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Physics · 2024 · 4 marks
Semiconductor Electronics: Materials, Devices and Simple CircuitsExtrinsic Semiconductor4 marksApplycase study
CBSE 2024 · Region 4 · Set 2 · Q30
A pure semiconductor like Ge or Si, when doped with a small amount of suitable impurity, becomes an extrinsic semiconductor. In thermal equilibrium,…
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)
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.
Initially during the formation of a p-n junction -
(A)
diffusion current is large and drift current is small.
(B)
diffusion current is small and drift current is large.
(C)
both the diffusion and the drift currents are large.
(D)
both the diffusion and the drift currents are small.
(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)
$\displaystyle 50 \mathrm{~Hz}, 100 \mathrm{~Hz}$
Marking-scheme solution
(i)(B)
0.$\displaystyle 01$ eV
(ii)(D)
$\displaystyle 5 \times 10^{22} \mathrm{~m}^{-3}$
(iii)(a)
(C) Electrons diffuse from n-region into p-region and holes diffuse from p-region to n-region.
OR
(b)(A)
Diffusion current is large and drift current is small.
(iv)(D)
$\displaystyle 50$ Hz, $\displaystyle 100$ Hz.
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