Class 12 Semiconductor Electronics Mock Test (CBSE)

Topics in Semiconductor Electronics

• Classification of metals, conductors and semiconductors
• Intrinsic semiconductor
• Extrinsic semiconductor
• p-n junction formation
• Semiconductor diode
• Forward and reverse bias
• Application of junction diode as a rectifier

Semiconductor Electronics practice questions with answers

1. Q1Multiple choice·1 mark
   When a pure germanium crystal is doped with a pentavalent impurity atom, the type of extrinsic semiconductor formed is:

   1. (a)intrinsic semiconductor
   2. (b)insulator
   3. (c)p-type semiconductor
   4. (d)n-type semiconductor
   Show answer and step marks

   1. Correct option: (d). Doping a pure semiconductor (group 14) with a pentavalent impurity (group 15) provides free electrons as majority charge carriers, resulting in an n-type semiconductor.+1

   Answer: (d)
2. Q2Multiple choice·1 mark
   At absolute zero temperature ($0\text{ K}$), an intrinsic semiconductor behaves as a/an:

   1. (a)P-type semiconductor
   2. (b)N-type semiconductor
   3. (c)Conductor
   4. (d)Insulator
   Show answer and step marks

   1. Correct option: (d). At $0\text{ K}$, the valence band is completely filled and the conduction band is completely empty with no thermally generated charge carriers, making an intrinsic semiconductor behave like an insulator.+1

   Answer: (d)
3. Q3Short answer·3 marks
   What is a semiconductor diode? Briefly describe its V-I characteristics in forward and reverse bias. Define its dynamic resistance.
   Show answer and step marks

   1. A semiconductor diode is a p-n junction with metallic contacts at its ends for applying an external voltage; it conducts mainly in one direction.+1
   2. Forward bias: the current is very small until the threshold (knee) voltage (about $0.7$ V for Si, $0.2$ V for Ge), after which it rises sharply (exponentially) with voltage.+0.5
   3. Reverse bias: a very small, nearly constant reverse saturation current (microamperes) flows until the breakdown voltage, where the current rises suddenly.+0.5
   4. Dynamic resistance ($r_d$) is the ratio of a small change in voltage to the resulting small change in current: $r_d = \frac{\Delta V}{\Delta I}$.+1

   Answer: A diode is a p-n junction with metal contacts for applying voltage. Forward bias: negligible current up to the knee voltage (about 0.7 V for Si), then a sharp rise. Reverse bias: a tiny, nearly constant saturation current until breakdown. Dynamic resistance $r_d = \frac{\Delta V}{\Delta I}$.
4. Q4Short answer·3 marks
   Distinguish between forward biasing and reverse biasing of a p-n junction diode based on the following parameters: (i) direction of the external electric field relative to the built-in electric field, (ii) change in the width of the depletion region, and (iii) nature of charge carriers responsible for current flow.
   Show answer and step marks

   1. (i) In forward bias, the external electric field opposes the built-in electric field; in reverse bias, it aids the built-in electric field.+1
   2. (ii) The width of the depletion region decreases in forward bias and increases in reverse bias.+1
   3. (iii) In forward bias, current is primarily due to the diffusion of majority charge carriers, whereas in reverse bias, a tiny current is due to the drift of minority charge carriers.+1

   Answer: (i) Forward bias: the external field opposes the built-in field; reverse bias: it aids the built-in field. (ii) The depletion region narrows in forward bias and widens in reverse bias. (iii) Forward-bias current is mainly diffusion of majority carriers; reverse-bias current is a tiny drift current of minority carriers.

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