31

Semiconductor Diodes

Structure

9.1 Introduction Objectives

9.2 Basics of Semiconductors Revisited

9.3 A p-n Junction Operation of a p-n Junction A Forward and Reverse Biased p-n Junction Identifying a Diode I-V Characteristics of a p-n Junction

9.4 Zener Diode Working of a Zener Diode

I-V Characteristics of a Zener Diode

9.5 Some Applications of Semiconductor Diodes Rectification of ac Zener Diode as Voltage Regulator

EXPERIMENT EXPERIMENTS USING SEMICONDUCTOR DIODES

9

32

Experiments with Electrical and Electronic Circuits

From your 10+2 physics course, you will recall that materials can be broadly

classified into conductors, insulators and semiconductors on the basis of their

resistivity. The resistivity of a conductor is of the order of 10−7 Ωm and that of

an insulator is of the order of 1012 −1024 Ωm. The resistivity of a

semiconductor lies in-between the resistivities of a conductor and an insulator.

Germanium (Ge) and Silicon (Si) are the most commonly used

semiconductors. At absolute zero, the semiconductor also acts as a near

perfect insulator. But with increase in temperature, the conductivity of the

semiconductor increases. This change in conductivity with temperature is

different for different semiconducting materials. The conductivity of a

semiconductor can also be influenced by doping it with some impurity

elements, called dopants like boron, phosphorus, arsenic etc. Depending on

the type of carrier added by a dopant, the semiconductor is classified as

p-type (hole carriers) or n-type (electron carriers). The p-type impurity is

acceptor type, whereas the n-type impurity is donor type.

A p-n junction is usually formed by doping a part of a pure semiconductor with

acceptor impurities and the remainder with donor impurities. Semiconductors

have very useful properties (small size, light weight and efficient operation)

and are being extensively used in electronic equipments. (These have

completely replaced vacuum tubes used earlier in electronic circuits.) You

may recall that microelectronic chips are the cores of computers and these are

also made using semiconductor junctions. Now this limit has been extended to

nano-electronic devices.

A p-n junction is also called a diode. There are various types of diodes. In

this experiment, you will draw the I-V characteristic curves of a p-n junction

and a zener diode. You will discover that the function of a device can be

influenced and determined by external conditions. While a p-n junction works

as rectifying diode, a zener diode acts as voltage regulator, depending on

biasing conditions.

Objectives

After performing this experiment, you should be able to:

• draw current-voltage (I-V) characteristic curves of a p-n junction and a

zener diode in forward and reverse bias conditions;

• determine the material of a diode from its I-V characteristic curves;

• devise a zener voltage regulator circuit and determine the range of

constancy; and

• measure the effects of variation in input voltage and load on the output of

a zener diode regulator.

When the anode is connected to +ve terminal of battery and the cathode

is connected to −ve terminal of the battery, the device is said to be forward-biased and vice versa. A p-n junction is said to be forward biased when p-type region is connected to +ve terminal of the battery and n-type region is connected

to −ve terminal of the battery.

99..11 IINNTTRROODDUUCCTTIIOONN

33

Semiconductor Diodes

You have learnt about semiconductors in your school physics. You have also

read about p-type and n-type semiconductors. However, for brevity, we

recapitulate the important characteristics of semiconductors.

Semiconductors are of two types: intrinsic and extrinsic. A pure

semiconductor is said to be an intrinsic semiconductor. But in practical

applications, intrinsic semiconductors are of little use due to their high

resistivity or low conductivity. In an electronic circuit, it is both necessary and

desirable to tailor their conductivity by doping an impurity. Doped

semiconductors are termed as extrinsic semiconductors. The most commonly

used semiconducting materials are crystalline silicon and germanium. In

recent years, compound semiconductors, amorphous semiconductors, and

semiconducting polymers have also been developed. In this experiment, we

will confine ourselves only to devices made of elemental semiconductors.

From the electronic configuration of Si ),(22622 3p3s2p2s1s you will recall

that in all 14 electrons are bound to the nucleus and revolve around it. Of

these, four electrons revolve in the outermost orbit. In an intrinsic silicon

semiconductor, the Si atom attains stability by sharing one outermost electron

each with four neighbouring Si atoms. (This is called covalent bonding.) The

same holds true for germanium whose electronic configuration is 1s2 2s2 2p6

3s2 3p6 3d10 4s2 4p2. When silicon (or germanium) is doped with a

pentavalent (five electrons in the outermost orbit) atom like phosphorus,

arsenic or antimony, four electrons form covalent bonds with the four

neighbouring silicon atoms, but the fifth (valence) electron remains unbonded

and is available for conduction, as shown in Fig. 9.1. Thus, when a silicon (or

germanium) crystal is doped with a pentavalent element, it develops excess

free electrons and is said to be an n-type semiconductor. Such impurities are

known as donor impurities.

Fig. 9.1: Covalent bonding in an n- type semiconductor

Doping is a process of adding small quantities of other elements, called impurity, in a pure semiconductor in order to modify its electrical conductivity.

99..22 BBAASSIICCSS OOFF SSEEMMIICCOONNDDUUCCTTOORRSS

RREEVVIISSIITTEEDD

34

Experiments with Electrical and Electronic Circuits

If silicon (or germanium) is doped with a trivalent (three electrons in the

outermost shell) atom like boron, aluminium, gallium or indium, three valence

electrons form covalent bonds with three silicon atoms and deficiency of one

electron is created. This deficiency (of an electron) is referred to as a hole. It

is shown in Fig. 9.2. Such a semiconductor is said to be a p-type

semiconductor and the impurities are known as acceptor impurities.

Fig. 9.2: Covalent bonding in a p-type semiconductor

Let us now discuss the formation of a p-n junction.

The most useful form of semiconductor devices is obtained when p- and

n- type semiconductors form a junction. This is achieved by introducing donor

impurities into one side and acceptor impurities into the other side of a single

semiconducting crystal, as shown in Fig. 9.3. Let us now understand how

charge carriers behave in such a situation.

Fig. 9.3: A p-n junction with depletion region

9.3.1 Operation of a p-n Junction

You now know that there is greater concentration of electrons in the n-region

of the crystal and of holes in the p-region. Because of this, electrons tend to

diffuse to the p-region and holes to the n-region. You may think that this

process will continue indefinitely. But it is not so. The movement of electrons

and holes creates (leaves behind) positively and negatively charged ions near

the junction in n- and p-regions, respectively. Due to accumulation of charges

99..33 AA pp--nn JJUUNNCCTTIIOONN

35

Semiconductor Diodes near the junction, an electric field is established. This gives rise to electrostatic

potential, known as barrier potential. This barrier has polarities, as shown in

Fig. 9.4. When there is no external electric field, this barrier prevents the

movement of charge carriers across the junction and a narrow region near the

junction is depleted in mobile charge carriers. It is about 0.5 µm thick and is

called the depletion region or space-charge region.

Fig. 9.4: Barrier potential due to depletion region

The barrier potential is characteristic of the semiconductor material. It is about

0.3 eV for Ge and about 0.7 eV for Si. The junction acts as a diode. It is

symbolically represented as shown in Fig. 9.5. Here A corresponds to p-region

and acts as an anode in a diode. Similarly, K indicates n-region and

corresponds to a cathode in a diode.

9.3.2 A Forward and Reverse Biased p-n Junction

When an external electric field is applied to a p-n junction, as shown in

Fig. 9.6a, the p-end becomes positively biased and the n-end becomes

negatively biased. The junction is then said to be forward biased. When the

bias exceeds barrier potential, holes cross the junction from the p-region to

the n-region. Similarly, electrons cross the junction in the reverse direction.

This sets in a forward current in the diode. The current increases with

voltage and is of the order of a few milliampere. Under the forward bias

condition, the junction offers low resistance to flow of current. The value of

junction resistance, called forward resistance, is in the range 10 Ω to 30 Ω.

Fig. 9.6: a) Forward biased; and b) reverse biased p-n junction

When the terminals of the battery are reversed, i.e. p- and n-ends are

connected to negative and positive terminals of the battery respectively as

Fig. 9.5: Symbol of a p-n

junction (diode)

36

Experiments with Electrical and Electronic Circuits

shown in Fig. 9.6b, the junction is said to be reverse biased. In this case,

holes in the p-region and electrons in the n-region move away from the

junction. Does it mean that no current shall flow in the circuit? No, a small

current flows because a few electron-hole pairs are generated due to thermal

excitations. This small current caused by the minority carriers is called

reverse saturation current or leakage current. In most commercially

available diodes, the reverse current is almost constant and independent of

the applied reverse bias. Its magnitude is of the order of a few nanoamperes

to microamperes.

A p-n junction offers low resistance when forward biased, and high resistance

when reverse biased. You can easily test it using a multimeter. This property

of p-n junction is used for ac rectification.

9.3.3 Identifying a Diode

Semiconductor diodes are designated by two letters followed by a serial

number. The first letter indicates the material: A is used for material with a

band gap of 0.6 eV to 1.0 eV such as germanium. B is used for material with a

band gap of 1.0 eV to 1.3 eV, such as silicon. The second letter indicates the

main application: A signifies detection diode, B denotes a variable capacitance

diode, E for tunnel diode, Y for rectifying diode and Z denotes zener diode.

The serial numbers specify the diodes with particular values of power rating,

peak reverse voltage, maximum current rating etc. For example, BY127 and

BZ148 respectively denote a silicon rectifier diode and a silicon zener diode.

You have to refer to manufacturer’s catalogue to know exact details.

To make visual identification of anode and cathode, the diode manufacturers

employ one of the following ways (typically shown in Fig. 9.7):

• the symbol is painted on the body of the diode;

• red and blue marks are used on the body of the diode. Red mark denotes

anode, whereas blue indicates the cathode;

• a small ring is printed at one end of the body of the diode that corresponds

to the cathode.

Always work within the specified range of diode ratings to avoid damages to

the device.

You are now ready to perform the first part of the experiment, i.e., to draw the

static characteristic curves of a p-n junction. You will need the following

apparatus.

Fig.9.7: Identification of a diode (Printed with permission from M/s Power Technology, New Zealand)

To test a p-n junction using a multimeter, set the multimeter on resistance measurement mode. Connect the junction in forward bias with the multimeter probes and measure its resistance. Next, reverse the multimeter probes to measure the resistance of the junction in the other direction. You will observe a large difference between these values.

Apparatus

A general purpose p-n junction diode, a variable power supply with voltage

range 0-10V, a voltmeter, a milliammeter (0-50mA), a resistance box, a

microammeter (0-50µA), and a multimeter.

37

Semiconductor Diodes 9.3.4 I-V Characteristics of a p-n Junction

First check that the junction is working properly using a multimeter. Next make

a circuit as shown in Fig. 9.8 for forward bias I-V characteristics. Vs is a

variable power supply. Keep the voltage control in the minimum position and

switch on the power supply. Increase the voltage in steps of 0.1V and note the

corresponding values of current, until an appreciable deflection is observed.

You will note that current in the circuit is small as long as the applied voltage

is less than the barrier potential. Once this potential is crossed, the current will

increase rapidly with small increase in voltage. The forward voltage required

to get the junction in conduction mode is called knee voltage. Beyond knee

voltage, current increases rapidly. Record your readings in Observation

Table 9.1. Note that in no case, you should exceed the maximum forward

current rating of the diode in the forward bias condition.

Fig. 9.8: Circuit diagram for I-V characteristics of a p-n junction in forward bias

Next decrease the voltage in same steps and note down the corresponding

current values. Record these also in Observation Table 9.1. Are the values of

current same in both cases? Calculate the mean value of current for each

value of V.

Observation Table 9.1: Forward biased junction characteristics

Forward current (mA) S.No. Forward voltage (V)

Increasing voltage

Decreasing voltage

Mean forward current

(mA)

1.

2.

3.

4.

.

.

0.0

0.1

0.2

0.3

.

.

To study the reverse bias characteristics, the circuit is made as shown in

Fig. 9.9. In this case, the connections of the p-n junction have been reversed.

Also, the milliammeter is replaced by a microammeter since the reverse

current is expected to be small.

38

Experiments with Electrical and Electronic Circuits

Fig. 9.9: Circuit diagram for I-V characteristics of a p-n junction in reverse bias

Vary the voltage (from minimum) in steps of one volt and record the

corresponding current values in Observation Table 9.2. Here, you should not

exceed the peak inverse voltage rating of the junction.

Observation Table 9.2: Reverse biased junction characteristics

Reverse current (µµµµA) S.No.

Reverse voltage (V)

Increasing voltage

Decreasing voltage

Mean reverse current

(µµµµA)

1.

2.

3.

.

.

.

0.0

1.0

2.0

.

.

.

Draw the characteristic curves for both forward and reverse biased conditions

by plotting voltage along x-axis and current along y-axis, as shown in

Fig. 9.10. From this graph, you can calculate the forward and reverse

resistances as well as knee voltage.

Extrapolate the linear part of the forward bias characteristic curve to meet the

x-axis. The intercept on the x-axis gives the value of knee-voltage.

Fig. 9.10: I-V characteristics of a p-n junction diode

39

Semiconductor Diodes Calculations: From your plot of I-V characteristics, you can easily calculate

forward resistance and reverse resistance using the following relations:

f

ff

VR

I∆

∆=

and

r

rr

VR

I∆

∆= .

Result:

Forward resistance =......................Ω.

Reverse resistance =......................Ω.

Knee-voltage =......................V.

You may now like to answer the following SAQ.

Let us now learn about a special kind of diode, called zener diode.

Zener diode allows current to flow not only in the forward direction like a

rectifying diode, but also in the reverse direction, when the voltage is more

than the breakdown voltage. This voltage is also called zener voltage.

9.4.1 Working of a Zener Diode

The p- and n- regions in a zener diode are heavily doped. These result in a

thin depletion layer, due to availability of a large number of carriers for

recombination near the junction. However, the minority carriers present in the

diode as a result of thermal excitations cannot cross the junction due to its

barrier potential. When a reverse bias is applied, a large electric field is

established across the junction. This field (i) accelerates the already available

minority carriers, which, in turn, collide with the atoms of the semiconductor

material and eject more electrons through energy transfer (avalanche effect),

and (ii) breaks covalent bonds resulting in creation of additional electron-hole

pairs in the junction region (zener effect). Both these processes give rise to

large reverse current even for a small increase in reverse bias voltage. This

process is termed as zener breakdown. However, since the (magnitude of)

A conventional solid state diode does not allow flow of significant current if reverse bias is below its reverse break down voltage. Once voltage across p-n junction exceeds reverse bias breakdown voltage, it is subject to high current flow due to Avalanche breakdown and can be permanently damaged.

Spend 4 min.

SSSSSSSSAAAAAAAAQQQQQQQQ 11111111 :::::::: DDDDDDDDiiiiiiiiooooooooddddddddeeeeeeee cccccccchhhhhhhhaaaaaaaarrrrrrrraaaaaaaacccccccctttttttteeeeeeeerrrrrrrriiiiiiiissssssssttttttttiiiiiiiiccccccccssssssss

a) You are given a resistor and a p-n junction. How would you identify these?

b) How will you determine whether a p-n junction is made of silicon or germanium? What was the material of the junction you characterised in this experiment?

Fig. 9.11: Symbol of zener diode

p

n

99..44 ZZEENNEERR DDIIOODDEE

40

Experiments with Electrical and Electronic Circuits

reverse voltage is small, the junction is not damaged. In silicon diodes, zener

effect dominates up to about 5.6V, and beyond this, avalanche effect prevails.

The symbol of zener diode is shown in Fig. 9.11.

A typical I-V characteristic plot of a zener diode is shown in Fig. 9.12. The

reverse breakdown voltage is indicated by Vz.

Fig. 9.12: I-V characteristics of a zener diode

The zener breakdown voltage (Vz) is of great significance in the operation of

zener diode as a voltage regulator. You will learn it in the later part of this

experiment.

9.4.2 I-V Characteristics of a Zener Diode

You now know that zener diode can sustain a constant voltage across it in

reverse breakdown condition. For this reason, it is always used as voltage

reference in reverse bias. Since resistance in breakdown region is very small,

the current through the diode has to be limited by varying the resistance in the

circuit. The value of resistor is chosen in such a way that the product of zener

breakdown voltage and reverse current through the zener, i.e. the power

dissipated across the junction, is within the power handling capability of the

diode. If this limit is exceeded, a large current may damage the diode.

We now list the apparatus with which you will work, in this part of the

experiment.

The circuit to study forward I-V characteristics of zener diode is shown in

Fig.9.13a. In this circuit, the value of resistor R is determined by the power

rating of the zener diode. The maximum current flowing through R should be

less than the diode current rating IZ.

Apparatus

Zener diode (with breakdown voltage in the range of 3 to 10V), variable

voltage supply, voltmeter, milliameter and a resistor.

41

Semiconductor Diodes

Fig. 9.13: Circuit diagram to determine I-V characteristics of zener diode in a) forward bias; and b) reverse bias

Take a variable dc voltage supply Vs in the range 0-15V. If zener breakdown

voltage (Vz) is 10V and maximum current rating (Iz), is 100 mA, the value of R

is given by

( )

.Ω50mA100

V1015=

−=

−=

z

zmax VVR

I

First connect zener diode in forward bias (anode to positive end and cathode

to negative end). Take observations using the procedure outlined for p-n

junction diode rectifier and record the readings in Observation Table 9.3.

Observation Table 9.3: Forward bias characteristics of zener diode

Forward current (mA)

S. No.

Forward voltage (V) With increasing voltage

With decreasing voltage

Mean forward current

(mA)

1.

2.

3.

.

0.0

0.1

0.2

.

Now reverse the zener diode bias by connecting the cathode to the positive-

end and the anode to the negative-end of supply. This configuration is shown

in Fig. 9.13b. Note that here also, you have to use a milliammeter. Start the

power supply from zero volt and increase voltage in steps of 1V. Note down

the voltage across the zener diode and the corresponding current flowing

through the circuit. Record your readings in Observation Table 9.4. Plot

forward and reverse bias I-V characteristic curves of zener diode. Do your

curves resemble the I-V characteristics shown in Fig. 9.12?

42

Experiments with Electrical and Electronic Circuits

Observation Table 9.4: Reverse bias I-V characteristic of a zener diode

Reverse current (mA)

S. No.

Reverse voltage (V) When increasing voltage

When decreasing voltage

Mean reverse current

(mA)

1.

2.

3.

.

.

.

0.0

1.0

2.0

.

.

.

Result: Knee voltage =.......................V

Forward resistance =.......................Ω Breakdown voltage =.......................V

Reverse resistance =.......................Ω You may now like to answer the following SAQ.

9.5.1 Rectification of ac

The general purpose p-n junction is used as a rectifier diode. From your

school physics classes, you may recall that conversion of ac voltage into dc

voltage is known as rectification. As you know, the ac voltage is sinusoidal

(Fig. 9.14a). When we place a diode in a circuit, it allows unidirectional current

in the circuit. As a result, negative half-cycle is eliminated and we obtain

pulsating dc (Fig. 9.14b). That is, the original signal has been modified

(rectified) to the extent that only one-half part of the input is being used here.

Obviously it is not only of little use but inefficient also. Therefore, in actual

practice, we use two diodes in such a way that the negative half cycle is also

made available in the circuit, as shown in Fig. 9.14c. Such an arrangement

constitutes a full-wave rectifier circuit (Fig. 9.15).

Fig. 9.14: a) ac signal,

b) half-wave rectification, c) full- wave rectification

Spend 4 min.

SSSSSSSSAAAAAAAAQQQQQQQQ 22222222 :::::::: ZZZZZZZZeeeeeeeennnnnnnneeeeeeeerrrrrrrr ddddddddiiiiiiiiooooooooddddddddeeeeeeee cccccccchhhhhhhhaaaaaaaarrrrrrrraaaaaaaacccccccctttttttteeeeeeeerrrrrrrriiiiiiiissssssssttttttttiiiiiiiiccccccccssssssss

Compare your results with those obtained in the previous part of the experiment and discuss the physics of differences.

99..55 SSOOMMEE AAPPPPLLIICCAATTIIOONNSS OOFF SSEEMMIICCOONNDDUUCCTTOORR DDIIOODDEESS

43

Semiconductor Diodes

Fig. 9.15: Full wave rectifier circuits: a) centre tapped and b) bridge rectifier

In the circuit shown in Fig. 9.15a, diode D1 conducts in the positive half cycle,

whereas diode D2 conducts in the negative half cycle. You must have

observed that here we need a centre-tapped transformer, which is fairly costly.

However, the same action can be achieved with a normal transformer in a

bridge circuit, which consists of four diodes (Fig. 9.15b). Diodes D1 and D3

conduct in the positive half cycle, while D2 and D4 conduct in negative half

cycle.

You will note that the circuit output exhibits fluctuations and can not be put to

any practical use. To minimise fluctuations, we use a pi-filter (π-filter) which

consists of an inductor and two capacitors.

To understand the rectification action, you should build the circuits shown in

Fig. 9.15 and observe the input and output waveforms using a cathode ray

oscilloscope (CRO).

9.5.2 Zener Diode as Voltage Regulator

While studying the I-V characteristics of zener diode, you must have noted

that in reverse bias condition, the voltage across zener diode remains

constant at Vz, and, independent of input voltage value when input is more

than Vz. This characteristic of zener diode gives rise to a very interesting

application in that it can be used as a constant voltage source. If we connect

a load across the zener diode, a constant voltage becomes available across it.

The circuit of zener regulated voltage supply is shown in Fig. 9.16. RL is the

load across which the stabilised voltage is obtained. In this circuit, the excess

voltage in dissipated across resistor R.

While a capacitor filters out ac component, an inductor allows maintenance of dc level. For details you should refresh your knowledge by reading your 12

th standard

physics book.

44

Experiments with Electrical and Electronic Circuits

Fig. 9.16: Zener diode as a voltage regulator

a) Line Regulation

Line regulation is a measure of regulation against any change in input

voltage. To study line regulation, the load is maintained at a fixed value

(say 1kΩ). The input voltage (Vi) to the regulator is varied in steps of about

20V with the help of multiple tappings in the secondary of the transformer.

Note the corresponding output voltages (Vo) and record these in

Observation Table 9.5. (You can also use a variable ac voltage supply in

the form of a dimmerstat in the primary of the transformer to vary the input

supply to the regulator.) Take at least 12 readings, starting from zero volt.

Now draw a graph by plotting input ac voltage (Vi) along x-axis and the

corresponding output voltage along y-axis. We expect you to obtain a

curve similar to that shown in Fig. 9.17. The percentage change in the

output voltage per unit change in the input ac voltage in the linear region

of the graph gives line regulation.

Observations Table 9.5: Line regulation by a zener diode

Load resistance, RL = 1kΩ

S.No. Input voltage, Vi (V) Regulated voltage, Vo (V)

1.

2.

3.

4.

5.

.

.

.

12.

Result: Line Regulation =.............%

Fig. 9.17: Line regulation

by a zener diode

45

Semiconductor Diodes b) Load Regulation

Load regulation is a measure of regulation against change in the load

resistance, i.e. the current drawn from the regulator circuit. To determine

load regulation, we begin with no load in the circuit. At constant input

voltage, measure output voltage VNL and record the value in Observation

Table 9.6. Next, connect a variable load resistance RL and decrease it

from 1 kΩ to 100 Ω in steps of 100 Ω. Note the output voltage VL in each

case. Calculate the load regulations using the relation

Load regulation .100×

−=

NL

RNL

V

VV

Next, plot a graph by taking RL along the x-axis and the corresponding

output voltage VR along the y-axis as shown in Fig. 9.18. The minimum

load resistance for the regulated output would be that value of RL for which

VR begins to drop significantly.

Observation Table 9.6: Load Regulation

Voltage without Load, VNL =……….V.

S.No. RL (Ω) Voltage VR (V) Percentage regulation

1

2

3

4.

5.

.

.

.

1000

900

800

700

600

100

Result: Load regulation at maximum loading condition (at the minimum value of RL) = ……………… %

Fig. 9.18: Load

regulation by a zener diode

46

Experiments with Electrical and Electronic Circuits

Spend 5 min.

SAQ 3SAQ 3SAQ 3SAQ 3 : Zener voltage regulator : Zener voltage regulator : Zener voltage regulator : Zener voltage regulator

In the circuit shown in Fig. 9.16, suppose that Vi = 12.4V, RL = 500 Ω,

VZ = 6 V and R = 100 Ω. Calculate

i) Current through R

ii) Current through zener diode

iii) Power dissipation in the diode and RL