Europe has currently no full supply chain of CMOS image sensors (CIS) for space use, certainly not in terms of image sensor manufacturing. Although a few commercial foundries in Europe manufacture CMOS image sensors for consumer and automotive applications, they are typically not interested in adapting their process flow to meet high-end performance specifications, mainly because the expected manufacturing volume for space imagers is extremely low.
This paper is a theoretical analysis and a comparison of imaging sensor technologies compatible with active imaging
applications that operate in the SWIR band. Two types of sensors are commonly used: detectors with gain induced by
internal photoelectron multiplication process (HgCdTe APD FPA and InGaAs/InP TE EBCMOS) and detectors without
internal gain (InGaAs FPA). Active imaging requires a sensitive high-speed sensor able to detect a short, weak pulse
backscattered from a target. The goal is to reach a sensitivity defined by a NEPh of 10 photons rms, and a SNR of 5-10
for an input signal of 100-200 photons/pulse/pixel. We demonstrate that the HgCdTe APD FPA detector with gain is
intrinsically the best technology to fulfill the aforementioned requirements. In comparison, the InGaAs/InP TE
EBCMOS also works well but suffers from limited quantum efficiency, less than 30% at the most. The main advantage
of detectors with internal gain is their potential to amplify the laser signal above the readout noise level, under the
condition that the excess noise factor induced by the multiplication process remains close to 1. The InGaAs FPA
detector without gain is lower cost, however it exhibits a NEPh of around one order of magnitude higher and a SNR 3 to
6 times lower. At a system level, a more powerful laser will compensate for this difference but will also increase the
cost. In conclusion to the tradeoff regarding performance versus cost, each specific system need should determine the
best matched technology.
Charge balance in the depletion region of a photodiode can be used to reduce the space charge effect. A partially depleted absorber (PDA) InGaAs/InP photodetecter with space charge balance and transit time balance has been demonstrated. A 1dB large signal compression current of 24mA was achieved at 48GHz for an 8μm × 8μm photodiode. A 100μm-diameter photodiode achieved a maximum saturation current of 199mA at 1GHz.