Signal and noise analysis of a-Si:H radiation detector-amplifier system

Abstract

Hydrogenated amorphous silicon (a-Si:H) has potential advantages in m',tking radiation detectors for many applications because of its deposition capability on a large-area substrate and its high radiation resistance. Position-sensitive radiation detectors can be made out of a 1-d strip or a 2-d pixel array of a-Si:H pin diodes. In addition, signal processing electronics can be made by thin..film transistors (amorphous silicon or polysilicon "ITTs) on the same substrate. The calculated radiation signal, based on a simple charge collection model agreed well with results from various wave-length light sources and I MeV beta particles on sample diodes. The total noise of the detection system was analyzed into (a) shot noise and (b) 1/f noise from a detector diode, and (c) thermal noise and (d) 1/f noise from the frontend TFr of a charge-sensitive preamplifier. The effective noise charge calculated by convoluting these noise power spectra with the transfer function of a CR-RC shaping amplifier showed a good agreement with the direct measurements of noise charge. The , derived equations of signal and noise charge can be,used to design an a-Si:H pixel detectoramplifier system optimally.Signals from a pixeI can be readout using switching TFTs, or diodes. Prototype tests of a double-diode readout scheme showed that the storage time and the readout time are limited by the resistances of the reverse-biased pixel diode and the tbrward-biased switching diodes respectively. A prototype charge-sensitive amplifier was made using 1 poly-Si TFTs to test the feasibility of making pixel-level amplifiers which would be required in small-signal detection. The measured overall gain-bandwidth product 'was -400MHz and the noise charge was~10,00 electrons at a 1 p.sec shaping time. When the amplifier is connected to a pixel detector of capacitance 0.2 pF, it would give a charge-tovoltage gain of~0.02 mV/electron with a pulse rise time less than 1O0 nsec and a dynamic range of 48 dB.