20 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 44, NO. 1, FEBRUARY 1997 CMOS Low-Noise Switched Charge Sensitive Preampliﬁer for CdTe and CdZnTe X-Ray Detectors Claudio G. Jakobson and Yael Nemirovsky Abstract— CdTe and CdZnTe X-ray detector arrays for imag- ing and spectroscopy provide low capacitance current sources with low leakage currents. The optimal shaping time for low- noise operation is relatively high in CMOS analog channels that provide the readout for these detectors. The shaper is centered at lower frequencies, and thus the 1/f noise from the electronics is the main noise source that limits the resolution of the channel. The optimal dimensions of the input stage MOSFET are determined by this noise. In this paper a design criterion for the optimization of the resolution and the power consumption in a 1/f noise dominated readout is introduced. A readout based on CMOS switched charge sensitive preampliﬁer without feedback resistor has been designed and fabricated in the CMOS 2- low-noise analog process provided by MOSIS. This design provides high sensitivity and the possibility to integrate a large number of channels with low power consumption. Measurements of the performance of a ﬁrst prototype chip are presented. Index Terms— CdTe, CdZnTe, charge sensitive preampliﬁers, CMOS ampliﬁers, low-noise ampliﬁers, X-ray detectors. I. INTRODUCTION C DTE and CdZnTe detector arrays have been recently attracting signiﬁcant attention for imaging and spec- troscopy [1]. A good performance can only be achieved with a carefully optimized analog channel for electronic readout that takes into consideration the unique features of these detectors. These features include a low capacitance source of charge packets (down to a few thousand electrons), a low leakage current, and a collection time of the order of 1 s. CMOS low-noise ampliﬁers for silicon microstrip readout have been presented in [2]–[6]. A general description of the electronic readout system which is based on a charge sensitive preampliﬁer (CSP) was previously reported [7]–[9]. This paper presents CMOS analog channels which provide the readout to CdTe and CdZnTe detectors. It is shown that the 1/f noise from the electronics is the main noise source that limits the resolution of the channel. This is due to several factors: i) the reduction of the detector leakage current provides a reduction in the frequency at which the optimal resolution is found; ii) the corner between 1/f noise and thermal noise in MOSFET’s is found at a relatively high frequency; and iii) the large Manuscript received May 30, 1996; revised September 5, 1996 and Septem- ber 17, 1996. This work was supported by the Kidron Foundation. The authors are with Kidron Microelectronics Research Center, Department of Electrical Engineering, Technion-Israel Institute of Technology, 32000, Haifa, Israel (e-mail: nemirov@ee.technion.ac.il). Publisher Item Identiﬁer S 0018-9499(97)01510-4. collection time of the CdTe and CdZnTe detectors of the order of 1 s forces a reduction in the center frequency of the shaper, regardless the optimum found from noise considerations, to reduce ballistic effects. In this paper a noise analysis is presented, based on these facts and on the gate voltage 1/f noise behavior of p-channel MOSFET’s, which has been recently conﬁrmed [10], [11]. Based on these unique features, expressions for the determi- nation of design parameters are derived (Section II). A ﬁrst prototype has been implemented through MOSIS [12] using the CMOS 2 low-noise analog process. The chip design and the measured characteristics are presented in Section III. In this design a CMOS switch is used to discharge the feedback capacitor. The preampliﬁer has high sensitivity that reduces the inﬂuence of the noise introduced by the shaper. Results of the operation of the ampliﬁer and switch with low feedback capacitance are reported. Noise and resolution measurements from the analog channel are presented in Section IV. The results conﬁrm the inﬂuence of the noise sources considered in this study. It is veriﬁed that the 1/f noise is the dominant noise source for the frequency range of interest, according to considerations in Section II. II. NOISE ANALYSIS The resolution of the X-ray channel is determined by three main noise sources: white noise from the detector, 1/f noise, and channel thermal noise from the input stage transistor of the electronics. The resolution of the channel, deﬁned as the minimum detectable variation of the measured parameter, is expressed in terms of the equivalent noise charge ENC. The expression for the equivalent noise charge for the noise sources mentioned have been calculated in [2] and [4] assuming that the noise ﬁltering is performed by a semi-Gaussian pulse shaper of order . It has been shown that an increasing of above 2 does not improve considerably the resolution [2]. Assuming and that the input MOSFET is working in saturation, the expressions for ENC are (1) 0018–9499/97$10.00  1997 IEEE