A new method for determining the Subgap Density of States in n-/p-Type Low-Temperature polycrystalline-silicon thin-film transistors

A new method for determining the acceptor-/donor-like density of states (DOS) over the entire bandgap using both the measured multi-frequency capacitance-voltage characteristics and the differential ideality factor in n-/p-Type low-Temperature polycrystalline-silicon thin-film transistors (LTPS TFTs) is proposed and verified. The density of deep states as a function of the gate voltage (Vgs) is obtained using the differential ideality factor, which is calculated from the transfer curves. The density of tail states as a function of Vgs is obtained using the subgap DOS-induced capacitance (CLOC), which is calculated from the multi-frequency capacitance-voltage and resistance-voltage characteristics. The relationship between Vgs and energy, which is used to convert the DOS as a function of Vgs into the distribution of the DOS as a function of energy for device simulation, is defined using the frequency-independent gate capacitance (CG), composed of the gate oxide capacitance (COX), CLOC and the free-carrier charge induced capacitance (CFREE). The device simulation results using the acceptor-/donor-like DOS obtained by the new method for n-/p-Type LTPS TFTs exhibit excellent agreement with the measured data.