Threshold Voltage and Space Charge in Organic Transistors

Abstract

We investigate rubrene single-crystal field-effect transistors, whose stability and reproducibility are sufficient to measure systematically the shift in threshold voltage as a function of channel length and source-drain voltage. The shift is due to space-charge transferred from the contacts, and can be modeled quantitatively without free fitting parameters, using Poisson's equation, and by assuming that the density of states in rubrene is that of a conventional inorganic semiconductor. Our results demonstrate the consistency, at the quantitative level, of a variety of recent experiments on rubrene crystals, and show how the use of FET measurements can enable the determination of microscopic parameters (e.g., the effective mass of charge carriers).Organic single-crystal field-effect transistors (FETs) are opening new possibilities for the detailed investigation of the intrinsic electronic properties of organic semiconductors and of their interfaces [1,2]. Transistors where a single-crystal was suspended on top of a gate electrode, have led to the observation of intrinsic transport properties, such as mobility anisotropy [3] and metallic-like temperature dependence [4]. Current work is aiming at the systematic study of microscopic electronic processes in these systems. Examples are the study of polaronic effects at the interface between organic crystals and highly polarizable dielectrics [5,6,7], the analysis of band-like transport at interfaces with low-materials [8,9], and the detailed investigation of electronic transport at metal/organic interfaces [10]. In most cases, a quantitative analysis of the data in terms of well-defined microscopic models has been possible, but the consistency of results obtained in different experiments remains to be verified.Virtually all experiments on single crystal FETs have focused on transport through a well-formed conducting channel, i.e. in the regime when the gate voltage is biased well above the threshold voltage V T . Here, we use rubrene single-crystal FETs for a systematic experimental investigation of the behavior of the threshold voltage itself. Specifically, we have measured the electrical characteristics of short channel transistors as a function of channel length L, and extracted the dependence of V T on L and on source-drain bias V DS . We find that V T systematically shifts to more positive values when L is decreased or V DS is increased, a behavior originating from changes in the space-charge transferred from the contacts into the semiconductor. We model the system using Poisson's equation, under the assumption that the density of states in the molecular crystals has the same functional dependence as in inorganic semiconductors, and find excellent quantitative agreement between experimental data and calculations, without introducing any adjustable parameter. Our results indicate that the effective mass of carriers in the rubrene valence band is close to the free electron mass, provide information about the low-energy density of states, and show that the physical...

Conduction Threshold in Accumulation-Mode InGaZnO Thin Film Transistors

“…The value of R C retrieved for the test structure considered here is 50 kΩ. The main-unknown in Equation (1) is μ b , which typically ranges from 10 to 30 cm 2 /V-s 7 9 . From Equation(1) and the measured I DS − V GS , Q free can be stated as a function of V GS , and related with the free carrier density (n free ) through Q free = q n free λ free .…”

“…The work is applicable not only to thin film field effect transistor (FET) but also the pervasive silicon metal-oxide-semiconductor FET (MOSFET). Given its significance as a crucial indicator of carrier transport and instability in FETs 4 5 6 7 8 9 10 , the conduction threshold has to be determined physically rather than by empirical means 11 .…”

“…In contrast to the silicon MOSFET, the TFT works in carrier accumulation mode regardless of the channel material 6 7 8 9 . This means that there is no polarity inversion of the induced carriers, so it is difficult to physically define the accumulation threshold.…”

Rubrene Microcrystals: A Route to Investigate Surface Morphology and Bulk Anisotropies of Organic Semiconductors

“…In contrast to inorganic semiconductors, the growth of molecular thin films commonly leads to the formation of polycrystalline layers which possess numerous defects such as stacking faults and grain or domain boundaries . Moreover, it has been shown that the molecular conformation and packing motif can be severely affected by the substrate so that the resulting characteristics of thin film devices are largely influenced by extrinsic, interface-related properties (e.g., trap-states) or space charge, and do not reflect the intrinsic properties of the OSC. Therefore, the main body of data on electronic properties of organic materials has been obtained from single crystal studies employing time-of-flight (TOF) measurements or single crystal field effect transistors (FET). , …”