Matteo Ghittorelli

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.

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.

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.

Analytical Physical-Based Drain-Current Model of Amorphous InGaZnO TFTs Accounting for Both Non-Degenerate and Degenerate Conduction

In this letter, we propose a physical-based analytical drain current model for amorphous indium-gallium-zinc oxide thin-film transistors (a-IGZO TFTs). As a key feature, the model accounts for both the non-degenerate and the degenerate conduction regimes, including the contributions of trapped and free charges. These two conduction regimes as well as the trapped and free charges are essential to consistently describe a-IGZO TFTs. The model is compared with both exact numerical calculations and measurements. It is continuous, symmetric, simple, and accurate. The model enables to gain physical insight on the material and device properties, and it is a valuable tool for fast process optimization and circuit design.

Physical Modeling of Amorphous InGaZnO Thin-Film Transistors: The Role of Degenerate Conduction

In amorphous indium-gallium-zinc oxide thin-film transistors (a-IGZO TFTs), the electron mobility easily exceeds 10 cm2/Vs and degenerate band conduction is observed. On the other hand, the field-effect mobility is gate voltage-dependent. Here, we propose a physical model for a-IGZO TFTs accounting for both the non-degenerate and degenerate conductions of trapped and free charges. The comparison between the model and the measurements shows that: 1) the shape of the drain current is almost completely defined by the localized density of states and 2) a transition from non-degenerate-to-degenerate conductions is always observed. This explains the measured gate voltage-dependent field-effect mobility and provides a simple and unified physical picture of the charge transport in a-IGZO TFTs.

Quantum tunnelling and charge accumulation in organic ferroelectric memory diodes

Non-volatile memories—providing the information storage functionality—are crucial circuit components. Solution-processed organic ferroelectric memory diodes are the non-volatile memory candidate for flexible electronics, as witnessed by the industrial demonstration of a 1 kbit reconfigurable memory fabricated on a plastic foil. Further progress, however, is limited owing to the lack of understanding of the device physics, which is required for the technological implementation of high-density arrays. Here we show that ferroelectric diodes operate as vertical field-effect transistors at the pinch-off. The tunnelling injection and charge accumulation are the fundamental mechanisms governing the device operation. Surprisingly, thermionic emission can be disregarded and the on-state current is not space charge limited. The proposed model explains and unifies a wide range of experiments, provides important design rules for the implementation of organic ferroelectric memory diodes and predicts an ultimate theoretical array density of up to 1012 bit cm−2.

Balancing Hole and Electron Conduction in Ambipolar Split-Gate Thin-Film Transistors

Complementary organic electronics is a key enabling technology for the development of new applications including smart ubiquitous sensors, wearable electronics, and healthcare devices. High-performance, high-functionality and reliable complementary circuits require n- and p-type thin-film transistors with balanced characteristics. Recent advancements in ambipolar organic transistors in terms of semiconductor and device engineering demonstrate the great potential of this route but, unfortunately, the actual development of ambipolar organic complementary electronics is currently hampered by the uneven electron (n-type) and hole (p-type) conduction in ambipolar organic transistors. Here we show ambipolar organic thin-film transistors with balanced n-type and p-type operation. By manipulating air exposure and vacuum annealing conditions, we show that well-balanced electron and hole transport properties can be easily obtained. The method is used to control hole and electron conductions in split-gate transistors based on a solution-processed donor-acceptor semiconducting polymer. Complementary logic inverters with balanced charging and discharging characteristics are demonstrated. These findings may open up new opportunities for the rational design of complementary electronics based on ambipolar organic transistors.

Physical-based analytical model of flexible a-IGZO TFTs accounting for both charge injection and transport

Here we show a new physical-based analytical model of a-IGZO TFTs. TFTs scaling from L=200 μm to L=15 μm and fabricated on plastic foil are accurately reproduced with a unique set of parameters. The model is used to design a zero-VGS inverter. It is a valuable tool for circuit design and technology characterization.

Electrical Characterization of PEDOT:PSS Strips Deposited by Inkjet Printing on Plastic Foil for Sensor Manufacturing

Inkjet printing is a viable method for rapid and low-cost manufacturing of flexible sensors. In this paper, we study a technique for inkjet printing of poly(3,4-ethylenedioxythio-phene):poly(styrene sulfonate) (PEDOT:PSS) strips. A low-cost inkjet desktop printer is used for the fabrication of PEDOT:PSS resistive strips on polyimide substrates. Accounting for several geometries of printed PEDOT:PSS strips, we assess the variability of the fabrication process. Owing to the printing process, we can easily choose the width, length, and thickness. We found that printed strips on polyimide foils show a conductivity equal to 115 S/cm, which linearly increases with the strip thickness. The maximum variability is lower than 13%. The frequency analysis shows a purely resistive impedance in the frequency range investigated (100 Hz-100 kHz). Moreover, the strips folded up to 1000 times shows a resistance variation lower than 6%. The study demonstrates that inkjet printing is a viable method for the simple, fast, reliable, and low-cost fabrication of PEDOT:PSS-based sensors on plastic substrates and circuit interconnections.