Design and operation of self-biased high-gain amplifier arrays for photon-counting sensors Bedabrata Pain and Eric R. Fossum Center for Space Microelectronics Technology Jet Propulsion Laboratory, California Institute of Technology MS 300-3 15, 4800 Oak Grove Drive, Pasadena, CA 91109 ABSTRACT Design and operation of high-gain (>1000), low-power (< 75 jtW), ultra low-noise amplifier arrays are presented. The amplifier array is operated in self-biased mode, such that all amplifiers are biased irrespective of threshold mismatches, and operate with low reset noise. The amplifiers are designed for possible incorporation as pixels of hybrid solid-state photon-counting sensor. The cell pitch is 30 jim in 1.2 m CMOS technology. Design and experimental results from small arrays of the two most promising amplifier circuits are reported. Design issues for obtaining sub-electron input-referred noise from these in-pixel amplifiers are discussed. A performance summary is incorporated. Keywords: Noise-reduction, JR readout, photon-counting, high-gain amplifier 2. INTRODUCTION Many space-based telescopes and spectrometers require ultra-low read noise [1] in order to observe a large number of astrophysical phenomena associated with galactic and stellar evolution, high red-shift objects, etc.[2}. Detection of ultra- low light level signals are also required in a large number of environments involving tactical and strategic military app1icitions, such as night vision. Detection of faint objects require either extremely long integration times to build enough signal to be above the system noise floor, or image intensification using photo-multiplier tubes or micro-channel plates (MCP). MCP approach suffers from the ungainly requirements of high voltage (—5000V), large mass, high power, high dead-times, small dynamic range, and "scrubbing" for stability {3]. On the other hand, flicker noise often limits the exposure time in a conventional JR readout, limiting delectability of ultra-low level JR signals. Jn a typical JR detection system, the analog nature of the signal makes it susceptible to noise pick-up along the entire path of the signal chain. The multiplexer noise, consisting of white noise in the MOS transistors and unwanted clock pick-up, is typically around 10-20 electrons in low-noise systems. Multiplexers with sub 10-electron read noise [4] are far and few between, and tend to suffer from a large response non-uniformity and non-linearity. Detection of faint objects will be greatly enhanced by having readouts with sub-electron read noise. JPL has been exploring a novel approach to ultra-low-noise sensor realization in which the limitation due to read noise can be overcome by counting photoelectrons within each pixel, and generating a one-bit digital signal from each pixel, making the readout system essentially noise free. The sensor has a hybrid structure, similar to conventional JR sensors, with the important difference that the readout chip consists of a novel multiplexer that is sensitive to single photo-electrons. Consequently, such a solid-state JR sensor enables in-pixel photon-counting, greatly erthancing ultra-low light level signal detection capability. Although in-pixel digitization has been previously demonstrated, it is limited to detection of large signal fluxes[5], and are not amenable for solid-state photon-counting. Figure 1 shows the schematic of a unit-cell of the solid-state photon-counting sensor under investigation, consisting of a photo-diode detector (PD) and cascade of high-gain amplifiers with gains Al and A2. The readout multiplexer senses the change in the voltage at the capacitance (C) of the hybrid bump bond (that includes the detector capacitance, bump-to- readout circuit capacitance, sense transistor gate capacitance, and parasitic capacitance) in order to detect the presence of photoelectrons. The circuit operates as follows. The photo-detector is reset using the switch () to bias it in the integrating mode. A photo-electron generated at the photo-diode, changes the potential at the input capacitance C1. This potential is buffered by 081942126X/96/$6.OO SPIE Vol. 2745 / 69