PHOTOCURRENT RESPONSE-TIMES IN AMORPHOUS-SILICON THIN-FILM TRANSISTORS

The transient photoconductivity in the source-drain channel of amorpho us silicon based thin film transistors (a-TFTs) was studied as a funct ion of the applied gate voltage. The response time, tau(R), measured u nder pulsed HeNe laser illumination increases from about 2 mu s at neg ative gate voltage which is typical for intrinsic a-Si:H films. to abo ut 10 mu s at large positive gate bias. In the latter case, the shape of the photocurrent decay is similar to that observed in doped samples . At low light intensities, tau(R), reaches values of several 1000 mu s. The results are interpreted on the basis of gate voltage dependent Fermi level position and a non-homogeneous carrier density profile acr oss the semiconductor film.