Semiconductor Electronics: Materials, Devices and Simple Circuits MCQs for NEET — Physics Questions with Answers

(b) In half wave rectifier ${\mathrm{V}}_{\mathrm{dc}}=\frac{{\mathrm{V}}_0}{\mathrm{\pi }}=\frac{10}{\mathrm{\pi }}$

(a) As we know current density J=nqv

$$\begin{gathered} \Rightarrow {\mathrm{J}}_{\mathrm{e}}={\mathrm{n}}_{\mathrm{e}}{\mathrm{qv}}_{\mathrm{e}} \mathrm{and} {\mathrm{J}}_{\mathrm{h}}={\mathrm{n}}_{\mathrm{h}}{\mathrm{qv}}_{\mathrm{h}} \\ \Rightarrow \frac{{\mathrm{J}}_{\mathrm{e}}}{{\mathrm{J}}_{\mathrm{h}}}=\frac{{\mathrm{n}}_{\mathrm{e}}}{{\mathrm{n}}_{\mathrm{h}}}\times \frac{{\mathrm{v}}_{\mathrm{e}}}{{\mathrm{v}}_{\mathrm{h}}}\Rightarrow \frac{3/4}{1/4}=\frac{{\mathrm{n}}_{\mathrm{e}}}{{\mathrm{n}}_{\mathrm{h}}}\times \frac{5}{20}\Rightarrow \frac{{\mathrm{n}}_{\mathrm{e}}}{{\mathrm{n}}_{\mathrm{h}}}=\frac{6}{5} \end{gathered}$$

(b) The conductance order of metals is 10⁶ to 10⁸ ohm^-1 cm^-1.

(d) The n-type semiconductors are obtained when Si or Ge are doped with elements of group 15, eg, arsenic (As), while p-type semiconductors are obtained when Si or Ge are doped witlh traces of elements of group 13, ie, indium (In), boron (B).

Note : The conductivity of semiconductors increases with rise in temperature

According to the NCERT text, 'In the energy band diagram of n-type Si semiconductor, the donor energy level $E_D$ is slightly below the bottom $E_C$ of the conduction band and electrons from this level move into the conduction band with very small supply of energy.' This placement facilitates electron donation to the conduction band.

The NCERT states, 'for p-type semiconductor, the acceptor energy level $E_A$ is slightly above the top $E_V$ of the valence band... With very small supply of energy an electron from the valence band can jump to the level $E_A$ and ionise the acceptor negatively.' This creates holes in the valence band, which are majority carriers in a p-type semiconductor.

The context mentions, 'at room temperature, most of the donor atoms get ionised but very few (~10^12) atoms of Si get ionised. So the conduction band will have most electrons coming from the donor impurities.' This highlights that doping is the dominant source of conduction electrons in n-type semiconductors.

The NCERT text explains, 'At room temperature, most of the acceptor atoms get ionised leaving holes in the valence band. Thus at room temperature the density of holes in the valence band is predominantly due to impurity in the extrinsic semiconductor.'

The NCERT states, 'In extrinsic semiconductors, because of the abundance of majority current carriers, the minority carriers produced thermally have more chance of meeting majority carriers and thus getting destroyed. Hence, the dopant, by adding a large number of current carriers of one type, which become the majority carriers, indirectly helps to reduce the intrinsic concentration of minority carriers.'

The passage confirms, 'the donor energy level $E_D$ is slightly below the bottom $E_C$ of the conduction band and electrons from this level move into the conduction band with very small supply of energy.' This small energy difference makes it easy for electrons to transition.