The graph given below represents I-V characteristics of zener diode. The part of the characteristics curve that is most relevant for its operation as a voltage regulator is

In n-type semiconductor, free electrons donated by the impurity atoms occupy energy levels in

In a pure silicon crystal electron-hole concentration is $10^{16}$ per $\mathrm{m}^3$ at 301 K . Now $10^{21}$ atoms of phosphorus are added per cubic metre. The new hole concentration in silicon is (in per $\mathrm{m}^3$ )

In a common emitter transistor amplifier, the output voltage and input voltage have a phase difference of