Glossary entry

PIN photodiode

A photodetector with an undoped (intrinsic) absorber region sandwiched between p-type and n-type contacts. The standard linear photodetector for optical communication and measurement.

A PIN photodiode has a wide intrinsic (lightly doped) absorber layer between p-doped and n-doped contact regions. Under reverse bias, the intrinsic layer is fully depleted, and incident photons absorbed in this region generate electron-hole pairs that are swept apart by the built-in electric field, producing photocurrent.

PIN structure is optimized over a simple p-n junction by:

Increasing absorber thickness → higher quantum efficiency, broader spectral coverage
Reducing junction capacitance → higher bandwidth
Separating absorption from doped contact layers → reduces minority carrier diffusion currents that limit speed

Responsivity for an ideal PIN ( $\eta_q = 1$ ) is set by the wavelength-to-energy conversion:

\mathcal{R}_\text{ideal} \;=\; \frac{q \lambda}{h c}.

Real PINs achieve 70–95% of this value, with the deficit due to surface reflection, incomplete absorption, and carrier recombination before collection.

Typical PIN specifications at 1550 nm:

Detector class	Responsivity	Bandwidth	Dark current
InGaAs, 100 μm diameter	0.85 – 0.95 A/W	$\sim$ 1 GHz	0.5 – 5 nA
InGaAs, 30 μm diameter	0.85 – 0.95 A/W	10 – 20 GHz	0.5 – 2 nA
InGaAs, high-speed (waveguide-coupled)	0.7 – 0.9 A/W	30 – 100 GHz	1 – 10 nA
Silicon (visible/NIR)	0.4 – 0.6 A/W	DC – 100s of MHz	0.1 – 5 nA
Germanium (NIR)	0.6 – 0.9 A/W	$\sim$ 1 GHz	100 nA – 1 μA

PIN photodiodes are inherently linear over many decades of input power (from below the dark current noise floor up to the onset of saturation at $\sim$ 1–10 mW). For coherent receivers, balanced PIN pairs cancel common-mode noise and double the signal swing.

For sensitivity beyond the PIN dark-current limit, avalanche photodiodes provide internal gain at the cost of additional noise and bandwidth-gain tradeoffs. PIN performance characterization is covered in Photodetector Characterization: Responsivity, NEP, and Bandwidth.