Calculate the photocurrent density in a PIN photodiode.
Consider a silicon PIN diode with an intrinsic region width of W=20 \mu \mathrm{m}. Assume that the photon flux is 10^{17} \mathrm{~cm}^{-2}-\mathrm{s}^{-1} and the absorption coefficient is \alpha=10^{3} \mathrm{~cm}^{-1}.