A new type ofphyphen;ihyphen;nIn0.53Ga0.47As photodiode having an optically transparent composite top electrode consisting of a thin semitransparent metal layer and a transparent cadmium tin oxide (CTO) layer was investigated. The composite functions as thenorpcontact, an optical window, and an antireflection coating. The transparent contact also prevents shadowing of the active layer by the top electrode, thus allowing greater collection of incident light. Since the CTO contact is nonalloyed, interdiffusion into theihyphen;region is not relevant avoiding an increased dark current. The photodiodes exhibited leakage currents of le;8 nA and some as low as 23 pA, with reverse breakdown voltages of ge;15ndash;17 V. Responsivity was measured using a 1.55 mgr;m InGaAsP diode laser focused onto an unpassivated 60 mgr;m diamphyphen;ihyphen;nphotodiode and was ge;0.41 A/W. Photoresponse of the diodes to 3 ps pulses from a Nd:YLF laser (lgr;=1.047 mgr;m) was 169 and 86 ps for the 60 and 9 mgr;m diodes, respectively. The maximum frequency response of the 9 mgr;m diode is packaging limited, and is expected to have an intrinsic response time of 20ndash;30 ps.
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