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 microns 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 and having a sensitive area 5 mm × 5 mm are: diode capacitance: 3pF; charge-collection time: 75% of charge collected in 25 ns at 500 volts. Predicted NEP: less than 10^-14watts/cycle^1/2at 1.65 µm and 77°K for frequencies up to 100 KHz, less than 0.5 × 10^-12watts/cycle^1/2at 1.65 µm and 195°K for frequencies up to 5 MHz. 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 hr without refilling, is described.