Trapped-Charge-Effect-Based Above-Threshold Current Expressions for Amorphous Silicon TFTs Consistent With Pao–Sah Model

Based on the charge analysis in the Pao-Sah model assuming the exponential deep and tail density of trap states (DOS), the above-threshold current expressions are presented. The trapped charge in the deep DOS is included in the threshold voltage. In particular, a trapped charge effect parameter β, which is determined mainly by the tail DOS, is introduced into the current expressions. The parameter clarifies the relationship between the free electrons and the trapped electrons concentration, and shows the relationship between the band mobility μ_b and the constant effective mobility μ_con. The expressions are consistent with the Pao-Sah model and verified by the experimental data. The different effects of the deep and tail DOS are clarified.