1. Introduction

Amorphous indium gallium zinc oxide (a-IGZO) is the most promising potential candidate for switching/driving devices and large area display devices, such as thin film transistor (TFT) backplanes in flat-panel displays. The advantages of its superior characteristics are low threshold voltage, high field-effect mobility, transparent and low-temperature deposition etc^{[1, 2]}. Several groups made an effort to increase the mobility of a-IGZO TFTs formed on flexible substrates by pulsed-laser deposition, the RF-sputtering method and the solution processed at a lower temperature^[3]. IGZO is promising material for electronic devices and circuits^[4]. An efficient drain current simulation model is useful for verifying the thin film transistor characteristics, such as the mobility and on/off ratio etc. Numerical simulation is an indispensable tool to understand the device physical structure and the TFT operation. There are remarkable advances in device fabrication, but there were limited numbers of publications related to the numerical simulation models of a-IGZO TFTs^[5]. Most of IGZO simulation models have been reported on device simulation^{[6, 7]}. The advantages of these types of device simulation models are used to predict electrical parameters without fabricating devices. The a-IGZO TFTs were also used in the application of a radiation harsh environment with a change in electrical parameters with respect to electron irradiation dosage^[8]. Although the effect of electron irradiation on metal oxide semiconductors has been known to be destructive, the investigations about the mechanism and changing tendencies of metal oxide semiconductor to irradiation are crucial^[9]. On the other hand the feasibility of IGZO TFTs and their compatibility of printed electronic elements for space application is challenging^[8]. Many of the papers have reported a variety of radiation effects on TFT’s^[10–12]. However, no simulation model has been reported on the effect of electron irradiation on a-IGZO TFTs. In this paper, a novel drain current simulation model has been developed for the first time to estimate the effect of the irradiation effect on IGZO TFTs. This mathematical model is helpful to predict the electron irradiation effect on electrical parameters of TFTs.

2. Experimental

2.1 Fabrication

A 200 nm thick MoW layer was grown by sputtering onto the top of the glass substrate for conventional bottom-gate TFT structure. IGZO samples with 3∶1∶2 ratios were prepared through sol–gel technique and TFTs are fabricated as in the reported paper^[13].

2.2 TFT Characteristics

The TFT characteristics were analysed by a DC probe station Agilent A 1500 at room temperature. For the operation of a thin film transistor, the following standard Eqs. (1)–(4) are used^[1]. The drain–source current (I_DS) is a function of gate voltage and device physical parameters as in the equation below:

$${I_{\rm DS}} = \frac{{{C_{\rm i}}{\mu _{\rm sat}}W}}{{2L}}{({V_{\rm GS}} - {V_{\rm th}})^2},\;\;\;\;{V_{\rm DS}} \geqslant {V_{\rm GS}} - {V_{\rm th}},$$

(1)

where width (W), length (L) and gate capacitance (C_i) of the device in the capacitance/unit area. Subthreshold swing (SS) is a gate voltage required for the decade increase in drain current at a constant drain voltage. Subthreshold swing is obtained from the equation given below:

$$\left. \text{SS} = \frac{\partial {{V}_{\text{GS}}}}{\partial (\log {{I}_{\text{DS}}})} \right|{_{{{V}_{\text{DS}}} = \text{Constant}}}.$$

(2)

The threshold voltage (V_th), of a thin film transistor is the minimum gate-to-source voltage that creates a conducting path between the source and drain terminals of the device. Threshold voltage is obtained graphically from the equation below:

$$\sqrt {{I_{\rm DS}}} = \sqrt {\frac{{{C_{\rm i}}{\mu _{\rm sat}}W}}{{2L}}} ({V_{\rm GS}} - {V_{\rm th}}).$$

(3)

Saturation mobility is defined as how quickly an electron can move through the active layer, which is obtained by the following equation.

$${\mu _{\rm sat}} = {\left[ {\frac{{{\rm d}\sqrt {{I_{\rm DS}}} }}{{{\rm d}{V_{\rm GS}}}}\frac{1}{{\sqrt {\frac{{{C_{\rm i}}W}}{{2L}}} }}} \right]^2}.$$

(4)

Drain current I_on/I_off ratio is used to examine the switching behavior of the TFT. It is the ratio between the highest measured current (the on-state current, I_on) to the lowest measured current (the off state current, I_off). For output characteristics, gate source voltage (V_GS) is varied from 0 to 10 V. The set of curves are obtained by measurement of the drain current as a function of drain to source voltage (V_DS). Fig. 1(a) shows the output characteristics. For transfer characteristics, drain to source voltage (V_DS) is varied from 0 to 10 V. The set of curves obtained by measurement of the drain current as a function of gate to source voltage (V_GS) are shown in Fig. 1(b) along with the transfer characteristics.

3. Analysis of parameters

3.1 Extraction of parameters from measured and simulation

Various parameters such as threshold voltage (V_th), saturation mobility (μ_sat), I_on/I_off, and subthreshold swing (SS) are extracted. Extraction is done by IGZO TFT values and compared with simulated values. The simulated values of V_th, μ_sat, I_on/I_off and SS for V_DS = 10 V are taken for comparison with practical values as shown in Figs. 2(a)–2(c). Table 1 shows a comparison of the simulated and practical values. There is a slight deviation in these parameters that may be due to the device fabrication process or non linearity in device physical parameters. Further, these TFTs are exposed to an 8 MeV electron source as given below.

3.2 Extraction of parameters effected by electron irradiation

To study the radiation hardened property of IGZO TFTs, these transistors were exposed to electron radiation. Transistors were irradiated with 8 MeV electrons at room temperature from a Microtron accelerator (Fig. 3). The features of the center of the Microtron for electron irradiation are detailed elsewhere^[14]. An irradiation dose of 1 kGy electron was exposed perpendicular to TFTs as shown in Fig. 4. The transistors parameters are extracted from I_DS–V_DS and I_DS–V_GS plots.

After the irradiation of 1 kGy on the sample, it is observed that the parameters such as Threshold voltage, Saturation mobility, SS, and I_on/I_off have been effected. In the case of X-ray and ion irradiated thin film transistors^[10–12] the drain current shift is different from that mentioned in Figs. 6 and 7. This is because electron irradiation stimulates the negatively charged interface trap and oxide trapped charges. Electron irradiation also shows the effect of degradation of these parameters as shown in Figs. 5, 6, and 7(a)–7(c). The parameters are compared with before and after irradiation as in Table 2.

4. Drain current model for irradiation effect

To analyse the effect of electron irradiation, a general model for drain to source current is considered, which is obtained by the linear fit to the curve plotted by taking I_DS on the y-axis and V_DS on the x-axis. A general model is given in Eq. (5)^[15].

$${I_{{\rm{DS}}}} = {P_0} + {P_1}\left[ {1 - \exp \left( {\frac{{ - x}}{{{t_1}}}} \right)} \right] + {P_2}\left[ {1 - \exp \left( {\frac{{ - x}}{{{t_2}}}} \right)} \right],$$

(5)

$${{P}_{0}}={{V}_{\text{th}}}={{V}_{\text{FB}}}+2{{\phi }_{f}}+\text{ }\sqrt{\frac{2{{E}_{\text{s}}}q{{N}_{\text{a}}}\left( 2{{\phi }_{\text{F}}}+{{V}_{\text{sb}}} \right)}{{{C}_{\text{ox}}}}},$$

(6)

$${P_1} = {P_2} = {\mu _{{\rm{sat}}}} = {m^2}\frac{{2L}}{{W{C_{{\rm{ox}}}}}},$$

(7)

$${t_1} = 2\left( {{V_{{\rm{GS}}}} - {V_{{\rm{th}}}}} \right) - V_{{\rm{DS}}}^2 - \frac{{2LW}}{{{C_{{\rm{ox}}}}}},$$

(8)

$${t_2} = \frac{{WL}}{{2{C_{{\rm{ox}}}}}} - {\left( {{V_{{\rm{GS}}}} - {V_{{\rm{th}}}}} \right)^{1/3}},$$

(9)

where V_th is the threshold voltage, μ_sat is the saturation mobility, V_GS is the gate to source voltage applied, and V_DS is the drain to source voltage applied. W and L are the channel width and length of the transistor. C_ox is the gate oxide capacitance per unit area. V_FB flat band voltage, ϕ_F is Fermi potential, V_sb is standby voltage and P₀, P₁, P₂, t₁ and t₂ are variables. Substituting Eqs. (6) to (9) in Eq. (5) we get the drain to source current as

$$\begin{split}{I_{{\rm{DS}}}} = & {V_{{\rm{th}}}} + {\mu _{{\rm{sat}}}}\left( {1 - {\rm{exp}}\frac{{2 - x{C_{{\rm{ox}}}}}}{{2\left( {{V_{{\rm{GS}}}} - {V_{{\rm{th}}}}} \right) - {V_{{\rm{DS}}}}^2 - W \cdot 2L}}} \right)\\ & + {\mu _{{\rm{sat}}}}\left(1 - {\rm{exp}}\frac{{2 - x{C_{{\rm{ox}}}}}}{{{{\left( {{V_{{\rm{th}}}} - {V_{{\rm{GS}}}}} \right)}^{1/3}}}} \right).\end{split}$$

(10)

With the same equation given in Eq. (5), analysis for the drain source current equation after electron irradiation is done. It is observed that there is no significant change in threshold voltage and saturation mobility. Saturation mobility can be estimated to have been decreased to a value known as effective saturation mobility, which is given by effective saturation = μ_sat − x, where x is the degradation factor, which depends upon the dosage of irradiation.

5. Conclusion

A drain current simulation model for solution processed, electron irradiated a-IGZO has been developed to estimate the effect of irradiation effect on IGZO TFTs. The experimental a-IGZO TFT electrical parameters were precisely reproduced by drain current simulation model, and the results suggest that the slight variation in electrical parameters is due to trap charges after electron irradiation. Parameters of drain to source current equations are linear as well as the saturation region being related to the variables of the drain current simulation model. The slight variation in stability of a-IGZO TFT due electron irradiation is reflected in the change in electrical parameters. The threshold voltage and subthreshold swing increase and saturation mobility and I_on/I_off decrease after 1 kGy electron irradiation. The difference in threshold voltage before and after irradiation is mainly due to latent trap charges due to electron irradiation. This model can be used to predict the effect of electron irradiation on a-IGZO TFTs.