A new germanium photodiode with extended long-wavelength response

A new germanium photodiode having an extremely uniform quantum efficiency from the visible to 1.65 micro. has been developed. The device consists of a lithium-drifted junction in which the light enters in a, direction parallel to the junction, thereby allowing absorption path lengths of several centimeters, if desired, with essentially zero dead layer. Sensitive areas up to about 10 mm in one dimension and several centimeters in the other dimension are possible. Typical characteristics for a device 2 cm long having a sensitive area 7 mm x 7 mm are: diode capacitance, 3 pF; charge collection time, 75% of charge collected in 25 nsec at 500 V; measured NEP, 3.7 x 10(-15) WHz(-(1/2)) at 1.625 microm and 77 K for frequencies up to 30 kHz, and 5.5 x 10(-13) WHz(-(1/2)) at 1.625 micron and 195 K. for frequencies up to 40 kHz. At low frequencies, the detectivity can be background limited by 300-K blackbody radiation. The device must be used and stored at reduced temperatures. A convenient cryostat, capable of maintaining the device at or near 77 K for 150 h without refilling, is described.