Contact-Controlled Thin-Film Transistors and Compact Circuits for Low-Power Sensors and Internet of Things

In order to deliver billions of cost-effective sensors for Internet of Things applications, large area electronics (LAE) would need to overcome challenges in high-throughput manufacturing methods, while providing power-efficient operation. Processes, such as roll-to-roll and/or inkjet printing, hold the most promise for future disposable and wearable technologies, however progress is limited despite growing demand. The reason is the fundamental electronic device at the heart of their circuits, the thin-film transistor (TFT). TFTs share the same device physics as silicon field-effect transistors found in chip technology, as well as their limitations, notably uniformity of operation. Ordinarily, device non-idealities are compensated through cascode circuits or gain stages, which increases circuit complexity. But these strategies reduce yield, as circuit failure increases with component count. Even though there have been many breakthroughs in material systems, the limitations have persisted. Thus, the ability to produce high yield in high-throughput methods requires TFTs with more robust and uniform operation that can tolerate imprecise processes.An alternative, the source-gated transistor (SGT) is a type of TFT that uses energy barriers at the source contact to control charge injection. As a contact-controlled architecture, the SGT provides uniform and robust operation with extremely high gain and power-efficient operation.In this thesis, SGT operation is further explored in light of off-state behaviour, which produces lower leakage current. The first source-gated transistor (SGT) circuits are also included and provide exemplary performance without support circuitry. Two-transistor (2T) circuits with polysilicon SGTs demonstrate: 49 dB gain in common-source amplifiers, a record for any polysilicon TFT-based amplifier; and current mirrors that have a tuneable temperature dependence by design of the source region, where positive, neutral or negative dependence of output current can be obtained.The SGT’s ability to provide superior high-gain and power-efficient performance in a compact footprint is only superseded by that of the newly invented multimodal transistor (MMT), which shares its benefits. The MMT is an evolution of the contact-controlled concept, where charge injection is separately controlled from channel conduction. As the channel in the MMT is not responsible for charge injection, it provides: faster digital switching (up to two orders-of-magnitude faster than SGTs and one order faster than regular TFTs); an alternative means of charge transport control to mitigate hot-carrier effects in high mobility materials, when doping strategies are unavailable; a unique sample-and-hold or enable line function, ensuring signal propagation when activated. Together with the inherent ability for the MMT to produce a directly proportional dependence of output current on input voltage, these functional benefits allow for extremely compact digital-to-analog conversion and multiplication. Examples of the MMT’s ability to reduce circuit complexity would allow for circuit designers to explore new avenues into developing future LAE applications

Contact Doping as a Design Strategy for Compact TFT-Based Temperature Sensing

Contact-controlled devices, such as source-gated transistors (SGTs), deliberately use energy barriers at the source, and naturally, the positive temperature dependence (PTD) of drain current can be utilized for temperature sensing. We exploit the difference in drain current activation energy, which arises with contact doping in polysilicon n-type contact-controlled transistors, to demonstrate output current with either a PTD or negative temperature dependence (NTD). The range over which output current varies linearly with temperature, as well as the sensitivity, can be tailored by the choice of reference current magnitude and relative source contact properties within the current mirror. The sensing scheme simplifies the circuit design because it relies solely on thin-film transistors and it has inherent immunity to output voltage variation. This ability to tune the sign of temperature dependence allows facile integration in applications requiring homeostasis via feedback, e.g., electronic skin, in a minimal layout area and potentially with convenient reduction of patterning steps during fabrication

Simulation study of overlap capacitance in source-gated transistors for current-mode pixel drivers

Contrary to conventional design principles, currentdriven pixel drivers based on source-gated transistors (SGTs) achieve their optimal drive current and speed with a deliberate 5 -10μm gate-source overlap. Total pixel circuit area need not increase, as the additional device area can be compensated by reducing the pixel storage capacitor. Numerical simulations demonstrate the viability of SGTs for emissive pixel drivers and high gain, low power, robust circuits for emerging sensor arrays

The secret ingredient for exceptional contact-controlled transistors

Contact-controlled transistors are rapidly gaining popularity. However, simply using arectifying source contact often leads to unsatisfactory operation, merely as a thin-filmtransistor with low drain current and reduced effective mobility. This may cause otherwisepromising experiments to be abandoned. Here, we analyse data from literature in conjunctionwith devices we have recently fabricated in polysilicon, organic and oxide semiconductors,highlighting the main factor in achieving good saturation, namely keeping saturationcoefficient γ well below 0.3. We also discuss secondary causes of suboptimal electricalcharacteristics. Correct design of these alternative device structures will expedite theiradoption for high gain, low-frequency applications in emerging sensor circuits

Extraordinarily Weak Temperature Dependence of the Drain Current in Small‐Molecule Schottky‐Contact‐Controlled Transistors through Active‐Layer and Contact Interplay

Abstract Low saturation voltages and extremely high intrinsic gain can be achieved in contact‐controlled thin‐film transistors (TFTs) with staggered device architecture, enabled by the energy barrier introduced at the source contact. The resulting device, the source‐gated transistor (SGT), is limited in its usefulness by the high temperature dependence of the drain current induced by the source energy barrier. Here, the interaction between the thermal characteristics of the source contact and the semiconductor to show drastically reduced temperature dependence for SGTs based on organic semiconductors (OSGTs) is exploited. This extraordinarily weak temperature dependence of the drain current is observed regardless of the height of the source energy barrier (27.8% in OSGTs with Ti contacts compared to 22.1% when using Au contacts, over a 34 K range). The reduction in mobility of the semiconductor offsets an increase in thermionic‐field emission of charge carriers at the source. This is a first for SGTs and provides a route to removing one of the last hurdles to their wider adoption. The OSGTs with Ti contacts also demonstrate: drain‐current saturation at very low drain‐source voltages (saturation factor of 0.22); noteworthy stability after 70 days; and minimal drain‐current variation with channel length or illumination

Suppression of Hot-Carrier Effects Facilitated by the Multimodal Thin-Film Transistor Architecture

International audienceHot-carrier effects are a persistent challenge for Ohmic contact, high carrier mobility thin-film transistors. As semiconductor properties are systematically improved, such phenomena (e.g., the kink effect) are becoming apparent even in materials such as InGaZnO. Few of the past solutions are practical in these low-complexity semiconductor systems. Here, it is shown that contact-controlled devices offer robust performance under extreme biasing conditions due to their distinctive charge injection processes. The recently-developed multimodal transistor (MMT) provides further control still, by separate regulation of current flow and magnitude. Internal electric field distributions in the source and drain regions are studied via technology computer-aided design simulations, and support the formulation of operational guidelines for the MMT's channel control gate for optimal characteristics in saturation. As MMT principles are universal, these findings should inform device design and operation in all high carrier mobility material systems

Compact Unipolar XNOR/XOR Circuit Using Multimodal Thin-Film Transistors

International audienceA novel compact realization of the XNOR/XOR function is demonstrated with multimodal transistors (MMTs). The multimodal thin-film transistors (MMT's) structure allows efficient use of layout area in an implementation optimized for unipolar thin-film transistor (TFT) technologies, which may serve as a multipurpose element for conventional and emerging large-area electronics. Microcrystalline silicon device fabrication is complemented by physical simulations

Multimodal transistors as ReLU activation functions in physical neural network classifiers

Artificial neural networks (ANNs) providing sophisticated, power-efficient classification are finding their way into thin-film electronics. Thin-film technologies require robust, layout-efficient devices with facile manufacturability. Here, we show how the multimodal transistor's (MMT's) transfer characteristic, with linear dependence in saturation, replicates the rectified linear unit (ReLU) activation function of convolutional ANNs (CNNs). Using MATLAB, we evaluate CNN performance using systematically distorted ReLU functions, then substitute measured and simulated MMT transfer characteristics as proxies for ReLU. High classification accuracy is maintained, despite large variations in geometrical and electrical parameters, as CNNs use the same activation functions for training and classification

Promoting Low-Voltage Saturation in High-Performance a-InGaZnO Source-Gated Transistors

As oxide semiconductors increase in popularity with emerging flexible electronics, advances in material performance may lead to parasitic and non-ideal effects becoming more prominent. Specifically, in source-gated transistors, and other contact-controlled devices, the influence of the lateral drain field produced by drain bias leads to an overwhelming increase in charge density at the source edge and obliterates their signature flat saturation at low drain voltages. Here, we present high current density amorphous InGaZnO (a-IGZO) source-gated transistors (SGTs) in a bottom contact architecture using Pt source and drain electrodes. The devices were fabricated by ensuring an oxygen-rich atmosphere to prevent the formation of oxygen vacancies at the Pt/a-IGZO interface. Incorporating a field relief structure within the source contact improves drain current saturation, towards behavior predicted by the saturation coefficient. In the device architecture considered, the screening was less effective for a field plate insulator thickness under 30 nm, possibly due to increased tunnelling. Field plate incorporation is one of the strategies that ensures flat saturation and the suitability of contact-controlled transistors for a multitude of current driving and amplification applications