Performance test for a pixelated silicon sensor with junction field effect transistor

Highlights

• Pixelated silicon sensors with JFET structure are manufactured by double-sided fabrication process.

• Characteristics of the fabricated pixelated sensors are described.

• The JFET switch performances are studied by measuring the drain currents as a function of the drain voltage for different gate voltages.

• The performance of the fabricated sensor is determined by measuring the sensor’s response to LEDs and X-rays.

• This sensor concept can be applied to the developments of medical and radiation detectors.

Abstract

We fabricated a pixelated silicon sensor with a junction field effect transistor (JFET) on a $650\mathrm{\mu }\mathrm{m}$ thick, high-resistivity ($>$ 5 k$\mathrm{\Omega }$ $\cdot$cm) n-type double-sided polished 6-in. silicon wafer using a double-sided fabrication process. The JFET, which had a cylindrical structure, acted as a switch to read out charges accumulated in the pixelated sensor. We investigated the electrical characteristics of the pixelated sensor, which had a size of $100\mathrm{\mu }\mathrm{m}$ $\times$ $100\mathrm{\mu }\mathrm{m}$. We also measured the drain current as a function of the drain voltage for different gate voltages to examine the switching performance of the JFET and optimized the design parameters of the pixelated sensor for the proper functioning of the switch. Furthermore, the pixelated sensor’s responses under illumination by a light-emitting diode and X-rays were measured. The electrical characteristics and responses of the pixelated sensors are presented in this paper.

Introduction

A scintillator coupled with a charge-coupled device (CCD) has been widely used as a detector in structural studies in X-ray protein crystallography [1], [2]. This detector is not an optimum in a synchrotron environment owing to the long CCD readout time, radiation intolerance of the scintillator, and non-uniformity of the fiber-optic taper. A conventional CMOS pixel sensor with metal–oxide–semiconductor field effect transistors had been used for visible light and X-ray imaging, but it has the disadvantage of being insensitive to X-rays owing to its shallow junction depth [3], [4]. To address these drawbacks, we studied a matrix pixel sensor with switch transistors on high-resistivity silicon material, which is sensitive to X-ray energies, typically between 8 and 12 keV, for use in protein crystallography.

The photon detection efficiency of silicon for low-energy X-ray radiation is very high, and therefore, a silicon sensor can be used in direct detection devices without employing a scintillator. Consequently, the problem of blurring of the position of the interaction point in the silicon caused by the scintillator can be avoided. Since the absorption length of silicon for the X-ray energy region of interest is between 66.3 and $215.8\mathrm{\mu }\mathrm{m}$, the thickness of a silicon sensor should be at least $450\mathrm{\mu }\mathrm{m}$, that is, twice the absorption length [5], [6]. A pixel-type sensor was adopted in the sensor design in order to improve the spatial resolution.

In the present study, a $650\mathrm{\mu }\mathrm{m}$ thick 6-in. silicon wafer was used to fabricate a pixelated sensor, and a double-sided fabrication process was employed for using the entire silicon as an active sensor. A junction field effect transistor (JFET) with a cylindrical structure was employed in the pixelated silicon sensor to read the charge accumulated in the sensor [7], [8].

The active volume of the silicon sensor was depleted by applying a negative bias voltage to the junction side. X-ray illumination of the junction side produced electron–hole pairs in the active sensor volume, and the electrons produced were swept to the other side of the sensor. Until being transferred during the readout cycle, the electrons were stored on a charge-storage capacitor which occupied most of the pixel area. During data accumulation the switches were closed by applying the reverse bias voltage to the gate terminal and then were selectively opened for data readout by removing the reverse bias voltage. These electrons provided the position information for the conversion point. X-ray illumination of the pixelated sensor integrated with the JFET resulted in the opening of the switches in every pixel, and the charges were transferred from the source to the drain in the JFET. All the pixels in a row were read in parallel, and after one row was read, the next row was selected by the controlling voltage [9], [10].

The concept of a pixelated silicon sensor with a JFET and simulation results for such sensors were presented in an earlier study [11]. In another study, the JFET design variables affecting the switching of the device were determined for a JFET fabricated by a single-sided fabrication process [12]. In this paper, we present the results of an investigation of a pixelated silicon sensor with a JFET. The sensor was fabricated by a double-sided fabrication process, and the investigation pertained to the switching performance of the JFET, the electrical characteristics of the sensor, and the sensor’s response to illumination by a light-emitting diode (LED) and X-rays [13], [14].