Uni-traveling-carrier photodiode (UTC-PD)
A photodiode in which only electrons cross the depleted collector, while holes are absorbed as majority carriers in a doped layer and never travel at all. The design removes the hole-transport bottleneck of PIN photodiodes, giving detectors that stay linear at high photocurrent with bandwidths beyond 300 GHz.
In a conventional PIN photodiode, light is absorbed in the depleted intrinsic region, and both photogenerated carriers must transit it: electrons one way, holes the other. Holes are the problem. They drift several times slower than electrons, so they set the transit-time limit, and at high optical power their accumulated space charge screens the bias field, slowing and saturating the response just when an application wants more photocurrent.
The uni-traveling-carrier photodiode, introduced at NTT in 1997, splits the two jobs that the PIN's intrinsic layer was doing at once.
How it works. Absorption happens in a thin p-doped InGaAs layer. There, photogenerated holes are majority carriers: they relax within the dielectric relaxation time (well under a picosecond) without going anywhere. The photogenerated electrons diffuse and drift, helped by a built-in quasi-field from doping and bandgap grading, into a separate wide-bandgap InP collector that is transparent at the signal wavelength and fully depleted. Only electrons cross the collector, at overshoot velocities near cm/s. The speed is set by fast-electron transit alone, and the space-charge saturation limit, now governed by electrons instead of holes, sits roughly an order of magnitude higher in current.
What that buys. UTC-PDs have demonstrated 3 dB bandwidths beyond 300 GHz, photomixing output past 1 THz at reduced power, and RF power on the order of 10 mW at 100 GHz, numbers a PIN structure cannot reach simultaneously. The high saturation current is as important as the bandwidth: a detector that stays linear at tens of milliamps can convert the full power of an optical signal into microwaves without distortion.
Where it shows up:
- THz communications and sensing: a UTC-PD illuminated by two lasers offset by the target frequency is the standard photonic THz transmitter.
- Low-noise microwave generation: photodetecting the pulse train of a frequency comb transfers optical stability to a microwave carrier, and the detector's linearity and saturation behavior set the achievable phase-noise floor.
- Analog and coherent links: high local-oscillator power with balanced detection pushes receivers toward the shot-noise limit, which requires photodiodes that tolerate the power.
The trade-off. A thin absorber intercepts little light, so surface-normal UTC-PDs have modest responsivity, typically 0.2 to 0.5 A/W. Waveguide-coupled and resonant designs recover much of it, and the modified UTC (MUTC), which lets part of the absorber deplete, balances responsivity, bandwidth, and power handling; MUTC structures are the usual choice for the highest-power microwave photonics work and for heterogeneous integration onto silicon photonics.
References: Ishibashi & Ito, J. Appl. Phys. 127, 031101 (2020); Nagatsuma, Ducournau & Renaud, Nat. Photonics 10, 371 (2016).