Hydrogenated amorphous silicon thin-film transistors with two-layer amorphous silicon nitride gate insulator

Abstract

A new type of hydrogenated amorphous silicon thin-film transistors (aSi:H TFT``s) with a two-layer amorphous silicon nitride (a-SiN) gate insulator is proposed and fabricated, and the properties of the amorphous silicon nitride film using ammonia/silane gases are investigated. The drain current characteristics of a-Si:H TFT are numerically calculated and the static C-V characteristics are also numerically examined in order to investigate the effects of the hydrogenated amorphous silicon/insulator interface states. In the fabricated a-Si:H TFT with a two-layer a-SiN gate insulator, the two-layer gate insulator is formed by PECVD method by depositing two layers of a-SiN in sequence with two different values of ammonia/silane volume ratio(R). The main advantageous characteristics which are obtained with the proposed structures are low subthreshold slope with high on-current and small threshold voltage shift ($\Delta{V_{th}}$) under a positive gate bias, while basically keeping the conventional a-Si:H TFT structure of the inverted-staggered electrode type. A quasi two dimensional numerical calculation of the drain current using the gradual channel approximation in the a-Si:H TFT with an inverted staggered structure has been performed. The effect of introducing fixed charge and surface states at the front interface between the a-Si:H layer and the a-SiN layer and also at the rear surface opposite to the front interface on the drain current characteristics is examined. From the calculation result, it is found that the field effect conductance is considerably sensitive to both the front interface and the rear surface qualities of the a-Si:H TFT so that on-current, off-current, and subthreshold current are largely affected by acceptorlike front interface state density ($S_{ifn}$), fixed rear surface charge density ($Q_{bf}$), and donorlike rear surface state density ($S_{bfp}$). Finally, the static C-V characteristics are numerically examined. The results indicate that...