CBSE 2024 · Region 4 · Set 2 · Q30 · 4 marks

A pure semiconductor like Ge or Si , when doped with a small amount of suitable impurity, becomes an extrinsic semiconductor. In thermal equilibrium, the electron and hole concentration in it are related to the concentration of intrinsic charge carriers. A p-type or n-type semiconductor can be converted into a $\displaystyle \mathrm{p}-\mathrm{n}$ junction by doping it with suitable impurity. Two processes, diffusion and drift take place during formation of a $\displaystyle \mathrm{p}-\mathrm{n}$ junction. A semiconductor diode is basically a $\displaystyle \mathrm{p}-\mathrm{n}$ junction with metallic contacts provided at the ends for the application of an external voltage. A p-n junction diode allows currents to pass only in one direction when it is forward biased. Due to this property, a diode is widely used to rectify alternating voltages, in half-wave or full wave configuration.

(i)

When Ge is doped with pentavalent impurity, the energy required to free the weakly bound electron from the dopant is about

(A)

0.$\displaystyle 001$ eV

(B)

0.$\displaystyle 01$ eV

(C)

0.$\displaystyle 72$ eV

(D)

1.$\displaystyle 1$ eV

(ii)

At a given temperature, the number of intrinsic charge carriers in a semiconductor is $\displaystyle 2.0 \times 10^{10} \mathrm{~cm}^{-3}$. It is doped with pentavalent impurity atoms. As a result, the number of holes in it becomes $\displaystyle 8 \times 10^{3} \mathrm{~cm}^{-3}$. The number of electrons in the semiconductor is

(A)

$\displaystyle 2 \times 10^{24} \mathrm{~m}^{-3}$

(B)

$\displaystyle 4 \times 10^{23} \mathrm{~m}^{-3}$

(C)

$\displaystyle 1 \times 10^{22} \mathrm{~m}^{-3}$

(D)

$\displaystyle 5 \times 10^{22} \mathrm{~m}^{-3}$

(iii)

(a)OR(b)

During the formation of a $\displaystyle \mathrm{p}-\mathrm{n}$ junction -

(A)

electrons diffuse from p-region into n -region and holes diffuse from n-region into p-region.

(B)

both electrons and holes diffuse from n -region into p -region.

(C)

electrons diffuse from n-region into p-region and holes diffuse from p-region into n-region.

(D)

both electrons and holes diffuse from p-region into n-region.

(iv)

An ac voltage $\displaystyle \mathrm{V}=0.5 \sin (100 \pi \mathrm{t})$ volt is applied, in turn, across a half-wave rectifier and a full-wave rectifier. The frequency of the output voltage across them respectively will be

(A)

$\displaystyle 25 \mathrm{~Hz}, 50 \mathrm{~Hz}$

(B)

$\displaystyle 25 \mathrm{~Hz}, 100 \mathrm{~Hz}$

(C)

$\displaystyle 50 \mathrm{~Hz}, 50 \mathrm{~Hz}$

(D)

$\displaystyle 50 \mathrm{~Hz}, 100 \mathrm{~Hz}$

CBSE Class 12 Physics past-paper question from the 2024board exam, with the answer as CBSE’s own marking scheme gives it. Where our answers come from.