Voltage drops, OFETs

As depicted in Figure 2.4.10(b), contact resistance at the source and drain electrodes results in a smaller than expected slope of the potential versus channel position profile. The profile is estimated by linear extrapolation between Vi and V2. Individual source and drain contact resistances are calculated by dividing the voltage drops AVs and AV by the source-drain current, respectively. By isolating the source and drain contact contributions to the total contact resistance, the gated four-probe technique provides more information than the transmission line technique, and it is possible to determine in one device (vs. several). An important caveat for the gated four-probe technique is that the extrapolated channel potential profile wiU only be valid for strict linear regime OFET operation (Vq V, ), where the channel potential profile can be expected to be linear and uniform. 

Alternatively, a local non-contact potentiometry of the transistor channel allows one to measure the potential distribution V (jc) across the entire OFET [70], unveiling thereby possible contact resistances. Due to the local determination of the surface potential, the voltage drops at the source and drain electrodes are accessible. By... 

V2) without passing any current. In the linear regime of OFET operation (Eg Vj,), the channel should be uniform in charge carrier density with a linear drop in electrostatic potential along L from source to drain. Therefore, any drops in electrostatic potential that occur at the contacts (due to contact resistance) will be manifested upon extrapolation of the channel potential profile based on the voltage-sensing probe measurements. 

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