode heats up to your finger temperature.  What is the new value?  For more dramatic results, dip the diode into liquid nitrogen, which you can obtain from the laboratory staff.  What is the forward voltage drop now?  Diodes are frequently used as temperature sensors by measuring this forward voltage drop.

Warning: Because diodes of the same type can have significantly different characteristics, use the same diode for all experiments in this lab. If you need to use your diode on another day, mark it with a piece of tape with your name and leave it in the storage cubbies in the back of the lab.

Problem 3.3 - Offset Adder Functionality I

Now examine the behavior of the Offset Adder circuit included on the breadboard box at your lab station. A picture of the offset adder, and a brief description of its functionality, cae be found in the Lab 1 manual immediately after Problem 1.1.3.

Temporarily ignore the input BNC jack. Measure the output voltage (from the BNC output jack) while turning the Offset Adjust knob. See how the voltage can be varied from approximately $-9$  to $+9\,\mathrm{V}$ . Try loading the output with several different resistor values. By plotting a V-I curve, prove that the circuit is a relatively stiff (low output impedance) voltage source  so long as the output current is kept below approximately 24 mA.

Problem 3.4 - Diode Characteristic Curve

DMM resistance measurements are a useful crude indicator of diode performance, and are often used to determine if a diode has been burnt out. However, DMM measurements are single current measurements, and do not determine the complete relation between the diodes forward voltage and the forward current.  This relation,  $I(V)$ , is called the diode characteristic curve, and is very useful for understanding how diodes work in a circuit  The most straightforward way to obtain this curve is to measure the current through the diode, while driving the diode with a variable voltage source and measuring the voltage across the diode.  

Obtaining enough points to carefully characterize the diode is tedious. Furthermore, the slow rate at which the data can be collected by hand causes the diode to heat up significantly  at the high current points, disturbing the measurement. A Curve Tracer is an instrument that automatically, and relatively quickly, measures characteristic curves.  Operating information about the curve tracer can be found here:  Curve Tracer Manual.

Use the Curve Tracer to find your diodes characteristic curve. Export the Curve  Tracer data to a file,  and p lot the points on a graph.  Save  this data for future use.  Add the points that you obtained in problem 3.4 to your plot.

Use the Curve Tracer's "Analyze Data" function to obtain the values of   $I_\mathrm{sat}$  and the voltage coefficient,  $e/nKT$ , for the plastic-stick-mounted 1N4448 diode from 3.2. Assuming that the diode is at room temperature, calculate   $n$ .

Repeat the measurement, this time with the diode immersed in liquid Nitrogen at $T=77\,\mathrm{K}$ .   Superimpose the cold diode data on your room temperature diode data graph.   Repeat the "Analyze Data" function, and c alculate the cold  $n$ .

Problem 3.6 - Diode Reverse Current

Problem 3.7 - Diode Equilibrium

Problem 3.8 - Offset Adder Functionality II

Connect  the signal generator to the input of the Offset Adder. Now, the output of the Offset adder will be the sum of the signal  connected to the Offset Adder’s input, and the internal offset set by the Offset Adjust knob.

Examine  the Offset Adder’s output on the scope, and play with different offsets and inputs until you understand the Offset Adder’s function.

Since diodes carry current only in one direction, they can be used to rectify   AC signals; rectify means to convert an AC signal into DC. Consequently, diodes are sometimes called rectifiers, especially when used in this application.

There are a few electronic circuits, like  light dimmers and some electric motor controllers, that run off of AC.  Most non-battery-powered electronics, however, require that the AC from a wall socket be converted to DC.

Problem 3.10 - Half Wave Rectification

Problem 3.12 - Light Emitting Diodes (LEDs)

LEDs are diodes made from the semiconducting material Gallium Arsenide (GaAs) rather than from silicon. GaAs junctions have the very useful property that they emit light when forward biased.

Problem 3.13 - LED Characteristic Curves

Compare the characteristic curves of red, green, and blue LEDs.   What fundamental constant partially explains your observations?

Problem 3.14 - Zener Diodes

Circuits frequently require DC voltages less than the circuit power supply voltage. Such voltages can be obtained with voltage dividers, but dividers are not stiff and, consequently, their output voltage will decrease when loaded. Furthermore, the divider voltage will follow any power supply voltage fluctuations.

Better schemes use a device called a Zener diode .  Zener diodes are diodes deliberately optimized for use in the reverse breakdown region. Use the Curve Tracer to obtain the characteristic curve of a 1N5234B $6.2\,\mathrm{V}$  Zener diode. (Make sure you switch from the Diode Tracer window to the Zener Diode Tracer window.)

Using a Zener diode in the lower leg, you can make a voltage divider-like circuit whose ouput voltage is quite stiff, i.e. independent (up to a point) of the load, and largely independent of the input voltage.  A resistor,  $R_s$ , should be used in the upper leg.

Design and build a circuit with the Zener that will reduce a voltage from 12V to 6.2V. The current going through $R_s$  should be limited to about  $15\,\mathrm{mA}$ . Hint: considering the Zener's characteristic curve, should the Zener be forward or reverse biased?  

What is the smallest load resistor  $R_\mathrm{L}$ , a resitor placed in parallel with the diode, that will not significantly decrease the circuit output voltage? [The resistor   $R_s$ is NOT the load resistor.  Do not try small values for   $R_s$ ]

Problem 3.15 - Frequency Doubling

Many nonlinear devices (including diodes) exhibit the very useful phenomenon of frequency doubling: when driven by a sufficiently high amplitude signal, they double the signal’s frequency.   Frequency doubling is for instance, employed to produce short-wavelength coherent light. Few lasers laze in blue or shorter wavelengths. Powerful red lasers, however, are easy to build. The output from such red lasers can be fed into a frequency doubling crystal, and blue light will come out. This blue light can then be fed into another crystal, yielding ultraviolet light. Although the process is lossy, frequency doubling is the most effective way of making intense, short-wavelength laser pulses.

We will study frequency doubling in a Spice diode circuit. Run MultiSim, and load  the  Desktop\Mulitisim\Lab 3 \ BiasDiod  schematic.

​In this circuit,  capacitor C1 is used to block the DC component of the signal across the diode.  Measure the output of the circuit across R2, the load. VAC sets the amplitude of the AC signal driving the diode, while VDC (1V) sets the DC bias. Both VAC and VDC can be changed by double clicking on the numeric value to their right.

Set  VAC to 0.00001. Here nonlinear effects are negligible.  Run  MultiSim, and  look  at the output by running an AC Analysis ( S imulate → A nalyses →  T ransient Analysis). The graphics program will pop up with a display of the input signal (at the amplitude 0.00001V) and the smaller output signal. As both waveforms look like perfect sine waves, it is difficult to determine the purity of the waves directly from these plots. The best way to determine their spectral content is to find their Fourier representation. Use  S imulate→  A nalyses→  F ourier Analysis The display will change to graphs of the Input and Output harmonic content. Both waves are nearly pure sine waves (note the log vertical scale.)  There will be some high order harmonics, but their amplitude will be many orders of magnitude lower. 

Now go back to the schematic and  adjust  VAC to 1V.  Rerun  the simulation, and note the gross distortion of the output wave.  Look  at the FFT display: there will now a whole series of slowly decreasing higher harmonics.  The first few harmonics will not be that much lower in amplitude.

Play  with VAC. What is the lowest amplitude input signal required for significant frequency doubling? (There is no hard threshold.) Are the harmonics ever stronger than the fundamental? Why do you think the frequency doubling occurs?

The remaining problems can be done away from the lab.

Problem 3.16 - Rectifier Ripple

Find  an (approximate) expression for the (peak-to-peak) amplitude of the ripple in the rectifier built in 3.11 as a function of the input voltage and frequency, load resistor, and filter capacitor.  Does your model agree with your observations?

Problem 3.17 - Small Signal Analysis I

Find the operating point for the circuit in 3.9 for all three offset voltages. Then perform a graphical perturbation analysis for the three offsets to predict the AC component of $V_{out}$ . Use the diode characteristic curve you obtained in 3.5. 

Problem 3.18  - Small Signal Analysis II

Now repeat 3.17 using the s mall signal impedance perturbation technique: ca lculate  the small signal impedance of the diode at each operating point; then use this impedance in a “linear” voltage divider analysis.  Show how the large-signal impedance would yield a voltage divider output much greater than the actual answer.

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Zener Diodes

Introduction

Diodes generally are known as a device that allows the flow of current in one direction (forward biased) and offers resistance to the flow of current when used in reverse bias. Zener Diode (Named after the American scientist C. Zener who first explained its operational principles) on the other hand, not only allow the flow of current when used in forward bias, but they also allow the flow of current when used in the reversed bias so far the applied voltage is above the breakdown voltage known as the Zener Breakdown Voltage . Or in other words Breakdown voltage is the voltage, on which Zener Diode starts conducting in reverse direction.

Operational Principle of Zener Diode:

In normal diodes , the breakdown voltage is very high and the diode gets damaged totally if a voltage above the breakdown diode is applied, but in Zener diodes, the breakdown voltage is not as high and does not lead to permanent damage of the zener diode if the voltage is applied.

As the reverse voltage applied to the Zener diode increases towards the specified Breakdown Voltage (Vz), a current starts flowing through the diode and this current is known as the Zener Current and this process is known as Avalanche Breakdown . The current increases to a maximum and get stabilized. This current remains constant over the wider range of applied voltage and allows the Zener diode to withstand with higher voltage without getting damaged. This current is determined by the series resistor.

Consider the Images below of a normal diode in action .

To show the operations of the zener diode , consider the two experiments (A and B) below.

In Experiment A , a 12V zener diode is connected in reversed biased as shown in the image and it can be seen that the zener diode blocked the voltage effectively because it was less/equal to the breakdown voltage of the particular zener diode and the lamp thus stayed off.

In Experiment B , a 6v Zener Diode used is conducting (the bulb comes on) in reverse biased because the applied voltage is greater than its breakdown voltage and thus shows that the breakdown region is the region of operation of the zener diode .

The current-voltage characteristic curve of the Zener diode is shown below.

From the graph, it can be deduced that the zener diode operated in the reverse bias mode will have a fairly constant voltage irrespective of the amount of current supplied.

Applications of Zener Diode:

Zener diodes are used in three main applications in electronic circuits;

1. Voltage Regulation

2. Waveform Clipper

3. Voltage Shifter

1. Zener Diode as Voltage Regulator

This is arguably the most common application of zener diodes.

This application of the zener diodes relies heavily on the ability of the zener diodes to maintain a constant voltage irrespective of variations in supply or load current. The general function of a voltage regulation device is to provide a constant output voltage to a load connected in parallel to it irrespective of variations in the energy drawn by the load (Load current) or variations and instability in the supply voltage.

The Zener diode will provide constant voltage provided current stays within the range of the maximum and minimum reverse current.

The circuit diagram showing the Zener diode being used as a Voltage regulator is shown below.

A resistor, R1 is connected in series with the zener diode to limit the amount of current flowing through the diode and the input voltage Vin (Which must be greater than the zener voltage)  is connected across as shown in the image and the output voltage Vout, is taken across the zener diode with Vout = Vz (Zener Voltage). Since the zener diode’s reverse bias characteristics are what is needed to regulate the voltage, it is connected in reversed bias mode, with the cathode being connected to the positive rail of the circuit.

Care must be taken when selecting the value of resistor R1 , as a small value resistor will result in a large diode current when the load is connected and this will increase the power dissipation requirement of the diode which could become higher than the maximum power rating of the zener and could damage it.

The value of resistor to be used can be determined using the formula below.

By using this formula it becomes easy to ensure that the value of the resistor selected doesn’t lead to the flow of current higher than what the zener can handle.

One little problem experienced with zener diode based regulator circuits is that the Zener sometimes generate electrical noise on the supply rail while making attempts to regulate the input voltage. While this may not be a problem for most applications, this problem may be solve by the addition of a large value decoupling capacitor across the diode. This helps stabilize the output of the zener.

2. Zener Diode as Waveform Clipper

One of the uses of normal diodes is in the application of clipping and clamping circuits which are circuits that are used to shape or modify an input AC waveform or signal , producing a differently shaped output signal depending on the specifications of the clipper or clamper.

Clippers circuits generically are circuits that are used to prevent the output signal of a circuit from going beyond a predetermined voltage value without changing any other part of the input signal or waveform.

These circuits along with clampers are widely used in Analog television and FM radio transmitters for the removal of interference (clamping circuits) and limiting noise peaks by clipping of high peaks.

Since Zener diodes generically behave like normal diodes when the applied voltage is not equal to the breakdown voltage, they are also thus used in clipping circuits.

Clipping circuits could be designed to clip the signal either in the positive, negative or both regions. Although the diode will naturally clip off the other region at 0.7V irrespective of whether it was designed as a positive or negative clipper.

For example, consider the circuit below.

The clipper circuit is designed to clip the output signal at 6.2v, so a 6.2v zener diode was used. The zener diode prevents the output signal from going beyond the zener voltage irrespective of the input waveform. For this particular example, a 20v input voltage was used and the output voltage on the positive swing was 6.2v consistent with the voltage of the zener diode.  During the negative swing of the AC voltage however, the zener diode behaves just like the normal diode and clips the output voltage at 0.7V, Consistent with normal silicone diodes.

To implement the clipping circuit for the negative swing of the AC circuit as well as the positive swing in such a way that the voltage is clipped at different levels on the positive and negative swing, a double zener clipping circuit is used. The circuit diagram for the double zener clipping circuit is shown below.

In the clipping circuit above, the voltage Vz2 represents the voltage on the negative swing of the AC source at which the output signal is desired to be clipped, while voltage Vz1 represents the voltage on the positive swing of the AC source at which the output voltage is desired to be clipped.

3. Zener Diode as Voltage Shifter

The voltage shifter is one of the simplest but interesting applications of the zener diode. If you have had experience especially with connecting a 3.3v sensor to a 5V MCU, and have seen first-hand the errors in readings, etc, that this can lead to them you will appreciate the importance of voltage shifters. Voltage shifters help convert signal from one voltage to another and with the ability of the zener diode to maintain steady output voltage in the breakdown region, it makes them Ideal component for the operation.

In a zener diode based voltage shifter , the circuit, lowers the output voltage, by a value equal to the breakdown voltage of the particular zener diode that is used. The circuit diagram for the voltage shifter is illustrated below.

Consider the experiment below,

The circuit describes a 3.3v zener diode based voltage shifter. The output voltage (3.72V) of the circuit is given by subtracting the breakdown voltage (3.3V) of the zener diode from the input voltage (7V).

Vout = Vin –Vz

Vout = 7 – 3.3 = 3.7v

The voltage shifter as describe earlier on has several applications in modern day electronic circuits design as the design engineer may have to work with up to three different voltage level at times during design process.

Types of Zener Diodes:

Zener diodes are categorized into types based on several parameters which include;

Nominal Voltage

• Power Dissipation
• Forward drive current
• Forward voltage
• Packaging type
• Maximum Reverse Current

The nominal Operation voltage of a zener diode is also known as the breakdown voltage of the zener diode, depending on the application for which the diode is to be used, this is often the most important criteria for Zener diode selection.

Power dissipation

This represents the maximum amount of power the zener current can dissipate. Exceeding this power rating leads to excessive increase in the temperature of the zener diode which could damage it and lead to the failure of the things connected to it in a circuit.  Thus this factor should be considered when selecting the diode with the use in mind.

Maximum Zener Current

This is the maximum current that can be passed through the zener diode at the zener voltage without damaging the device.

Minimum Zener Current

This refers to the minimum current required for the zener diode to start operating in the breakdown region.

Other parameters that serve as the specification for the diode all need to be fully considered before a decision is made on the type on the kind of zener diode needed for that peculiar design.

Conclusion:

Here are 5 points you should never forget about the zener diode.

• A zener diode is like an ordinary diode only that it has been doped to have a sharp a breakdown voltage.
• The Zener diode maintains a stable output voltage irrespective of the input voltage provided the maximum zener current is not exceeded.
• When connected in forward bias, the zener diode behaves exactly like the normal silicone diode. It conducts with the same 0.7v voltage drop that accompanies the use of the normal diode.
• The zener diode default operational state is in the breakdown region (reversed biased). It means it actually starts to work when the applied voltage is higher than Zener Voltage in reverse biased.
• The zener diode is mostly used in applications involving, voltage regulation, clipping circuits and Voltage shifters.

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Zener Diode Basics (A Beginner’s Guide)

In this guide, you’ll learn exactly how a Zener diode works and how to use it in circuits.

Did you know that some of the common things you can build with Zener diodes include simple power supplies and guitar pedals?

Sounds interesting? Let’s jump in!

What Is a Zener Diode?

A Zener diode is a type of diode that is often used for voltage regulators and shaping waveforms.

Its symbol is an arrow pointing towards a crooked line. There are actually three different ways you can draw the Zener diode symbol in schematics :

While a normal diode only allows current to flow through a circuit when it is forward-biased (current going from anode to cathode), the Zener diode also works when it is reverse-biased (current going in the opposite direction).

With standard diodes, if you place it in reverse, no current flows.

At least, so it appears. But actually, if you apply enough voltage in reverse, current will start to flow.

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This voltage is called the breakdown voltage of the diode.

For example, the rectifier diode 1N4001 has a breakdown voltage of 50V.

The Zener diode is basically the same as the standard PN junction diode. However, it is specially designed to work in reverse bias with a low and specified reverse breakdown voltage.

So, why is that interesting?

Because you’re not limited to the standard forward voltage of 0.7V. You can design the breakdown voltage to be for example 3.3V or 12V – or many other voltages. The manufacturers call this Zener Voltage (Vz) .

This means a Zener diode can act as a voltage regulator because it keeps the breakdown voltage at a nearly constant value across its terminals.

How To Make A Zener Diode Voltage Regulator

Making a voltage regulator is easy with the Zener: Just add a resistor!

Here’s how it works:

When a voltage (must be higher than the Zener voltage) is applied across the resistor and Zener diode, the diode starts conducting in reverse and keeps the voltage drop across it at a constant value of 3.3V.

The rest of the voltage drops across the resistor. This means the resistor acts as a current-limiting resistor so that you can easily calculate the current by using Ohm’s law .

The resistor (R S ) limits the maximum current that can flow through the circuit. If there is no load connected to the circuit all the current flows through the Zener diode, causing it to dissipate its maximum power.

A smaller value of resistor R S gives you a higher maximum current. But at the same time, you have to make sure that you don’t exceed the maximum power rating of the Zener. Therefore, it’s important to choose the right value of series resistance.

Example: Choosing a resistor for a 12V to 3.3V voltage regulator

In this example, you’re going to use a Zener diode that can handle up to 2W of power. What’s a suitable value for the resistor (R S )?

First, you need to find the maximum current that can flow through the Zener diode:

That means that the minimum value of the resistor is:

So a value of 14.5 Ω is the absolute minimum. But you can use higher resistances of course.

Next, you need to check how much current your load needs. The load resistor is 1 kΩ, so its current is calculated as follows:

That’s much lower than the 0.6A you’d get from the 14.5 Ω resistor, so no problem here.

If you want to use a higher resistor value, just use the following formula with your chosen resistor value and make sure that it’s above the current you need for your load:

For example, if you want to use a 100 Ω resistor, this would give you a maximum current of:

Still, high above the needed 0.0033A for the load, so you’re good to go.

That said, Zener diodes have their disadvantages. Check out this article about why Zener diodes make lousy voltage generators .

Wave-Shaping With the Zener

In the previous example, you saw how the Zener works with DC power. However, what happens when it’s used with AC power? How would it react to a constantly changing signal such as an audio signal?

Check out the following circuit:

In the above diagram, the Zener diode has a V Z  of 5V, so if the waveform exceeds this limit, the diode will “clip off” the excess voltage from the input, resulting in a waveform with a flat top maintaining a constant output of 5V.

When forward-biased, the Zener diode behaves like a regular diode. So when the waveform reaches negative values below 0.7V, the Zener acts like a typical rectifier diode, resulting in output clipping at -0.7V.

Zener Diode Example: Distortion Guitar Pedal

Diode clipping and clamping circuits can shape a waveform to produce an output waveform with a different shape. A clipper circuit clips off the positive or negative part of an AC signal, which is why they are commonly used as waveform shaping circuits.

When you cut off the top of an audio signal, it sounds distorted. This is exactly what you want in a distortion guitar pedal!

But the output signal of an electric guitar isn’t high enough to be clipped. So in order to clip it, you must amplify it first. Here’s a basic guitar distortion pedal circuit that uses Zener diodes to distort the sound:

So, if you build the circuit above the output waveform will be clipped at the V Z voltage plus 0.7V, which is the forward voltage drop of the other diode.

Zener Diode Characteristics (The I-V Graph)

The I-V characteristics curve of a Zener diode is shown in the image above. By studying this graph, it becomes clear why Zener diodes are employed in reverse bias.

By observing the behavior of a Zener diode, you can notice that as the reverse voltage rises, the reverse current also increases gradually until it hits the Zener knee current, I Z (min). At this point, the breakdown effect starts, and the Zener impedance (Z Z ), which is the internal resistance of the diode, begins to rapidly decrease as the reverse current increases.

In general, the breakdown voltage of the Zener (V Z ) is fairly constant, although it increases slightly with increasing Zener current (I Z ). V Z is commonly set at a value of the Zener current known as the I Z (min).

A Zener diode’s ability to maintain almost constant voltage in its breakdown region makes it suitable for regulating voltage even in the simplest voltage regulator applications.

A voltage regulator’s main role is to deliver a steady output voltage to a load connected in parallel. This is even when the supply voltage has ripples or the load current varies. Zener diodes can maintain a constant voltage output as long as their reverse breakdown region holding current does not drop below I Z (min).

Common Zener Diode Voltages

Here you have a table with the most common Zener voltages in diodes of 0.3 W and 1.3 W.

More Diodes Tutorials

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Zener Diode: Theory, Applications, and Advancements

• September 2023
• 11(9):1392-1398

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Experiment to Study V-I characteristics of Zener Diode.

V-I characteristics of Zener Diode.

Objectives:

• To study p type and n type semiconductor.
• To understand reverse biasing.
• To understand breakdown voltage.
• Perform the experiment on the trainer kit and plot the graph of V-I characteristics of Zener diode.

Components and equipments required: Zener diode, multimeter, connecting wires.,power supply.

General Instructions: You will plan for Experiment after self study of Theory given below, before entering in the Lab.

Zener diode A zener diode is a special kind of diode which allows current to flow in the forward direction in the same manner as an ideal diode, but will also permit it to flow in the reverse direction when the voltage is above a certain value known as the breakdown voltage, "zener knee voltage" or "zener voltage." The device was named after Clarence Zener, who discovered this electrical property. Many diodes described as "zener" diodes rely instead on avalanche breakdown as the mechanism. Both types are used. Common applications include providing a reference voltage for voltage regulators, or to protect other semiconductor devices from momentary voltage pulses. A Zener Diode is a special kind of diode which permits current to flow in the forward direction as normal, but will also allow it to flow in the reverse direction when the voltage is above a certain value - the breakdown voltage known as the Zener voltage.

The Zener voltage of a standard diode is high, but if a reverse current above that value is allowed to pass through it, the diode is permanently damaged. Zener diodes are designed so that their zener voltage is much lower - for example just 2.4 Volts. When a reverse current above the Zener voltage passes through a Zener diode, there is a controlled breakdown which does not damage the diode. The voltage drop across the Zener diode is equal to the Zener voltage of that diode no matter how high the reverse bias voltage is above the Zener voltage.

The illustration above shows this phenomenon in a Current vs. Voltage graph. With a zener diode connected in the forward direction, it behaves exactly the same as a standard diode - i.e. a small voltage drop of 0.3 to 0.7V with current flowing through pretty much unrestricted. In the reverse direction however there is a very small leakage current between 0V and the Zener voltage - i.e. just a tiny amount of current is able to flow. Then, when the voltage reaches the breakdown voltage (Vz), suddenly current can flow freely through it.

Uses of Zener Diodes Since the voltage dropped across a Zener Diode is a known and fixed value, Zener diodes are typically used to regulate the voltage in electric circuits. Using a resistor to ensure that the current passing through the Zener diode is at least 5mA (0.005 Amps), the circuit designer knows that the voltage drop across the diode is exactly equal to the Zener voltage of the diode.

Procedure:-

• Do the connections of trainer kit.
• After increasing the battery of Vb to 1v.
• Measure the current and voltage across Zener diode.
• Repeat the step 2 and 3 for voltage 2v-10v with the increase in steps of 1v.

Observation Table

Do and Don’ts to be strictly observed during experiment:

Do (also go through the General Instructions):

• Before making the connection, identify the components leads, terminal or pins before making the connections.
• Before connecting the power supply to the circuit, measure voltage by voltmeter/multimeter.
• Use sufficiently long connecting wires, rather than joining two or three small ones.
• The circuit should be switched off before changing any connection.
• Avoid loose connections and short circuits on the bread board.
• Do not exceed the voltage while taking the readings.
• Any live terminal shouldn't be touched while supply is on.

Conclusion:

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Electronic Devices & Circuits Lab Experiment list

• 1 Experiment to Study Half Wave vs Full Wave Rectifier
• 2 Study Input vs Output Characteristics of Transistor in Common Emitter Configuration
• 3 Experiment to Study V-I characteristics of Zener Diode.
• 4 V-I Characteristics of p-n-Junction Diode
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Zener Diode as Voltage Regulator

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Zener Diode

Physics Practicals Class 12

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About Simulation

• Zener diode experiment is one of the important class 12 physics practicals that you can learn in our virtual lab.
• In this simulation, you will get the knowledge of the basics of Zener diode, its working system, plotting characteristics of a Zener diode and calculating its breakdown voltage.
• You will learn about Avalanche and Zener breakdown in PN junction.
• You will study about the application of Zener diode as a voltage regulator.
• All the experiment steps and procedures like determining the least counts, arranging the experimental setup, adjusting the rheostat, noting down the readings, plotting the graphs, etc., and many more are highly interactive and have been simulated in a very similar manner as you do in a physics lab.

• This interaction provides a very immersive environment and gives you a real-lab-like experience while conducting or performing experiments.
• This will help learners to better prepare for various competitive exams such as IIT-JEE (JEE Main & Advanced), NEET, and Olympiads.

Simulation Details

Description

A Zener diode is a silicon semiconductor device that permits current to flow in either a forward or reverse direction. The Zener diode has a reverse-breakdown voltage, at which it starts conducting current, and continues operating continuously in the reverse-bias mode without getting damaged. The following two mechanisms can cause a breakdown in a junction diode:

(i) Avalanche breakdown

With increasing reverse bias voltage, the electric field across the junction of p-n diode increases. At a certain reverse bias, the electric field imparts a sufficiently high energy to a thermally generated carrier crossing the junction. This carrier disrupts a covalent bond and produces an electron-hole pair. This process is cumulative and produces an avalanche of carriers in a very short time. This mechanism is known as avalanche multiplication, causes large reverse current and the diode is said to work in the region of avalanche breakdown.

(ii) Zener breakdown

In a Zener diode, both the p and n-sides are heavily doped. Due to the high dopant densities, the depletion layer junction width is small. This high junction field may strip an electron from the valence band, which can tunnel to the n-side through the thin depletion layer. Such a mechanism of emission of electrons after applying a certain electric field is termed as internal field emission which gives rise to a high reverse current or breakdown voltage. This breakdown is termed as Zener breakdown.

To protect a diode from damage, we connect a protective resistance to a Zener diode which limits maximum current. Also, Zener diode is used as a voltage regulator.

Requirements for this Science Experiment

⦁ Zener Diode ⦁ Voltmeter ⦁ Milliammeter ⦁ Plug Key ⦁ Rheostat ⦁ Battery ⦁ Resistance

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Zener Diode

Zener Diode is one of the most important semiconductor diodes used in our daily life. It is a specific diode that works in reverse bias conditions. It allows current to flow from anode to cathode and it also works in the reverse direction. Let’s learn about Zener Diodes their function, and their construction, in detail in this article.

A heavily doped p-n junction diode that works in reverse bias conditions is called a Zener Diode. They are special semiconductor devices that allow the current to flow in both forward and backward directions. For the Zener diode, the voltage drop across the diode is always constant irrespective of the applied voltage. Thus, Zener diodes are used as a voltage regulator.

 What is Zener Diode?

A Zener diode can be considered as a highly doped p-n junction diode which is made such that it works in reverse bias condition.

A Zener diode which is also called a Breakdown diode works in reverse bias conditions. An electrical breakdown occurring in the reverse-biased condition of the PN junction diode is called the Zener effect. In this condition when the electric field increases to a high value it enables the tunnelling of electrons from the valence band to the conduction band of a semiconductor, which suddenly increases the reverse current.

History of Zener Diodes

A theoretical physicist working at Bell Labs was the first man to describe the electrical properties of Zener Diode. His name was Clarence Melvin Zener, he was the first to tell about this special diode that works on reverse bias conditions so the diode is named after him Zener Diode. He first postulated the breakdown effect in a paper published in 1934.

Zener Diode Explanation

Zener diode that is also known as a breakdown diode is a heavily doped semiconductor device that has been specially designed to operate in the reverse direction. When the potential reaches the Zener voltage which is also known as Knee voltage and the voltage across the terminal of the Zener diode is reversed, at that point time, the junction breaks down and the current starts flowing in the reverse direction. This effect is known as the Zener effect.

Zener Diode Circuit Diagram

The figure given below is the circuit diagram of the Zener diode. The Zener diode has applications in various electronic devices and it works in reverse biasing conditions. In reverse biasing, the P-type material of the diode is connected with the negative terminal of the power supply, and the n-type material is connected with the positive terminal of the power supply. The diode consists of a very thin depletion region as it is made up of heavily doped semiconductor material.

A Zener diode can be packed in many ways. Some Zener diodes are used where high levels of power dissipation are required. The Zener diode which is the most commonly used is contained within a small glass encapsulation having a band around one end marking the cathode side of the diode.

There are two tags at the end of the bar in the circuit symbol of the Zener diode, one in the upward direction and the other in the lower direction, as shown in the figure given below. In this way, we can easily distinguish between the Zener diode and other diodes.

Zener Diode Working

High-level impurities are added to a Zener diode to make it more conductive and thus the Zener diodes can easily conduct electricity compared to other p-n junction diodes. These impurities reduce the depletion layer of the Zener diode and make it very thin. Thus, this diode also works even if the voltage applied is very small.

In no biassing condition of the Zener diode, all the electrons accumulate in the valence band of the p-type semiconductor material and thus no current flow occurs through the diode.

In reverse bias conditions, if the Zener voltage is equal to the supplied voltage, the diode conducts electricity in the direction of reverse bias. When the Zener voltage equals the supplied voltage the depletion layer vanishes completely.

Zener Diode Working in Reverse Biased

In forward-biased conditions, the Zener Diode works like any normal diode but in the reverse-bias condition, a small leak current flows through the diode. As we keep increasing the reverse voltage it reaches a point where the reverse voltage equals the breakdown voltage. The breakdown voltage is represented as V z and in this condition the current start flowing in the diode. After the breakdown voltage the current increase drastically until it reaches a stable value. 

In reverse bias condition, two kinds of breakdowns occur for Zener Diode which are,

Avalanche Breakdown

Zener breakdown.

The phenomenon of Avalanche breakdown occurs both in the ordinary diode and Zener Diode at high reverse voltage. For a high value of reverse voltage, the free electron in the PN junction diode gains energy and acquires high velocity and these high-velocity electrons collide with other atoms and knock electrons from that atoms. This collision continues and new electrons are available for conducting current thus the current increase rapidly in the diode.

This phenomenon of a sudden increase in the current is called the Avalanche breakdown. This phenomenon damages the diode permanently whereas the Zener diode is a specific diode that is made to operate in this reverse voltage area. 

If the reverse voltage is greater than 6V the avalanche breakdown happens in the Zener diode.

Zener breakdown happens in heavily doped PN junction diodes. In these diodes, if the reverse bias voltages reach closer to Zener Voltage, the electric field gets stronger and is sufficient enough to pull electrons from the valance band. These electrons then gain energy from the electric field and break free from the atom. 

Thus, for these diodes in the Zener breakdown region, a slight increase in the voltage causes a sudden increase in the current.

Avalanche Breakdown vs Zener Breakdown

There is a clear difference between Avalanche Breakdown and Zener Breakdown which can easily be understood by the table discussed below,

VI Characteristics of Zener Diode

The graph given underneath shows the V-I characteristics of the Zener diode.

 V-I characteristics of a Zener Diode can be studied under the following two headings,

Forward Characteristics of Zener Diode

Forward characteristics of the Zener Diode are similar to the forward characteristics of any normal diode. It is clearly evident from the above diagram in the first quadrant that the VI forward characteristics are similar to other P-N junction diodes.

Reverse Characteristics of Zener Diode

In reverse voltage conditions a small amount of current flows through the Zener diode. This current is because of the electrons which are thermally generated in the Zener diode. As we keep increasing the reverse voltage at any particular value of reverse voltage the reverse current increases suddenly at the breakdown point this voltage is called Zener Voltage and is represented as V z .

Applications of  Zener Diode

Zener diode is a very useful diode. Due to its ability to allow current to flow in reverse bias conditions, it is used widely for various purposes. Some of the common uses of Zener Diode are discussed below,

Zener diode as Voltage Regulator

Zener diode is utilized as a Shunt voltage controller for managing voltage across little loads. The breakdown voltage of Zener diodes will be steady for a wide scope of current. The Zener diode is associated with corresponding to the heap to make it switch predisposition and when the Zener diode surpasses knee voltage, the voltage across the heap will become consistent.

Zener Diode in Over-Voltage Protection

At the point when the info voltage is higher than the Zener breakage voltage, the voltage across the resistor drops bringing about a short-out. This can be kept away from by utilizing the Zener diode.

Zener Diode in Clipping Circuits

Zener diode is utilized for adjusting AC waveform cutting circuits by restricting the pieces of it is possible that one or both the half patterns of an AC waveform.

Zener Diode Specifications

Zener Diode is one other most commonly used diode and some of the specifications of Zener diode are,

• Zener Voltage: The voltage at which Zener breakdown occurs in the Zener diode is called as Zener Voltage. It is denoted by V z generally it ranges from 2.4 volts to 200 volts.
• Current I z (max): The maximum current that the diode can achieve at the Zener Voltage is called max current. It ranges from 200μA to 200 A
• Current I z (min): The minimum current required for the diode to break down is called min current.
• Power Rating: The maximum power the Zener diode can dissipate is the power rating of that diode. Power is calculated by taking the product of the breakdown voltage and the value of current at that time.
• Temperature Stability: Temperature stability of the Zener diode is greatest at 5V.
• Voltage Tolerance: Voltage Tolerance for any Zener diode is normally ±5%
• Zener Resistance (R z ): The resistance exhibited by the Zener diode is called Zener Resistance.

Diode p-n Junction Diode Difference Between Diode And Zener Diode

FAQs on Zener Diode

Question 1: what is a zener diode.

A Zener Diode, otherwise called a breakdown diode, is a highly doped diode that is intended to work in reverse bias cndition.

Question 2: Why is Zener Diode used as a regulator?

The voltage across Zener Diode always remains constant and thus Zener diode is used as a voltage regulator. Zener diode also works in reverse bias conditions.

Question 3: Who was the first person to describe the electrical properties of the Zener diode?

The first person to describe the electrical properties of the Zener diode was an American scientist Clarence Melvin Zener working at Bell Labs. Zener diode is named in his honour.

Question 4: What are the types of breakdowns for a Zener Diode?

The two types of breakdowns for a Zener Diode are, Avalanche Breakdown Zener Breakdown

Question 5: What is the other name of the Zener diode?

The other name of the Zener diode is Breakdown Diode.

Question 6: What is the difference between a Zener diode and a normal diode?

The difference Zener diode and an normal diode is that a normal diode allows to flow the current  in one direcrtion whereas the zener diode allow the current to flow in both directions.

Question 7: What is the voltage tolerance of a Zener diode?

The voltage tolerance of a Zener diode is close to ±5%.

Question 8: What is Avalanche Breakdown in the Zener diode?

Avalanche breakdown happens in presence of a high electric field. In a reverse biased condition if a high electric field is applied, the electrons start to gain high kinetic energy. These energised electrons breaks other covalent bonds and creates electron-hole pairs which cause a sudden surge in current this is called Avalanche Breakdown.

Question 9: What are the Applications of the Zener diode?

Following are the applications of Zener diode: Zener diode is used as a voltage regulator Zener diode in over-voltage protection Zener diode in clipping circuits

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