A p-n photodiode is fabricated from a semiconductor with band gap of $2.8eV$. Can it detect a wavelength of $6000nm$?

Energy band gap $E=2.8eV$

$E=\frac{hc}{\lambda }$

$\Rightarrow \frac{6.6\times 10^{-34}\times 3\times 10^8}{6000\times 10^{-9}\times 1.6\times 10^{-19}}=0.207eV$

The energy of a signal of wavelength $6000nm$ is $0.207eV$, which is less than $2.8eV$, the energy band gap of a photodiode. Hence, the photodiode cannot detect the signal.