ANALYSIS OF BIAS STRESS ON UNPASSIVATED HYDROGENATED AMORPHOUS-SILICON THIN-FILM TRANSISTORS

Both the subthreshold slope and the threshold voltage in inverted-stag gered amorphous silicon thin-film transistors (a-Si:H TFT's) are vulne rable to metastable changes in the density of states (DOS) due to Ferm i level displacement. In previous work, we have used passivated and un passivated TFT's to distinguish between the effects of bulk states and interface states at the top passivating nitride interface [1], [2], H ere we report the results of experimental measurements and two-dimensi onal (2-D) simulations on unpassivated TFT's, Since there are no top i nterface states, all the observed changes are due solely to the bulk D OS. The subthreshold current activation energies in a-Si:H TFT's are c ompared for n-channel nonpassivated TFT's before and after bias stress . The experimental results agree well with the 2-D simulations, confir ming that the dependence of subthreshold current activation energy on gate bias reveals the distribution of the DOS in energy but cannot res olve the magnitude of features in the DOS. This type of analysis is no t accurate for TFT's with a top passivating nitride, since the activat ion energies in such devices are affected by the interface states.