What type of $p-n$ junction diode does not require forward bias to operate?

Explain why elemental semiconductors cannot be used to make visible $LEDs$.

$A$ light emitting diode $(LED)$ is fabricated using $GaAs$ semiconducting material whose band gap is $1.42 \,eV$. The wavelength of light emitted from the $LED$ is: (in $\,nm$)

Which wavelength of the Sun is primarily used to generate electric energy in solar cells?

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 one will be able to detect light of wavelength $600 nm$?

Explain the construction and working of a photodiode.