(Invited) Contact Effects in Organic Thin Film Transistors with Different Device Structures

The electrical characteristics of organic thin film transistors (OTFTs) are frequently affected by contact effects, which can seriously reduce the transistor performance. The “parasitic” voltage drop at the contacts, induced by contact resistance, decreases the effective drain-source and gate-source voltages applied to the intrinsic channel of the transistor, reducing the device current. The importance of the contact resistance, R_c, is more relevant in the case of high carrier mobility and/or small channel length devices, where its value may become comparable or even larger than the channel resistance. R_c appears to be strongly affected by the device architecture and much higher R_c-values are typically observed, at low drain voltages, in coplanar structures (i.e. Bottom Gate/Bottom Contact, BGBC devices) than in staggered structures (i.e. Top Gate/Bottom Contact, TGBC devices). The presence of Schottky barriers, trap states, field dependence of carrier mobility and defected regions near the electrodes have been suggested as the origin of R_c.

In this work, the contacts effects in devices with different architectures (staggered and coplanar) will be discussed. Devices characteristics have been measured at different temperature and the results have been analysed by using 2D numerical simulations (fig. 1 and 2). The electrical characteristics of both type of devices can be reproduced considering a Schottky barrier at the source contact with an “effective” barrier lowering, that takes into account different field-dependent effects occurring at the contacts (Shottky effect, tunnelling, trap assisted tunneling) that can increase the current injected by the electrode (fig.1 and 2). In the case of staggered devices, the detailed analysis of the current density shows that, at low V_ds and for a given V_gs, the current is mainly injected from an extended source contact region overlapped by the gate electrode, inducing a current spreading along the contact (I_c - current in fig.3). The distribution of the current spreading remains basically constant for increasing V_ds up to a value V_dsat1 (fig. 1), when the depletion layer of the Schottky contact expands  reaching the insulator-semiconductor interface and causing the pinch-off of the channel at the source end. For V_ds>V_dsat1 the current injected from the edge of the source contact rapidly increases (I_edge in fig.3a), due to field enhanced injection mechanisms, while the current injected from the remaining part of the source contact basically saturates. At higher drain voltage (V_ds>V_dsat2, see fig.1), pinch off at the drain occurs. In the case of coplanar devices, current spreading, obviously, does not occurs and even at low V_ds the current increase is related to the field enhanced injection mechanisms that take place at the edge of the source contact (fig.3b). The lack of the extended contact contribution to the injected current explains the higher contact resistance usually observe in coplanar devices at low V_ds compared to staggered OTFTs. On the other hand, at high V_ds (saturation condition) higher electric fields are present, in coplanar structure, near the source, that is in direct contact to the channel region. This results in a more efficient carrier injection that allows obtaining an almost ohmic contact condition in coplanar devices in saturation condition.