There is no universally best photodiode. Start by matching the detector’s specified response to your wavelength, then balance sensitivity, speed, noise, active area, bias, and readout. Silicon PIN devices are a common choice for visible light; InGaAs PIN devices suit many 0.9–1.7 µm applications; avalanche photodiodes add internal gain when the readout noise justifies their added complexity. The right choice depends on the whole optical and electrical system, not one headline specification.
Photodiode technologies differ along three separate axes
“Photodiode technology” can refer to the semiconductor material, the junction or gain structure, or the way the detector is operated and packaged. These are not interchangeable categories: a PIN diode can be made from silicon or InGaAs, for example, while an amplified module may contain a PIN detector plus its own readout circuit.
- Material: silicon, germanium, InGaAs, GaAs, and infrared-sensitive materials such as InAs, InAsSb, PbS, PbSe, and HgCdTe determine the broad spectral response and influence noise and temperature behavior.
- Structure: PN and PIN junctions collect photogenerated charge; APDs multiply charge through avalanche gain; Schottky and MSM structures target specialized speed or spectral needs.
- Operation or package: photovoltaic and reverse-biased photoconductive modes describe electrical operation. A bare diode, amplified module, array, SPAD, and SiPM are different implementation choices.
A detector’s wavelength range is the first gate: inadequate responsivity at the operating wavelength makes otherwise attractive speed or price irrelevant. Hamamatsu’s photodetector selection guide likewise treats wavelength as a central selection factor.
Compare the main technologies at a glance
| Technology | Typical use | Main strengths | Main limitations |
|---|---|---|---|
| Silicon PN | Basic visible-light sensing | Low cost; suitable low dark current | Generally less effective collection and speed than optimized PIN designs |
| Silicon PIN | Visible and near-infrared sensing | Linear, widely available, often low dark current, high-speed options | No internal gain; response falls off near its long-wavelength limit |
| InGaAs PIN | Near-infrared, including many 1.31/1.55 µm links | Good NIR response and speed | More costly and typically higher dark current than silicon |
| Germanium | Broad near-infrared detection | Useful spectral breadth; may be economical | Often higher dark current and poorer noise performance than InGaAs in demanding telecom use |
| APD | Low-light links, ranging, and time-of-flight systems | Internal avalanche gain | High-voltage bias, gain control, temperature dependence, and excess noise |
| SPAD / SiPM | Photon counting and timing | Single-photon sensitivity or multi-cell photon detection | Dark counts, afterpulsing or crosstalk, dead time, and limited linear range |
| Schottky / MSM | Specialized UV or very-high-speed detection | Low-capacitance, fast structures in suitable designs | Responsivity and noise suitability vary; not universal PIN replacements |
| Extended InGaAs and other IR materials | SWIR through mid- or long-wave infrared, depending on detector | Access to wavelengths beyond ordinary silicon or InGaAs | Often greater dark current, cost, cooling, or readout complexity |
These are broad tendencies, not guaranteed device specifications. A specific part’s response curve and datasheet conditions take precedence.
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What the key specifications tell you
A photodiode converts incident optical power into photocurrent. Its usefulness depends on the signal produced, noise under actual operating conditions, and the speed and range of the readout chain. RP Photonics summarizes core photodiode parameters including responsivity, quantum efficiency, bandwidth, dark current, and material choice in its photodiode reference.
- Responsivity, R (A/W): photocurrent divided by incident optical power at a stated wavelength. Compare values at the same wavelength; an APD’s multiplied responsivity should not be compared as though it were PIN responsivity.
- External quantum efficiency: the fraction of incident photons that generate collected charge. Responsivity and quantum efficiency are related, but depend on wavelength.
- Dark current: current with no intended illumination. It contributes shot noise and commonly rises with detector area, reverse bias, and temperature.
- Noise-equivalent power (NEP): optical input power that produces a signal equal to the detector system’s RMS noise in a 1-Hz bandwidth under specified conditions.
- Specific detectivity (D*): a normalized sensitivity metric that helps account for active area and bandwidth when comparing devices.
- Bandwidth and response time: describe how quickly the detector and readout can follow changing light; these depend on the diode, package, bias, amplifier, and load.
- Capacitance, shunt resistance, linearity, and saturation: affect amplifier stability, low-frequency behavior, usable signal range, and maximum output.
NEP and D* are only comparable when wavelength, temperature, bias, bandwidth, area, load, and amplifier inclusion are aligned. Noise also depends on the measurement bandwidth: unnecessary bandwidth admits noise without helping a slower signal. Hamamatsu’s detector selection guide discusses detector and readout noise considerations.
Material choices by wavelength and application
Silicon: visible light and much of the near infrared
Silicon is the usual general-purpose choice for visible-light sensing when cost, low dark current, linearity, and straightforward electronics matter. Representative silicon devices respond from the UV or visible region toward approximately 1.1 µm, but construction, surface treatment, coatings, and depletion depth affect the usable range. The approximate cutoff follows silicon’s bandgap; Hamamatsu explains the relationship in its silicon photosensor overview.
Silicon PIN products serve photometry, encoders, imaging, laser monitoring, and industrial sensing. UV response may require a specially treated device. Larger-area versions ease optical alignment but usually carry more capacitance, making a high-speed design harder. Vishay’s photo-detector catalog and PIN parametric tables show the range of commercial silicon parts; their listed peaks and ranges are product-specific, not universal silicon limits.
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InGaAs: a common choice around 1.3 and 1.55 µm
Standard InGaAs PIN photodiodes commonly cover roughly 0.9–1.7 µm and are widely used for fiber-optic communications, optical power measurement, spectroscopy, and NIR sensing. Product families extend beyond that interval, but extended-wavelength versions involve different trade-offs. Hamamatsu’s photodiode catalog lists multiple InGaAs families and ranges.
Active area can change performance substantially. As one manufacturer-specific example, Hamamatsu specifies the 1-mm G8370-81 for 0.9–1.7 µm, with typical peak photosensitivity of 1.1 A/W at 1.55 µm, maximum dark current of 5 nA, and typical cutoff frequency of 35 MHz under its stated conditions. The 5-mm G8370-85 family member has typical cutoff frequency of 0.6 MHz and maximum dark current of 125 nA. These figures are part-specific, not general InGaAs limits; the manufacturer’s product pages provide the 1-mm specifications and 5-mm specifications.
Germanium and GaAs: narrower reasons to choose them
Germanium offers useful NIR response and can make sense where broad spectral coverage or cost outweighs the need for minimum dark current. In many low-noise telecom systems, InGaAs is the stronger choice because germanium typically has higher dark current and poorer noise performance. This is an application comparison, not a claim that germanium is unsuitable for every NIR measurement.
GaAs can serve visible-to-NIR applications and specialized very-fast detectors. Representative applications cited by RP Photonics place response around 400–870 nm, while engineered structures can achieve very short carrier lifetimes. Actual wavelength and speed depend strongly on the device design.
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Beyond standard InGaAs: infrared detectors are not interchangeable
For wavelengths beyond ordinary InGaAs, select from device-specific families rather than assuming that a material name guarantees a fixed range. Extended InGaAs can reach farther into the SWIR, commonly with increased dark current and reduced sensitivity. Representative InAs devices cover approximately 1–3.8 µm; InAsSb, PbS, PbSe, and HgCdTe cover differing infrared regions depending on composition and construction. HgCdTe can be tuned across a broad infrared span and is often cooled. Hamamatsu’s semiconductor detector guide compares infrared materials and detector behavior.
PbS and PbSe photoconductors can be useful for infrared spectroscopy, but slower response and low-frequency 1/f noise can make them poor fits for fast modulation. Thorlabs discusses product-specific infrared detector characteristics in its infrared detector material. For any of these technologies, verify spectral response, peak wavelength, cutoff, temperature, bias, and speed for the exact part.
Junction structures and gain: PN, PIN, APD, SPAD, Schottky, and MSM
PN versus PIN
A PN photodiode collects carriers at a p–n junction’s depletion region. It is simple and can be low cost, but a smaller depletion region and diffusion of carriers may limit collection efficiency or speed. A PIN diode inserts an intrinsic or lightly doped region between p and n layers. Its wider depletion region supports efficient collection and, under reverse bias, can reduce junction capacitance. PIN is an architecture, not a material: silicon PIN and InGaAs PIN devices have very different spectral and dark-current characteristics.
PIN versus avalanche photodiode
An APD uses reverse bias near avalanche conditions so impact ionization multiplies photogenerated carriers. That gain can help when the transimpedance amplifier’s input-referred noise limits detection. It also multiplies noise, and dark current, gain, breakdown margin, and temperature behavior need careful management. Hamamatsu describes APD operation on its optical sensor pages; Excelitas lists silicon and InGaAs APD families.
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Consider an APD when the optical signal is weak, amplifier noise is a major limitation, and the system can regulate high-voltage bias and temperature. Prefer a PIN device when linearity, broad dynamic range, low complexity, or low cost is more important and the amplifier can meet the sensitivity target. APD gain alone does not establish better system signal-to-noise ratio.
SPADs and SiPMs: photon-counting avalanche devices
A SPAD operates above breakdown in Geiger mode, producing discrete avalanche pulses rather than a continuously proportional photocurrent. A silicon photomultiplier (SiPM) combines many Geiger-mode microcells to detect weak light and estimate photon counts or intensity. They suit photon counting and timing tasks such as time-of-flight, fluorescence, and quantum optics, but have dark counts, afterpulsing, dead time, and limited linear range; SiPM arrays can also exhibit optical crosstalk and saturation. Hamamatsu’s MPPC/SiPM guide covers gain, photon-detection efficiency, and linearity.
Schottky and MSM structures
Schottky photodiodes use a metal–semiconductor barrier and can provide fast carrier collection in suitable designs, including specialized UV detectors. Metal–semiconductor–metal (MSM) detectors use two Schottky contacts. Their low capacitance can enable very high speed, but many designs trade away responsivity or low-noise sensitivity. RP Photonics describes these specialized photodetector structures; choose them for a concrete speed or fabrication need, not as automatic PIN upgrades.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Bias mode and the readout circuit change the result
Photovoltaic mode
In photovoltaic operation the diode has no external reverse bias. This avoids bias-related dark current and is often useful for low-frequency, low-noise sensing. The lower electric field and junction capacitance can limit speed, depending on detector and readout.
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Photoconductive mode
Applying reverse bias increases the depletion region and generally lowers junction capacitance, which can improve speed and linearity. The cost is higher dark current and its associated shot noise, plus bias leakage and protection requirements. Check the device’s voltage rating before applying bias; exceeding limits can damage it. Thorlabs’ photodiode data sheet illustrates reverse-bias effects.
Transimpedance amplifier and total bandwidth
A transimpedance amplifier (TIA) converts photodiode current into voltage. Its input noise, feedback network, stability, and bandwidth can dominate a practical detector system. A diode’s bandwidth rating is not the whole system bandwidth: package parasitics, bias circuit, TIA, cable, termination, filters, oscilloscope or ADC, and software all contribute. Integrated amplified detectors add their own gain, output impedance, saturation, supply, and bandwidth constraints.
Speed is commonly limited by the RC time constant, carrier transit time, diffusion tails from carriers generated outside the depletion region, package parasitics, and amplifier bandwidth. A large active area is easier to align but usually has more capacitance; a thicker absorbing region can aid long-wavelength absorption but increase transit time. Choose only as much bandwidth as the signal needs, then design the TIA and detector together.
Noise, linearity, and optical geometry set usable performance
Build a noise budget, not a dark-current ranking
Relevant noise sources include shot noise from photocurrent and dark current, Johnson noise in resistors, amplifier voltage and current noise, background-light fluctuations, and readout or ADC noise. APDs add excess avalanche noise. Some infrared photoconductors have important 1/f noise at low frequencies. A detector that looks quiet in darkness may be noisy under ambient, pump, or stray illumination; optical filtering, modulation, shielding, lock-in detection, or balanced detection may address that better than changing material.
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Check the actual linear range
PIN devices are generally linear over their rated range, but excessive photocurrent, space charge, load resistance, reverse-bias limits, temperature, optical heating, or amplifier saturation can reduce linearity. APD usable range is more complex because multiplication, excess noise, saturation, and breakdown margin vary with bias and temperature. Datasheets may state maximum photocurrent, maximum linear optical power, damage threshold, maximum average power, or maximum pulsed power—these are not interchangeable limits.
Match active area to the optics
For a fiber-coupled or tightly focused beam, a small-area detector can provide lower capacitance and higher speed, provided alignment is controlled. A larger diode better captures a divergent or diffuse beam and tolerates alignment error, but generally brings more capacitance and may increase dark current and total noise. Consider whether the system is free-space, fiber-coupled, imaging, or diffuse-light before selecting area.
Quick Recap
Choose a detector in a practical sequence
- Specify wavelength. Confirm the actual source wavelength, tolerances, harmonics, and detector response curve. Visible/UV work often points to silicon or a specialized UV part; 1.3/1.55 µm commonly points to InGaAs; beyond 1.7 µm requires extended InGaAs or dedicated IR technology.
- Define optical power and dynamic range. Record minimum, typical, and maximum continuous and pulsed power, background illumination, and required linear range.
- Set the bandwidth. Match detector and TIA bandwidth to signal modulation or pulse timing; avoid excess bandwidth without a reason.
- Decide whether gain is necessary. Start with PIN unless low-light performance and amplifier noise justify APD complexity. Use SPAD or SiPM for photon counting rather than ordinary linear measurement.
- Choose active area and coupling. Balance alignment tolerance against capacitance, speed, and noise.
- Calculate the noise budget. Include diode dark and photocurrent noise, TIA components, bias network, optical background, and ADC.
- Check required conditions. Confirm zero or reverse bias, load, temperature stabilization, and test conditions behind the quoted specifications.
- Choose package and integration. Check optical window/coating, TO-can or surface mount, fiber connector, integrated amplifier, balanced output, cooling, EMI shielding, and qualification needs.
Read datasheets without comparing unlike conditions
- Compare spectral-response curves at the operating wavelength, not just nominal material ranges or peak response.
- For responsivity, record wavelength and whether avalanche gain is included.
- For dark current and noise, align active area, temperature, bias, bandwidth, load, and amplifier inclusion.
- For bandwidth, check the measurement circuit, load, reverse bias, and whether the figure is typical or guaranteed.
- Distinguish maximum photocurrent, linear-power limit, damage threshold, and pulse rating.
- For modules, check gain, bandwidth, saturation, output impedance, power supply, and termination requirements.
- Inspect package, window, coating, active area, and operating-temperature requirements alongside electrical parameters.
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