1

EXPERIMENT NO. : 1

DATE:

CHARACTERISTICS OF PN DIODE

AIM

To plot the forward and reverse VI characteristics of a PN diode and to calculate the following parameters

1. Forward resistance

2. Reverse resistance

3. Cut in voltage

APPARATUS REQUIRED

EQUIPMENT / RANGE
Power supply / (0-10)V
Ammeter / (0-30)mA, (0-100)µA
Volt meter / (0-1)V, (0-10)V
Resistors / 1K
Diode / 1N 4001
Bread board & wires / -

THEORY

In a piece of semiconductor material, if one half is doped by P-type impurity and the other half is doped by N-type impurity, a PN junction is formed. The plane dividing the two halves or zones is called PN junction. The N-type material has high concentration of free electrons, while P-type material has high concentration of holes. Therefore, at the junction there is a tendency for the free electrons to diffuse over the P-side and holes the N-side. This process is called diffusion. As the free electron moves across the junction from N-type to P-type, the donor irons become positively charged. Hence a positive charge is built on the N-side of the junction. The free electrons that cross the junction uncover the negative acceptor ions by filling in the holes. Therefore, a net negative charge is established on the P-side of the junction. This net negative charge on the P-side prevents further diffusion of electron in to the P-side. Similarly, the net positive charge on the N-side repels the holes crossing from P-side to N-side. Thus a barrier is set-up near the junction which prevents further movement of charge carriers. As the consequence of the induced electric field across the depletion layer, an electrostatic potential difference is established between P and N region, which is called the potential barrier, junction barrier, diffusion potential, or contact potential, VO. The magnitude of the contact potential VO varies with doping levels and temperature. VO is 0.3 V for germanium and 0.72 V for silicon.

Forward bias

When positive terminal of the battery is connected to the P-type and negative terminal to the N-type of the PN diode, the bias is known as forward bias. Under the forward bias condition, the applied positive potential repels the holes in P-type region so that the holes move towards the junction and the applied negative potential repels the electron in the N-type region and the electron move towards the junction. Eventually, when the applied potential is more than the internal barrier potential the depletion region and internal potential barrier disappear. A feature worth to be noted in the forward characteristics is the cut in or threshold voltage VR below which the current is very small. It is 0.3 V for GE and 0.7 V for Si respectively. At the cut in voltage, the potential is overcome and the current through the junction starts to increase rapidly.

SYMBOL

PIN DIAGRAM

CONSTRUCTION

CIRCUIT DIAGRAM

FORWARD BIAS

REVERSE BIAS

Reverse bias

When the negative terminal of the battery is connected to the P-type and positive terminal of the battery is connected to the N-type of the PN junction, the bias applied is known as reverse bias. Under applied reverse bias, holes which form the majority carriers or the P-side moves towards the negative terminal of the battery and electron which form the majority carrier of the N-side are attracted towards the positive terminal of the battery. Hence, the width of the depletion region which is depleted of the mobile charge carriers increases. Thus the electric field produced by applied reverse bias, is in the same direction as the electric field of the potential barrier. Hence, the resultant potential barrier is increased which prevents the flow of majority carriers in both the directions. Therefore, theoretically no current should flow in the external circuit. But in practice, a very small current of the order of a few microamperes flows under reverse bias.

Electron forming covalent bonds of the semiconductor atoms in the P and N-type regions may absorb sufficient energy from heat and light to cause breaking of some covalent bonds. Hence electron-hole pairs are continually produced in both the regions. Under the reverse bias condition, the thermally generated holes in the P-region are attracted towards the negative terminal of the battery and the electrons in the N-region are attracted towards the positive terminal of the battery. Consequently, the minority carriers, electron on the P-region and holes in the N-region, wander over to the junction and flow towards their majority carrier side giving rise to a small reverse current. This current is known as reverse saturation current, IO. The magnitude of reverse current depends upon the junction temperature because the major source of minority carriers is thermally broken covalent bonds.

For large applied reverse bias, the free electrons from the N-type moving towards the positive terminal of the battery acquire sufficient energy to move with high velocity to dislodge valence electron from semiconductor atoms in the crystal. These newly liberated electrons, in turn, acquire sufficient energy to dislodge other parent electrons. Thus, a large number of free electrons are formed which is commonly called as an avalanche of free electrons. This leads to the breakdown of the junction leading to very large reverse current. The reverse voltage at which the junction breakdown occurs is known as breakdown voltage, VBD.

PN diode applications

An ideal PN diode is a two terminal polarity sensitive device that has zero resistance when it is forward biased and infinite resistance when it is reverse biased. Due to this characteristic the diode finds number of applications as given below.

  1. Rectifier
  2. Switch
  3. Clamper
  4. Clipper
  5. Demodulation detector circuits

PROCEDURE

Forward bias

  1. Connect the circuit as per the circuit diagram.
  2. Vary the power supply voltage in such a way that the readings are taken in steps of 0.1 V in the voltmeter.
  3. Note down the corresponding ammeter readings.
  4. Plot the graph: V against I
  5. Calculate the forward resistance RF = ∆VF / ∆IF

TABULATION

FORWARD BIAS

Sl. No. / SUPPLY
VOLTAGE / VF
(V) / IF
(mA)
1
2
3
4
5
6
7
8
9
10

REVERSE BIAS

Sl. No. / SUPPLY
VOLTAGE / VR
(V) / IR
(µA)
1
2
3
4
5
6
7
8
9
10

PROCEDURE

Reverse bias

  1. Connect the circuit as per the circuit diagram.
  2. Vary the power supply voltage in such a way that the readings are taken in steps of 0.5 V in the voltmeter.
  3. Note down the corresponding ammeter readings.
  4. Plot the graph: V against I
  5. Calculate the forward resistance RR = ∆VR / ∆IR

VIVA-VOCE QUESTIONS

  1. Explain how a potential barrier is created within a PN diode.
  1. Define peak inverse voltage of a diode.
  1. Mention the characteristics of an ideal diode.
  1. What are the sources of reverse current in a diode?
  1. What are the transition and diffusion capacitances?

MODEL GRAPH

CALCULATION

Forward resistance RF = ∆VF / ∆IF

=

Reverse resistance RR = ∆VR / ∆IR

=

VIVA-VOCE QUESTIONS

  1. Write the diode equation.
  1. Why silicon devices are more popular than germanium devices?
  1. What do you mean by valance electron?
  1. Give the barrier potential for silicon and germanium.
  1. Write the other name of PN diode.

RESULT

Thus the characteristics of PN diode were plotted and the following results were obtained.

Forward resistance RF
Reverse resistance RR
Cut In voltage

EXPERIMENT NO. : 2

DATE:

CHARACTERISTICS OF ZENER DIODE

AIM

To plot the forward and reverse VI characteristics of a zener diode and to calculate the following parameters

1. Forward resistance

2. Reverse resistance

3. Break down voltage

APPARATUS REQUIRED

EQUIPMENT / RANGE
Power supply / (0-30)V
Ammeter / (0-30)mA, (0-100)mA
Volt meter / (0-1)V, (0-10)V
Resistors / 1K
Diode / SZ 5.6
Bread board & wires / -

THEORY

Zener diode is heavily doped PN diode. When the reverse voltage reaches breakdown voltage in normal PN diode the current through the junction and the power dissipated at the junction will be high. Such an operation is destructive and the diode gets damaged. Diodes can be designed with adequate power dissipation capabilities to operate in the breakdown region. One such diode is known as Zener diode.

From the VI characteristics of the Zener diode, it is found that the operation of Zener diode is same as that of ordinary PN diode under forward biased condition. Under reverse biased condition, breakdown of the junction occurs. The breakdown voltage depends upon the amount of doping. If the diode is heavily doped, depletion layer will be thin and, consequently, breakdown occurs at lower reverse voltage and the breakdown voltage is sharp. The sharp increasing current under breakdown conditions is due to the following two mechanisms.

  1. Avalanche breakdown
  2. Zener breakdown

Avalanche breakdown

As the applied reverse bias increases, the field across the junction increases correspondingly, thermally generated carriers while traversing the junction acquire a large amount of kinetic energy from this field. As a result the velocity of these carriers increases. These electrons disrupt covalent bond by colliding with immobile ions and create new electron-hole pairs. These new carriers again generating energy from the field and collide with other immobile ions thereby generating further electron-hole pairs. This process is cumulative in nature and results in generation of an avalanche of charge carriers within a short time. This mechanism of carrier generation is known as avalanche multiplication. This process results in flow of large amount of current at the same value of reverse bias.

SYMBOL

PIN DIAGRAM

CONSTRUCTION

CIRCUIT DIAGRAM

FORWARD BIAS

REVERSE BIAS

Zener breakdown

When the PN regions are heavily doped, direct rupture of covalent bonds takes place because of strong electric fields, at the junction of PN diode. The new electron-hole pair so created increases the reverse current in a reverse biased PN diode. The increase in current takes place at a constant value of reverse bias typically below 6V for heavily doped diodes. As a result of heavy doping of P and N regions, the depletion regions, the depletion region width becomes very small and for applied voltage of 6V or less, the field across the depletion region becomes very high, of the order of 107 V/m, making conditions suitable for Zener breakdown. For lightly doped diodes, Zener breakdown voltage becomes high and breakdown is predominantly by Avalanche multiplication. Though Zener breakdown occurs for lower breakdown voltage and Avalanche breakdown occurs for higher breakdown voltage, such diodes are normally called Zener diodes.

Application

Zener diode can be used as a voltage regulator.

PROCEDURE

Forward Bias

  1. Connect the circuit as per the circuit diagram.
  2. Vary the power supply voltage in such a way that the readings are taken in steps of 0.1 V in the voltmeter.
  3. Note down the corresponding ammeter readings.
  4. Plot the graph: V against I
  5. Calculate the forward resistance RF = ∆VF / ∆IF

Reverse Bias

  1. Connect the circuit as per the circuit diagram.
  2. Vary the power supply voltage in steps of 0.5 V.
  3. Note down the corresponding ammeter readings.
  4. Plot the graph: V against I
  5. Calculate the forward resistance RR = ∆VR / ∆IR

TABULATION

FORWARD BIAS

Sl. No. / SUPPLY
VOLTAGE / VF
(V) / IF
(mA)
1
2
3
4
5
6
7
8
9
10

REVERSE BIAS

Sl. No. / SUPPLY
VOLTAGE / VR
(V) / IR
(mA)
1
2
3
4
5
6
7
8
9
10

VIVA-VOCE QUESTIONS

  1. How Zener diode is formed?
  1. What is Avalanche breakdown?
  1. Mention few differences between PN diode and Zener diode.
  1. What are the general applications of Zener diodes?
  1. What is Zener breakdown?

MODEL GRAPH

CALCULATION

Forward resistance RF = ∆VF / ∆IF

=

Reverse resistance RR = ∆VR / ∆IR

=

RESULT

Thus the characteristics ofZener diode were plotted and the following results were obtained.

Forward resistance RF
Reverse resistance RR
Breakdown voltage

EXPERIMENT NO. : 3

DATE:

CHARACTERISTICS OF CE TRANSISTOR

AIM

To plot the input and output characteristics of an NPN transistor in common emitter configuration and to find its

1. Input resistance

2. Output resistance

APPARATUS REQUIRED

EQUIPMENT / RANGE
Power supply / (0-30)V
Ammeter / (0-500)µA,(0-10)mA
Voltmeter / (0-5)V, (0-10)V
Resistors / 500Ω, 1K
Bread board & wires / -
Transistor / BC 107

THEORY

Transistor can be connected in a circuit in any of the three different configurations namely; common emitter, common base, and common collector. Common emitter (CE) configuration is the most frequently used configuration because it provides voltage, current, power gain more than unity.

The name CE is because the emitter of the transistor is common to the input and output circuits. Input is applied across the base and emitter, and the output is taken across the collector and emitter. CE configuration is also called grounded emitter configuration.

Input Resistance

Input resistance can be calculated from the input characteristic curves. It is given by the ratio of small change in base to emitter voltage to corresponding base current; keeping collector to emitter voltage constant.

Ri= ∆VBE / ∆IB; keeping VCE constant.

Output Resistance

Output resistance can be calculated from the output characteristic curves. It is given by the ratio of small change in collector to emitter voltage to corresponding collector current; keeping base current constant.

Ro = ∆VCE / ∆IC; keeping IB constant.

Application

The CE configuration is used for audio frequency circuits.

SYMBOL

PIN DIAGRAM

CONSTRUCTION

CIRCUIT DIAGRAM

PROCEDURE

Input Characteristics

  1. Rig up the circuit as per circuit diagram.
  2. Set VCE and vary VBE insteps of 0.1 V and note down the corresponding IB. Repeat the above procedure for various values of VCE.
  3. Plot the graph: VBEvs IB for constant values of VCE.
  4. Find the input resistance Ri= ∆VBE / ∆IB; keeping VCE constant.

Output Characteristics

  1. Rig up the circuit as per circuit diagram.
  2. Set IB and vary VCE insteps of 1 V and note down the corresponding IC. Repeat the above procedure for various values of IB.
  3. Plot the graph: VCEvs IC for constant values of IB.
  4. Find the output resistance Ro = ∆VCE / ∆IC; keeping IB constant.

VIVA-VOCE QUESTIONS

  1. What is the importance of a CE amplifier?
  1. Mention the regions of operation of a transistor.
  1. In which region of operation a transistor acts as an amplifier.
  1. What is the use of CC amplifier stage?
  1. What is an emitter follower and why it is called so

TABULATION

INPUT CHARACTERISTICS

SL. NO. / VCE = / VCE = / VCE =
VBE
(V) / IB (µA) / VBE
(V) / IB
(µA) / VBE (V) / IB
(µA)
1
2
3
4
5
6
7
8
9
10

OUTPUT CHARACTERISTICS

SL. NO. / IB = / IB = / IB =
VCE (V) / IC (mA) / VCE (V) / IC
(mA) / VCE (V) / IC
(mA)
1
2
3
4
5
6
7
8
9
10

VIVA-VOCE QUESTIONS

  1. What is application of CB amplifier?
  1. What the arrow in the symbol of transistor indicates.
  1. Why the emitter of a transistor is highly doped.
  1. Why the area of collector of transistors is largest.
  1. Define transistor.

MODEL GRAPH

INPUT CHARACTERISTICS

MODEL GRAPH

OUTPUT CHARACTERISTICS

CALCULATION

Input resistance RI= ∆VBE / ∆IB

Output resistance Ro = ∆VCE / ∆IC

RESULT

Thus the characteristics of CE transistorwere plotted and the following results were obtained.

Input resistance RI
Output resistance Ro

EXPERIMENT NO. : 4

DATE:

CHARACTERISTICS OF CB TRANSISTOR

AIM

To plot the input and output characteristics of an NPN transistor in common base configuration and to find its

1. Input resistance

2. Output resistance

APPARATUS REQUIRED

EQUIPMENT / RANGE
Power supply / (0-5)V, (0-30)V
Ammeter / (0-100)mA
Voltmeter / (0-5)V, (0-10)V
Resistors / 1K, 500 ohm
Bread board & wires / -
Transistor / BC 107

THEORY

In common base configuration, the base of the transistor is common to the input and output circuits. Input voltage is applied between emitter and base. Output is taken across collector and base. CB configuration is also called as grounded base configuration. In this set up, an increase in emitter current causes an increase in collector current. Collector current is given by the expression IC = IE – IB. since IB is in the order of micro amperes. Collector current is almost same as the emitter current in spite of the variations in the collector – base junction. Another important expression for a common base transistor configuration is IC = α IE + ICBO where ICBO is the current flowing through the collector circuit when the collector – base junction is reverse biased and emitter - base junction is open circuited.

Transistor offers medium input resistance and very high output resistance when it is in CB configuration. It provides almost unit current gain and high voltage gain.

Input Resistance

Input characteristics are plotted between the emitter current IE and emitter to base voltage VBE for a constant value of collector to base voltage VCB. The reciprocal of the slope of the curve s gives the value of dynamic input resistance Ri and is given by the expression Ri = ∆VBE/∆IE.

Output Resistance

Output characteristics are plotted between the collector current IC and collector to base voltage VCB for a constant value of emitter current IE. The output resistance can be obtained from the curves and is given by the expression Ro = ∆VCB/∆IC keeping IE constant. Ro has rather high values since the curves are almost flat.

Application

The CB configuration is used for high frequency circuits.

SYMBOL

PIN DIAGRAM

CONSTRUCTION

CIRCUIT DIAGRAM

PROCEDURE

Input Characteristics

  1. Rig up the circuit as per circuit diagram.
  2. Set VCB and vary VEB insteps of 0.1 V and note down the corresponding IE. Repeat the above procedure for various values of VCB.
  3. Plot the graph: VEBvs IE for constant values of VCB.
  4. Find the input resistance Ri= ∆VEB / ∆IE; keeping VCB constant.

Output Characteristics

  1. Rig up the circuit as per circuit diagram.
  2. Set IE and vary VCB insteps of 1 V and note down the corresponding IC. Repeat the above procedure for various values of IE.
  3. Plot the graph: VCBvs IC for constant values of IB.
  4. Find the output resistance Ro = ∆VCB / ∆IC; keeping IE constant.

VIVA-VOCE QUESTIONS

  1. What s the current amplification factor for CB stage?
  1. Mention the characteristics of CB configuration.
  1. Mention the application of CB amplifier?
  1. Which configuration is good as a constant current source? Why?
  1. What is collector power dissipation of transistor?

TABULATION

INPUT CHARACTERISTICS

SL. NO. / VCB = / VCB = / VCB =
VEB
(V) / IE (mA) / VEB
(V) / IE
(mA) / VEB (V) / IE
(mA)
1
2
3
4
5
6
7
8
9
10

OUTPUT CHARACTERISTICS

SL. NO. / IE = / IE = / IE =
VCB (V) / IC (mA) / VCB (V) / IC
(mA) / VCB (V) / IC
(mA)
1
2
3
4
5
6
7
8
9
10

VIVA-VOCE QUESTIONS

  1. What is a bipolar junction transistor?
  1. List the difference between NPN and PNP transistor.
  1. What are the different BJT configurations?
  1. Explain about early effect.
  1. List the two types of bread down in transistors.
  1. Draw the pin diagram of BC 107 transistor.
  1. What is meant by base-width modulation?
  1. Explain about punch through.
  1. Give the application of CC configuration.

MODEL GRAPH