1. M. Mahdouani a, W. Boukhili a, C. Tozlu b, R. Bourguigaa
Investigation of negative output differential resistance effect in organic thin-film transistors based on pentacene
M. Mahdouani a, W. Boukhili a, C. Tozlu b, R. Bourguigaa
aLaboratoire de Physique des Matériaux: Structure et Propriétés, Groupe Physique des Composants et Dispositifs Nanométriques, Faculté des Sciences de Bizerte, 7021 Jarzouna-Bizerte, Université de Carthage, Tunisia.
b Department of Energy System Engineering, Faculty of Engineering, Karamanoğlu Mehmetbey University, Karaman, Turkey.
*Corresponding author: 
ABSTRACT: Conjugated small molecule pentacene based p-channel organic thin film transistors (OTFTs) have been fabricated and characterized. In this work, the effect of channel length variation on the electrical parameters of the pentacene-OTFTs is presented. As the channel length (L) of the pentacene-TFTs is decreased from 20 μm to 2.5 μm, a negative differential resistance (NDR) behavior is obtained in the saturation region of the output characteristics. This NDR effect is attributed to the trapping and de-trapping mechanism of the mobile charges at the pentacene–metal electrodes interface. We conclude that the main macroscopic consequence of the channel length variation of OTFTs is NDR effect in its output characteristics.
Fig. 1: Schematic diagram of the bottom-gate top-contact OTFTs and 3D chemical structure of pentacene.
Output characteristics and negative output differential resistance effect
Fig. 2 (a)–(d): Output characteristics curves of pentacene-TFTs with different channel lengths ((a): L =2.5µm, (b): 5µm, (c): 10µm and (d): 20µm.
As shown in Fig. 2, a negative slope, i.e. the NDR effect, in the ID –VD curves was observed clearly from –40V to –100V.
The NDR effect on the fabricated devices is controlled by the channel length variation.
The variation of channel length value from 20 to 2.5µm results in a clear NDR effect in the saturation region of the output characteristics of the OTFTs as seen in Fig. 2(a)-(d).
As seen in Fig. 2 (a)–(d), the drain current of OTFTs increases by applying negative drain voltage at constant gate voltage and reaches saturation region due to the pinch-off phenomena of the accumulation layer.
It can be noted that the fabricated devices are off for low applied VG. This is a major benefit to be considered for low power consumption organic devices.
Conclusions
** Pentacene based organic thin film transistors with different channel lengths were successfully fabricated and characterized.
** We conclude that the main macroscopic consequence of the channel length variation is NDR effect in output characteristics of OTFTs because of the trapping and de-trapping mechanism of the mobile charges.
** As a general conclusion, the variation of the channel length has an important role in the appearance of the NDR effect leading to significant differences in the output curves.
Fig. 3 : Relationship between the NDR effect and the channel length at high gate voltage.
The NDR behavior became more severe for the small channel lengths with the value of L=2.5 and L=5µm. This is essential for integration of organic transistors into electronic circuits.
The NDR effect in the pentacene-OTFTs is attributed to the trapping and de-trapping mechanism of the holes at the interface between the source/drain electrodes and the pentacene active layer .