Single-charge-carrier-type sensing with an insulated Frisch ring CdZnTe semiconductor radiation detector

Abstract

Performance optimization of an insulated Frisch ring design was investigated for a 3x3x6 mm CdZnTe planar semiconductor detector. The Frisch ring was composed of copper and was insulated from the detector surface with Teflon. Optimization variables included the Frisch ring length and the bias voltage. Optimized overall device performance was found using a 5 mm long Frisch ring extending from the cathode toward the anode, leaving a 1 mm separation between the Frisch ring and the anode. The best energy resolution observed was 1.7% full-width at half-maximum (FWHM) at 662 keV with the ring extending 4 mm from the cathode toward the anode.CdZnTe has shown potential as a room temperature semiconductor radiation detector, but the effect of severe 'hole' trapping inhibits the ability to efficiently collect the total charge. Conventional planar geometry radiation detectors require efficient charge collection of both electrons and holes to produce good energy resolution; hence hole trapping poses a serious problem for CdZnTe radiation spectrometers. Single charge carrier device designs alleviate many of the problems caused by hole trapping. Such 'electron-only' devices rely mostly on the transport of electrons to induce a signal and, by negating the deleterious effects of hole trapping, give improved energy resolution.Frisch-grid-based designs have become a popular choice for semiconductor single carrier radiation detectors [1][2][3][4][5]. The Frisch grid is a conductive screen structure originally fashioned for gas-filled ion chambers and is usually located near the anode [6]. Generally, a potential is applied to the device such that negative charges (electrons) drift through the grid toward the anode. A signal is induced at the anode by the charge motion between the grid and the anode, whereas the Frisch grid screens out the induced signal from slow moving positive ions drifting towards the cathode. Placing the grid near the anode ensures that the origin of induced signal is from those electrons that drifted from the detector volume into the measurement region, thereby causing the signal to form mainly from electron motion.Several methods of creating a Frisch grid effect without an embedded grid have been studied in semiconductor detectors [1][2][3][4][5]. Methods showing promise include "co-planar" and "small-pixeleffect" devices [1][2][3]. A simple method of achieving single-carrier performance was demonstrated with side contacts acting as the Frisch grid [4,5,7]. Unfortunately, all of the aforementioned designs suffer from problems, which include leakage current between the anode and grid, processing difficulties, or electric field distortions.The non-contacting Frisch ring detector eliminates grid-to-anode leakage current while still achieving single-carrier performance [7]. The design utilizes a bar-shaped detector inserted into a conductive ring, which also allows for an insulator filling between the Frisch ring and the detector body [7]. The conductive ring, when connected into the circuit between the a...

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