Glossary entry

Photodiode

A semiconductor diode designed to convert incident photons into electrical current via the photoelectric effect. The fundamental optical receiver element.

A photodiode is a p-n or p-i-n semiconductor diode operated under reverse bias (or zero bias) such that absorbed photons generate electron–hole pairs that are swept out of the depletion region by the built-in or applied electric field, producing a measurable photocurrent.

Operating modes.

Photoconductive mode (reverse-biased): faster response, lower capacitance, larger linear range. Standard for telecom and high-speed receivers.
Photovoltaic mode (zero bias): lower dark current and noise, slower response. Standard for solar cells and low-light instrumentation.
Avalanche mode (avalanche photodiode): biased near breakdown to multiply photocarriers internally, providing gain $M = 10 - 100$ (linear-mode) or $> 10^4$ (Geiger-mode single-photon counting).

Physical structure choices.

Structure	Description	Common applications
p-n photodiode	Simple junction without intrinsic layer	Solar cells, low-cost slow detectors
PIN photodiode	p–intrinsic–n with wide intrinsic absorption region	Telecom, instrumentation (10 MHz – 50 GHz)
APD	PIN with separate multiplication region	High-sensitivity receivers, single-photon detection
Schottky photodiode	Metal–semiconductor junction	UV detection, high-speed millimeter-wave
MSM (metal–semiconductor–metal)	Interdigitated contacts on semiconductor	Very high speed ( $> 100$ GHz), waveguide-coupled
Waveguide photodiode	Lateral absorption along guided wave	Silicon photonic and InP PIC receivers
Resonant-cavity-enhanced (RCE)	Photodiode inside a thin FP cavity	Wavelength-selective receivers
Uni-traveling-carrier (UTC)	Engineered to use only electron drift	Highest-speed photodiodes ( $> 300$ GHz)

Material selection. The semiconductor bandgap sets the absorption cutoff wavelength: photons with $\lambda > hc/E_g$ pass through unabsorbed.

Material	Useful wavelength range	Common substrate
Si	200 – 1050 nm	Silicon
GaAs	600 – 870 nm	GaAs
InGaAs (lattice-matched to InP)	900 – 1700 nm	InP
Ge	800 – 1800 nm	Si (in Ge-on-Si waveguide PDs) or bulk Ge
InAs	1.0 – 3.6 μm	InAs / InP
HgCdTe (variable composition)	1 – 25 μm (cryogenic)	CdZnTe

Key performance specifications.

Parameter	Definition	Typical range
Responsivity	A/W	0.5 – 1.1 A/W at peak
Dark current	nA at $-5$ V bias	pA to nA
Bandwidth	RC-limited or transit-time-limited	DC to $> 100$ GHz
Active area	mm to μm scale	varies with bandwidth
Junction capacitance	Sets RC bandwidth limit	pF (large area) to fF (high speed)
NEP	Sensitivity floor	$10^{-15}$ to $10^{-12}$ W/ $\sqrt{\text{Hz}}$

Speed–area tradeoff. Bandwidth is limited either by RC time (capacitance times load resistance) or by carrier transit time across the absorption region. Reducing area lowers capacitance for higher speed but reduces the optical alignment tolerance. Telecom PDs are typically 30–50 μm diameter for moderate speed, $< 10$ μm for $> 50$ GHz operation.

For PIC integration, photodiodes are typically waveguide-coupled (lateral absorption along the guided wave rather than vertical absorption from a surface beam), allowing decoupling of absorption length from junction area and enabling both high speed and high responsivity simultaneously.