characteristics of the devices include drain current Id and gate current Ig as a function of
gate voltage Vg at a fixed drain voltage Vd, drain current on-and-off ratio (IoN/IoFF) and
field-effect mobility. When the current carriers are confined within a narrow channel
layer additional scattering mechanisms have to be considered. The location of the carriers
at the oxide-semiconductor interface introduces additional scattering mechanisms like
Coulomb scattering from oxide charges and interface states, as well as surface roughness
scattering. Generally, field-effect mobility can be obtained both from the
transconductance value and from the saturation current. In field-effect transistors,
transconductance is defined by [134]
BIDS
ngm --s VD (4-1)
GS
The field-effect mobility 1- FE is given by
Lg ,
SFE W V (4-2)
WC OX DS
Where L is the channel length, W is the channel width, Cox is the capacitance of
gate oxide, VDS is the source-drain voltage. The turn-on voltage need not be known for
the determination of LtFE. Another way to calculate field-effect mobility is fitting straight
line to the plots of the square root of drain current vs gate-drain voltage, while the drain
current in the saturated region, Idsat is given by [128]
W
cit = )iFECi (VGS Vth) (4-3)
(h, > -
Where Ci is the capacitance per unit area of the gate oxide.