- PIN Photodiode Definition: The PIN photodiode is a device that converts optical signals into electrical signals through a structured design of p, intrinsic, and n regions.
- Operating Mechanism: Operates by applying a reverse bias voltage that enlarges the space charge region to fully encompass the intrinsic region, where light-induced electron-hole pairs are generated.
- Speed Enhancement: The response speed of a PIN photodiode can be enhanced by minimizing the minority carrier lifetime and maximizing the depletion region width.
- Avalanche Photodiode Difference: Unlike the PIN photodiode, the avalanche photodiode operates at a higher reverse bias, causing impact ionization that quickly multiplies the charge carriers.
- Voltage Requirements for Avalanche: Avalanche photodiodes need between 100 and 200 volts of reverse bias, depending on the semiconductor material, to facilitate impact ionization.
PIN Diode
A PIN photodiode converts incident optical power into photocurrent without internal avalanche gain.
Its name describes the sequence of p-type, intrinsic and n-type semiconductor regions.
The wide, lightly doped intrinsic region separates the p and n regions. Under reverse bias, most photon-generated carriers are created in or reach the depletion region and are collected by its electric field. Compared with simple p-n diodes, this structure can provide lower junction capacitance and a wider absorption region. The exact layer thickness and doping depend on wavelength, speed and sensitivity requirements for the pn junction detector.
A PIN photo diode is often operated with reverse-bias voltage so the intrinsic layer is depleted and its electric field sweeps photon-generated electron-hole pairs to the contacts. Reverse bias also reduces junction capacitance, but it increases dark current and must stay within the device rating. Response speed depends on the circuit RC time constant, carrier diffusion outside the depletion region and carrier transit time through it.
A wider intrinsic layer lowers junction capacitance, which can improve the RC-limited response. It also increases the distance that carriers cross, which can increase transit time. High-speed design therefore balances depletion width, active area, load resistance, bias and absorption depth rather than maximising one dimension. A PIN photodiode also avoids slow diffusion by placing much of the light-absorbing volume inside the electric field. The diagram below shows its basic structure.
Avalanche Photodiode
An avalanche photodiode is a light detector designed for internal carrier multiplication, while an avalanche diode is normally specified for voltage regulation or protection rather than optical detection. In either device, a strong electric field can accelerate a carrier enough to create another electron-hole pair through impact ionisation in the diode material.
An Avalanche photodiode includes a light-absorption region and a high-field multiplication region. Its layer structure is more specialised than a basic PIN photodiode, whose named regions are
- p-region,
- intrinsic region,
- n-region.
An APD operates at a reverse bias high enough to produce controlled multiplication but, in linear mode, below breakdown. Material and device design determine the required voltage, so the 100 to 200 V range does not apply to every APD. A photon first generates an electron-hole pair. One carrier then enters the high-field region, where impact ionisation creates additional carriers and increases photocurrent. This internal gain can improve receiver sensitivity, but the multiplication process adds excess noise and is sensitive to bias and temperature. The manufacturer’s rating and bias circuit for each diode must be followed.
