Photodiodes & Photodetectors

A photodiode reverses the LED principle: instead of injecting carriers to generate light, incident light is absorbed and converted into an electrical current. When a photon with sufficient energy (E ≥ E_g) strikes the semiconductor, an electron is lifted from the valence band into the conduction band, creating an electron-hole pair. In the depletion region of a reverse-biased pn junction, the electric field present there immediately separates this pair before it can recombine – electron and hole drift to their respective contacts and generate a measurable photocurrent, proportional to the incident optical power.

In a simple pn diode, however, the depletion region is very thin (typically < 1 µm), so most of the light is absorbed deeper in the field-free bulk material, where generated carrier pairs must reach the contacts by slow diffusion rather than fast drift – this limits both sensitivity and switching speed. The pin photodiode solves this problem by inserting a thick, intrinsic (undoped) i-layer between the p- and n-layers. The electric field then extends over a substantially larger distance (often 10–50 µm), essentially all incident light is absorbed in the high-field region, and carriers drift rather than diffuse – this substantially increases quantum efficiency and enables bandwidths in the gigahertz range.

Two quantities describe how efficiently a photodiode converts light into current. Quantum efficiency η indicates what fraction of incident photons actually generates a detectable electron-hole pair (values near 1 for well-designed devices). The more practically relevant quantity is responsivity R = I_ph/P_opt in A/W, which directly relates the generated photocurrent to the incident optical power: R = η·e/(h·f) = η·λ/1240 nm (with R in A/W and λ in nm). A silicon photodiode typically reaches R ≈ 0.5–0.6 A/W at 850 nm, an InGaAs photodiode about R ≈ 0.9–1.0 A/W at 1550 nm.

Responsivity is wavelength-dependent and drops sharply above a material-specific cutoff wavelength (set by the bandgap, λ_g ≈ 1240 nm/E_g), since photons below the bandgap energy can no longer be absorbed. Toward short wavelengths, responsivity also declines, since high-energy photons are already absorbed very close to the surface, where surface recombination destroys part of the generated carriers before they can be separated. Each material therefore has a characteristic responsivity window that defines its usable detection bandwidth.