Photodetectors (photodiode detectors and modules)

01

Check photosensitivity, quantum efficiency, noise and linearity

Hamamatsu's technical note on Si photodiodes defines quantum efficiency as photocurrent electrons per incident photon, QE = S × 1240 ÷ λ × 100%, with S the photosensitivity in A/W and λ the wavelength in nm. It defines noise equivalent power (NEP) as the light level giving a current equal to the noise current, NEP = noise current ÷ S, in W/Hz½, and says photocurrent is linear over more than nine orders of magnitude, from about 10⁻¹² to 10⁻² W depending on the device and circuit, with the upper limit set by load resistance and reverse voltage. [1]

Hamamatsu's datasheet for its S5971, S5972 and S5973 Si PIN photodiodes gives peak photosensitivity of 0.64 A/W at 920 nm (S5971), 0.57 A/W at 800 nm (S5972) and 0.52 A/W at 760 nm (S5973), dark currents of 0.07 nA, 0.01 nA and 0.001 nA (typ.), and NEPs of 7.4, 3.1 and 1.1 × 10⁻¹⁵ W/Hz½; the violet-enhanced S5973-02 reaches 0.3 A/W and 91% QE at 410 nm. [2]

Editorial calculation: the QE formula reproduces the datasheet figure, 0.3 × 1240 ÷ 410 ≈ 91%; for the S5971 at its 920 nm peak, 0.64 × 1240 ÷ 920 ≈ 86%. Compare detectors at the wavelength you will use, not at each one's peak.

An avalanche photodiode adds internal gain. Excelitas' C30902SH silicon reach-through APD (data sheet Rev. 2022.02) has a 0.5 mm active diameter, 400–1,100 nm response and 84% quantum efficiency at 800 nm. In linear mode at its recommended gain of 250 it lists responsivity as 117 and 135 A/W, 0.4 ns rise and fall time, 800 MHz bandwidth, capacitance as 1.5 and 1.8 pF (the extracted table does not show which of its three columns each of these two figures belongs to; confirm with Excelitas), dark current of 7 nA (typ.) and 10 nA maximum, and NEP of 0.74 fW/√Hz, falling to 0.29 and 0.15 fW/√Hz with one- and two-stage thermo-electric coolers. [3] Breakdown voltage is 185–260 V (225 V typ.) with a temperature coefficient of 0.5–0.9 V/°C, and it can also run above breakdown in Geiger mode to count single photons. [3]

Editorial calculation: dividing the APD's 135 A/W by its gain of 250 gives a unity-gain responsivity of about 0.54 A/W at 800 nm, close to a PIN photodiode's; the APD's advantage is gain ahead of the amplifier noise. Excelitas gives a breakdown temperature coefficient of 0.7 V/°C (typ.) [3]; at that value a 10 °C rise moves breakdown by about 7 V, so gain must be held by bias or temperature control.

Give the supplier the wavelength range, optical power range, required bandwidth or rise time, free-space or fibre input (fibre type and connector), active area, output (current, voltage, amplified gain), noise requirement, single or balanced detection, supply, operating temperature and mounting.

02

Work out rise time and bandwidth from capacitance and load

Hamamatsu's note explains that rise time (10% to 90%) depends on three factors: the time constant of terminal capacitance and load resistance, t₁ = 2.2 × Ct × RL; the diffusion time of carriers generated outside the depletion layer, which can run to microseconds; and carrier transit time in the depletion layer. Rise time is approximately √(t₁² + t₂² + t₃²), and cutoff frequency fc ≈ 0.35 ÷ tr. Applying reverse voltage reduces capacitance and speeds response but increases dark current. [1]

The S5971, S5972 and S5973 series reach cutoff frequencies of 0.1 GHz and 0.5 GHz at 10 V reverse bias and 1 GHz at 3.3 V, with terminal capacitance of 3 pF (S5971/S5972, 10 V) and 1.6 pF (S5973, 3.3 V); photosensitive areas are 1.2, 0.8 and 0.4 mm diameter, and the maximum reverse voltage is 20 V. [2]

Editorial calculation: with Ct = 3 pF into a 50 Ω load, t₁ = 2.2 × 3 pF × 50 Ω ≈ 0.33 ns, a capacitance limit of about 1 GHz; into a 10 kΩ load, t₁ ≈ 66 ns and fc ≈ 5 MHz. Larger load resistance or amplifier gain raises signal but cuts bandwidth, so state the bandwidth at the gain you need. [1]

A single InGaAs product family shows active area trading against speed. OSI Optoelectronics lists its FCI-InGaAs series in TO-18 packages with the same responsivity, 0.9 A/W at 1310 nm and 0.95 A/W at 1550 nm, across four active-area diameters, while capacitance, dark current and rise time all rise with area. [4]

Editorial calculation: values in the first five columns are from OSI [4]; the last column applies Hamamatsu's fc ≈ 0.35 ÷ rise time [1]. Going from the 75 µm to the 500 µm device raises capacitance 13-fold and rise time 50-fold. Using Hamamatsu's QE formula, 0.95 A/W at 1550 nm is a quantum efficiency of 0.95 × 1240 ÷ 1550 ≈ 76%, and 0.9 A/W at 1310 nm is about 85%. [4] [1]

03

PIN, APD, balanced and 10 Gbps modules in the listings

Listings include PIN, avalanche (APD), amplified and balanced photodetectors and high-speed 10 Gbps modules for fibre systems, offered alongside lasers, modulators and fibre sensing equipment. Ask each supplier for responsivity at your wavelength, NEP, bandwidth at the stated gain, saturation power and the fibre or free-space input.

04

Photodiode, window, bias, amplifier and package

The bias row works differently for the two detector types on this page: reverse voltage on a PIN photodiode lowers capacitance at the cost of more dark current, while an APD is biased near its breakdown voltage. [1] [3]

05

Photodetector datasheet entries to request, from spectral range to output gain

State bandwidth at the gain you will run: in the speed section a 3 pF diode drops from about 1 GHz into 50 Ω to about 5 MHz into 10 kΩ. [1]

06

Send the optical, speed and output data together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Light: [wavelength; power range; free-space or fibre]
Speed: [bandwidth or rise time]
Noise: [NEP; dark current]
Detector: [PIN; APD; balanced; area]
Output: [current; amplified; gain]
Mechanical: [package; connector; mounting]
Environment: [temperature]

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