NEET Practice Questions (MCQs) with Answers & Solutions

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.

The NCERT explicitly states for p-type semiconductors, and generally applies, 'Note that the crystal maintains an overall charge neutrality as the charge of additional charge carriers is just equal and opposite to that of the ionised cores in the lattice.'

The text states, 'At room temperature, most of the donor atoms get ionised but very few (~10^12) atoms of Si get ionised.' This implies a significantly lower number of intrinsic Si atoms ionising compared to donor atoms contributing to free charge carriers.

The NCERT states, 'With very small supply of energy an electron from the valence band can jump to the level $E_A$ and ionise the acceptor negatively. (Alternately, we can also say that with very small supply of energy the hole from level $E_A$ sinks down into the valence band. Electrons rise up and holes fall down when they gain external energy).'

The NCERT explicitly provides the relationship: 'The electron and hole concentration in a semiconductor in thermal equilibrium is given by $n_e n_h = n_i^2$ (14.5).'

The NCERT states, 'The semiconductor’s energy band structure is affected by doping. In the case of extrinsic semiconductors, additional energy states due to donor impurities ($E_D$) and acceptor impurities ($E_A$) also exist.'

The donor impurity atoms, being pentavalent, have an extra electron that is weakly bound and can easily be donated to the conduction band, making them the source of majority carriers (electrons) in n-type semiconductors, as implied by $E_D$ being slightly below $E_C$.