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(Experiment 1)

Electronics Lab

Advanced Electrical Engineering Lab Course IISpring 2006

PN Junction Diodes

Instructor: H. Elgala, Dr. D. Knipp

The experiment has been carried out by

Date:

Group number:

http://www.faculty.iu-bremen.de/course/c300221a/EE_module_2

Ta

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Electronics Lab, Advanced Electrical Engineering Lab course II, Spring 2006, International University Bremen

Lab Guidelines

Safety

Read the safety instructions before attending the lab.

Attendance

Absence of a group member will be accepted only after providing a medicalcertificate.

Preparations

Each group has to be prepared before attending the lab. All group members haveto be familiar with the subject, the objectives of the experiment and have to beable to answer questions related to the experiment handout and prelab.

If a member of the group or the whole group is not prepared, the student or thegroup will be excluded from the lab for this specific experiment.

The prelab has to be prepared in a written form by the group and has to be

presented in before attending the Lab.Supplies

All equipment, cabling and component you need should be in your work area. Ifyou cant find it, ask your lab instructor or teaching assistant, dont take it fromanother group. Before leaving the lab, put everything back, where you found it!

Please bring your notebook so that you can readout the oscilloscope via theRS232 interface.

Prelab and Lab Report

A lab report has to be prepared after each lab. The lab report has to be written insuch a form that the instructors and the teaching assistants can follow it. The lab

report includes the experimental data taken during the lab, the analysis of thedata, a discussion of the results and answer of all questions. You should alsoinclude your PSpice netlists/schematics and all required plots, sketches andhardcopies. All group members are in charge of preparing and finalizing the Labreport. Please divide the workload amongst the group members.

The lab report and the prelab have be submitted one week after the experiment.This is a hard deadline. If you miss the deadline, the experiment will bedowngraded by 10% (1 point) each day (excluding the weekends).

Grading

Each experiment will count for 10% of the overall grade. All members of a group

will get the same grade for an experiment (prelab + lab report).The final exam will count for 30% of the overall grade. The final exam will begraded on an individual bases. You have to get at least 50% in the final exam inorder to pass the lab course.

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3rd edition (February 2006)

The handout PN Junction Diode is part of the Electronics Lab, AdvancedElectrical Engineering Lab Course II. The lab course is mandatory for all 2nd yearElectrical Engineering and Computer Science students at the International

University Bremen.

Do not hesitate to contact the instructors of the course to make suggestions andprovide feedback on how the experiment can be improved.

Bremen, February 2006

H. Elgala and D. Knipp

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Objectives of the Experiment

The objective of experiment 1 of the Electronics Lab (Advanced ElectricalEngineering Lab Course II) is to become familiar with semiconductor diodes andtheir application. The handout introduces the properties and the device behavior

of different diodes like rectifier diodes and Zener diodes.Throughout the experiment, several applications like rectifiers, voltage regulators,clampers and clippers will be examined.

Introduction

A diode is one of the simplest electronic devices, which has the characteristic ofpassing current in only one direction. However, unlike a resistor, a diode does notbehave linearly with respect to applied voltages (the diode has an exponential I-Vrelationship) and hence is not simply described by an equation such as Ohm'slaw for resistors.

The diode is considered a passive element; we do not expect it to amplify power.

There are two operating regions for the diode, reverse biased region, and forwardbiased region.

The diode is a semiconductor pn junction. In addition to being applied as a diode,the pn junction is the basic element of bipolar-junction transistors (BJTs) andfield-effect transistors (FETs). Thus, an understanding of the physical operation ofpn junctions is important to understand the operation of diodes, BJTs and FETs.

Theoretical Background

Diode Structure

The semiconductor diode is a pn junction as shown in Fig. 1.1. As indicated, thepn junction consists of p-type semiconductor material in contact with n-typesemiconductor material.

A variety of semiconductor materials can be used to form pn junctions like silicon,germanium, or gallium arsenideetc. However, we will concentrate on silicon, asthis is the most widely used material in microelectronics.

In actual practice, both the p and n regions are part of the same silicon crystal.The pn junction is formed by creating regions of different doping (p and n regions)within a single peace of silicon. The material is doped by bringing in additionalatoms (impurities). The impurities can be either donors or acceptors atoms.

p-type

region

n-type

region

AnodeCathode

PN junction

Metal

contactMetal

contact

Fig. 1.1 pn junction diode structure

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The words acceptor and donor can be associated with donating and acceptingelectrons. In the case of donor atoms, the material gets n-type doped, whereas inthe case of acceptor atoms the material gets p-type. External wire connections tothe p and n regions (diode terminals) are made through metal (e.g. aluminum)contacts.

pn Junction

To understand how a pn junction is formed we will start by imagining two separatepieces of semiconductor, one n-type and the other p-type as shown in Fig.1.2.Now we bring the two pieces together to make one piece of semiconductor. Thisresults in the formation of a pn junction (Fig.1.3).

Where, free electrons donor atoms free holes acceptor atoms

The periodic structure of the semiconductor (in our case silicon) leads to theformation of energy levels. Only two of these energy levels are of interest to us:the conduction and the valence band. These energy levels can be now occupiedor unoccupied. The conduction band in a semiconductor (the semiconductor is

assumed to be undoped) is typically empty, whereas the valence band iscompletely filled with electrons.

By introducing donors or acceptors, the situation can be changed. Introducingdonors leads to an increase of the concentration of electrons in the conductionband. Electrons are free to move in the conduction band up on an electric field.Introducing acceptors leads to a decrease of the electrons concentration in thevalence band, the missing electrons in the valence band are the holes, which arenow free to move in the valence band.

Free electrons on the n-side and free holes on the p-side can initially diffuseacross the junction because of the presence of a concentration difference at theboundary. Holes will diffuse from p-side to n-side leaving uncompensated bound

negative acceptor ions behind. Thus, the region directly to the right of theboundary will be negatively charged. Similarly, a positively charged layer in then-side of the boundary will be built up from the donor ions.

When a free electron meets a free hole recombination occurs (Fig. 1.4), thismeans the hole and electron cancel each other. As a result, the free electronsnear the junction tend to cancel each other, producing a region depleted of anymoving charges. This creates what is called the depletion region (Fig. 1.5).

Fig. 1.2 Separate pieces Fig. 1.3 One crystal

VB

CB

p-typen-type

Band gap VB

CB

Band gap

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The charges on both sides of the depletion region cause an electric field to beestablished across the region; hence a potential difference results across thedepletion region, with n-side at positive voltage relative to p-side (Fig. 1.6).

Thus, the resulting electric field opposes the diffusion of holes into the n-regionand electrons into the p-region. In fact, the voltage drop across the depletion

region act as a barrier that has to be overcome for holes to diffuse into then-region and electrons to diffuse into the p-region, blocking any charge flow(current) across the barrier. The larger the barrier voltage the smaller the numberof carrier that will be able to overcome the barrier, and hence the lower themagnitude of diffusion current. We represent this barrier by bending theconduction and valence bands as they cross the depletion region (Fig. 1.7).

A free charge now requires some extra energy to overcome the barrier to be ableto cross the depletion region. A suitable positive voltage (forward bias) appliedbetween the two ends of the pn junction diode can supply free electrons andholes with the energy required. However, applying a negative voltage (reversebias) results in pulling the free charges away from the junction.

Forward/Reverse Bias Characteristics

If a negative voltage is applied to the pn junction, the diode is reverse biased. Inresponse, free holes and electrons are pulled towards the end of the crystal andaway from the junction. The result is that all available carriers are attracted awayfrom the junction, and the depletion region is extended. There is no current flowthrough under such conditions. We are here considering an ideal diode. In real

Depletion region

Fig. 1.4 Electrons-holes recombination Fig. 1.5 Depletion region

Fig. 1.6 Generated electric field Fig. 1.7 Bending of the energy bands

p-typen-type

E

Emax

+

+

+

+

+

Barrier

XY

VB

CB

Band gap

VB

CB

Band gap

Energy

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life, the diode cannot be perfect, and some current (reverse current) does flow.This is known as reverse bias applied to the semiconductor diode (Fig.1.8).

If the applied voltage is positive, the diode operates in forward bias. This has theeffect of shrinking the depletion region. As the applied voltage supplies enoughenergy to the free charge to overcome the barrier, carriers of both types can crossthe junction into the opposite ends of the crystal. Now, electrons in the p-type endare attracted to the positive applied voltage, while holes in the n-type end areattracted to the negative applied voltage. This is the condition of forward bias(Fig. 1.9).

Because of this behavior, an electrical current can flow through the junction in theforward direction, but not in the reverse direction. This is the basic nature of anordinary semiconductor diode.

Diode Characteristics

Figure 1.10 shows the diode I-V characteristics.

Where,

Vf Forward voltage If Forward current

Vr Reverse voltage Ir Reverse current

Vcut-in Cut-in voltage VB Breakdown Voltage

IS Saturation current

When forward-biased, a cut-in voltage Vcut-inhas to be overcome for the diode tostart conduction. In silicon, this voltage is about 0.7 volts. When reverse-biased,the current is limited to IS. For higher reverse voltages Vr, the junction breaks

down.

Depletion region

+_

Fig. 1.8 Reverse bias

Depletion region

+ _

Fig. 1.9 Forward bias

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Diode Equation

The diode equation gives a reasonably good approximated representation of thediode I-V characteristics.

Forward Bias Condition

In the forward bias condition current through a diode varies exponentially with theapplied voltage and the I-V relationship is closely approximated by

= 1expII TnV

V

S (1.1)

Where IS is the saturation current, which is constant for a given diode at a giventemperature. The voltage VT is called the thermal voltage, given by

q

KTVT = (1.2)

K= Boltzmanns constant = 1.38 x 10-23

joules/KelvinT = the absolute temperature in Kelvin

q = the magnitude of electronic charge = 1.602 x 10-19

As.At room temperature (300K), the value ofVTis taken to be 26mV.

In the diode equation, the constant n varies between 1 and 2, depending on thematerial, the temperature and the physical structure of the diode.

Note that this equation characterizes the basic features of the diode I-V curve,but leaves out some details like reverse breakdown (the equation says nothingabout the possibility of reverse bias breakdown), junction capacitanceetc.

For V VT in equation 1.1, the exponential relationship can be approximated by

Vcut-in

If

Vf

Ir

Vr

IsVB

Fig. 1.10 I-V Characteristics of a diode

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TnV

V

S expII (1.3)

Reverse Bias Condition

Using equation 1.1 we can predict that the diode current is approximated by

SII forV0 (1.4)

Where V is negative and a few times larger than VT(26 mV) in magnitude so theexponential term becomes negligibly small compared to unity.

Breakdown Region

The breakdown region is entered when the magnitude of the reverse voltageexceeds a threshold value specific to the particular diode and called thebreakdown voltage. As we can see from Fig. 1.10, in the breakdown region thereverse current increases rapidly. For a general-purpose diode, we should avoidreaching the breakdown region. If the power dissipated exceeds the diodespower rating, immediate destruction of the diode can result.

While for the general-purpose diode it is very important to operate below thisvoltage, special diodes are manufactured to operate in the breakdown region andare called Zener diodes. The Zener diodes can handle breakdown without failingcompletely as in the case of general-purpose diodes.

Zener Diode

The Zener diode is like a general-purpose diodes consisting of a silicon pnjunction. When forward-biased it behaves like general-purpose diodes. In case ofreverse-biased, if the reverse voltage is increased the saturation current remainsessentially constant until the breakdown voltage is reached where the currentincreases dramatically. This breakdown voltage is the Zener voltage for Zenerdiodes. When reverse voltages greater than the breakdown voltage are applied

the voltage drop across the junction (Zener diode) remains almost constant over awide range of currents.

From the I-V characteristics in Fig.1.11 after the breakdown voltage the I-V curveis almost a straight line providing almost constant voltage as its current changes.

The fact that in the breakdown region the voltage across the diode is almostconstant turns out to be an important application of diodes that is the voltageregulator. Basically, the function of the regulator is to provide constant outputvoltage to a load connected in parallel in spite of the ripples in the supply voltageand the variation in the load current.

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Diode Equivalent Circuit

A small signal equivalent model for forward biased diode is shown in Fig.1.12.

The resistor rd models the change in the diode voltage Vd that occurs when Idchanges.

Differentiating equation 1.1 we get

T

d

T

V

V

Sd

d

d V

I

V

exp

IdV

dI

r

1 Td

===(1.5)

The capacitor Cd is called the diffusion capacitance. This capacitive effect ispresent when the junction is forward biased. It is called diffusion capacitance toaccount for the time delay in moving charges across the junction by diffusionprocess. It varies directly with the magnitude of forward current.

The capacitorCj is called the junction capacitance. A reverse-biased pn junctioncan be compared to a charged capacitor. The p and n regions act as the plates of

Fig. 1.12 Equivalent model Fig. 1.13 Accurate model

Fig. 1.11 Zener diode

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the capacitor while the depleted region as the insulating dielectric. The value ofthe capacitance depends on the width of the space charge region. Thus, itdepends on reverse voltage. As the reverse voltage increases, the space chargeregion becomes wider, effectively increasing the plate separation and decreasingthe capacitance.

For more accurate modeling of the diode, it is necessary to add a seriesresistance due to the bulk (p-and n-type semiconductors material of which thediode is made of) and the metal contacts. In addition, a shunt resistance is addeddue to parasitic effects in the material. The shunt resistance of crystalline silicondiodes is typically very high. The Equivalent circuit of the accurate model isshown in Fig. 1.13.

Where,

V = voltage across the entire real diode in forward bias

I = current through the entire real diode in forward bias

Vd = voltage across the ideal diode (due to the drop across the pn-junction)

Id = current through the ideal diode

Diode Application

A diode can be used in several applications as follows:

A. Rectifier Circuit

A diode rectifies an ac voltage, so that it can be smoothed and converted into adc voltage. The basic half wave rectifier is shown in Fig. 1.14. The diodeeliminates the negative cycles of the input voltage. The capacitor acts as asmoothing filter so that the output is nearly a dc voltage. As filtering is not perfect,there will be a remaining voltage fluctuation known as ripple, on the outputvoltage.

In case of half wave rectifier, an approximate expression for the peak-to-peakripple voltage is

L

p

rfCR

VV = , where Vp is the peak value of the input sinusoidal voltage, RL is the

load resistance and f is the frequency of the input voltage.

The amount of ripple can be reduced by a factor of two by using the full waverectifier shown in Fig. 1.15. Here, four diodes are connected as a bridge, to invertthe negative cycles and make them positive. See reference [1] for moreinformation on how to derive the ripple voltage equation.

Fig. 1.14 Half wave rectifier Fig. 1.15 Full wave rectifier

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B. Voltage Regulator Circuit

A voltage regulator is designed to keep the output voltage of a circuit at aconstant value, independent of the input voltage and also independent of theload current. A Zener diode connected in parallel to the load is the simplest formof such a voltage regulator circuit as shown in Fig. 1.16.

If the voltage across the load tries to rise then the Zener takes more current.The increase in current through the resistor R causes an increase in voltage

dropped across the resistorRand causes the voltage across the load to remainat its correct value. Similarly, if the voltage across the load tries to fall, then theZener takes less current. The current through the resistor R and the voltageacross the resistor both fall. The voltage across the load remains at its correctvalue.

C. Clipper Circuit

These circuits clip off portions of signal voltages above or below certain limits, i.e.the circuits limit the range of the output signal. The level at which the signal isclipped can be adjusted by adding a dc bias voltage in series with the diode asshown in Fig. 1.17.

D. Clamper CircuitThere are circuits used to add a dc voltage level to a signal. A positive clampercircuit (Fig. 1.18) adds positive dc voltage level (the output waveform will beidentical to that of the input but the lowest peak clamped to zero) while negativeclamper circuit adds negative dc voltage level. A dc bias voltage can be added toraised or lowered the signal to a reference voltage. The clamper circuits can beused to restore dc levels in communication circuits that have passed differentfilters.

Fig. 1.17 Biased clipper

Fig. 1.16 Voltage regulator

Fig. 1.18 positive Clamper

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Practical Background

Diode Identification

Many diodes are identified as 1Nxxxx. The cathode terminal of a diode isidentified with a dark line on its package as indicated in Fig. 1.19.

Safety Precautions

The following is a list of some of the special safety precautions that should betaken into consideration when working with diodes:

1. Never remove or insert a diode into a circuit with voltage applied.

2. When testing a diode, ensure that the test voltage does not exceed thediode's maximum allowable voltage.

3. Ensure a replacement diode into a circuit is in the correct direction.

Meter Check of Diode

Using the multimeter as an ohmmeter, place the positive lead of the multimeter onthe anode of the diode and the negative lead of the multimeter on the cathode ofthe diode. Record the resistance value you measure, the meter should show avery low resistance (forward resistance). Be sure to have the multimeter on themost sensitive scale the meter will allow. Then reverse the leads. Record theresistance value you measure, the meter should show a very high resistance

(reverse resistance).Two high-value resistance measurements indicate that the diode is open or has ahigh forward resistance. Two low-value resistance measurements indicate thatthe diode is shorted or has a low reverse resistance. A normal set ofmeasurements will show a high resistance in the reverse direction and a lowresistance in the forward direction.

The function diode test of the multimeter tests the semiconductor junction bysending a current through the junction, then measuring the junctions voltagedrop. A good silicon junction drops between 0.5 V and 0.8 V.

References

1. Adel S. Sedra, Kennth C. Smith, Microelectronic Circuits, Saunders CollegePublishing, 3rd ed., ISBN:0-03-051648-X, 1991.

2. David J. Comer, Donald T. Comer, Fundamentals of Electronic Circuit Design,John Wiley & Sons Canada, Ltd.; ISBN: 0471410160,2002

_+

Fig. 1.19 Standard diode symbol

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Prelab

Problem 1

Using PSpice, implement the circuits in Fig. 1.20 using a 1N4002 diode and a1K resistor.

Perform a DC sweep analysis in linear variation to,

1. Display a plot for the diode current id vs. the diode voltage vd using a linearscale.

2. Display a plot for the diode current id vs. the diode voltage vd using a semi logscale ( log10 (id) vs. vd).

3. From the graph of log10 (id) vs. vd , show how can you extract the values of theconstant n and the saturation current Is in the diode equation (equ. 1.3). TakeVT=26mV at 25

oC and log10 (e) = 0.43.

Problem 2

Using PSpice, implement the circuits in Fig. 1.21 and Fig. 1.22 using 1N4002diodes, R1 a 3K resistor, RL a 40K resistor and a 1F capacitor. The input is

sinusoidal source of frequency 100Hz and 10V peak.

Fig. 1.21 Half wave rectifier

Fig. 1.20 Simple diode circuit

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Perform a transient analysis for about 5 cycles of the sinusoidal input and displaythe voltage across the load resistor for both circuits under the following cases:

1. Simulate the output signal of the half wave and the full wave rectifier with out

R1 and C1. The load resistance RL is connected to the rectifier.2. Add now the capacitor C1 to the circuit and simulate again the output voltage.

3. Now add the resistor R1 to the circuit and simulate again the output voltage.

Problem 3

Using PSpice, implement the Zener regulator circuit in Fig. 1.23 using a D1N750Zener diode and R=RL=500. The input voltage is the dc source (20V) and asinusoidal signal using the sinusoidal source (5V peak, 60Hz) is superimposed onthe dc value and considered to be the ripple voltage Vr.

1. Perform a transient analysis for about 5 cycles of the sinusoidal input and plotthe input voltage (dc voltage + ac voltage) and the output voltage across theload resistor RL.

2. What is the change in the regulated voltage for the change in the inputvoltage?

Fig. 1.23 Voltage regulator

Fig. 1.22 Full wave rectifier

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