InGaAs photodetector

The alloy In₀.₅₃Ga₀.₄₇As has the same lattice constant as indium phosphide, so it can be grown on InP substrates without strain or dislocations, and its bandgap of about 0.75 eV puts the absorption edge at 1.65 µm. Every telecom band from 1260 nm to the end of the L-band lies inside its response, and InP, with a 1.34 eV gap, is transparent at those wavelengths and serves as the window and contact layers. That combination made InGaAs the detector material of fiber communication from the 1980s onward.

The common structure is a PIN photodiode: an undoped InGaAs absorption layer between p- and n-doped InP. Light passes through the InP and is absorbed in the InGaAs, and the depleted layer sweeps the carriers out. Responsivity follows from $R = \eta\lambda/1.23984$ A/W with $\lambda$ in micrometers, so a quantum efficiency of 80% gives 1.00 A/W at 1550 nm and 0.85 A/W at 1310 nm. The absorption-layer thickness trades efficiency against transit time: a few micrometers absorb nearly all of the light at 1550 nm, while high-speed receivers use thinner layers, waveguide coupling, or the UTC structure to reach bandwidths beyond 50 GHz.

The same material serves in other detector types. Avalanche photodiodes keep InGaAs for absorption and move multiplication into a separate InP or InAlAs layer, where the field can be high without tunneling in the narrow-gap material. Gated InGaAs/InP SPADs count single photons at 1550 nm. Two-dimensional InGaAs arrays bonded to silicon readout circuits make the short-wave infrared cameras used for inspection and beam profiling at telecom wavelengths. Extended-wavelength InGaAs, with more indium and a strained or graded buffer, reaches about 2.6 µm at the cost of much higher dark current.

The narrow bandgap brings thermally generated dark current. Dark current rises steeply with temperature and with area, which is why large-area InGaAs detectors in power meters and cameras are often cooled; the smallest high-speed photodiodes have dark currents of nanoamperes or less and are rarely limited by it. On silicon photonic chips, germanium photodetectors take InGaAs's place, because germanium can be grown in a CMOS process; InGaAs remains the choice where the highest responsivity and lowest dark current at 1550 nm are needed. Measurement of these figures is covered in Photodetector characterization.

References: S. M. Sze, K. K. Ng, Physics of Semiconductor Devices, 3rd ed. (Wiley, 2007); G. P. Agrawal, Fiber-Optic Communication Systems, 5th ed. (Wiley, 2021).