Pumped channel MOS photodetector

A new type of photodetector, based on an MOS capacitor structure, is described. It is based on the fact that surface-states under the gate-electrode can act as very efficient recombination centers when the device is operated with a large ac signal. Thus a surface inversion-layer can be depleted of carriers by pumping a gate-electrode located within a much larger oxidized area. The structure investigated here consisted of an SiO2mesa of 0.2 cm diameter on a p-type silicon substrate with two gate-electrodes, each of 3.8×10^-2cm diameter. The conditions for observation of the pumping effect are as follows: (1) Inversion of the oxidized portion by built-in positive charge, (2) moderate surface-state density, and (3) low thermal generation-rate of minority carriers. The pumping effect is detected by measuring the capacitance of the second gate-electrode while the pumping signal is applied to the first electrode. In the pumped condition the entire oxidized surface is very tight-sensitive. The device can be operated in two modes, the integrating mode, where the filling of the initially depleted channel is observed, and the compensating mode, where the pumping frequency is varied to compensate the light-intensity. Cooling to 77°K makes the generation-rate so low that the device stays in the pumped state for many hours. Therefore very low light-levels can be integrated ever this time. Quantum-efficiency for photons arriving at the Si surface is unity under these conditions. Advantages of the device are simplicity of design, large sensitive area, no conductive semitransparent layers over the active part of the device, and sensitivity far into the ultraviolet range.