TFT electrical characteristics

The description of the current-voltage characteristics is essentially the same as for the equivalent crystalline silicon devices, with some modifications to account for the localized states near the band edges. The accumulation charge, Q/, per unit area of channel is 

Saturation of occurs when d7i,/dl = 0 and from this condition the saturation current is 

Fig 10.9 shows the output current plotted in both forms, Eqs (10.14) and (10.16). The mobility is about 1 cm V s which is comparable to the drift mobility of bulk a-Si H. It is cAddent that the Fermi energy in the channel remains within the band tail states and that the band tails are not much perturbed by the close proximity of the dielectric layer. If a nitride thickness of 3000 A with a dielectric constant of 7 is assumed, then the gate capacitance is 2 x 10 F cm . With parameters WjL= 10 and = 1 cm V s , the channel conductance is 10 2 at a gate voltage of 5 V. This value is about 1000 times smaller than crystalline silicon, because of the lower mobility. 

The only other approach to increasing the channel conductance 

One strategy for making L smaller without the need for more precise processing is to reconfigure the device as a vertical structure, and two examples are illustrated in Fig. 10.10. The channel length is now defined by the thickness of the film rather than by the accuracy of the photolithographic patterning. A film thickness of about 1000 A potentially increases the transconductance by a factor of 100. Improved values of have been demonstrated in vertical TFT structures, 

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