Abstract: An avalanche photodiode is deposited and integrated directly on top of CMOS readout circuitry. The anode of the avalanche photodiode may be independently biased at high voltage so that the avalanche photodiode may be operated in an avalanche multiplication mode. The avalanche photodiode has a multi-layered structure which is not pixilated; and photo-carrier generation and carrier multiplication may take place in the same layer or in different layers. A constant-gate-bias transistor isolates the high-voltage avalanche photodiode from the low-voltage the CMOS readout circuitry.