Photodiodes — Abridged Guide

Quick-reference guide to photodiodes — types, responsivity, noise, APDs, readout, and selection. For full derivations and worked examples, see the Comprehensive Guide.

Comprehensive Photodiodes Guide →

1.Introduction

Photodiodes convert light to current via the internal photoelectric effect with linearity spanning 8+ decades, bandwidths from DC to tens of GHz, and room-temperature operation from UV to mid-IR.

Commercial photodiode products come in four tiers — bare elements, biased modules, amplified (TIA-integrated) detectors, and calibrated power sensors. Match the tier to your integration effort and measurement need.

2.Types and Structures

Cutoff Wavelength

\lambda_c = \frac{1.240}{E_g \;\text{(eV)}} \;\mu\text{m}

PIN photodiodes are the default choice — they offer the best combination of speed, linearity, and simplicity. APDs add internal gain for low-light applications at the cost of high-voltage bias and excess noise.

Choose material by wavelength: Si for 350–1100 nm, InGaAs for 900–1700 nm, Ge for broadband 800–1800 nm (higher noise than InGaAs). UV work requires UV-enhanced Si or GaP.

Wavelength Range	Material	Peak R (A/W)
190–550 nm	GaP	0.10
190–1100 nm	Si (UV-enhanced)	0.50
350–1100 nm	Si (standard)	0.50
800–1800 nm	Ge	0.70
900–1700 nm	InGaAs	1.00
900–2600 nm	Extended InGaAs	0.80

3.Operating Principles

Responsivity

R(\lambda) = \eta \frac{\lambda \;(\mu\text{m})}{1.240} \;\text{A/W}

Photovoltaic mode (zero bias) minimizes noise for precision measurement. Photoconductive mode (reverse bias) maximizes speed and linearity for dynamic signals.

If your measurement is DC or slow (< 1 kHz) and sensitivity matters, use zero bias. If you need bandwidth above 1 MHz, always reverse-bias the photodiode.

4.Key Parameters

Rise Time ↔ Bandwidth

t_r = \frac{0.35}{f_{3\text{dB}}}

RC-Limited Bandwidth

f_{RC} = \frac{1}{2\pi R_L C_j}

Bandwidth is limited by the slower of the RC time constant and the carrier transit time. For most lab setups using 50 Ω termination, the RC limit dominates.

Always check whether the datasheet specifies C_j at the bias voltage you plan to use. Capacitance at 0 V can be 5–10× higher than at 10 V reverse bias.

Active Area	Typical C_j (5 V)	f_3dB (50 Ω)	Use Case
0.05 mm	0.5 pF	>5 GHz	Ultrafast, telecom
0.5 mm	5 pF	~600 MHz	General high-speed
1 mm	20 pF	~160 MHz	Moderate speed
5 mm	200 pF	~16 MHz	Power measurement

Photodiode Signal & Noise Calculator →

5.Noise and Sensitivity

NEP

\text{NEP} = \frac{i_n}{R(\lambda)} \quad \text{[W/}\sqrt{\text{Hz}}\text{]}

Shot Noise

i_{\text{shot}} = \sqrt{2q(I_{ph} + I_d) \cdot B}

NEP is the most important sensitivity specification on a detector datasheet. Lower NEP = higher sensitivity. Compare detectors at the same wavelength and bandwidth.

For the lowest NEP: use a small-area detector, operate at zero bias, and use a high-value TIA feedback resistor (reduces thermal noise — bandwidth drops, but sensitivity improves).

6.Avalanche Photodiodes

McIntyre Excess Noise Factor

F(M) = kM + (1-k)\left(2 - \frac{1}{M}\right)

APDs improve SNR only when amplifier noise dominates. If the detector is already shot-noise limited, APD gain adds noise without benefit. Si APDs (k ≈ 0.02) outperform InGaAs APDs (k ≈ 0.5) in noise at the same gain.

Start with a PIN + low-noise TIA. Switch to APD only if the noise budget analysis shows the TIA noise floor limits your measurement.

APD Gain Optimizer →

7.Readout Electronics

Conversion Gain

G_{\text{conv}} = R(\lambda) \times Z_T \quad \text{[V/W]}

Conversion gain (V/W) is the system-level spec that matters. It combines responsivity and transimpedance into a single number for comparing amplified detectors.

A 50 Ω terminated cable halves the output voltage of detectors with a 50 Ω series output resistor. Always check whether the datasheet quotes conversion gain into 50 Ω or high impedance.

Gain Setting	Transimpedance	Bandwidth	Best For
0 dB	1.5 kV/A	11 MHz	Fast signals, high power
30 dB	47 kV/A	1 MHz	Moderate signals
50 dB	475 kV/A	100 kHz	Weak signals, slow
70 dB	4.75 MV/A	3 kHz	Ultra-weak, DC

8.Practical Considerations

Dark current doubles every 8–10°C in silicon. For precision work, temperature-compensated detectors or controlled environments are essential near the bandgap edge.

For fiber-coupled measurements, use FC/APC connectors to minimize back-reflection. Ensure the fiber mode-field diameter is smaller than the detector active area.

9.Calibration

NIST-traceable calibration with annual recalibration ensures absolute power accuracy of ±1–3%. The calibration data is stored in an EEPROM module that the power meter reads automatically.

OD3 attenuators extend the calibrated range by 3 decades (30 dB). Remove the attenuator for signals below ~1 mW to maximize SNR.

10.Selection Workflow

Follow this sequence: spectral range → material → bandwidth → noise budget → PIN vs. APD → single vs. balanced → product tier → free-space vs. fiber → verify datasheet → calibration needs.

For most laboratory applications, an amplified InGaAs (900–1700 nm) or Si (350–1100 nm) photodetector with switchable gain is the most versatile first purchase.

Continue Learning

Comprehensive Guide →Signal & Noise Calculator →APD Gain Optimizer →

The Comprehensive Guide includes 7 worked examples, 5 SVG diagrams, 4 data tables, and 10 references.