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

Generally, the gain of a transistor amplifier falls at high frequencies due to the

  1. internal capacitances of the device

  2. coupling capacitor at the input

  3. skin effect

  4. coupling capacitor at the output
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

The gain of amplifier is

$$A_i = \dfrac{-g_m}{g_o + j\omega C}$$

Thus the gain of a transistor amplifier falls at high frequencies due to the internal capacitance that are diffusion capacitance and transition capacitance.

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 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

Directions: Consider the CMOS circuit shown below, where the gate voltage v0 of the n-MOSFET is increased from zero, while the gate voltage of the p −MOSFET is kept constant at 3 V. Assume that, for both transistors, the magnitude of the threshold voltage is 1 V and the product of the trans-conductance parameter is 1 mAV-2.

For small increase in VG beyond 1 V, which of the following gives the correct description of the region of operation of each MOSFET?

  1. Both the MOSFETs are in saturation region.

  2. Both the MOSFETs are in triode region.

  3. n -MOSFET is in triode and p −MOSFET is in saturation region.

  4. n -MOSFET is in saturation and p −MOSFET is in triode region.
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

For small increase in $v_G$ beyond 1V the n-channel MOSFET goes into saturation as $V_{GS} \rightarrow +ive \ and\ p-MOSFET$ is always in active region or triode region.

Multiple choice

Directions: Consider the CMOS circuit shown, where the gate voltage v0 of the n-MOSFET is increased from zero, while the gate voltage of the p−MOSFET is kept constant at 3 V. Assume that, for both transistors, the magnitude of the threshold voltage is 1 V and the product of the trans-conductance parameter is 1 mA V-2.

Estimate the output voltage V0 for VG = 15 V.
  1. 4 - $\dfrac{1}{\sqrt2}$
  2. 4 + $\dfrac{1}{\sqrt2}$
  3. 4 - $\dfrac{\sqrt3}{2}$
  4. 4 + $\dfrac{\sqrt3}{2}$
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

Vo = 4 + √3/2 = 4 + 0.866 = 4.866 V This voltage is near to supply voltage, while when it increases to more than 15 V, then the output will decrease.

Multiple choice

Two identical NMOS transistors M1 and M2 are connected as shown below. V bias is chosen so that both transistors are in saturation. The equivalent gm of the pair is defined to be $ \dfrac{\partial I_{out}}{\partial V_i}$at constant Vout The equivalent gm of the pair is

  1. the sum of individual gm's of the transistors

  2. the product of individual gm's of the transistors

  3. nearly equal to the gm of M1
  4. nearly equal to $\dfrac{g_m}{g_o}$of M2
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

The current in both transistor are equal. Thus $g_m$ is decided by $M_1$

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