Suspended Gate Field Effect Transistor Research Articles

Polymer Ring-Flexure-Membrane Suspended Gate FET Gas Sensor: Design, Modelling and Simulation.

This work reports the design, modelling, and simulation of a novel polymer MEMS gas sensor platform called a ring-flexure-membrane (RFM) suspended gate field effect transistor (SGFET). The sensor consists of a suspended polymer (SU-8) MEMS based RFM structure holding the gate of the SGFET with the gas sensing layer on top of the outer ring. During gas adsorption, the polymer ring-flexure-membrane architecture ensures a constant gate capacitance change throughout the gate area of the SGFET. This leads to efficient transduction of the gas adsorption-induced nanomechanical motion input to the change in the output current of the SGFET, thus improving the sensitivity. The sensor performance has been evaluated for sensing hydrogen gas using the finite element method (FEM) and TCAD simulation tools. The MEMS design and simulation of the RFM structure is carried out using CoventorWare 10.3, and the design, modelling, and simulation of the SGFET array is carried out using the Synopsis Sentaurus TCAD. A differential amplifier circuit using RFM-SGFET is designed and simulated in Cadence Virtuoso using the lookup table (LUT) of the RFM-SGFET. The differential amplifier exhibits a sensitivity of 2.8 mV/MPa for a gate bias of 3 V and a maximum detection range of up to 1% hydrogen gas concentration. This work also presents a detailed fabrication process integration plan to realize the RFM-SGFET sensor using a tailored self-aligned CMOS process adopting the surface micromachining process.

A Closed-Loop In-Plane Movable Suspended Gate FET (CLIP-SGFET) Sensor With a Dynamically Reconfigurable Charge Pump

This article proposes a single-axis, highperformance Complementary Metal Oxide Semiconductor- Microelectromechanical System (CMOS-MEMS) closed-loop accelerometer, implemented using in-plane movable suspended gate-field-effect transistor (IP-SGFET). The IPSGFET’s gate is embedded within a movable proof mass supported by flexible folded beams. The external acceleration displaces the proofmass, which changes the drain current of IP-SGFET. The change in current is read out with the help of a simple differential amplifier which is constructed with the IP-SGFET. The differential amplifier’s output voltage is used througha closed-loopintegral controland a high-voltage dynamically reconfigurable charge pump, which generates a high-voltage differential drive of up to 10.5 ± 5.12 V, to drive an actuator that brings the gate back to its resting position. The actuator used is a comb-drive type electromechanical actuator. In this work, two tools, viz., MEMS+ and Technology Computer Aided Design (TCAD) device simulation tools are used in a sequential manner to handle the electrostatics coupled with the micro-mechanical domain present in the suspended gate field effect transistors (SGFET). The results obtained are then used as look-up-tables for the design and simulation of the entire system, including the control circuits, in SPECTRE. Appropriate design of the MEMS suspended structure, the field effect transistor (FET) device, and interface circuit results in a open-loop sensitivity of 927 mV/g with a near-zero cross-axis sensitivity. Using the actuator and the reconfigurable charge pump, closed-loop operation is realized with a dynamic range of ±4 g at a supply voltage of 3.3 V.

CMOS-MEMS Accelerometer With Stepped Suspended Gate FET Array: Design & Analysis

This article presents a novel stepped suspended gate field-effect transistor (SSGFET) array-based <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${z}$ </tex-math></inline-formula> -axis accelerometer with an enhanced detection range. The stepped gate electrode structure of SGFET aids in extending the stable driving range beyond 33.33% of the initial air gap. The stable driving range is extended to 50% of the initial air gap with ~90% increase in the pull-in voltage. Mechanical, electrical, and electromechanical analytical models are developed. These models are validated through microelectromechanical systems (MEMS) simulations using CoventorMP and transistor simulations in Synopsys TCAD. An SSGFET-based common source (CS) amplifier with diode-connected p-MOSFET load is designed and simulated in Cadence Virtuoso using the lookup table (LUT) approach. The <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$z$ </tex-math></inline-formula> -axis accelerometer exhibits a sensitivity of 38 (mV/g) with a supply voltage of 3.3 V for a dynamic range (DR) of ±6 g with the nonlinearity of about 5.3% comparedtoSGFETswith a planar gate electrode,whichcan detect up to ±4 g with the same sensitivity. The 3-dB bandwidth of the accelerometer is 412 Hz with a noise-limited resolution of 109.31 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu$ </tex-math></inline-formula> g/(Hz) <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1/2</sup> . This article also presents a detailed analysis of the relation between the number of gate fragments, the pull-in voltage, stable driving range, and the DR along with a feasible fabrication integration plan.

A CMOS-MEMS Accelerometer With U-Channel Suspended Gate SOI FET

There have always been demands for the development of microscale sensors with integrated miniature electronics circuitry. Requirement of external amplification for the conventional MEMS accelerometer poses limitation on scaling, design, fabrication and on-chip Integrated Circuit (IC) compatibility. These limitations could be surpassed by the implementation of integrated CMOS-MEMS architectures for sensors and actuators. Here a novel U-channel suspended gate silicon on insulator field effect transistor (USG-SOIFET) is proposed to circumvent the pseudo short channel effects (P-SCE) present in the suspended gate field effect transistor (SGFET). In the proposed USG-SOIFET, the source and drain are separated by an air-gap and thereby ensuring effective screening of drain electric field penetration into source region. The gate length (L) of USG-SOIFET is 4 times lower than the conventional planar channel SGFET for the same performance of the device. Since the same performance is achieved with a smaller length, this results in a smaller device. Finite Element Modeling (FEM) of USG-SOIFET z-axis accelerometer using CoventerMP1.3 and TCAD process, device modeling and simulation of the USG-SOIFET using Synopsys Sentaurus are presented here. The sensitivity of the accelerometer is 4.185 μA/g with a non-linearity of 4.96% for ± 5 g detection range. The bandwidth of the accelerometer is 100 Hz and the cross-axis sensitivity is found to be 3.52%. A fabrication process integration scheme for realizing USG-SOIFET has also been discussed along with process simulation to demonstrate the feasibility of realizing this new device architecture. This device platform is a potential candidate for monolithic integration with CMOS.

A new smart nanoforce sensor based on suspended gate SOIMOSFET using carbon nanotube

This paper presents a new nanoforce sensor based on a suspended carbon nanotube gate field-effect transistor. To do so, a numerical investigation of Suspended Gate Silicon-on-Insulator MOSFET (SG-SOIMOSFET) is carried out using ATLAS 2D simulator. Based on the relationship between the nanotube’s deflection and the applied force, a comprehensive study of the proposed nanoforce sensor behavior is performed. Moreover, we describe the evolution of the drain current characteristics as a function of the applied force while examining the influence of capacity variation of the insulating gate on the drain current in the saturation region. It is found that the sensor has a good sensitivity of 230.68 ln(A)/pN. Our second contribution in this paper is to develop a model based on artificial neural networks (ANNs). We successfully integrate our neural model of nanoforce sensor as a new component in the ORCAD-PSPICE electric simulator library; this component must accurately express the behavior of the sensor. A second model based on neural networks, which deals with correction and linearization of the sensor output signal, is designed and implemented into the same simulator. The proposed device can be considered as a potential alternative for CMOS-based nanoforce sensing.

Stability Constraints Define the Minimum Subthreshold Swing of a Negative Capacitance Field-Effect Transistor

The current-voltage characteristics of a classical field-effect transistor (FET) is dictated by thermal injection of charge carriers over a gate-controlled energy barrier. It is well known that the subthreshold swing <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="TeX">\((S)\) </tex-math></inline-formula> associated with these transistors cannot be reduced below the Boltzmann limit of 60 mV/decade, which in turn defines the lower limit of power dissipation. Therefore, a number of groups have recently proposed to use negative capacitance (NC) gate insulators in FETs to reduce <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="TeX">\(S\) </tex-math></inline-formula> below the Boltzmann limit. The ferroelectric-FET (FE-FET) and suspended-gate FET (SG-FET) are two examples. It is now well accepted that NC-FETs can lower <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="TeX">\(S\) </tex-math></inline-formula> , but given a specific device architecture, the question of whether there is a lower limit of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="TeX">\(S\) </tex-math></inline-formula> (and if so, the conditions that define the limit) has not been answered. In this paper, we: 1) demonstrate that the fundamental constraints of stability and hysteresis-free operation dictate that there is a lower limit of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="TeX">\(S\) </tex-math></inline-formula> associated with each NC-FET technology; 2) provide a general algorithm to calculate the minimum subthreshold swing <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="TeX">\((S_{\rm min})\) </tex-math></inline-formula> ; and 3) illustrate the concept using SG-FET and FE-FET for two different channel configurations, namely, a bulk FET and an FET with constant channel capacitance. Our results broaden the understanding of field-effect action in NC-FETs and should inform research on these devices.

Prospects of Hysteresis-Free Abrupt Switching (0 mV/decade) in Landau Switches

The sub-threshold swing (S) defines the sharpness of ON-OFF switching of a field effect transistor (FET), with S=0 corresponding to abrupt switching characteristics. While thermodynamics dictate S ≥ 60 mV/decade for classical FETs, a new class of switches which we call “Landau switches” e.g., suspended gate FET (SG-FET) and ferroelectric FET (FE-FET) use inherently unstable (negative capacitance) gate insulators to achieve abrupt switching. Unfortunately, S=0 switching in Landau switches is always achieved at the expense of an intrinsic hysteresis, making them unsuitable for low-power applications. The fundamental question therefore is: under what conditions, hysteresis-free abrupt switching can be achieved in a Landau switch? In this paper, we first provide an intuitive classification of all semiconductor FETs in terms of their energy landscapes and identify a two-well energy landscape as the characteristic feature of all Landau switches. We then use the SG-FET as an illustrative example to conclude that a flat energy landscape is essential for hysteresis-free abrupt switching. In contrast, a hysteresis-free smooth switching (S ≤ 60 mV/decade) is less restrictive, and is obtained by stabilizing the unstable gate insulator in its negative capacitance regime. Our conclusions have broad implications for the analysis and design of ultralow power sub-60 mV/decade switches.

Suspended gate field effect transistor type microelectromechanical systems resonators modelling with micro-Raman spectroscopy measured residual stress

Presented is an improved model of doubly-clamped microelectromechanical systems (MEMS) resonators implemented in very high-speed integrated circuit hardware description language for analogue and mixed-signal (VHDL-AMS). The model includes the effect of residual stress, which may severely shift the resonant frequency of the MEMS resonator from the analytical pre-designed value if the magnitude of intrinsic residual stress is imprecisely predicted. As the stress is not only determined by the fabrication process but also related to the structural dimensions of the resonators, in this work micro-Raman spectroscopy was utilised to measure and characterise the residual stress of a series of fabricated doubly-clamped polysilicon suspended gate field effect transistor type resonators with varying sizes. Combined with the experimentally determined residual stress, the proposed VHDL-AMS analytical model provides an error of <; 3.5% resonant frequency shift with respect to the experimental result.

Sensors based in suspended gate field effect transistors for pH measurements

This work presents a new pH sensor based on the architecture SGFET (Suspended Gate Field Effect Transistor). The pH variations are obtained using the threshold voltage and the transfer characteristics shift of the SGFET. To evaluate the accuracy and reliability of the pH sensor, pH variations were measured directly in the solution using both the SGFET and a commercial pH-meter.

Suspended Gate Field Effect Transistor with an Integrated Micro-Fluidic Channel Performed by Surface Micromachining for Liquids Sensing

This paper presents original designs of micro-fluidic sensors. Targeted applications are pH-meters devices or chemical sensors. The sensing part of the device consists of a Suspended Gate Field Effect Transistor (SGFET). Chemicals to test are brought inside the device by a micro-fluidic channel. The floor of the channel consists on the drain, source and also active area of the transistor, whereas the top of the buried channel corresponding of the gate of the transistor. By this way products that flow in the micro-fluidic channel directly module the concentration charge inside the transistor's gap and thus induce changes in transfer characteristics. This paper describes the fabrication process and presents first electrical tests performed with liquid flow injection in the device showing the interest of the system.

Transient Operation Techniques for Gas Sensor Applications

This paper provides an update, how transient operation methods can improve the performance of gas sensors and sensor systems for specific applications. Even new applications can be enabled, if existing state-of-the-art sensors are operated in transient modes adapted to the needs of the application in mind. Several parameters are available for transient variation, depending on the particular sensing technology: Suspended Gate FET sensors deliver a simultaneous readout of work function and capacitive effects. By applying pulsed gate voltages these effects can be interpreted separately and used e.g. for compensation of humidity effects and improved selectivity. Modulation of the operating temperature of μ-machined metal oxide sensors is creating a type of virtual sensor array. In this case, the dynamic behavior of adsorption and desorption at changing temperature levels delivers a fingerprint for application relevant gas mixtures. The pulsed application of charge voltage on standard exhaust lambda probes and the recording of the related discharge characteristic over time provide an exhaust NOx sensor based on a standard sensor setup. The charging creates a misbalance in oxygen-related surface species not available under continuous operation conditions. The related discharge characteristic is strongly influenced by the interaction of these oxygen species with NOx.

PH Meter Based in Suspended Gate Field Effect Transistors to Application in Monitoring of Water Quality

The protocol to monitor the water pH variation during a long time using a full electronic pH-meter consisting of a SGFET (Suspended Gate Field Effect Transistor) is presented after the analysis of the transconductance and the drain current drifts under pH. The main advantages of using this device are its high sensitivity and the ability to integrate it with the electronic treatment into a miniaturized system.

Modeling the high pH sensitivity of Suspended Gate Field Effect Transistor (SGFET)

High sensitivity to chemical species of sub-micron gap Suspended-Gate FETs (more than 200 mV/pH for example) is explained from the charge distribution induced by the high field in the sub-micronic gap under the gate-bridge. Modeling of Metal–Electrolyte–Insulator–Silicon (MEIS) capacitor, which is the basic vertical structure of the transistor, is performed to highlight this effect through the response to the pH change of the solution filling the gap. The analytical model is based on the 2D-numerical resolution of Poisson's equation. The response of quasi-static C( V) plots versus pH is simulated using both electrolyte charge distribution and site-binding theory considering the influence of sites densities on silicon nitride. Device modeling and simulated/experimental electrical characteristics are presented. Effect of the gap thickness on the pH sensitivity is also discussed in this study.

Humidity Sensor Thanks Array of Suspended Gate Field Effect Transistor

This work deals with a sensor using array of Suspended Gate Field Effect Transistor (SGFET) to measure the relative humidity (%RH). Technics of microtechnology are used to fabricate a polysilicon bridge with a gap de 0,5 µm which acts as transistor gate. Electrical measures have been realized by sampling mode, involving a sensitivity of -5 µA/%. Also the SGFET humidity sensors are very interesting due to the simplicity to polarize and to analyze the output signal.

Scaling the Suspended-Gate FET: Impact of Dielectric Charging and Roughness

Suspended gate field-effect transistors (SG-FETs) with switching gates are interesting as digital logic switches because of their high I on/I off current ratio and their infinite subthreshold slope. However, the limits of scalability of the SG-FETs are still unclear. This paper investigates two effects that could limit scaling: the dielectric charging and the dielectric roughness. To do so, a surface-potential-based model for suspended gate transistors with a mechanically switching gate is presented and validated using experimental data. Devices fabricated in a standard complimentary metal-oxide-semiconductor process are used for the model assessment. The model reproduces the effect of a fixed charge and the effect of a nonideal contact of the gate after pull-in. We show that, at the device dimensions required to follow the International Technology Roadmap for Semiconductors, these effects will be critical.

Investigation on suspended gate field effect transistor as humidity sensor

This work deals with SGFET (Suspended Gate Field Effect Transistor) for humidity rate (%HR) measurement. SGFET devices have already been used to detect gas, biological species, and also to measure pH [1]. This structure has already proved its good sensitivity to atmosphere and charge variation [2]. The following results have been obtained with SGFET realized in CMOS 0,8 μm technology and which have been reported and bonded on PCB card after all wires have been isolated from ambient sources. This paper has investigated especially the sensitivity to %HR. SGFET measurements realized in this work have been performed by sampling the drain current few seconds. By this way it becomes possible to presents for the first time results involving SGFET obtained during several hours of quasi continuous test. With this work and all results issue from the %HR measurement, the hysteresis and the reliability of SGFET’s structures could be evaluated especially in monitoring applications.

Direct Electrical Detection of Biological Species

Suspended-Gate FETs, namely SGFET, with sub-micron gap, is shown to be able to detect biological species with very high sensitivity in a large range of concentration. Examples of detection of DNA (through the DelF508 mutation of the cystic fibrosis gene that is widely disseminating in Europe and North America, and one mutation of BRCA1 gene that is the main indication of the possibility for a woman to have breast cancer) and proteins (through the transferrin that is the only carrier of iron in blood) are presented. This very sensitive electrical detection without any labelling is shown to deliver directly readable signal.

Finite element analysis and analytical simulations of Suspended Gate-FET for ultra-low power inverters

This paper proposes, the investigation of the Suspended Gate Field-Effect Transistor (SG-FET) small-slope switch based on a hybrid numerical simulation approach combining ANSYSTM Multiphysics and ISE-DESSISTM in a self-consistent system. The proposed numerical simulations uniquely enable the investigation of the behavior and the physics of complex micro-electro-mechanical/solid-state devices, such as the SG-FET. Abrupt switching as well as the effect of trapped charges in the gate dielectric are demonstrated. The numerical data serve to calibrate an analytical EKV-based SG-FET model, which is then used to design and originally simulate a sub-micron (90nm) scaled SG-FET complementary inverter. It is shown that, due to abrupt switching in the subthreshold region and electro-mechanical hysteresis, the SG-FET inverter could deliver a significant power saving (1–2 decades reduction of inverter peak current and practically no leakage power) compared to traditional CMOS inverter.

Analytical Modeling of the Suspended-Gate FET and Design Insights for Low-Power Logic

An analytical model for the suspended-gate field-effect transistor (SGFET), dedicated to the dc analysis of SGFET logic circuits, is developed. The model is based on the depletion approximation and expresses the pull-in voltage, the pull-out voltage, and the stable travel range as a function of the structural parameters. Gate position is explicitly expressed as a function of the gate voltage, thus enabling the convenient integration of the analytical SGFET relationships into the standard MOSFET models. Starting from the new SGFET model, the influence of the mechanical hysteresis on the circuit steady-state behavior is discussed, the potential of using the SGFET as an ultra-low power switch is demonstrated, and the operation of the complementary SGFET inverter is analyzed.

Study of mechanical stability of suspended bridge devices used as pH sensors

This study presents the processing of micro-bridges used in suspended gate field effect transistor. Micro mechanical structures were processed using different materials. To ensure the mechanical stability, boron doped polysilicon films, showing positive mechanical stress, had to be embedded between two negatively stressed silicon nitride layers. These micro-bridges are used as suspended gate with a gap as low as 500nm in the field effect transistor process. The pH sensitive membrane is a silicon nitride layer. The device characteristics are sensitive to any electrical charge in the air-gap because of the local high electrical field. It is then possible to detect any charge variation with higher sensitivity than the one reached using other non electrical technique. For example the present structure was used as pH sensor with a sensitivity five times greater than the ISFET usual Nernstian response.