Explanation: Fig A represents constant voltage drop model of a diode. In this model, the diode is assumed to be a perfect insulator in reverse bias. On forward bias up to the cut-in voltage, it is assumed to be an insulator and after it becomes perfect conductor. Fig B represents an ideal diode, which is in forward bias, it is a perfect conductor and in reverse bias, it is perfect insulator. We can also depict the piecewise linear mode of diode through Fig C. This assumption is very close to constant voltage drop model except the fact that after cut-in voltage instead of perfect insulator diode is assumed as a ohmic device. Fig D represents real diode. The diode in real life is similar to the characteristics. That is, after the cut-in voltage, graph is exponential and in reverse bias mode current is in micro or nano ampere range.

Explanation: Fig B denotes a Si diode because the cut-in voltage is 0.7V and after this the current is exponential. Also in reverse bias there is a nano ampere range current it must also be accounted. The cut-off of 0.3V is correct for Germanium and not for a Si diode. Also, fig A, fig C & fig D which are seemingly linear are the piecewise linear model, not a real representation for the diode. Therefore, fig B is the best.

Explanation: Fig A denotes a Germanium diode because the cut-in voltage is 0.3V and after this the current is exponential. Also in reverse bias there is a nano ampere range current it must also be accounted. The cut-off of 0.7V is correct for Silicon and not for a Germanium diode. Also, fig B, fig C & fig D which are seemingly linear are the piecewise linear model, not a real representation for the diode.

Explanation: Voltage rating is the voltage at which reverse breakdown occurs. Voltage rating of a zener diode indicates the voltage beyond which current increases rapidly. Beyond this point increase in reverse bias voltage will only reflect on current which means voltage drop beyond this point is almost constant. At forward bias zener diode behaves as a normal diode. So the appropriate graph is fig B.