Heterojunction III—V alloy photodetectors for high-sensitivity 1.06-µm optical receivers

A new type of 1.06-µm solid-state detector is discussed, the inverted hetrojunction III-V alloy mesa photodiode, which offers quantum efficiencies near 1.00 percent, extremely low capacitance and transit time, and low dark currents. The characterstics of these detectors allow their use in sensitive 1.06-µm optical receivers which promise better signal-to-noise ratios in a number of applications than any other available 1.06-µm photodetector. In particular, an optimization procedure for selecting photodiode and preamplifier parameters to give the best signal-to-noise ratio under signal conditions is discussed and this technique is applied to a proposed system application. It is shown that in this laser-illuminated airborne night imaging system, a small area heterojunction III-V alloy photodiode detector in an optimized receiver should be able to give signal-to-noise ratios much higher than any other 1.06-µm detector approach, even though the other 1.06-µm detectors may have lower noise equivalent power (NEP) values than this receiver. This is an illustration of the fact that such "magic numbers" for detector comparision as NEP are applicable only to comparing similar types of detectors in certain specific types of applications (such as comparing IR photoconductors in a high background application), and are of very little value in determining the relative performance of different types of detectors for a given system application (such as comparing photomultipliers, avalanche photodiodes, and low-noise photodiodes for this application).