Ohmic currents transistor

When the current is switched very rapidly, the rectifier may not be able to respond with infinite speed. The electrical contacts to the rectifiers or transistors or vacuum tubes discussed here are, by preference, nonrectifying or ohmic, which occurs when the Fermi levels bend, but meet with conduction bands on both sides of the junction. When, however, the motion of electrons (or holes) from semiconductor to metal encounter a barrier that is higher in one direction than in the opposite direction, then the junction is called rectifying, and one speaks of a Schottky34 barrier [17]. 

Illustrated in Figure 24.4 is the output characteristic of a pentacene OFET with Au drain-source electrodes and a 200 nm Si02 dielectric [32]. The OFET exhibits unipolar p-type behaviour with a hole mobility = 0.165 cmWs, a threshold of = -4.5 V as well as an On/Off ratio of >10. These parameters have been derived from the respective transfer characteristics. The absence of an s-shaped feature in the linear range of the characteristic indicates ohmic contacts between the Au electrodes and the pentacene active layer. This is attributed to the good matching of the ionisation potential of the organic semiconductor and the Au work frmction. However, employing a Ca drain-soirrce metallisation, with an otherwise identical OFET device structure, the transistor did not exhibit any current in the electron accumulation mode. This is unexpected, since the metal work frmction is well matched to the electron affinity of pentacene. 

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