<title>Junction CCD-based x-ray sensor for dental applications</title>

Abstract

ABSTRACTCharge-Coupled Devices are used as x-ray sensors. For dental purposes it is important to have an image area ofroughly 3x4cm2. In case of direct imaging with an MOS Charge-Coupled Device one is restricted to image areas of about 1 .5x2.5 cm2. The reason for this is that in commercially available CCDs one aims at getting a high spatial resolution on asmall area. This article will show, that Junction Charge-Coupled Devices do not have this restriction, which makes thesedevices perfectly suited for dental purposes.1. INTRODUCTIONThe charge-tranfer channel in a Junction Charge-Coupled Device (JCCD) is formed by potentials applied on the gates,the epitaxial layer and the substrate. Unlike MOS CCDs, the gates in a JCCD are p-type diffused areas in an n-typeepitaxial layer on a p-type substrate. The gates are impurity doped regions instead of metallization. As no gate oxide ispresent, there can no charge, generated by radiaton, be trapped in it. This gives the JCCD an advantageous position forradiation detecton. As the gates in a JCCD can not be overlapping, the pixel size in a JCCD must be larger than in anMOS CCD. This means, that with the same number of pixels the image area of a JCCD will be larger than that of anMOS CCD. The large size makes the JCCD ideal for direct imaging in dentistry. The fabrication process of the JCCDallows a good yield for these large JCCDs.2. JUNCTION CHARGE COUPLED DEVICE2.1. Operation of a JCCDA Junction Charge-Coupled Device (JCCD) is a buried channel CCD'. The gates are p-type diffused areas in an n-type epitaxial layer (n-epilayer), which has been grown on a p-type substrate. Fig. 1 shows this structure and theelementary gate structure is shown in Fig. 2a. To create the channel, the gates and the substrate are kept at groundpotential and a positive voltage V is applied to the epitaxial layer (Fig. 2b). This will result in depleted areas around thejunctions. When the positive voltage on the epitaxial layer is high enough, both depleted regions will touch, thus creatingthe channel (Fig. 2c). The maximum potential below the gate is called the channel potential. When a positive voltage isapplied to a gate, while the adjacent gates are kept at zero Volts, a potential well for electrons is created, as under gate Bin Fig. 3. If electrons are present in this potential well and the voltages at the gates are clocked in a certain sequence, thecharge packet of electrons will be transferred synchronously through the JCCD.2.2. Typical properties of a Junction CCDThe Junction CCD differs in some respect from an MOS CCD. The most obvious difference with an MOS CCD is the374