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SCIENCE CHINA Information Sciences, Volume 61, Issue 2: 022401(2018) https://doi.org/10.1007/s11432-016-9049-2

A threshold voltage and drain current model for symmetric dual-gate amorphous InGaZnO thin film transistors

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  • ReceivedNov 24, 2016
  • AcceptedMar 10, 2017
  • PublishedAug 25, 2017

Abstract

Based on the drift-diffusion theory, a simple threshold voltage and drain current model for symmetric dual-gate (DG) amorphous InGaZnO (a-IGZO) thin film transistors (TFTs) is developed. In the subthreshold region, most of the free electrons are captured by trap states in the bandgap of a-IGZO, thus the ionized trap states are the main contributor to the diffusion component of device drain current. Whereas in the above-threshold region, most of the trap states are ionized, and free electrons increase dramatically with gate voltage, which in turn become the main source of the drift component of device drain current. Therefore, threshold voltage of DG a-IGZO TFTs is defined as the gate voltage where the diffusion component of drain current equals the drift one, which can be determined with physical parameters of a-IGZO. The developed threshold voltage model is proved to be consistent with trap-limited conduction mechanism prevailing in a-IGZO, with the effect of drain bias being also taken into account. The gate overdrive voltage-dependent mobility is well modeled by the derived threshold voltage, and comparisons of the obtained drain current with experiment data show good verification of our model.


Acknowledgment

This work was supported by National Natural Science Foundation of China (Grant No. 61274085) and Science and Technology Research Projects of Guangdong Province (Grant No. 2015B090909001).


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  • Figure 1

    Schematics of (a) DG a-IGZO TFT and (b) sub-gap DOS model of a-IGZO, the dotted and solid lines are sketches of free-carrier and trap-state distributions in the bandgap of a-IGZO.

  • Figure 2

    Current-voltage characteristics of DG a-IGZO TFTs from this analytical model (solid lines) and TCAD simulations (symbols) at $T$=300 K. (a) and (b) are transfer curves with $\varphi_{\rm~t}$ set as 0.05 V and 0.065 V; (c) and (d) are output curves with $\varphi_{\rm~t}$ set as 0.05 V and 0.065 V. The dashed and dotted lines represent the drift and diffusion components of drain current respectively.

  • Figure 3

    Variations of $\textit{n}\rm_{free}$, $\textit{n}\rm_{trap}$ and $\lambda$ with the gate voltage.

  • Figure 4

    Comparison of threshold voltage obtained by different definition methods under various (a) $\varphi\rm_{t}$ and (b) $\textit{V}\rm_{DS}$.

  • Figure 5

    Comparison of transfer curves between this model (solid lines) and experiment data (symbols) in (a) [10], (b) [16], with the dashed and dotted lines being the drift and diffusion drain current respectively. Values of parameters in [10]: $\textit{t}\rm_{IGZO}$=30 nm, $\textit{C}\rm_{ox}$=1.73$\times$10$^{-8}$ F$\cdot~{\rm~cm}^{-2}$, $\mu\rm_{0}$=13 cm$^{2}\cdot~{\rm~V}^{-1}\cdot~{\rm~s}^{-1}$, $\textit{g}\rm_{t}$=8.5$\times$10$^{17}$ cm$^{-3}$eV$^{-1}$, $\varphi\rm_{t}$=0.08 V, $\varphi\rm_{F0}$=0.5 V, $\textit{V}\rm_{fb}$=0.4 V, $\textit{m}$=0.7, $\gamma$=0.14. Values of parameters in [16]: $\textit{W}$/$\textit{L}$=40 $\mu$m/5 $\mu$m, $\textit{t}\rm_{IGZO}$=70 nm, $\textit{C}\rm_{ox}$=1.73$\times$10$^{-8}$ F$\cdot~{\rm~cm}^{-2}$, $\mu\rm_{0}$=6.5 cm$^{2}\cdot~{\rm~V}^{-1}\cdot~{\rm~s}^{-1}$, $\textit{g}\rm_{t}$=8.0$\times$10$^{16}$ cm$^{-3}$eV$^{-1}$, $\varphi\rm_{t}$=0.085 V, $\varphi\rm_{F0}$=0.2 V, $\textit{V}\rm_{fb}$=1.8 V, $\textit{m}$=0.9, $\gamma$=0.10.

  • Table 1   Parameters and the calculated threshold voltage in this model
    Symbol $\textit{V}\rm^{lin}\rm_{th}$ $\textit{V}\rm^{sat}\rm_{th}$ W/L $\textit{t}\rm_{IGZO}$ $\textit{C}\rm_{ox}$ $\mu\rm_{0}$ $\textit{g}\rm_{t}$ $\varphi\rm_{t}$ $\varphi\rm_{F0}$ $\textit{V}\rm_{fb}$ m
    (unit) (V) (V) ($\rm{\mu}$m/$\rm{\mu}$m) (nm) (F$\cdot$cm$^{-2}$) (cm$^{2}\cdot~{\rm~V}^{-1}\cdot~{\rm~s}^{-1}$) (cm$^{-3}\cdot~{\rm~eV}^{-1}$) (V) (V) (V)
    TFT in Figure 2(a) and (c) 0.08 $-$0.01 20/20 50 1.73$\times$10$^{-8}$ 15 1$\times$10$^{18}$ 0.05 0.3 0 0.8
    TFT in Figure 2(b) and (d) 0.11 0.14 20/20 50 1.73$\times$10$^{-8}$ 15 1$\times$10$^{18}$ 0.065 0.3 0 0.9

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