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A charge-accumu1ationand..mu1tip1ication photodetector (CHAMP) has been developed. The unit cell contains three coupled MOS capacitors designated as detector, transfer, and avalanche gates. The photocu.rrent at the detector gate is first integrated with the transfer gate closed and with the avalanche gate ramping toward avalanche voltages. The charge is then transferred to the avalanche gate where charge multiplication occurs. These processes induce a displacement current pulse for readout which is proportional to the photocurrent, the integration time, and the multiplication gain factor, and is inversely proportional to the transfer time. A tunable current gain capable ofexceeding the avalanche gain is achieved. A cooled CHAMP device is suitable for detecting photons ofextremely low flux. The internal current gain and noise issues are discussed.