Readout circuit design for noise-based photodetection

Signal from a photoconductive detector is usually provided by assessing the electrical current (photocurrent) that is obtained from the photo-excited carriers arising from the absorption process in the presence of an applied bias field. Usually when analyzing the signal from a photodetector the photocurrent noise is treated as an undesirable degree of imprecision carrying no information. We describe in details a readout circuit designed to access the electronic noise induced by the photo-absorption process in the photodetector, at zero bias voltage, and to convert it into a useful detection signal. The main required attributes for the electronics and the proper operational conditions are analysed and exemplified by simulation and experimental results. A basic readout circuit was implemented and their output-signal to imput-noise equivalence curve was characterized using a calibrated white-noise source. We propose and simulate a simple readout-integrated-circuit (ROIC) cell that can be scaled-down to a micrometer pixel level. The simulation results suggests the proposed ROIC cell allows the infrared-focal-plane-array (IRFPA) to be fabricated in a conventional 0.25 μm CMOS fabrication technology. The spectral response of two quantum-well infrared photodetectors (QWIPs), operating in the mid-wavelength infrared, could be obtained when two readout circuits was directly applied as interfaces of a FTIR spectrometer. Since noise is also present at equilibrium, when no bias voltage is applied and no net current flows through the device, this concept expand the use of photodetectors that have high internal quantum efficiency (good absorption) but not necessarily a high external efficiency (large photocurrent) and opens new possibilities for the design of novel types of photodetector heterostructures.