In this section, output characteristics of a P3HT nanoOFETs fabricated on Silicon wafer (n-doped) are compared. The nanoOFETs are fabricated such that the channel length to oxide thickess (L/tox) varies between 1-10. The effect of decreasing channel length on the performance of a nanoOFET will be clearly observed as the non-saturating output curve in very small channel OFET (L = 100 nm) as compared to relatively larger channel OFET (L = 1 um). As already discussed in very short-channel OFET with L = 70 nm, the output current changes non-linearly with the applied drain-potential in short-channel OFETs. Smaller the channel is the probability of space charge carriers in reaching the drain contact becomes very high. Also, the potential at the drain terminal is also influenced by the potential at the source terminal, thus channel fails to pinch-off. At higher drain potential, space-charge carriers easily reach the drain terminal from the source terminal and results in the non-linear output characteristics.
Figure (1): Output characteristics of four different nanoOFETs with channel length (L = 100 nm (top-left), L = 180 nm (top-right), L = 250 nm (bottom-left) and L = 1000 nm (bottom-left)). All the devices belong to the same Silicon wafer sample (DDE058 with oxide thickness of 100nm), fabricated with the e-beam lithography.
It must be noted that, the current itself between these devices cannot be compared because they have different channel width. Still, the shape of the output curve can be compared to identify the appearance of short channel behavior in these OFETs. In L = 1000 nm OFET (bottom-right), at lower Vgs (-1 V or Vgs = -5 V), the Id has saturated, independent of increasing Vds. However, at higher Vgs (-9, -13 V), the output currects exponentially increases with increasing Vds. Hence, for L = 1000 nm and L/tox = 10, the short-channel behavior when transistor is operated at Vgs = -9 V. It can be concluded that, a P3HT nanoOFET with L/tox < 10 depicts short-channel effect.
Further, when the channel length is smaller than 1000 nm, Id changes non-linearly with Vds which is independen on the applied Vgs. Serious, short channel behavior is observed in L = 100 nm (top-left), where at very low Vgs (Vgs = -1 V), trainsistor output current no longer saturates. Most importantly, in the same device (L = 100 nm) the change in the drain current at Vgs = -1 V and Vgs = -9 V(for Vds = -10 V) is the lowest which is only the factor of 1.2. In the same case, for L = 1000 nm, the change of drain current at Vgs = - 1V and Vgs = -9V (for Vds = -10 V) is the heighest which is the factor of 3.
Graudal loss of gate effect, non-saturating output curves are the primary symptoms of short channel OFETs. They could be minimized by operating a transistor in relatively lower Vds, decreasing the oxide thickness (produces same effect as increasing channel length). Serious, short channel effect is observed in sub-100 nm (L = 70 nm) OFET.