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

For semiconductors, the text states: 'Here a finite but small band gap ($E_g < 3 \text{ eV}$) exists. Because of the small band gap, at room temperature some electrons from valence band can acquire enough energy to cross the energy gap and enter the conduction band. These electrons (though small in numbers) can move in the conduction band.'

The context mentions: '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.'

The text states: 'For C (diamond), Si and Ge, the energy gaps are 5.4 eV, 1.1 eV and 0.7 eV, respectively. Sn also is a group IV element but it is a metal because the energy gap in its case is 0 eV.' Therefore, $E_g > 3 \text{ eV}$ for Sn is incorrect.

The text defines: 'The energy band which includes the energy levels of the valence electrons is called the valence band.'

The context 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 text clearly explains this for semiconductors: 'Here a finite but small band gap ($E_g < 3 \text{ eV}$) exists. Because of the small band gap, at room temperature some electrons from valence band can acquire enough energy to cross the energy gap and enter the conduction band.' This implies easier transition for smaller $E_g$, leading to conduction.

The 'POINTS TO PONDER' section states: 'The energy bands ($E_C$ or $E_V$) in the semiconductors are space delocalised which means that these are not located in any specific place inside the solid. The energies are the overall averages. When you see a picture in which $E_C$ or $E_V$ are drawn as straight lines, then they should be respectively taken simply as the bottom of conduction band energy levels and top of valence band energy levels.'

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.'

The NCERT states, 'for p-type semiconductor, the acceptor energy level $E_A$ is slightly above the top $E_V$ of the valence band as shown in Fig. 14.9(b).'

The NCERT text mentions that '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...'