How it works
A photodiode is a PN junction operated in reverse bias. Photons with energy E = hf greater than the bandgap (1.12 eV for silicon, requiring λ < 1100 nm) create electron-hole pairs in the depletion region. The built-in electric field sweeps electrons toward the N side and holes toward the P side, producing photocurrent I_ph = R · P_opt, where R is responsivity in A/W. In photoconductive mode (reverse biased), response is fast and linear but dark current exists. In photovoltaic mode (zero bias), dark current is eliminated — this is how solar cells operate — but response is slower due to diffusion-dominated carrier collection.
Key points to remember
Responsivity R (A/W) = η·q·λ/(h·c), where η is quantum efficiency; silicon peaks around 0.5–0.7 A/W at 800–900 nm. Rise time in photoconductive mode depends on junction capacitance and load resistor: t_r ≈ 2.2·R_L·C_j — the BPW34 has C_j ≈ 7 pF at −10 V, giving sub-nanosecond response with a 50 Ω load. Increasing reverse bias reduces C_j and speeds up response but increases dark current. The PIN photodiode has a wide intrinsic layer between P and N, dramatically reducing C_j and extending bandwidth to GHz range for fibre optic receivers.
Exam tip
The examiner always asks you to compare photoconductive and photovoltaic modes of photodiode operation — be specific: photoconductive mode is faster and more linear, photovoltaic mode has zero dark current but slower response due to minority carrier diffusion.