The device that can act as a complete electronic circuit is

The figure shows an intrinsic semiconductor $S$ connected in series with a variable resistor $R$ and a constant voltage source $V$. The value of $R$ can be adjusted to maintain a constant current in the ammeter $A$. Under what condition will the semiconductor $S$ get heated? Give reasons.

$A$ thermoelectric refrigerator works on

Given below are two statements:
Statement $I$: Photovoltaic devices can convert optical radiation into electricity.
Statement $II$: Zener diode is designed to operate under reverse bias in the breakdown region.
In the light of the above statements,choose the most appropriate answer from the options given below:

In which of the following circuits is the junction reverse biased?

Which of the following figures correctly shows the phase relation between the input signal and the output signal of a common-emitter transistor amplifier?