Band-Bending Effect in the Characterization of Subgap Density-of-States in Amorphous TFTs Through Fully Electrical Techniques

We report a model for the band-bending effect (BBE) for improved extraction of the subgap density-of-states (DOS) in amorphous semiconductor thinfilm transistors (TFTs). In previous works, the potential (Psi(x)) across the amorphous active layer was assumed to be the same as the surface potential (Psi S) over the active layer without the BBE. Due to the distributed DOS (g(E)) over the bandgap and the modulation of the quasiFermi level (E-F(x,V-GS)) by the gate bias, the non-uniform potential distribution should be considered in the characterization of DOS. We propose an empirical quadratic potential model (Psi(x,V-GS) =Psi S(V-GS)(1 - x/t(IGZO))(2)) for the BBE and extract a corrected distribution of DOS [ g(E)]. We applied the BBE model to the amorphous indiumgallium- zinc oxide TFTs through the differential ideality factor technique. We extracted a corrected DOS as a superposition of two exponential functions with tail and deep state densities (N-To = 8.8 x 10(17) eV(-1)cm(-3), and N-Do = 9x10(16) eV(-1)cm(-3)) at the tail and deep state characteristic energy (kT(T) = 18 meV and kT(D) = 280 meV), respectively. We confirmed the potentialmodel and extracted DOS parameters by the TCAD simulation.