Physics

Semiconductors and Diodes

461 Questions

Semiconductors and diodes form the foundation of modern electronics, covering topics like intrinsic carrier concentration and Zener breakdown. Questions often explore the characteristics of bipolar junction transistors (BJT) and the properties of doped silicon materials. This topic is crucial for physics and electronics engineering competitive exams.

Zener diode breakdownBJT circuit analysisIntrinsic carrier concentrationDoping in semiconductorsEmitter follower circuit

Semiconductors and Diodes Questions

Multiple choice

A Silicon PN junction diode under reverse bias has depletion region of width 10 m. The relative permittivity of Silicon, $\epsilon_r = 11.7$ and the permittivity of free space $\epsilon_0 = 8.85 \times 10^{-12} F/m$. The depletion capacitance of the diode per square metre is
  1. $100 \mu F$
  2. $10 \mu F$
  3. $1 \mu F$
  4. $20 \mu F$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

$C = \frac{\epsilon_0 \epsilon_r A}{d} \\ or \hspace{0.2cm} \frac{C}{A} = \frac{\epsilon_0 \epsilon_r}{d} = \frac{8.85 \times 10^{-12} \times 11.7}{10 \times 10^{-6}} = 10.35 \mu F$

Multiple choice

The band gap of Silicon at room temperature is

  1. 1.3 eV

  2. 0.7 eV

  3. 1.1 eV

  4. 1.4 eV
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

For Silicon at 0 K,
At any temperature
$E_{gT} = E_{g0} - 3.6 \times 10^{-4}T$
At T = 300 K,
$E_{g300} = 1.21 - 3.6 \times 10^{-4} \times 300 = 1.1 eV$
This is standard value, that must be remembered.

Multiple choice

An n-channel depletion MOSFET has following two points on its ID -VGS curve: (i) VGS = 0 at ID = 12 mA and (ii) VGS = - 6 volts at ID = 0

Which of the following Q-points will give the highest trans-conductance gain for small signals?

  1. VGS = - 6 Volts

  2. VGS = - 3 Volts

  3. VGS = 0 Volts

  4. VGS = 3 Volts
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

 The transconductance for the junction FET is given by gm = (2 Idss/|Vp |) [1 - (Vgs/Vp)] Vp is the pinch off voltage. Max transconductance can occur between pinch off voltage and at origin. So it lies between - 6V and 0. So Q point = - 3 V

Multiple choice

In the transistor amplifier circuit shown in the figure below, $\beta_{DC} = 60, V_{BE} = 0.7V, h_{ie} \rightarrow \infty, h_{fe} \rightarrow \infty$ The capacitance Cc can be assumed to be infinite.

If $\beta_{DC}$ is increased by 10%, the collector-to-emitter voltage drop (Note: The ground has been shown by the symbol $\nabla$.)

  1. increases by less than or equal to 10%

  2. decreases by less than or equal to 10%

  3. increases by more than 10%

  4. decreases by more than 10%
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Multiple choice

In a transconductance amplifier, it is desirable to have

  1. a large input resistance and a large output resistance

  2. a large input resistance and a small output resistance

  3. a small input resistance and a large output resistance

  4. a small input resistance and a small output resistance
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

In the transconductance amplifier, it is desirable to have large input resistance and large output resistance. ideally, Id = Gm.Vs Id = Gm.Vi(Rd/Rd+RL) if Rd>>RL, Id = Gm Vi Vi = Vs if Ri >> Rs

Multiple choice

Group I lists four different semiconductor devices. Match each device in Group I with its characteristic property in Group II

Group I Group II
(P) BJT (1) Population inversion
(Q) MOS capacitor (2) Pinch-off voltage
(R) LASER diode (3) Early effect
(S) JFET (4) Flat-band voltage

  1. P-3, Q-1, R-4, S-2

  2. P-1, Q-4, R-3, S-2

  3. P-3, Q-4, R-1, S-2

  4. P-3, Q-2, R-1, S-4
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

In BJT as the B-C reverse bias voltage increases, the B-C space region width increases which $x_B$ (i.e neutral base width) > A change in neutral base width will change the collector current. A reduction in base width will causes the gradient in minority carrier concentration to increase, which in turn causes an increased in the diffusion current. This effect is known as base modulation as early effect.

In JFET the gate to source voltage that must be applied to achieve pinch off voltage is described as pinch off voltage and is also called as turn voltage or threshold voltage.

In LASER population inversion occurs on the condition when concentration of electrons in one energy state is greater than that in lower energy state, i.e a non equilibrium condition.

In MOS capacitor, flat band voltage is the gate voltage that must be applied to create flat ban condition in which there is no space region in semiconductor under oxide.

Therefore BJT : Early effect

MOS capacitor : Flat - band voltage

LASER diode : Population inversion

JFET : Pinch-off voltage

Multiple choice

Which of the following is NOT associated with a p - n junction?

  1. Junction Capacitance

  2. Charge Storage Capacitance

  3. Depletion Capacitance

  4. Channel Length Modulations
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

Channel length modulation is not associated with p - n junction. It is being associated with MOSFET in which effective channel length decreases, producing the phenomenon called channel length modulation.

Multiple choice

A silicon wafer has 100 nm of oxide on it and is furnace at a temperature above 10000 C for further oxidation in dry oxygen. The oxidation rate

  1. is independent of current oxide thickness and temperature

  2. is independent of current oxide thickness but depends on temperature

  3. slows down as the oxide grows

  4. is zero as the existing oxide prevents further oxidation
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

Oxidation rate is zero because the existing oxide prevents the further oxidation.