Matteo Rapisarda

Contact effects in high performance fully printed p-channel organic thin film transistors

Contact effects have been investigated in fully printed p-channel organic thin film transistors with field effect mobility up to 2 cm2/Vs. Electrical characteristics of the organic thin film transistors, with channel length <200 μm, are seriously influenced by contact effects with an anomalous increase of the contact resistance for increasing source-drain voltage. Assuming that contact effects are negligible in long channel transistors and using gradual channel approximation, we evaluated the current-voltage characteristics of the injection contact, showing that I-V characteristics can be modeled as a reverse biased Schottky diode, including barrier lowering induced by the Schottky effect.

Evidence of Correlated Mobility Fluctuations in p-Type Organic Thin-Film Transistors

We report on the results of noise measurements in p-type organic thin-film transistors (TFTs) extending from the subthreshold region into the strong accumulation region over four decades of drain current values. The low-frequency noise produced by the devices can be successfully interpreted in the context of a multitrap correlated number fluctuation-mobility fluctuation (CMF) theory, while neither phonon-induced mobility fluctuation nor carrier number fluctuation mechanisms are capable of justifying the observed noise behavior. The Coulomb scattering parameter is found to be in the order of 107 Vs/C, about three orders of magnitude larger with respect to crystalline silicon MOSFETs and comparable with what already reported in hydrogenated amorphous silicon TFTs, suggesting a much more relevant contribution coming from CMF in disordered materials.

Modeling of Capacitance Characteristics of Printed p-Type Organic Thin-Film Transistors

We studied the capacitance characteristics of printed p-type organic thin-film transistors (OTFTs) under various frequencies and dc bias conditions. The experimental results show that the device capacitance is largely influenced by parasitic capacitances, related to the large gate-active layer overlap areas required by the printing processes. We developed a nonquasi-static small-signal capacitance model that adopts a transmission line approach and considers the specific layout of the OTFTs, taking into account for the parasitic capacitances and, hence, is particularly well suited for printed devices. In addition, the model included parasitic impedance at the metal-organic semiconductor contacts, related to the nonohmic behavior of source-drain contacts. The model has been shown to nicely reproduce the experimental capacitance characteristics in all their features. It should be pointed out that the proposed model allows the reproducing of any device layout and could be implemented in device simulator to analyze small-signal dynamic characteristics.

Self-heating effects on the electrical instability of fully printed p-type organic thin film transistors

Bias stress instability has been investigated in printed p-channel organic thin film transistors. The observed instability is related to two mechanisms: one, dominating at low T and causing “mobile ions” like threshold voltage variations is probably due to creation/annihilation of acceptor-like states; the second one, causing charge-trapping like instability, dominates at high T. High drain voltage bias stress experiments, inducing device self-heating, present threshold voltage variations, suggest a channel temperature rise ranging from 50 to 60 °C. The results point out the role of self-heating on the bias-stress instability, which is related to a combination of bias and temperature conditions.

The Role of Defective Regions Near the Contacts on the Electrical Characteristics of Bottom-Gate Bottom-Contact Organic TFTs

We studied, by 2D numerical simulations, the effects of poor semiconductor morphology near the source and drain contacts of BGBC-OTFTs. The variations of the electrical characteristics and of the path of the injected carriers in the transistor channel have been analyzed considering different defective regions, parameters (mobility, density of states) and contact thicknesses. The results showed that 100 nm wide defective regions can induce high contact resistance, resulting in large variation in the electrical characteristics. However, the typical S-shape in the low-Vds output characteristics is clearly observed only considering a combination of highly defected regions and Schottky barrier at the contacts. Furthermore, the simulations showed that most of the current is injected and extracted, at the source and drain contact, within a few nanometers from the semiconductor-dielectric interface. This explains the small influence of the contact thickness on the simulated electrical characteristics, at least for a contact thickness down to 10 nm.

Correlated Mobility Fluctuations and Contact Effects in p-Type Organic Thin-Film Transistors

Low-frequency noise (LFN) has been used in order to gain insight into the physical properties of the materials involved in organic thin-film transistors (OTFTs) fabrication, often with contradictory results. Besides the physical origin of noise, contact effects on noise have been a source of concern and discussion. In this paper, we report on accurate LFN measurements in p-type staggered top-gate OTFTs over four decades of channel current, from the subthreshold to the strong accumulation region. The measured spectra follow a clear 1/f behavior attributed to the trapping/detrapping of channel charge carriers into interface and oxide defects, while the influence of noise sources at contacts is found to be negligible. However, contacts affect the measured noise by a nonnegligible differential resistance. Noise data are interpreted in the context of a multitrap correlated mobility fluctuations (CMFs) model, showing that noise is dominated by acceptor-like traps. Despite the low mobility (μeff ~ 2 cm2/V/s), the large scattering parameter (α ~ 107 Vs/C) produces an increase of the noise at the higher currents due to CMFs. The product αμeff ≈ 2·107 cm2/C, which measures the strength of CMFs, is similar to what was reported for a-Si:H and much higher with respect to crystalline silicon MOSFETs revealing a strong correlation between CMFs and the state of disorder of the active layer.

Reduction of Short Channel Effects and Hot Carrier Induced Instability in Fully Self-Aligned Gate Overlapped Lightly Doped Drain Polysilicon TFTs

Electrical characteristics of fully self-aligned gate overlapped lightly doped drain (FSA-GOLDD) polysilicon thin-film transistors (TFTs), fabricated with a spacer technology and providing submicron (0.35 μm) LDD regions, have been analyzed. Device characteristics show negligible series resistance of the LDD region while effective drain field relief has been demonstrated by a reduced kink effect and off-current, if compared to conventional self-aligned (SA) devices. Short channel effects are also mitigated by the LDD region, while substantial reduction in the hot-carrier induced instability is found, when compared with conventional SA devices. Optimum doping dose of the LDD region has been identified to be 9 × 1012 cm2.

Invited) Short Channel Effects and Drain Field Relief Architectures in Polysilicon TFTs

Applications of polycrystalline silicon (polysilicon) thin film transistors (TFTs) to active matrix organic light emitting displays require further performance improvement. The biggest leverage in circuit performance can be obtained by reducing channel length from the typical current values of 3-6μm to 1μm, or less. However, short channel effects and hot-carrier induced instability in scaled down conventional self-aligned polysilicon TFTs can substantially degrade the device characteristics. To reduce these effects and allow proper operation of the circuits, drain field relief architectures have to be introduced. In this work we show that a fully self-aligned gate overlapped lightly doped drain (LDD) structure, with submicron LDD regions, can provide an excellent solution, allowing effective short channel effect control and improved electrical stability.

A large signal non quasi static compact model for printed organic thin film transistors

In this work we present a large signal non quasi static (LSNQS) compact model based on the discretization of the current continuity equation using a spline collocation approach. The underlying charge/current model is based on the VRH theory. The LSNQS model takes in consideration the presence of parasitic regions and is particularly suited for simulations of printed organic thin film transistors. Some results obtained by a Verilog-A implementation are presented.

Water stable organic thin film transistors (TFTs) made on flexible substrates

We have investigated the stability of the electrical performances in staggered organic thin film transistors (OTFTs) manufactured by using a CYTOP fluoropolymer as a gate dielectric, after the devices underwent reliability cycles in a high moisture and temperature controlled environment chamber. The results of such testing cycles showed a very high stability against the environmental conditions that might be related to the device top-gate structure, working as an efficient encapsulation, and to the hydrophobic properties of the fluoropolymer used as gate dielectric.