Design and fabrication of single grain TFTs and lateral photodiodes for low dose x-ray detection

Abstract

Design, fabrication and measurement results of single grain (SG) lateral PIN photodiodes and SG thin film transistors (TFT) are reported in this paper. Devices were developed to be used in indirect X-ray image sensor pixel design. We have controlled position of 6 μm x 6 μm silicon grains with excimer-laser crystallization of a-Si film. Lateral PIN photodiode (PD) arrays were designed inside the single grain with 1 μm, 1.5 μm and 2 μm intrinsic region length and 4 μm width. The gate length and the width of the fabricated TFTs are 1.5 μm and 4 μm, respectively. Devices were fabricated using a-Si, SOI and crystalline silicon layers and electrical measurement results were compared. 100 μm x 100 μm sizes SG-photodiodes have dark and saturation currents on the order of 0.1 nA and 10 nA resulting in a light sensitivity of 200 with an exposure of white light. Fabricated NMOS and PMOS TFTs inside the grains have field effect mobility of 526 cm²/Vs and 253 cm²/Vs, respectively.