A physics-based model of flat-band capacitance for metal oxide thin-film transistors

Wei-Feng Chen,Lei Zhou,Wei-Jing Wu,Jun-Biao Peng,Miao Xu,Guo-Ming Qin,Lei Wang,Jian-Hua Zou

doi:10.1063/1.5023032

Abstract

This paper proposes a physics-based model of flat-band capacitance Cfb for metal oxide thin-film transistors, in which the influences of free carriers and electrons trapped in deep/tail states are taken into account. Hereinto, a parameter Ls is introduced to characterize the screening length in the metal oxide semiconductor, which is similar to the extrinsic Debye length (Ld) for conventional MOS structure. Based on the proposed model, the flat-band voltage Vfb can be consequently determined as the gate voltage corresponding to Cgs equal to Cfb. It is shown that the value of Vfb determined by the proposed model is consistent with that extracted from experimental data of I-V and C-V or the simulated results of 2D device simulator ATLAS. Moreover, we investigate the effect of the parameters of density of states on the flat band capacitance and Cgs-Vgs characteristics of metal oxide TFTs by ATLAS in details. It is found that both tail states and deep states should be simultaneously taken into account for determining the flat band capacitance and flat band voltage of metal oxide TFTs. The proposed model for the flat band capacitance of metal oxide TFTs is expected to be useful for device characterization since it is analytical and physically meaningful.

Highlights

For the conventional silicon-based metal oxide semiconductor (MOS) structure, the flat band capacitance Cfb can be computed from capacitance-voltage (Cgs-V gs) curve based on the analytical solution of surface charge Qs, the flat-band voltage V fb can be determined as the gate voltage corresponding to Cgs=Cfb.[4,5]
An analytical expression of the channel capacitance per unit area Cs at flat band is obtained by taking the limit, in which the influences of free carriers and electrons trapped in deep/tail states are taken into account
We introduce a parameter Ls to characterize the screening length in the metal oxide semiconductor, which is similar to the extrinsic Debye length (Ld) for conventional MOS structure

Summary

INTRODUCTION

Metal oxide semiconductor thin-film transistors (TFTs) have emerged as the most promising technology for flat panel display (FPD) due to moderate mobility, good uniformity, transparency and low temperature fabrication process.[1,2,3] It is important to determine flat-band capacitance (Cfb) and flat-band voltage (V fb) of TFTs for extracting oxide charge, surface potential and density of states (DOS), which are significant parameters for device design, process characterization, or modeling. Migliorato et al.[13] presented an approach to extract V fb of metal oxide TFTs based on the combined analysis of transfer and capacitance voltage characteristics. This method was independent from the DOS information and required no fitting parameters. The calculated results are confirmed to be consistent with the results extracted by other methods and 2D device simulation

MODEL OF FLAT-BAND CAPACITANCE

RESULTS AND DISCUSSIONS

CONCLUSION