Semiconductor Diodes

EXPERIMENT
EXPERIMENTS USING
9 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

31
Experiments with
Electrical and 9.1 INTRODUCTION
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
When the anode is influenced and determined by external conditions. While a p-n junction works
connected to +ve terminal
of battery and the cathode as rectifying diode, a zener diode acts as voltage regulator, depending on
is connected to −ve terminal biasing conditions.
of the battery, the device is
said to be forward-biased
Objectives
and vice versa. A p-n
junction is said to be
forward biased when p-type After performing this experiment, you should be able to:
region is connected to +ve
terminal of the battery and • draw current-voltage (I-V) characteristic curves of a p-n junction and a
n-type region is connected
to −ve terminal of the
zener diode in forward and reverse bias conditions;
battery.
• 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.

32
 Semiconductor Diodes
9.2 BASICS OF SEMICONDUCTORS
REVISITED

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
Doping is a process of
semiconductors are termed as extrinsic semiconductors. The most commonly adding small quantities of
used semiconducting materials are crystalline silicon and germanium. In other elements, called
impurity, in a pure
recent years, compound semiconductors, amorphous semiconductors, and
semiconductor in order to
semiconducting polymers have also been developed. In this experiment, we modify its electrical
will confine ourselves only to devices made of elemental semiconductors. conductivity.

From the electronic configuration of Si (1s 2 2s 2 2p 6 3s 2 3p 2 ), 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

33
Experiments with If silicon (or germanium) is doped with a trivalent (three electrons in the
Electrical and
Electronic Circuits 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.

9.3 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
34 the junction in n- and p-regions, respectively. Due to accumulation of charges
near the junction, an electric field is established. This gives rise to electrostatic Semiconductor Diodes
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 Fig. 9.5: Symbol of a p-n
and acts as an anode in a diode. Similarly, K indicates n-region and junction (diode)

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 35
Experiments with shown in Fig. 9.6b, the junction is said to be reverse biased. In this case,
Electrical and
Electronic Circuits 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
To test a p-n junction using when reverse biased. You can easily test it using a multimeter. This property
a multimeter, set the
multimeter on resistance of p-n junction is used for ac rectification.
measurement mode.
Connect the junction in 9.3.3 Identifying a Diode
forward bias with the
multimeter probes and Semiconductor diodes are designated by two letters followed by a serial
measure its resistance.
Next, reverse the number. The first letter indicates the material: A is used for material with a
multimeter probes to band gap of 0.6 eV to 1.0 eV such as germanium. B is used for material with a
measure the resistance of
the junction in the other
band gap of 1.0 eV to 1.3 eV, such as silicon. The second letter indicates the
direction. You will observe a main application: A signifies detection diode, B denotes a variable capacitance
large difference between
diode, E for tunnel diode, Y for rectifying diode and Z denotes zener diode.
these values.
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
Fig.9.7: Identification of a to the cathode.
diode (Printed
with permission Always work within the specified range of diode ratings to avoid damages to
from M/s Power the device.
Technology, New
Zealand)
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.

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.

36
9.3.4 I-V Characteristics of a p-n Junction Semiconductor Diodes

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
S.No. Forward voltage (V) Forward current (mA) Mean
Increasing Decreasing forward
voltage voltage current
(mA)

1. 0.0

2. 0.1

3. 0.2

4. 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.
37
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 (µ
µA) Mean
S.No. Reverse voltage (V) reverse
Increasing Decreasing current
voltage voltage (µ
µA)

1. 0.0

2. 1.0

3. 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

38
Calculations: From your plot of I-V characteristics, you can easily calculate Semiconductor Diodes
forward resistance and reverse resistance using the following relations:

∆Vf
Rf =
∆ If
A conventional solid state
and diode does not allow flow of
significant current if reverse
∆Vr bias is below its reverse
Rr = . break down voltage. Once
∆I r
voltage across p-n junction
exceeds reverse bias
Result: breakdown voltage, it is
subject to high current flow
Forward resistance =......................Ω. due to Avalanche
breakdown and can be
Reverse resistance =......................Ω. permanently damaged.
Knee-voltage =......................V.
You may now like to answer the following SAQ.

SAQ 1 : Diode characteristics Spend

4 min.

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?

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

9.4 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 p
minority carriers, which, in turn, collide with the atoms of the semiconductor
n
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 Fig. 9.11: Symbol of
zener diode
process is termed as zener breakdown. However, since the (magnitude of)
39
Experiments with reverse voltage is small, the junction is not damaged. In silicon diodes, zener
Electrical and
Electronic Circuits 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.

Apparatus

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

voltage supply, voltmeter, milliameter and a resistor.

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.
40
 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

V − V z (15 − 10 ) V
R = max = = 50 Ω.
Iz 100 mA

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) Mean

forward
S. Forward voltage (V) With increasing With decreasing current
No. voltage voltage
(mA)

1. 0.0

2. 0.1

3. 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?
41
Experiments with Observation Table 9.4: Reverse bias I-V characteristic of a zener diode
Electrical and
Electronic Circuits
Reverse current (mA) Mean
reverse
S. Reverse voltage (V) When increasing When current
No. voltage decreasing voltage
(mA)

1. 0.0

2. 1.0

3. 2.0

. .

. .

. .

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

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

You may now like to answer the following SAQ.

Spend SAQ 2 : Zener diode characteristics

4 min.

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

9.5 SOME APPLICATIONS OF

SEMICONDUCTOR DIODES
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
Fig. 9.14: a) ac signal, (rectified) to the extent that only one-half part of the input is being used here.
b) half-wave Obviously it is not only of little use but inefficient also. Therefore, in actual
rectification,
c) full- wave
practice, we use two diodes in such a way that the negative half cycle is also
rectification made available in the circuit, as shown in Fig. 9.14c. Such an arrangement
constitutes a full-wave rectifier circuit (Fig. 9.15).
42
 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.
While a capacitor filters out
However, the same action can be achieved with a normal transformer in a ac component, an inductor
bridge circuit, which consists of four diodes (Fig. 9.15b). Diodes D1 and D3 allows maintenance of dc
level. For details you should
conduct in the positive half cycle, while D2 and D4 conduct in negative half refresh your knowledge by
th
cycle. reading your 12 standard
physics book.
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.
43
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
Fig. 9.17: Line regulation
by a zener output voltage per unit change in the input ac voltage in the linear region
diode 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 =.............%

44
b) Load Regulation Semiconductor Diodes

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
Fig. 9.18: Load
 V − VR  regulation by a
Load regulation =  NL  × 100. zener diode
 VNL 

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 1000

2 900

3 800

4. 700

5. 600

. 100

Result: Load regulation at maximum loading condition (at the minimum

value of RL) = ……………… % 45
Experiments with
Electrical and
Electronic Circuits SAQ 3 : Zener voltage regulator
Spend
5 min. 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

46