The transition frequency ft indicates the upper limit for the operative frequency of a transistor and represents an important figure of merit for the evaluation of the device performance in a real application. A detailed investigation of this parameter is important in the evaluation of an OFET ac operation.
By modulating the gate/source voltage with a small ac signal νgs around a bias point VGS, the current flowing in the channel is the sum of a static component ID and an ac component id, proportional to νgs:
in which gm is the transconductance of the device and it is proportional to the mobility μ and to L-1.
However, during the frequency characterization of an OFETs in top gate, bottom staggered electrodes, in which the gate electrode overlap both the source and drain contacts, the id depends on gm as well as on the paracitic overlap capacities and channel capacity.
We can then introduce the following gate capacitive current:
ig = iGS + iGD = j2πf(CGS + CGD)νgs
in which CGS = Cgs + CCH-s and CGD = Cgd + CCH-d . The total current that flows from source to drain finally is:
iD = id - ig
At this point we can define the transition frequency ft as the frequency where id = ig: below this frequency the id current dominates and iD is constant in frequency (ideal behaviour), and above it current increases with frequency as it is dominated by ig.
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