Dual-gate thin-film transistors, integrated circuits and sensors

Geometrical structure and interface dependence of bias stress induced threshold voltage shift in C60-based OFETs

The influence of the nature of interface between organic semiconductor and gate dielectric on bias stress electrical stability of n-type C60-based organic field effect transistors (OFETs) was studied. The bias stress induced threshold voltage (Vth) shift was found to depend critically on the OFET device structure: the direction of V(th) shift in top-gate OFETs was opposite to that in bottom-gate OFETs, while the use of the dual-gate OFET structure resulted in just very small variations in V(th). The opposite direction of Vth shift is attributed to the different nature of interfaces between C60 semiconductor and Parylene dielectric in these devices. The V(th) shift to more positive voltages upon bias stress in bottom-gate C60-OFET was similar to that observed for other n-type semiconductors and rationalized by electron trapping in the dielectric or at the gate dielectric/C60 interface. The opposite direction of Vth shift in top-gate C60-OFETs is attributed to free radical species created in the course of Parylene deposition on the surface of C60 during device fabrication, which produce plenty of hole traps. It was also realized that the dual-gate OFETs gives stable characteristics, which are immune to bias stress effects.

Fundamental limitations for electroluminescence in organic dual-gate field-effect transistors

A dual-gate organic field-effect transistor is investigated for electrically pumped lasing. The two gates can independently accumulate electrons and holes, yielding current densities exceeding the lasing threshold. Here, the aim is to force the electrons and holes to recombine by confining the charges in a single semiconducting film. It is found that independent hole and electron accumulation is mutually exclusive with vertical recombination and light emission.

Highly stable carbon nanotube top-gate transistors with tunable threshold voltage

Carbon-nanotube top-gate transistors with fluorinated dielectrics are presented. With PTrFE as the dielectric, the devices have absent or small hysteresis at different sweep rates and excellent bias-stress stability under ambient conditions. Ambipolar single-walled carbon nanotube (SWNT) transistors are observed when P(VDF-TrFE-CTFE) is utilized as a topgate dielectric. Furthermore, continuous tuning of the threshold voltages of both unipolar and ambipolar SWNT thin-film transistors (TFTs) is demonstrated for the first time.

Performance enhancement of capacitive-coupling dual-gate ion-sensitive field-effect transistor in ultra-thin-body

Recently, thin-film transistor based-ISFETs with the dual-gate (DG) structures have been proposed, in order to beat the Nernst response of the standard ISFET, utilizing diverse organic or inorganic materials. The immutable Nernst response can be dramatically transformed to an ultra-sensing margin, with the capacitive-coupling arisen from the DG structure. In order to advance this platform, we here embedded the ultra-thin body (UTB) into the DG ISFET. The UTB of 4.3 nm serves to not only increase its sensitivity, but also to strongly suppress the leakage components, leading to a better stability of the DG ISFET. In addition, we first provide a comprehensive analysis of the body thickness effects especially how the thick body can render the degradation in the device performance, such as sensitivity and stability. The UTB DG ISFET will allow the ISFET-based biosensor platform to continue enhancement into the next decade.

Through thick and thin: tuning the threshold voltage in organic field-effect transistors

Organic semiconductors (OSCs) constitute a class of organic materials containing densely packed, overlapping conjugated molecular moieties that enable charge carrier transport. Their unique optical, electrical, and magnetic properties have been investigated for use in next-generation electronic devices, from roll-up displays and radiofrequency identification (RFID) to biological sensors. The organic field-effect transistor (OFET) is the key active element for many of these applications, but the high values, poor definition, and long-term instability of the threshold voltage (V(T)) in OFETs remain barriers to realization of their full potential because the power and control circuitry necessary to compensate for overvoltages and drifting set points decrease OFET practicality. The drifting phenomenon has been widely observed and generally termed "bias stress." Research on the mechanisms responsible for this poor V(T) control has revealed a strong dependence on the physical order and chemical makeup of the interfaces between OSCs and adjacent materials in the OFET architecture. In this Account, we review the state of the art for tuning OFET performance via chemical designs and physical processes that manipulate V(T). This parameter gets to the heart of OFET operation, as it determines the voltage regimes where OFETs are either ON or OFF, the basis for the logical function of the devices. One obvious way to decrease the magnitude and variability of V(T) is to work with thinner and higher permittivity gate dielectrics. From the perspective of interfacial engineering, we evaluate various methods that we and others have developed, from electrostatic poling of gate dielectrics to molecular design of substituted alkyl chains. Corona charging of dielectric surfaces, a method for charging the surface of an insulating material using a constant high-voltage field, is a brute force means of shifting the effective gate voltage applied to a gate dielectric. A gentler and more direct method is to apply surface voltage to dielectric interfaces by direct contact or postprocess biasing; these methods could also be adapted for high throughput printing sequences. Dielectric hydrophobicity is an important chemical property determining the stability of the surface charges. Functional organic monolayers applied to dielectrics, using the surface attachment chemistry made available from "self-assembled" monolayer chemistry, provide local electric fields without any biasing process at all. To the extent that the monolayer molecules can be printed, these are also suitable for high throughput processes. Finally, we briefly consider V(T) control in the context of device integration and reliability, such as the role of contact resistance in affecting this parameter.

Unique device operations by combining optical-memory effect and electrical-gate modulation in a photochromism-based dual-gate transistor

We demonstrate a new device that combines a light-field effect and an electrical-gate effect to control the drain current in a dual-gate transistor. We used two organic layers, photochromic spiropyran (SP)-doped poly(triarylamine) (PTAA) and pristine PTAA, as top and bottom channels, respectively, connected to common source and drain electrodes. The application of voltage to the top and bottom gates modulated the drain current through each layer independently. UV irradiation suppressed the drain current through the top channel. The suppressed current was then maintained even after the UV light was turned off because of an optical memory effect induced by photoisomerization of SP. In contrast, UV irradiation did not change the drain current in the bottom channel. Our dual-gate transistor thus has two organic channels with distinct photosensitivities: an optically active SP-PTAA film and an optically inactive PTAA film. This device configuration allows multi-level switching via top- and bottom-gate electrical fields with an optical-memory effect.

Toward printed integrated circuits based on unipolar or ambipolar polymer semiconductors

For at least the past ten years printed electronics has promised to revolutionize our daily life by making cost-effective electronic circuits and sensors available through mass production techniques, for their ubiquitous applications in wearable components, rollable and conformable devices, and point-of-care applications. While passive components, such as conductors, resistors and capacitors, had already been fabricated by printing techniques at industrial scale, printing processes have been struggling to meet the requirements for mass-produced electronics and optoelectronics applications despite their great potential. In the case of logic integrated circuits (ICs), which constitute the focus of this Progress Report, the main limitations have been represented by the need of suitable functional inks, mainly high-mobility printable semiconductors and low sintering temperature conducting inks, and evoluted printing tools capable of higher resolution, registration and uniformity than needed in the conventional graphic arts printing sector. Solution-processable polymeric semiconductors are the best candidates to fulfill the requirements for printed logic ICs on flexible substrates, due to their superior processability, ease of tuning of their rheology parameters, and mechanical properties. One of the strongest limitations has been mainly represented by the low charge carrier mobility (μ) achievable with polymeric, organic field-effect transistors (OFETs). However, recently unprecedented values of μ ∼ 10 cm(2) /Vs have been achieved with solution-processed polymer based OFETs, a value competing with mobilities reported in organic single-crystals and exceeding the performances enabled by amorphous silicon (a-Si). Interestingly these values were achieved thanks to the design and synthesis of donor-acceptor copolymers, showing limited degree of order when processed in thin films and therefore fostering further studies on the reason leading to such improved charge transport properties. Among this class of materials, various polymers can show well balanced electrons and holes mobility, therefore being indicated as ambipolar semiconductors, good environmental stability, and a small band-gap, which simplifies the tuning of charge injection. This opened up the possibility of taking advantage of the superior performances offered by complementary "CMOS-like" logic for the design of digital ICs, easing the scaling down of critical geometrical features, and achieving higher complexity from robust single gates (e.g., inverters) and test circuits (e.g., ring oscillators) to more complete circuits. Here, we review the recent progress in the development of printed ICs based on polymeric semiconductors suitable for large-volume micro- and nano-electronics applications. Particular attention is paid to the strategies proposed in the literature to design and synthesize high mobility polymers and to develop suitable printing tools and techniques to allow for improved patterning capability required for the down-scaling of devices in order to achieve the operation frequencies needed for applications, such as flexible radio-frequency identification (RFID) tags, near-field communication (NFC) devices, ambient electronics, and portable flexible displays.

Dual channel operation upon n-channel percolation in a pentacene-C60 ambipolar organic thin film transistor

Thickness resolved measurements of ambipolar thin-film transistor characteristics track the charging of an organic-organic heterojunction. Combined with structural investigation methods such as AFM and GIXS, this leads to a better understanding of the physics in state of the art devices such as organic solar cells, organic light emitting diodes and light emitting TFTs.

Self-assembled dual in-plane gate thin-film transistors gated by nanogranular SiO2 proton conductors for logic applications

Phosphorus (P)-doped nanogranular SiO(2) films are deposited by plasma-enhanced chemical vapor deposition at room temperature, and a high proton conductivity of ~5.6 × 10(-4) S cm(-1) is measured at room temperature with a relative humidity of 70%. The accumulation of protons at the SiO(2)/indium-zinc-oxide (IZO) interface induces a large electric-double-layer (EDL) capacitance. Thin-film transistors (TFTs) with two in-plane gates are self-assembled on transparent conducting glass substrates. The large EDL capacitance can effectively modulate the IZO channel with a current ON/OFF ratio of >10(7). Such TFTs calculate dual input signals at the gate level coupled with a floating gate, analogous to that of neuron MOS (vMOS). AND logic is demonstrated on the neuron TFTs. Such neuron TFTs gated by P-doped nanogranular SiO(2) shows an effective electrostatic modulation on conductivities of oxide semiconductors, which is meaningful for portable chemical-biological sensing applications.

In-situ probe of gate dielectric-semiconductor interfacial order in organic transistors: origin and control of large performance sensitivities

Organic thin film transistor (OTFT) performance is highly materials interface-dependent, and dramatic performance enhancements can be achieved by properly modifying the semiconductor/gate dielectric interface. However, the origin of these effects is not well understood, as this is a classic "buried interface" problem that has traditionally been difficult to address. Here we address the question of how n-octadecylsilane (OTS)-derived self-assembled monolayers (SAMs) on Si/SiO(2) gate dielectrics affect the OTFT performance of the archetypical small-molecule p-type semiconductors P-BTDT (phenylbenzo[d,d]thieno[3,2-b;4,5-b]dithiophene) and pentacene using combined in situ sum frequency generation spectroscopy, atomic force microscopy, and grazing incidence and reflectance X-ray scattering. The molecular order and orientation of the OTFT components at the dielectric/semiconductor interface is probed as a function of SAM growth mode in order to understand how this impacts the overlying semiconductor growth mode, packing, crystallinity, and carrier mobility, and hence, transistor performance. This understanding, using a new, humidity-specific growth procedure, leads to a reproducible, scalable process for highly ordered OTS SAMs, which in turn nucleates highly ordered p-type semiconductor film growth, and optimizes OTFT performance. Surprisingly, the combined data reveal that while SAM molecular order dramatically impacts semiconductor crystalline domain size and carrier mobility, it does not significantly influence the local orientation of the overlying organic semiconductor molecules.