Learning Objectives:

1. Explain basic terms related with diodes.
2. Understand operating principle of diodes.
3. Recognizes types and the construction of diodes.
4. Explains ideal diode, equivalent circuit and dc characteristic of a diode.
5. Tells the operating principles of diodes and zener diodes.
6. Recognizes half-wave, full-wave and bridge rectifier circuits.
7. Explains the operation of these circuits.
8. Analyzes and measures parameters in basic diode circuits.

• Diodes allow current to flow freely in one direction, but not the other.
• A diode is simply a p-n junction.
• Zener diodes, a special kind of diode that can recover from breakdown caused when the reverse-bias voltage exceeds the diode breakdown voltage.

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Solved Example: 9999-01

In a junction diode:

A. The depletion capacitance increases with increase in the reverse bias

B. The depletion capacitance decreases with increase in the reverse bias

C. The depletion capacitance increases with increase in the forward bias

D. The depletion capacitance is much higher than the depletion capacitance when it is forward biased

Depletion capacitance has other names such as space charge capacitance, transition capacitance or barrier capacitance. This capacitance occurs when the junction is reverse biased. Junction behaves as a parallel plate capacitance whose capacitance is given by \[C_T = \dfrac{\epsilon A}{W}\] Where,

A – cross section area of junction

W – Thickness of space charge

$\epsilon$ - Dielectric constant

Correct Answer: B

Solved Example: 9999-02

The diffusion potential across a p n-junction:

A. Decreases with increasing doping concentration

B. Increases with decreasing band gap

C. Does not depend on doping concentrations

D. Increases with increases in doping concentration

The term diffusion potential also has other names such that Barrier potential, Built in potential, and contact potential.

$N_D$ – Donor concertation on n – side

$N_A$ – Acceptor concentration on p – side

If temperature is constant and ni (intrinsic concentration) is constant

Diffusion potential $\propto \ln(𝑵_𝑨𝑵_𝑫)$

Correct Answer: D

Solved Example: 9999-03

A Zener diode works on the principle of:

A. Tunnelling of charge carriers across the junction

B. Thermionic emission

C. Diffusion of charge carriers across the junction

D. Hopping of charge carriers across the junction

Zener diode has heavily doped p – n junction. It operates under the reverse bias.

When heavily doped junction is reverse biased energy bands become crossed at relatively low voltages i.e. n side conduction band appears opposite to p – side valence band. Crossing of bands allows large number of empty states is n side conduction band opposite to many filled states of the p – side valence band. If barrier is narrow, tunnelling of electrons can occur.

Tunnelling of electrons from p – side valence band to n – side conduction band constitutes a reverse current from n to p this is Zener effect

Correct Answer: A

Solved Example: 9999-04

For small signal AC operation, a practical forward biased diode can be modelled as:

A. A resistance and a capacitance

B. An ideal diode and resistance in parallel

C. A resistance and an ideal diode in series

D. A resistance

For small signal AC operation the practical forward biased diode can be modelled as a resistance ($𝒓_{𝒂𝒄}$) given by the reciprocal of the slope of the characterises at that point

Correct Answer: D

Solved Example: 9999-05

The direction of the arrow represents the direction of __________ when the diode is forward biased.

A. P-type material

B. N-type material

C. P-N Junction

D. Conventional current flow

Correct Answer: D

Solved Example: 9999-06

Which of the following diodes is also known as a 'voltacap' or 'voltage-variable capacitor diode?

A. Varactor diode

B. Step recovery diode

C. Schottky diode

D. Gunn diode

Correct Answer: A

Solved Example: 9999-07

Among these alternatives, the PIV rating of which diode is lower than that of equivalent vacuum diode?

A. PN junction Diode

B. Crystal diode

C. Tunnel diode

D. Small single diode

Correct Answer: B

Solved Example: 9999-08

Which of the following devices is used in voltage regulators and voltage limiters as a fixed reference voltage in the network?

A. PIN diode

B. Varactor diode

C. Zener diode

D. Tunnel diode

Correct Answer: C

Learning Objectives:

1. Explain the operation of p-n-p and n-p-n transistors.
2. Sketch the (CB) common-base, (CE) common-emitter and (CC) common-collector characteristics.
3. Study the different transistor configurations.
4. Explain different bias circuits
5. Calculate voltage gain and current gain for common-emitter amplifier.

A Transistor is an electronic device made of three layers of semiconductor material that can act as an insulator and a conductor. The three layered transistor is also known as Bipolar Junction Transistor. (BJT). A Transistor is used as a switch, but also as an amplifier.

There are two types of transistors, the N-P-N type and P-N-P type. A transistor has three regions:

• Emitter- Heavily doped
• Base- Lightly doped
• Collector- Moderatly doped

• Common Base Configuration - has Voltage Gain but no Current Gain.
• Common Emitter Configuration - has both Current and Voltage Gain.
• Common Collector Configuration - has Current Gain but no Voltage Gain.

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Solved Example: 9998-01

A BJT is said to be operating in the saturation region if:

A. Both junctions are reverse biased

B. Base – emitter junction is reverse biased and base collector junction is forward biased

C. Base – emitter junction is forward biased and base – collector junction reverse biased

D. Both the junctions are forward biased

A BJT has four modes for operation depending on polarities of emitter base junction and collector base junction:

Correct Answer: D

Solved Example: 9998-02

The Early – Effect in a bipolar junction transistor is caused by:

A. Fast – turn – on.

B. Fast – turn – off.

C. Large collector – base reverse bias.

D. Large emitter – base forward bias.

When the effective base width of the transistor is changed by varying the collector junction voltage is called base width modulation or Early effect. This happens for transistor of$ p^+ - n – p^+$

Correct Answer: C

Solved Example: 9998-03

Consider the following statements $S_1$ and $S_2$

$S_1$: The $\beta$ of a bipolar transistor reduces if the base width is increased.

$S_2$: The $\beta$ of a bipolar transistor increases if the doping concentration in the base is increased.

Which one of the following is correct?

A. $S_1$ is FALSE and $S_2$ is TRUE

B. Both $S_1$ and $S_2$ are TRUE

C. Both $S_1$ and $S_2$ are FALSE

D. $S_1$ TRUE and $S_2$ is FALSE

Note the relation between $\alpha$ and $\beta$ current gains

If base width of transistor increases, recombination in base region increases, and thus $\alpha$ decreases and hence $\beta$ decreases.

Thus $S_1$ is TRUE.

If doping of base region increases, then recombination is base increases and $\alpha$ decreases thereby decreasing $\beta$

Thus $S_2$ is FALSE.

Correct Answer: D

Solved Example: 9998-04

As compared to power MOSFET, a BJT has:

A. Higher switching losses but lower conduction losses

B. Higher switching losses and higher conduction losses

C. Lower switching losses and Lower conduction losses

D. Lower switching losses but higher conduction losses

Correct Answer: A

Learning Objectives:

1. Describe the construction and working principles of SCR
2. Describe methods of SCR triggering.
3. Specify SCR specifications VI characteristics of SCR.
4. TRIAC , UJT, DIAC.

• Thyristor is a device, that is it will only conduct current in one direction only.
• However, unlike a diode, the thyristor can be made to operate as either an open-circuit switch or as a rectifying diode depending upon how the thyristors gate is triggered.
• 4-layered semiconductor device of alternating p and n material.
• thyristor has four states: Reverse blocking mode, reverse conduction mode, forward blocking mode and forward conducting mode.

Three Important Thyristor Specifications:

• Forward break-over voltage V$_{BR}$: This is the voltage at which the SCR enters the forward-conduction region.
• Holding current, I$_H$: This is the value of anode current below which the SCR switches from the forward conduction region to the forward blocking region.
• Gate trigger current, I$_{GT}$: This is the value of gate current necessary to switch the SCR from the forward-blocking region to the forward-conduction region under specified conditions.

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Solved Example: 9974-01

Three SCRs are connected together to form a series string. The voltages across the thyristors are 350V, 300V and 250V respectively. If the currents in the thyristors are 6A, 9A, and 12A respectively, what will be the value of equalising resistance to be used across each thyristor?

A. 26.23$\Omega$

B. 50$\Omega$

C. 16.67$\Omega$

D. 33.33$\Omega$

Correct Answer: C

Solved Example: 9974-02

What does the amp$^2$-sec rating of the SCR specify?

A. The energy that the device can absorb while operating in the forward blocking mode

B. The energy that the device can absorb before the fault is cleared

C. The energy dissipated by the device when the fault occurs

D. The power dissipated by the device when fault occurs

Correct Answer: B

Solved Example: 9974-03

For the power semiconductor devices IGBT, MOSFET, Diode and Thyristor, which one of the following statements is TRUE?

A. All the four are majority carrier devices.

B. All the four are minority carrier devices.

C. IGBT and MOSFET are majority carrier devices, whereas Diode and Thyristor are minority carrier devices.

D. MOSFET is majority carrier device, whereas IGBT, Diode Thyristor are minority carrier devices.

Correct Answer: D

Solved Example: 9974-04

Three SCRs are connected together to form a series string. The voltages across the thyristors are 350V, 300V and 250V respectively. If the currents in the thyristors are 6A, 9A, and 12A respectively, what will be the value of equalising resistance to be used across each thyristor?

26.23 $\Omega$ 50 $\Omega$ 16.67 4\Omega$ 33.33 $\Omega$

Correct Answer: C

Solved Example: 9974-05

Snubber circuits are used with thyristors to:

A. See that SCR turns ON at a voltage much less than its forward break over voltage

B. To protect the gate circuit

C. To limit the rate of rise of voltage dv/dt.

D. To limit the rate of rise of current di/dt.

Correct Answer: C