A high-speed photodiode in which only electrons are the active (fast) carriers — the absorption layer is p-doped so photogenerated holes relax quickly via majority-carrier response, while electrons traverse a separate depleted collection layer. This sidesteps the slow-hole space-charge bottleneck that limits conventional PIN photodiodes at high optical power, giving simultaneously high bandwidth (200+ GHz demonstrated) and high output power/linearity.
UTC-PDs are usually built in III-V (InGaAs/InP), with modified-UTC (MUTC) variants and silicon-integrated demonstrations. They dominate mmWave/THz photonics, optical-to-electrical conversion for wireless, and high-linearity instrumentation; they are a candidate structure for the next datacom bandwidth generation but, like the rest of the receiver layer, the value tends to integrate into the silicon-photonics platform or stay a III-V specialist part rather than a standalone business — see Photonic Photodetection Layer.