Photocurrents in poly-Si TFTs

Steady-state photocurrents have been measured in poly-Si TFTs fabricated from columnar poly-Si as well as from material crystallised from amorphous LPCVD and PECVD precursors. With top face white light illumination, through the poly-Si gate, the photocurrents from all three technologies were comparable and showed similar trends of increasing photocurrent with film thickness. The photocurrents measured in the thinnest films (600 Å thick) were ~7×10^-14 A/μm of channel width/klux. With back face illumination the currents were approximately four times larger. When compared with dark current values of ~4×10^-14 A/μm in high quality TFTs, it is apparent that in high light environments, such as LCTV projectors, the photocurrents in unshielded TFTs can be two to three orders of magnitude greater than the dark currents. From the channel length and gate and drain bias dependences, the photocurrent was identified as arising from both optical generation in the drain space charge region and diffusion from the bulk channel. The weak dependence of the photocurrent on drain bias meant that field relieving structures, used for the reduction of dark currents, had a more limited effect upon photocurrents. Spectral photocurrent measurements yielded a value for the recombination lifetime, in a columnar poly-Si TFT, of ~2×10^-10 s. The detailed spectral response was shown to be due to interference effects in the multiple layer thin film structure