PHOTOCURRENTS IN POLY-SI TFTS

Steady-state photocurrents have been measured in poly-Si TFTs fabricat ed from columnar poly-Si as well as from material crystallised from am orphous LPCVD and PECVD precursors. With top face white light illumina tion, through the poly-Si gate, the photocurrents from all three techn ologies were comparable and showed similar trends of increasing photoc urrent with film thickness. The photocurrents measured in the thinnest films (600 angstrom thick) were approximately 7 x 10(-14) A/mum of ch annel width/klux. With back face illumination the currents were approx imately four times larger. When compared with dark current values of a pproximately 4 x 10(-14) A/mum in high quality TFTs, it is apparent th at in high light environments, such as LCTV projectors, the photocurre nts in unshielded TFTs can be two to three orders of magnitude greater than the dark currents. From the channel length and gate and drain bi as dependences, the photocurrent was identified as arising from both o ptical generation in the drain space charge region and diffusion from the bulk channel. The weak dependence of the photocurrent on drain bia s meant that field relieving structures, used for the reduction of dar k currents, had a more limited effect upon photocurrents. Spectral pho tocurrent measurements yielded a value for the recombination lifetime, in a columnar poly-Si TFT, of approximately 2 x 10(-10) s. The detail ed spectral response was shown to be due to interference effects in th e multiple layer thin film structure.