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 () is set by the wavelength-to-energy conversion:
Real PINs reach roughly 70–80% of this value at 1550 nm, where the ideal is 1.25 A/W. The deficit comes from reflection at the surface, light that passes through the absorber without being absorbed, and carriers that recombine before they are collected.
Typical PIN specifications at 1550 nm, except for silicon, which does not absorb at 1550 nm and is listed near its peak around 900 nm:
| Detector class | Responsivity | Bandwidth | Dark current |
|---|---|---|---|
| InGaAs, 100 μm diameter | 0.85 – 0.95 A/W | 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 | 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 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.
When the photodiode is the back-facet monitor of a laser diode, the stability of its reading relative to the front-facet output is the laser's tracking error.
References: S. M. Sze and K. K. Ng, Physics of Semiconductor Devices, 3rd ed. (Wiley, 2007). G. P. Agrawal, Fiber-Optic Communication Systems, 4th ed. (Wiley, 2010), chapter 4. B. E. A. Saleh and M. C. Teich, Fundamentals of Photonics, 3rd ed. (Wiley, 2019). J. Michel, J. Liu, and L. C. Kimerling, "High-performance Ge-on-Si photodetectors," Nature Photonics 4, 527 (2010).