Highly Sensitive Swir Detector Array Based On Nanoscale Phototransistors Integrated On Cmos Readout

Ultra-sensitive and fast infrared imaging has become increasingly important in applications that require high frame rates at low light levels, such as exoplanet imaging. The sensitivity of conventional short-wave infrared cameras is limited by their readout noise level. This limitation can be addressed by the internal gain of the sensors, but only if fast response time and low dark current are achieved simultaneously. Recent theoretical predictions suggested that reducing the internal capacitance of detectors with internal gain can increase their sensitivity. Here, we show the experimental validation of this prediction for III-V heterojunction phototransistors. We have fabricated a 320x256 array of InGaAs/InP infrared phototransistors integrated with a conventional silicon readout circuit. The array is made of two groups of pixels: 50% are devices with a 1 mu m base diameter and the other 50% with a 2 mu m base diameter. Characterization of a large number of pixels shows that 1 mu m devices have significantly higher sensitivity than 2 mu m devices. These have an average noise equivalent photon sensitivity of about 20 photons at a camera frame rate of similar to 500 frames per second, which is better than the best existing infrared cameras with a similar cutoff wavelength and frame rate. Interestingly, the processing variation in the 1 mu m devices resulted in variation in sensitivity, and a good number of devices show sensitivity to less than 10 photons. These results suggest that the proposed phototransistors are promising for ultra-sensitive short-wavelength infrared cameras.