When light shines on a $p-n$ junction diode,the current $I$ versus voltage $V$ is observed as in the figure below. In which quadrant$(s)$ does the diode generate power,so that it can be used as a solar cell?

For three circuit elements $A, B$ and $C$,following observations were made in lab:
$(a)$ element $A$ cannot conduct $d.c.$ but allows $a.c.$ through it.
$(b)$ element $B$ conducts both $AC$ and $DC$ but in one direction only.
$(c)$ Element $C$,when connected across $DC$,conducts only for one polarity and emits light whose intensity first increases and then decreases with increase in applied voltage.
The elements $A, B$ and $C$ are respectively identified as

$A$ photodiode is fabricated from a semiconductor with a bandgap of $2.5 \ eV$. Can it detect a wavelength of $6000 \ \mathring{A}$?

Identify the correct circuit diagrams for the normal operation from the following.

$A$ Light Emitting Diode $(LED)$ has a voltage drop of $2\,V$ across it and passes a current of $10\,mA$ when it operates with a $6\,V$ battery through a limiting resistor $R$. The value of $R$ is.........$k\,\Omega$.

Which type of semiconductor device does not need any bias voltage?