Study of Electric Field Modulation in Organic Field-Effect Transistors

Abstract

A solid-state test platform has been designed and fabricated that allows characterization of candidate organic semiconductor materials used in organic field-effect transistors. Origins of electric-field modulation in these devices have been investigated. Using a modified four-point-probe technique, it has been found that in addition to the resistance of the organic semiconductor, the resistances of the source and drain contacts are also modulated by the gate electric field. A systematic experimental protocol has been outlined that allows the separation and study of contribution of modulated contact resistances and of the organic film resistance to the overall response. From these measurements the true carrier mobility of the organic semiconductor can be determined.There has been an explosion in the number of the reports on organic electronics, specifically on organic field-effect transistors ͑OFETs͒. 1-6 Their performance and utility are usually measured against conventional silicon-based FETs. The prevailing opinion is that OFETs will not compete with silicon-based transistors in most electronic applications but will have some attractive features that will make them complementary in some aspects. The main, often quoted attractive features of OFETs which appeal to the "disposable electronics" market are their low fabrication cost, mechanical flexibility, and low weight. The low cost is because OFETs are usually fabricated in a one-mask lithographic process that does not require expensive mask alignment. Thus, it is claimed that functional fieldeffect devices can be mass produced by a planar process in which the metal electrodes are printed on flexible dielectric substrates. The deposition of the organic semiconductor ͑OS͒ and the realization of the contacts do not require patterning and therefore bypass the alignment step. The final realization of the device then comes out in two basic forms: OFET with either the bottom or the top metal contacts.The original and the broadest definition of the term "transistor" is a "voltage-controllable resistor." 7 It is a three-terminal device consisting of a pair of metal electrodes called drain ͑D͒ and source ͑S͒ that contact the semiconductor. The third gate ͑G͒ electrode, which controls the current between D and S, is separated by a dielectric through which a dc current cannot pass. Thus, it is the applied electric field from the gate that controls the drain-source current. Hence the device is called insulated gate field-effect transistor ͑IGFET͒. The theory of FET operation has been developed in minute detail for silicon-based devices and has been extended to the operation of many other inorganic semiconductor field-effect devices such as III-V compound semiconductor FETs, 8 etc. The theories developed for the operation of IGFETs have been adopted for the operation of OFETs with only marginal justification and success. 9 When fabricating the devices in a one-mask process, the S and D contacts lie in the same plane as the OS conducting channel. They are therefore modulated ...

Semiconductor Junction Gas Sensors

“…If current is passing through a forward biased junction with a low doped semiconductor or a semiconductor exhibiting low mobility, a depletion layer will be formed in front of the ohmic contact of the Schottky barrier diode, as discussed by Chen et al 85 It can be described in analogy to the depletion polarization effect in an electrochemical experiment by a limited charge carrier transport process inside the semiconductor toward the ohmic contact. The width of the mass-transport limited contact resistance can be modulated by the external field.…”

Field‐Effect Transistors with Mixed Ionic‐Electronic Gate

“…There was no indication of any field-modulation of the overall resistance, up to À 15 V of applied gate voltage. In order to further ascertain the possibility of field modulation we have used an interdigitated coplanar test structure that allows separation of contribution of the contact resistances from the resistance of the active material [4]. It can be also used to study the effect of added Field-Effect Transistors with Mixed Ionic-Electronic Gate perpendicular electrical field on the value of such resistances.…”

“…It is a very different situation from that when current is passed through the OFET with a genuine organic semiconductor, such as pure, undoped poly (3-hexyl thiophene) active layer. In such polymer the depletion layer was formed at the drain contact upon passage of drain current, resulting in formation of the diffusion-depletion space charge, which is then modulated by the gate electric field [4,12].…”

Gas Sensing with Conducting Polymers

“…In that case, the junction resistance becomes much smaller than the polarization resistance, which then dominates the overall resistance of the junction contact. This situation is common to both chemiresistors and field-modulated chemiresistors (aka OFETs) [38].…”

“…It is obvious that, because of the low mobility of the charge carriers in conducting polymers, the polarization space charge region (expressed as polarization resistance R P ) is created by the depletion of the charge carriers at the vicinity of the drain electrode, analogous to the formation of a diffusion depletion layer in the electrochemical experiment. It has been shown that the effect of gate voltage on the modulation of the contact resistance of the drain electrode greatly exceeds the modulation of the interfacial conductivity of the organic semiconductor itself [25,38,41], and the device functions as a field-modulated junction.…”