Which semiconductor device converts electrical energy into light?

$(i)$ 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 energy gap of an $LED$ is $2.4 eV$. When the $LED$ is switched $ON$,the momentum of the emitted photons is

Three photodiodes $D_1, D_2,$ and $D_3$ are made of semiconductors having band gaps of $2.5 \ eV, 2 \ eV,$ and $3 \ eV,$ respectively. Which ones will be able to detect light of wavelength $6000 \ \mathring{A}$?

In an $LED$,visible light is produced by:

The photocurrent in a photodiode depends on