Undoped Amorphous Silicon TFTs with n-Channel OR p-Channel Device Operation for the Determination of the Gap States Distribution.

Hydrogenated amorphous silicon thin-film transistors (TFTs) were made with either n-channel or p-channel device operation. Layers of undoped amorphous silicon were deposited in the same run and appropriate contact implantation techniques were used. These devices offer the possibility of investigating unipolar conduction of either electrons or holes. In this way ambiguities in the field-effect (FE) analysis of the density of states (DOS) can be avoided. We present the DOS distribution over the entire band gap region of undoped amorphous silicon deduced from a pair of transfer characteristics (one forward and one in reverse). Both types of TFTs are subject to degradation under continuous accumulation conditions. The rate of current decay under hole accumulation appears to be larger than under electron accumulation, as is expected from the width of the respective tail-states distributions. The ON/OFF current ratios obtained for the p- and n-channel devices were 105 and 107, respectively, and both devices showed good pinchoff behaviour. Therefore, these devices are in principle attractive for application in novel amorphous silicon integrated logic circuits (IC).