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10.41

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Gate Voltage [Volts]

Figure 1: Transfer characteristics of an n-channel a-Si:H TFT. (W=1501.nm, L=60pin, d•=300nm.) Symbols are experinmental data and solid lines are calculated curves (relevant parameters for the model are discussed in [1]). (a) Vs=0.l, 1,5, 10V, T=300K; (b) Vd=10V, T=300, 350, 400K.

below-threshold, when the electron quasi-Fermi level is in the deep states; and above-threshold, when the Fermi level enters the tail states [5]. Following this approach, an a-S:H TFT 1(V) model is derived in [1] by considering the sheet carrier density in the amorphous silicon both above and below threshold. In the below threshold regime, the sheet carrier density is found as a function of the Fermi level position, taking into account the density of deep localized states. In the above threshold regime, the sheet carrier density is given by a modified charge control model which relates the ratio of induced charge in the conduction band to the total induced charge. The complete room temperature model is described in [1]. Here we only describe the enhancements to the model which allow us to describe the temperature dependencies of the current voltage character-

istics and the leakage current at large negative gate biases. The transfer characteristics are shown for both the above- and below-threshold regimes in Fig. 1 (a) for varying Vd at T=300K, and in Fig. 1 (b) for varying temperature, Vds=lOV. The effect of the temperature can be very clearly seen in the subthreshold and leakage regimes. The resultant increase in the density of states causes a severe reduction in the subthreshold slope at high temperatures. This effect will be able to be included in the model through the density of deep states, as described in [6]. Fig. 2 (a)-(c) show the above-threshold i(V) characteristics for a 60getm a-Si:H TFT for varying V and for increasing temperature. There is good agreement between the experimental and modeted data for low temperatures (T=300, 350K), but at T--400K, there are discrepancies between the experimental and modeled curves. This could possibly be explained by the effect of

the temperature stress on the Tcu. Although stress effects are not as severe or as apparent in the above-threshold regime as below threshold (Fig. 1 (b)), they also have an effect on the abovethreshold characteristics. A.Temperature Dependent Model Parameters s(V) model is incorporatendthrough the temperatureThe temperature dependence of the dependent threshold voltage, the leakage current, and the field effect mobility. The threshold voltage, VT, (found from the channel capacitance, [3]) decreases nearly linearly as the temperature increases, as shown in Fig. 3. This relationship was used to model the data shown in Figs. 1 and 2. The leakage current at large negative gate biases is temperature activated with activation

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