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.