What kind of diode is used as a photo detector?

The material suitable for making a solar cell is

The photodiode is made of a material with a band gap of $2.0\, eV$. The minimum frequency of the radiation that can be absorbed by the material is nearly

$(i)$ Name the type of diode whose characteristics are shown in figure $(a)$ and figure $(b)$.
$(ii)$ What does the point $P$ in figure $(a)$ represent?
$(iii)$ What do the points $P$ and $Q$ in figure $(b)$ represent?

The schematic symbol of a light emitting diode $(LED)$ is:

Which of the following semiconductor devices requires a reverse bias to operate?