PHOTODETECTOR CIRCUITS

A photodetector circuit incorporates an APD detector structure ( 10 ) comprising a p- silicon handle wafer ( 12 ) on which a SiO2 insulation layer ( 14 ) is deposited in known manner. During manufacture a circular opening ( 16 ) is formed through the insulation layer ( 14 ) by conventional photolithography and etching, and an annular p+ substrate contact ring ( 18 ) is implanted in the handle wafer ( 12 ) after opening of the window ( 16 ). The APD itself is formed by implantation of a p region ( 20 ) and an n+ region ( 22 ). After the various implantation steps a metallisation layer is applied, and annular metal contacts are formed by the application of suitable photolithography and etching steps, these contacts comprising an annular contact ( 26 ) constituting the negative terminal and connected to the p+ substrate contact ring ( 18 ), an annular metal contact ( 28 ) constituting the positive terminal and connected to the n+ region ( 22 ) of the APD, and source and drain contacts ( 30 ) and ( 32 ) (not shown in FIG. 1 ) connected to the source and drain of one or more CMOS MOSFET devices of the associated CMOS readout circuitry fabricated within a Si layer ( 34 ) formed on top of the insulation layer ( 14 ). Such an arrangement overcomes the problem of combining APDs with CMOS circuits in that APDs operate at relatively high reverse bias (15-30V) and CMOS circuits operate at low voltage (SV), and the arrangement must be such as to prevent the high bias voltage from affecting the operation of adjacent CMOS transistors.