Fabrizio Torricelli

Reconfigurable Complementary Logic Circuits with Ambipolar Organic Transistors

Ambipolar organic electronics offer great potential for simple and low-cost fabrication of complementary logic circuits on large-area and mechanically flexible substrates. Ambipolar transistors are ideal candidates for the simple and low-cost development of complementary logic circuits since they can operate as n-type and p-type transistors. Nevertheless, the experimental demonstration of ambipolar organic complementary circuits is limited to inverters. The control of the transistor polarity is crucial for proper circuit operation. Novel gating techniques enable to control the transistor polarity but result in dramatically reduced performances. Here we show high-performance non-planar ambipolar organic transistors with electrical control of the polarity and orders of magnitude higher performances with respect to state-of-art split-gate ambipolar transistors. Electrically reconfigurable complementary logic gates based on ambipolar organic transistors are experimentally demonstrated, thus opening up new opportunities for ambipolar organic complementary electronics.

Transport Physics and Device Modeling of Zinc Oxide Thin-Film Transistors Part I: Long-Channel Devices

Thin-film transistors (TFTs), which use zinc oxide (ZnO) as an active layer, were fabricated and investigated in detail. The transport properties of ZnO deposited by spray pyrolysis (SP) on a TFT structure are studied in a wide range of temperatures, electrical conditions (i.e., subthreshold, above-threshold linear, and saturation regions), and at different channel lengths. It is shown that ZnO deposited by SP is a nanocrystalline material; its field-effect mobility is temperature activated and increases with carrier concentration. On the basis of this analysis, we propose the multiple-trapping-and-release (MTR)-transport mechanism to describe the charge transport in ZnO. By means of numerical simulations, we prove that MTR is a suitable approach, and we calculate the density of states. We show that the tail states extend in a wide range of energy and that they strongly influence the transport properties. Finally, an analytical physical-based DC model is proposed and validated with experiments and numerical simulations. The model is able to reproduce the measurements on devices with different channel length in a wide range of bias voltages and temperatures by means of a restricted number of parameters, which are linked directly to the physical properties of the ZnO semiconductor. For the first time, the charge transport in the ZnO is investigated by means of the MTR, and a consistent analysis based on experiments, numerical simulations, and analytical modeling is provided.

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.

Accurate Analytical Physical Modeling of Amorphous InGaZnO Thin-Film Transistors Accounting for Trapped and Free Charges

A physical-based and analytical drain current model of amorphous indium gallium zinc oxide (a-IGZO) thin-film transistors (TFTs) is proposed. The model considers the combined contribution of both trapped and free charges that move through the a-IGZO film by multiple-trapping-and-release and percolation in conduction band. The model is compared with both measurements of TFTs fabricated on a flexible substrate and numerical simulations. It is accurate in the whole range of a-IGZO TFTs operation. The model requires only physical and geometrical device parameters. The resulting mathematical expressions are suitable for computer-aided design implementation and yield the material physical parameters that are essential for process characterization.

Space-charge-limited current in organic light emitting diodes

A physically based mathematical model for the dc current of single-carrier organic light emitting diodes is presented. The model accounts for the most important physical quantities that influence the carrier mobility and thus the device current itself: temperature, charge carrier concentration, and electric field. It is rigorously developed basing on the variable range hopping transport theory and extends the pioneering work of Mark and Helfrich [J. Appl. Phys. 33, 205 (1962)] to large electric fields typical of light emitting diodes. It was validated on experimental data collected from devices of different materials in a wide range of operating conditions. Thanks to the effective electric field approach, the mathematical expression is simple, accurate and suitable for CAD applications.

A Charge-Based OTFT Model for Circuit Simulation

In this paper, a mathematical model for the dc/dynamic current of organic thin-film transistors is proposed. The model is based on the variable-range hopping transport theory, i.e., thermally activated tunneling of carriers between localized states, and the mathematical expression of the current is formulated by means of the channel accumulation charge. It accurately accounts for below-threshold, linear, and saturation operating conditions via a single formulation, and does not require the explicit definition of the threshold and saturation voltages. Basing on the charge control approach, the dc model is straightforwardly generalized to dynamic conditions; the resulting mathematical expressions are simple and suitable for CAD applications.

Ultra-high gain diffusion-driven organic transistor

Emerging large-area technologies based on organic transistors are enabling the fabrication of low-cost flexible circuits, smart sensors and biomedical devices. High-gain transistors are essential for the development of large-scale circuit integration, high-sensitivity sensors and signal amplification in sensing systems. Unfortunately, organic field-effect transistors show limited gain, usually of the order of tens, because of the large contact resistance and channel-length modulation. Here we show a new organic field-effect transistor architecture with a gain larger than 700. This is the highest gain ever reported for organic field-effect transistors. In the proposed organic field-effect transistor, the charge injection and extraction at the metal–semiconductor contacts are driven by the charge diffusion. The ideal conditions of ohmic contacts with negligible contact resistance and flat current saturation are demonstrated. The approach is general and can be extended to any thin-film technology opening unprecedented opportunities for the development of high-performance flexible electronics.

Single-transistor method for the extraction of the contact and channel resistances in organic field-effect transistors

A simple and accurate method for the extraction of the contact and channel resistances in organic field-effect transistors (OFETs) is proposed. The method is of general applicability since only two measured output-characteristics of a single OFET are needed and no channel-length scaling is required. The effectiveness of the method is demonstrated by means of both numerical simulations and experimental data of OFETs. Furthermore, the provided analysis quantitatively shows that the contact resistance in OFETs depends on both VG and VD , and, in the case of non-linear injecting contact, the drain-source voltage (viz., the electric field along the channel transport direction) plays a major role.

Ambipolar Organic Tri-Gate Transistor for Low-Power Complementary Electronics

Ambipolar transistors typically suffer from large off-current inherently due to ambipolar conduction. Using a tri-gate transistor it is shown that it is possible to electrostatically switch ambipolar polymer transistors from ambipolar to unipolar mode. In unipolar mode, symmetric characteristics with an on/off current ratio of larger than 10(5) are obtained. This enables easy integration into low-power complementary logic and volatile electronic memories.

Transport Physics and Device Modeling of Zinc Oxide Thin-Film Transistors—Part II: Contact Resistance in Short Channel Devices

Short-channel zinc oxide (ZnO) thin-film transistors (TFTs) are investigated in a wide range of temperatures and bias conditions. Scaling down the channel length, the TFT performance is seriously affected by contact resistances, which depend on gate voltage and temperature. To account for the contact resistances, the transistor is ideally split in three parts. The contact regions are modeled as two separate transistors with a fixed channel length and an exponential distribution of localized states, whereas the channel is treated as reported in Part I. The overall model reproduces the measured characteristics at different channel length, with a single set of physical and geometrical parameters. It can be readily implemented in a circuit simulator. Numerical simulations confirm the validity of the model approach and are used to evaluate the impact of nonidealities at the electrode/semiconductor interface.