Subthreshold swing

K is a constant that depends on materials and device structure n is the ideality factor k is Boltzmann s constant T is the absolute temperature 

Gate currents are often ignored, but for the polymer dielectrics and low-temperature inorganic dielectrics often used in OTFTs, the gate current can be significant. 

In a MOSFET, the channel capacitance is a complicated bias-dependent quantity. The standard model is as follows [5]. For Vqs V, the channel capacitance is zero, but for Vqs Vi it is greater than zero and depends on Vps- When Vps = 0, the channel capacitance is simply the parallel plate expression WLC , which by symmetry is partitioned equally between source and drain terminals so that Cgo = WLCJ2. The situation is more complicated for DS 0 because symmetry no longer holds. In saturation, Cc5j,i,a ei = 2/3 WLC and = 0. For values of 

Vj s between zero and the voltage required for saturation, there is a smooth transition between these regions. 

In display design, one is often concerned with the charge injected into a node of a circuit when a TFT used as a switch is shut off. Thus, one is led to consider not the small-signal capacitances, but rather the charges go Go and Qs associated 

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Figure 9.7a shows an example of a-Si H TFT transfer characteristics in linear regime with different W/L ratios and an example of a-Si H TFT characteristics in saturation regime with W/L = 170 pm/6 kpm. A threshold voltage (Vth) of 10 = 11 V, a field-effect mobility (p) of 0.2 0.3 cm2/(V s), a subthreshold swing slope of 0.8 dec/V, and a current ON/OFF ratio of larger than 106 for VGS from —10 to 30 V were obtained from these curves for a-Si H TFTs. These devices were used in 4-a-Si H TFTs AM-PLEDs. The electrical properties of a-Si H TFTs used in 3-a-Si H TFTs AM-PLEDs are described in Ref. [18]. 

Fig. 6.9. Statistical distribution of (a) mobility, (b) switch-on voltage, and (c) subthreshold swing of 49 pentacene TFTs on the same substrate.

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