Accumulation Mode, Linear Regime

Because there is no depletion layer between the substrate and the conducting channel, the equations of the current-voltage curves are in fact simpler in the TFT than in the MISFET, provided the mobility can still be assumed constant (which is not actually the case in most devices, as will be seen below). Under such circumstances, the charge induced in the channel is given, in the case of an n-channel, by Eq. (14.23). In the accumulation regime, the surface potential Vg x) is the sum of two contributions (i) the ohmic drop in the accumulation layer, and (ii) a term V(x) that accounts for the drain bias. The first term can be estimated from Eqs. (14.15), (14.16) and (14.19). In the accumulation regime, and provided Vs kT/q, the exponential term prevails in Eq. (14.16), so that Eq. (14.15) reduces to 

Substituting (14.42) into Eq. (14.19), we see that, once more, the linear term Vg can be neglected with respect to the exponential term. In other words, most of the gate-source ohmic drop occurs in the insulator, so that Eq. (14.23) becomes simply 

43) requires accumulation to extend all along the channel, namely Vd Vg. As the channel is directly in contact with the rest of the semiconductor layer, we have also to accoimt for the bulk conductivity of the semiconducting film when integrating the drain current, which leads to an equation of the form 

is the bulk charge, where wq is the bulk carrier density and ds 

In tlie case of an -type semiconductor in the accumulation regime, the TFT 

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