# Physics Class 12 NCERT Solutions: Chapter 14 Semiconductor Electronics Materials Devices and Simple Circuits Part 3

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Q: 12. The number of silicon atom per is . This is doped simultaneously with atoms of Arsenic and atoms of Indium. Calculate the number of electrons and holes. Given that . Is the material or ?

Answer:

Number of silicon atoms,

Number of arsenic atoms,

Number of silicon atoms,

Number of thermally-generated electrons,

Number of electrons,

Number of holes

In thermal equilibrium, the concentrations of electrons and holes in a semiconductor are related as:

Therefore, the number of electrons is approximately and the number of holes is about. Since the number of electrons is more than the number of holes, the material is an simiconductor.

Q: 13. In an intrinsic semiconductor the energy gap is . Its whole mobility is much smaller than electron mobility and independent of temperature. What is the ratio between conductivity at and that at? Assume that the temperature dependence of intrinsic carrier concentration is given by

Where is a constant

Answer:

Energy gap of the given intrinsic semiconductor,

The temperature dependence of the intrinsic carrier-concentration is written as:

Where,

Initial temperature,

The intrinsic carrier-concentration at this temperature can be written as:

Final temperature,

The intrinsic carrier-concentration at this temperature can be written as:

The ratio between the conductivities at is equal to the ratio between the respective intrinsic carrier-concentrations at these temperatures.

Therefore, the ratio between the conductivities is .

Q: 14. In a p-n junction diode, the current I can be expressed as

Where is called the reverse saturation current, is the voltage across the diode and is positive for forward bias and negative for reverse bias, and is the current through the diode, is the Boltzmann constant and is the absolute temperature. If for a given diode, then

(A) What will be the forward current at a forward voltage of?

(B) What will be the in the current if the voltage across the diode is increased to?

(C) What is the dynamic resistance?

(D) What will be the current if reverse bias voltage changes from?

Answer:

In a p-n junction diode, the expression for current is given as:

Where,

(A) Forward voltage,

Therefore, the forward current is about.

(B) For forward voltage,, we can write:

Hence, the increase in current,

(C) Dynamic resistance

(D) If the reverse bias voltage changes from, then the current (I) will almost remain equal to in both cases. Therefore, the dynamic resistance in the reverse bias will be infinite.