Junction Field Effect Transistor X-Ray Detectors
- PDF / 292,128 Bytes
- 6 Pages / 414.72 x 648 pts Page_size
- 54 Downloads / 217 Views
JUNCTION FIELD EFFECTTRANSISTOR X-RAY DETECTORS J.C. LUND*, F. OLSCHNER* AND L. REHN *RMD, Inc. 44 Hunt St., Watertown, MA 02172 **InterFET Scientific, 322 Gold St., Garland, TX 75042
ABSTRACT We describe the theory of operation, design, and estimated performance of an n-channel JFET designed to be operated as a detector in an X-ray spectrometer system. We estimate that a room temperature (300 K) JFET detector can be built with performance comparable to a small area, cryogenically cooled Si(Li) detector.
INTRODUCTION In another paper1 we demonstrated that commercially available JFETs, designed as amplifying elements, could be configured as detectors in an X-ray spectrometer system. However, JFETs designed for amplifier applications are less than ideal X-ray detectors due to their very small area and thickness. In this paper we discuss the design of a JFET specifically intended for us as an X-ray detector. Our calculations indicate that such a device would be capable of performance similar to a conventional small area Si(Li) detectors but capable of room temperature operation.
PRINCIPLE OF OPERATION The basic operation of a FET detector is illustrated in Figure 1. When X-rays interact with the silicon in the depletion region of the gate, they produce electron-hole pairs which drift in the electric field of the depletion region, producing a measurable current pulse. Thus, the principle of operation of a JFET detector is the same as a conventional semiconductor detector attached to a WFET input charge sensitive preamplifier. The only difference between a JFET and a conventional system is that the detector and front end amplifier in a JFET detector are merged into one entity.
DESIGN ISSUES The utility of a JFET detector is determined largely by its energy resolution and sensitivity to X-rays. The central problem in designing a WFET detector is trading off increased X-ray sensitivity with energy resolution, because energy resolution worsens with increasing device volume. Energy Resolution The energy resolution of a WET detector is determined largely by noise processes. As in a conventional detector system, the noise in a JFET detector can be broken down into two components, parallel and series, depending on how the noise generator may be represented in an equivalent circuit model of the device 2 .
Mat. Res. Soc. Symp. Proc. Vol. 302. @1993 Materials Research Society
550
Figure 1. Diagrammatic representation of the operation of an n-channel JFET radiation detector. Charge pairs generated by X-ray interaction in the gate depletion region, alter the potential of the gate. The change in gate potential, in turn, produces a relatively large increase in the magnitude of the drain current. Series Noise The dominant form of series noise in a JFET detector system is due to Johnson noise in the channel of the JFET. The equivalent noise charge, ENC, (in units of electrons rms) 2 of the series noise in a FET amplifier (and hence in a FET detector) can be shown to be : ENCseries = (2 Ct/q)(kTan/gmtm)'/ 2
(1)
where tm is the
Data Loading...
