Design and characterization of integrating silicon junction field-effect transistor amplifiers for operation in the temperature range 40-77 K.

Abstract

The very low photon backgrounds to be achieved by future cryogenic astronomical telescopes present the ultimate challenge to the sensitivity of infrared detectors and associated readout electronics. Cooled silicon JFETs, operated around 70 K in transimpedance amplifiers, have shown excellent performance and stability. However, due to Johnson noise in the feedback resistor, the read noise in one second achieved by such amplifiers is about 500 electrons per second. A drastic improvement in sensitivity was demonstrated using a simple form of integrating JFET amplifiers. Therefore, the excellent performance obtained with cooled silicon JFETs has led to the investigation of their properties in the temperature range 33-77 K to explore their full potential and improve the performance of the integrating amplifier. The freezeout effect in silicon JFETs has been characterized both experimentally and theoretically using a simple analytical simulation program. The effect of variation in device parameters on the freezeout characteristic has been studied, and test results showed that an effective channel mobility must be used instead of a bulk mobility in order to simulate accurately the device current and transconductance freezeout at low temperatures. Many types of commercially available JFETs have been characterized below 77 K and measurements revealed that a balanced source follower or a common-source amplifier with active load can operate well down to 38 Kelvin with extremely low power dissipation. The open gate equivalent input noise voltage was found to be optimum below 77 K, due to a decrease in the gate leakage current, in agreement with theoretical prediction. Based on the superior performance of the balanced source follower with active load, a single channel hybrid integrating JFET amplifier with a JFET reset and a compensation capacitor was developed for operation in the temperature range 40-77 K. Read noise as low as 10 electrons in 128 seconds integration was achieved when the integrator was operated at an optimum temperature of about 55 K. Using a similar design, a 16-channel monolithic integrating amplifier array was designed and built. Preliminary test results at 77 K showed noise performance comparable to the single channel hybrid integrator.