  In this paper, two dimensional modulation of the potential in sexithiophene
(T6) / N,N-bis(n-octyl)-dicyanoperylenediimide (PDI-8CN2) heterojunction field
effect transistors due to the specific microstructure at the interface is used
to explain the negative transconductance effect (NTC) experienced in
sexithiophene (T6) / N,N-bis(n-octyl)-dicyanoperylenediimide (PDI-8CN2)
heterojunction field effect transistors. The NTC effect has been experienced in
tunnel devices, such as the tunnel diode, the resonant tunneling field effect
transistors (RT-FETs), resonant tunneling double barrier devices. In grid-gate
modulation-doped field effect transistors, instead, a periodic potential
barriers in the direction of the transport of charges was used to explain the
negative transconductance (NDR). Since in T6 / PDI-8CN2 heterojunction field
effect transistors the NTC effect is irrespective of the order of the
semiconductor layer and since the modulation of the transport properties is
deeply influenced by the island dimension of the semiconductor layer, we argue
that the origin of the NTC effect resides in the achievement of a specific
microstructure of the heterostructure in the charge transport plane.
