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Analog Electronics

Module 1: Semiconductor Diodes

PREPARED BY

Academic Services Unit

August 2011

© Applied Technology High Schools, 2011

ATE1210 – Analog Electronics

Module 1: Semiconductor Diode 2

Module 1: Semiconductor Diodes

Module Objectives Upon successful completion of this module, students should be able to:

1. Identify the purpose of the diode and give a brief description of its construction.

2. Sketch the schematic symbol of a diode. 3. Identify the diode by its number code or package type. 4. Distinguish between the terminals of a diode. 5. Recognize whether a diode is forward biased or reverse biased by

observing the voltage polarity. 6. Explain how the diode functions in the forward or reverse bias direction. 7. Test a diode and check its condition. 8. Explain the operation of a half-wave rectifier. 9. Explaing the operation of a full-wave bridge rectifier. 10. Build half-wave and full-wave rectifier circuits and test their functioning.

Module Contents:

Topic Page No.

1.1 Introduction 3

1.2 Diode identification 5

1.3 Diode Operation 6

1.4 Testing a Diode 7

1.5 Diode Application 9

1.6 Lab Activity 1 13



1.9 Review Exercise 23



1.1 INTRODUCTION

The invention of semiconductor devices was a revolution, which lead to an

impressive technological achievement that would endlessly alter modern

society. Semiconductor devices, made electronics physically very small,

including computers, certain types of medical diagnostic and treatment

equipment, and popular telecommunication devices. A diode is a

semiconductor device that allows current to flow in only one direction.

A semiconductor in its pure form does not conduct current well because it

has limited number of free electrons and holes. Doping is the process of

increasing the conductivity of the semiconductor where in small amounts of

impurities are added to a pure semiconductor. An N-type semiconductor has

more number of electrons (negative charges) and a P-type semiconductor

has more number of holes (positive charges). Sandwiching, or joining a P-

type semiconductor with N-type semiconductor forms a diode. The diode is

contained in a small capsule made of glass or plastic and has two terminals

or electrodes and hence the name diode (di two and ode electrode). The

electrode connected to the P-type is called the Anode “A”, and the electrode

connected to the N-type is called the Cathode “K”. The cathode (K) is

marked with a silver color band as shown in Figure 1(c). Figure 1(a) and

1(b) shows the structure and symbol of the PN junction diode.

K

b) Diode Symbol

A

Anode

(A) Cathode

(K) N-TYPE

Electrons

+

a) Diode Structure

P-TYPE

Holes

c) Typical Diode

Cathode (K)

Figure 1: Diode structure and Symbol



Diode Types

According to the type of semiconductors they are classified into:

Germanium diodes

Silicon diodes

According to their application, they are classified into the following:

Rectifier diodes. Zener diodes.

Light emitting diodes (LEDs). Photo diodes.

Figure 2 shows commonly used diodes of different types.

Photo DiodeZener Diode

Low Power Diode

Rectifier diode

LED

Figure 2 Typical Diodes of different types

Diode Packing

Diode is generally mounted in one of three basic packages shown in figure

3. These are designed to protect the diode from mechanical stress and the

environment. The size of the package indicates the current rating (larger

size means higher current rating). Note that (DO) refers to Diode Outline.

DO-5

Figure 3 Diode packing types

DO-8 DO-41



1.2 Diode Identification There are hundreds of diodes, of different types and characteristics

according to the way they are designed and made. In order to distinguish

between them, each type is given a unique code so that it can be identified.

In fact there are two main semiconductor numbering systems in use:

1) Pro-Electron System: This numbering system originated in Europe

and is widely used for semiconductors developed and manufactured by

them.

2) JEDEC System: This numbering system originated in the USA and is

widely used for semiconductors manufactured in North America.

1.1 Pro-Electron Numbering or Coding System

First Letter Specifies Semiconductor Material

Second Letter Specifies type of Diode

A Germanium A Low power or signal

B Silicon P Light detector

Q Light Emitting Diode (LED)

Y Rectifier

EXAMPLE: BZY74-C6V3

B = Si, Z = Zener diode, Y74 = Commercial or

Industrial use, C = 5% of rating voltage.

6V3 = 6.3 Volt (Voltage rating) Z Zener (voltage reference)

1.2 JEDEC Numbering or Coding System

First Number Second Letter Subsequent numbers

1 Diode N Semiconductor Serial number of device

2 Bipolar Transistor

3 FET

4&5 Photo Coupler

Example: This code “1N4001” means:

1 Diode, N Semiconductor.

4001 Serial number.



1.3 Diode Operation

Forward biased:

A diode conducts only in one direction, and the conduction is from anode to

cathode. When a diode is connected to the power supply such its anode (A)

is connected to the positive terminal, and its cathode (K) is connected to the

negative terminal as shown in figure 4, we say that it is forward biased

(FB). The diode in FB-direction allows the current to flow from A to K as

shown in Figure 4.

Figure 4 Diode Operated in FB Direction

DC+

RThe Diode will conduct IF IF

K

A

Reverse biased:

A diode is reverse biased (RB) when its anode (A) is negative with respect

to the cathode (K). The diode in RB-direction does not allow the current to

flow as shown in Figure 5.

Figure 5 Diode Operated in RB Direction

DC+

RThe Diode will not conduct

K

A



1.4 Testing a Diode

Testing the diode in forward direction with an Ohmmeter:

Connect the positive meter-lead (Red) to the anode (A) and connect the

negative meter-lead (Black) to the cathode (K) as shown in Figure 6a. A

good diode must display low resistance (typically < 10) in FB. Note that

the ohmmeter consists of an internal battery (1.5 V), which can FB or RB a

diode.

Figure 6a Testing the Diode in FB Direction

IN 4001

(A) Anode

(K) Cathode

Ohm

met

er

Testing the diode in reverse direction with an Ohmmeter:

Connect the positive meter-lead (Red) to the cathode (K), and the negative

meter-lead (Black) to the anode (A) as shown in Figure 6b. A good diode

must display a very high resistance (>1000 M) in RB. If both bias

conditions display low resistance, then the diode is shorted, and if both bias

conditions display very high resistance, then the diode is considered to be

open.

Figure 6b Testing the Diode in RB Direction

IN 4001

(A) Anode

(K) Cathode

Ohm

met

er



Testing the diode in forward direction with a Digital Multimeter:

Set the digital Multimeter knob to ‘diode testing’ function. Now, connect the

positive lead (Red) to the anode (A) and the negative lead (Black) to the

cathode (K) as shown in Figure 7a. For a diode in a good condition the

reading will be in the range of 0.3V for Ge and 0.7V for Si.

Dig

ital M

ultim

eter

Figure 7a Testing the Diode in FB Direction

IN 4001

(A) Anode

(K) Cathode

Testing the diode in reverse direction with a Digital Multimeter:

Connect the positive lead to the cathode and negative lead to the anode as

shown in Figure 7b. For a diode in good condition, the reading will be 1.5V

for both types. If in both cases the reading is 0V, then the PN junction is

shorted, and if it is 1.5V the PN junction is open.

IN 4001

(A) Anode

(K) Cathode

Dig

ital

Mul

tim

eter

Figure 7b Testing the Diode in RB Direction



1.5 Diode Application

The diode has a unique ability to offer very little resistance to current flow in

the forward-bias direction, but maximum resistance to current flow when

reverse biased. For this reason, the diodes are used in rectification.

Rectification is the process of converting ac to pulsating dc, and the diodes

used for this purpose are called rectifier diodes.

AC current behavior:

During the positive alteration, AC current flows first in one direction,

reaches the maximum (positive) and decreases to zero.

During the negative alteration, the current follows the same manner in

the opposite direction.

In each alteration, AC current reverses its direction as shown in Figure 8.

-ve

I

+ve

-ve

+ve

I

Positive half-cycle

Negative half-cycle

Figure 8 Alteration of AC current between (+ve) and (-ve)

Half Wave Rectifier

The half-wave rectifier circuit is constructed simply by connecting a diode

between the secondary of a transformer and the load as shown in Figure 9.

Figure 9 Half Wave Rectifier Circuit

IF A

Load AC

K



During the positive half-cycle, the diode conducts when the input voltage

exceeds the barrier potential (0.7V for Silicon and 0.3V for Germanium

diodes) and current flows through the load and returns along the 0V line.

Only the positive half-cycle appears across the load as shown in Figure 10a.

IF A K + +

+ AC Input

Load

Output

Figure 10a Operation during Positive Half-Cycle During the negative half-cycle, the diode turns OFF (RB). This will prevent

any current from flowing, and no voltage appears across the load as shown

in Figure 10b.

IR A K

+ +

+ AC Input

Load

Output

Figure 10b Operation during Negative Half-Cycle

Output Voltage: The net result is that only the positive half-cycles of the AC

input voltage appear across the load, providing a pulsating DC voltage at

the output as shown in Figure 11. However, the amplitude of the output

voltage is less than the input amplitude, and this is because of the forward

voltage drop across the diode. Because this circuit produces output current

only during one cycle, it is called a half-wave rectifier.

Secondary Voltage (VS)

Voltage across the Load

Figure 11 Output of Half-Wave Rectifier Voltage

VRL

VS

Conduct lab activity 1.



Full Wave Rectifier

The circuit in figure 12 rectifies AC by using a bridge of four diodes. It is

therefore called a full-wave bridge rectifier.

Figure 12: Bridge Rectifier

During the positive half cycle, diodes D1 and D2 are forward biased, so they

conduct. Diodes D3 and D4 are reverse biased and do not conduct. Current

flows through the load as shown in figure 13.

Figure 13: Conduction during positive half cycle



During the negative half cycle as shown in figure 14, diodes D1 and D2 are

reverse biased, so they do not conduct. Diodes D3 and D4 are forward

biased and conduct.

Figure 14: Conduction during negative half cycle

The result is that the current continues to flow through the load in the same

direction in both the half cycles. The output and input are shown in figure

15. The rectifier produces output during both half cycles, and therefore it is

100% efficient. In each half-cycle, current flows through two diodes, and

therefore the output voltage is two voltage drops less than the input

voltage.

Figure 15: Input and Output Waveforms



1.6 Lab Activity 1

Objective

To determine the diode polarity and understand the need for proper

connection.

Background Information

A semiconductor diode (or diode) is made from a piece of P-type

semiconductor and a piece of N-type semiconductor joined together as

shown in figure 16.

Figure 16: Diode construction

The amount of current that flows through a diode when it conducts depends

on the size and polarity of the applied voltage. Figure 17(a) shows the diode

symbol, and figure 17(b) shows the types of diodes supplied in the feedback

kit.

Figure 17(a): Diode symbol

Figure 17(b): Diode types



Equipment Required

Apparatus Quantity

Electricity & Electronics Constructor, EEC470

1

Basic Electricity and Electronics Kit EEC471-2

1

Power supply unit 0 to 20 V (Feedback Power Supply 92-445)

1

Digital Multimeters 2

Diode (1N4007), R = 4.7k 1-each

Procedure

1. As shown in the patching diagram of figure 18, construct the circuit of

figure 19(a). (Note that the resistor limits the current to a safe value).

Figure 18: Constructor-EEC470

Figure 19(a): Diode’s Circuit diagram



2. Move knob of the power supply control to 0V position.

3. Switch on the power supply.

4. Set the power supply control to give 10 V on the meter.

5. Record the current measurement in the first row of the result table figure

20.

6. Now, switch off the power supply.

7. Reverse the 1N4007 diode to give the circuit of figure 19(b).

Figure 19(b): Diode’s Circuit diagram

1. Switch on the power supply and readjust the voltage to 10 V.

2. Read the new value of diode current and record it in the second row of

the result table figure 20.

3. Study your results and answer the questions on the next page.

Observations:

Write your observation in table below.

Circuit Current (mA)

Figure 19(a) IF = ……………….

Figure 19(b) IR = ……………….

Figure 20: Observation Table



Questions 1) Which side of a diode should be connected to the positive voltage

supply to make it conduct current?

…………………………………………………………………………………………………………

…………………………………………………………………………………………………………

2) When the diode was connected the opposite way round was the

current?

a) slightly smaller

b) much smaller

c) too small to measure

3) Which side of a diode should be connected to the positive voltage

supply to make it stop current?

…………………………………………………………………………………………………………

…………………………………………………………………………………………………………

Conclusion:

The diode conduct current when the Anode is connected to the

……………………… and the Cathode is connected to the ………………………

The diode does not conduct current when the Anode is connected to

the ………………… and the Cathode is connected to the ………………………



1.7 Lab Activity 2

Objectives:

1. To learn to recognize a half-wave rectified sinusoidal voltage.

2. To understand the term ‘mean value’ as applied to a rectified

waveform.

3. To understand the effect of a reservoir capacitor upon the rectified

waveform and its mean value.

Equipment Required

Apparatus Quantity

Electricity & Electronics Constructor, EEC470.

1

Basic Electricity and Electronics Kit EEC471-2.

1


1

Power supply unit. ac supply; 12 V rms; 50 or 60 Hz.

1

Digital Multimeters. 2

Oscilloscope. 1

Diode (SY356/6), R = 10k, and Capacitor. 1-each

Procedure

1. Switch on the oscilloscope and the sinusoidal supply.

2. With the oscilloscope dc coupled adjust the time-base and the Y

amplifier sensitivity to obtain a steady trace of about 4 cm vertical and

5 ms/cm horizontal. You should see a waveform as in figure 21.

Figure 21: Half-wave Rectified Waveform



3. As shown in the patching diagram of figure 22, construct the circuit of

figure 23.


Figure 23: Half-wave Rectification

4. Measure and record the time T and the ge Vpk

5. Sketch the waveform and label it to show the periods when the diode

is conducting and those when it is not.

6. Time T depends upon the frequency of your power supply. For a 50Hz

supply it should be 20 ms and for 60 Hz it should be 17ms.

7. Confirm this: Vpk should be very nearly equal to the peak voltage of

the alternating supply.



Questions

1) Why will Vpk not be exactly equal to this voltage?

…………………………………………………………………………………………………………

…………………………………………………………………………………………………………

2) How much will it differ?

Hint: The mean value of a half-sinusoid can be shown by geometry to

be:

…………………………………………………………………………………………………………

…………………………………………………………………………………………………………

…………………………………………………………………………………………………………

3) Note the mean voltage indicated by the voltmeter, and compare it with the

calculated value. ……………………………………………………………..………………

4) The mean voltage you obtain is positive relative to zero. How could

you obtain a negative voltage? (Confirm your answer by experiment).

…………………………………………………………………………………………………………

Conclusion:

A simple diode circuit can convert a/an ………………………… voltage to

a/an ……………………………voltage.

The mean value of the rectified voltage can be increased by using a

……………………………………………………… across the load.

A half-wave rectified voltage gives appreciable ripple which however,

can be reduced by ……………………………………………………….



1.8 Lab Activity 3

Objective: To understand the working of a diode bridge circuit as a full-wave rectifier

and its advantage over half-wave rectifier.

Equipment Required

Apparatus Quantity

Electricity & Electronics Constructor, EEC470.

1

Basic Electricity and Electronics Kit EEC471-2.

1


1

Power supply unit. ac supply; 12 V rms; 50 or 60 Hz.

1

Digital Multimeters. 2

Oscilloscope. 1

Bridge-Rectifier, R = 10k, and Capacitor. 1-each

Procedure: Select the Bridge Rectifier from the component kit. It appears

as in figure 24(a) and figure 24(b). Note how the rectifier terminals are

labeled.

(a) Bridge Module

(b) Circuit

Figure 24: Bridge Rectifier

1. Note: Prior to connecting an ac power supply to the board, ensure the

supply is isolated from ground.



2. With the oscilloscope dc coupled adjust the time-base and the Y

amplifier sensitivity to obtain a steady trace of about 4 cm vertical and

5 ms/cm horizontal. You should see a waveform as in figure 25. Time

'T' will be 10 ms for 50 Hz supply, and 8.5 ms for 60 Hz.

Figure 25: Full-wave Rectified Waveform

3. Construct the circuit of figure 26 as in the patching diagram of figure 25


Figure 26: Test Circuit (full-wave rectifier)



4. Measure and record the time T and the peak voltage Vpk

5. Sketch the waveform and label it to show the periods of each 2-diode.

6. Note the value of Vpk.

7. Note the mean value of output voltage indicated on the voltmeter.

8. Compare these values with those obtained in Lab activity-2.

9. Confirm this: Vpk should be very nearly equal to the peak voltage of

the alternating supply.

Conclusion:

A bridge ……………………… rectifier gives a greater mean value and fewer

ripples for a given load and reservoir capacitor than a ………………………

rectifier.

The alternative full-wave circuit using a centre-tapped transformer and two

diodes is less efficient than the bridge circuit because it requires a bigger

transformer for a given output power.



1.9 Review Exercise

1. The cathode of a diode is made 4V positive of its anode.

a) What is the name of this type of bias?

____________________________________________________________________

b) Does current flow through the diode in this case?

____________________________________________________________________

2. Draw a circuit of a half-rectifier, and draw a sketch of 3 cycles of it output waveform.

Circuit Diagram:

Waveforms:



3. Draw the output voltage waveform of the rectifier for the indicated input voltage, as shown in the figure below. The 1N4003 is a specific rectifier diode with a barrier potential/forward voltage drop of 0.7V.

4. Draw the bridge rectifier circuit, and sketch 3 cycles of its input and output waveforms.

Circuit Diagram:

Waveforms:



Notes

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