A novel radiation imaging detector with proportional charge gain

Using electric field to partition the selenium layer into a low field charge drift region and a high field avalanche gain region was first proposed in 2005⁽¹⁾. Engineering and fabricating such a grid structure on a TFT array have been a challenge. High dielectric strength material (up to several hundred volts/um) is required. Furthermore, it is very difficult to achieve or control a stable and uniform avalanche gain for imaging without too much excess noise from the elevated grid structure about the pixel plane. Image charge gain is non-uniform depending on the distance from the center of the avalanche well. A novel coplanar detector structure is now being tested. All image charges collected on a dielectric pixel surface will transfer to the central pixel readout electrode along a converging field. Uniform gain via a stable avalanche process can be achieved. This new structure does not require a conventional TFT platform and higher temperature fabrication process can be used. Imaging charges generated from x-ray are first directed to a dielectric charge collection interface surface. During the sequential rolling image readout, imaging charges in each line are re-directed to an orthogonal lines of central readout electrode by a convergent field with high electric field strength at the rim of each pixel central electrode. All accumulated image charges need to pass through the end point of this converging field and therefore undergo a uniform impact ionization charge gain. This gain mechanism is similar to a proportional counter in radiation detection.