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

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 $E_C$.

The text 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.' When an electron leaves the valence band to fill an acceptor level, it leaves behind a hole in the valence band. So, the holes in the valence band are due to acceptance of electrons by acceptor impurities.

The context states: 'From the V-I characteristic of a junction diode we see that it allows current to pass only when it is forward biased. So if an alternating voltage is applied across a diode the current flows only in that part of the cycle when the diode is forward biased. This property is used to rectify alternating voltages and the circuit used for this purpose is called a rectifier.' This directly explains why the diode's unidirectional conduction property is crucial for rectification.

The NCERT text says: 'If an alternating voltage is applied across a diode in series with a load, a pulsating voltage will appear across the load only during the half cycles of the ac input during which the diode is forward biased. Such rectifier circuit, as shown in Fig. 14.18, is called a half-wave rectifier.' This confirms that the output is a pulsating DC voltage present only during half-cycles.

The context states: 'When A is negative, the diode is reverse-biased and it does not conduct. The reverse saturation current of a diode is negligible and can be considered equal to zero for practical purposes.' This means during the negative half-cycle, the diode is reverse-biased, and virtually no current flows through the load.