InAs charge-storage photodiode infrared vidicon targets

InAs charge-storage photodiode arrays for electron-beam vidicon applications are being developed. The InAs array diodes exhibit sufficiently low reverse leakage currents for the diode to be operated in a charge-storage mode with an electron beam scannig at the conventional TV scanning rate of 1/30 s, at a diode temperature of 77 ° K. The meaured typical reverse leakage current density near liquid-nitrogen temeperature is in the range of 5 × 10^{-10} A/cm^{2} at the reverse bias of 5.5 V with minimum background radiation on the diode. For the low-bias region, a study of the reverse current as a function of temperature indiactes that the current transport mechanism is thermal in nature, and that it is dominated by the generation current in the space-charge region of the junction, The target structure is very similar to the silicon diode array vidicon target; the n-type substrate side of the target receives the infrared image, while the array side is scanned by an electron beam. The spectral sensitivity for radiation passing through the n-type substrate of an InAs photodiode is narrow-band nature and can be greatly increased by the use of a heavily doped n^{+} -substrate, which causes the absorption edge to shift to the higher energy. Such an infrared vidicon target structure can be very effective, for it automatically eliminates large amounts of unwanted background radiation without use of a lossy filter. InAs charge-storage photodiodes are also proven to be very, sensitive high-gain infrared, detectors, because of a carrier multiplication gain which results in quantum efficiencies greater than unity. The internal carrier multiplication is an integral part of the charge-storage array diodes in vidicon operation. InAs charge-storage photodiode arrays have been measured, with a demountable electron-beam TV system and preliminary test results have been obtained. Measurement of a line array with electron beam reveal a high sensitivity at a wavelength of 3.25 µ. The results indicate that very sensitive infrared imaging systems using a beam readout technique are realizable.