Design & Simulation of Multi Gate Piezoelectric FET Devices for Sensing Applications

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1 Design & Simulation of Multi Gate Piezoelectric FET Devices for Sensing Applications Sunita Malik 1, Manoj Kumar Duhan 2 Electronics & Communication Engineering Department, Deenbandhu Chhotu Ram University of Science & Technology, Murthal, Sonepat, India Abstract The smart structural system, based on piezoelectric elements, have diverse applications as sensors as well as actuators utilizing direct piezoelectric effect to sense deformation and inverse effect to activate the structure. Simulation of pressure sensor device based on double gate and triple gate structures have been performed using Sentaurus TCAD tool for design and optimization of the sensors. The proposed structures have been analyzed for their improved sensitivity behaviour by measuring the change in drain current when force is applied. Better control ability of the gate on the threshold voltage in case of multi gate PE-FET makes them good candidates to replace single gate device in terms of improved current driving capabilities and sub threshold slope. Further better drain current values are obtained through proposed double and triple gate structures for PE-FET makes. Keywords PE-FET (Piezoelectric Field Effect Transistor); DG PE-FET; TG PE-FET; PVDF; MEMS; I. INTRODUCTION Sensors and actuators, also termed as transducers, serve the purpose of transformation of energy from one domain to another. There are many applications of piezoelectric materials including sensors to measure various physical parameters (like temperature, applied force, strain etc.), ultrasonic transducers, ultrasound system, microphones etc. Recent advancement in the field of sensing and actuation leads to development of new smart structures. A smart structure senses and controls its own characteristics. Among the entire range of materials used in sensing and actuation application, piezoelectric materials show the promising characteristics. Many researchers have used piezoelectric elements as sensors [1]. Several investigations have been carried out on touch sensing technologies in recent years. Many pressure sensors have been developed using different materials and transduction methods like resistive/piezoresistive, capacitive, optical, ultrasonic, piezoelectric etc. Most of these sensors are not suitable because of their size where high density of sensors is required. For increased integration density, revolutionary advances are required in transduction techniques. In wearable electronics, the flexibility requirement of tactile or pressure sensing can be met through piezoelectric material having high sensitivity, deformability, high thermal stability and low permittivity. These sensors detect the mechanical deformation caused by applied force and measure the required electrical parameters to provide measurable information [3], [4]. The piezoelectric sensors can be either ceramic or polymeric. The research related to the use of piezoelectric polymer in pressure sensor, tactile sensors has been growing rapidly. The piezoelectric polymer PVDF (Polyvinylidene fluoride) and its copolymers are the most promising material to be used in tactile sensor because of their ultra sensitivity and high deformability [5]. The piezoelectric property and sensitivity of these materials can improve the performance of pressure sensor. A device working as pressure sensor or tactile sensor can be designed utilizing the distributed sensor strategy, implemented by integration of number of devices on a substrate which can be rigid or flexible. The pressure sensor developed by integration of CMOS and MEMS (Micro-electro-mechanical systems) Technology results in reduced area and high Page 112

2 sensitivity of the sensor. For the design of flexible sensor array, organic FET based sensors have also been reported by researchers [7]. These sensors are having advantages of flexibility and low fabrication cost, but still not preferred over silicon based sensors for faster response applications. Low mobility of organic FET based sensor limits their use for measuring slow varying forces [5]. To introduce flexibility in sensor array, one possible alternate can be the design of sensors at micro or nano scale and connecting them through flexible interconnects. For such devices, integration of nano scale transducers with required signal conditioning electronic circuitry needs to be explored. A. Piezoelectric Material PVDF is a semi crystalline piezoelectric polymer, available in different phases (α, β) whose piezoelectric properties depend on polar crystalline phase. The most investigated phase is the β phase which is electrically active having highest electrical dipole moment and could be directly fabricated without any post treatment [3]. The PVDF material used in pressure sensors shows high piezoelectric coefficients which makes them favourable to be used in such applications. PVDF is also a promising piezoelectric polymer having wide range of applications in various other fields like acoustics, non destructive testing, health monitoring and artificial skin etc. Piezoelectricity in PVDF was first discovered in 1969, since then the researcher explored several other polymers and copolymers. The strain coefficient d 33 of piezoelectric polymers has lower value in comparison with piezoelectric ceramics, but having lower value of permittivity also leads to high value of voltage coefficient g 33, which are inversely proportional to permittivity. Researchers have shown that piezoelectricity also scales well with decreasing dimension for micro and nano sensors, provided polling control can be achieved in deposited piezoelectric films. It has been demonstrated literature that piezoelectric activity can be retained in piezoelectric film of nano scale thickness and energy density also scales well with small volume structures. This enables high integration density and reduces the voltage requirement for signal conditioning electronics. II. CONCEPT OF MULTI GATE PE-FET The electrostatic integrity of MOSFET can be enhanced by using multi gate structures as compared to planer bulk MOSFET structure which further results in lower drain voltage and threshold voltage variability. Many researches, found in literature, have shown that multi gate transistors offer excellent performance in terms of sub threshold slope, drain current and transconductance. Due to improved gate controllability, double Gate MOSFET has lower threshold voltage making it suitable for nano electronics application. The integration of piezoelectric capacitor with MOSFET, PE-FET device, has already been investigated for local amplification of charge generated by the piezoelectric capacitor when used in force sensing mode [1]. Further if the dimensions are to be scaled, different PE- FET structures need to be investigated. Moreover, the gate voltage, applied for local amplification of generated charge, can alter the piezoelectric and dielectric properties of the material. To evade these changes in material properties, multi gate PE-FET structures have been proposed. In multi gate structures, asymmetric gate strategy has been employed. Contact force to be measured has been applied at one gate and voltage for amplification of charge at another. Different structures that are simulated to analyze change in drain current with and without external applied force are discussed in next section. Page 113

3 III. SIMULATION OF SINGLE GATE PE-FET DEVICE In piezoelectric materials, a potential is created when mechanical stress is applied due to piezoelectric effect. This potential can control the transport phenomena of charge carriers, if used as gate voltage of underlying MOSFET device and can be used as force sensing element while embedded with MOSFET. Transducer action of the piezoelectric-fet device is demonstrated by converting applied pressure into electrical voltage/charge generated. There is a linear relationship between voltage generated by piezoelectric capacitor and various input pressure applied [1]. In this simulation, the device with channel length 1µm, effective channel length 200nm, oxide thickness 10nm and different thickness of PVDF layer have been considered. The simulation has been done for range of forces applied at gate area. The transfer curve shows the change in drain current when different forces are applied and also shift in threshold voltage that occurs because of inclusion of piezoelectric layer which provide local amplification of charge as well as improvement in sub threshold slope due to negative capacitance of piezoelectric material. This change in current can further be used to calculate contact forces by using signal processing circuitry. (a) (b) Fig 1: (a) Geometrical illustration of PE-FET device. (b) Transfer characteristics of PE-FET device showing change in drain current when force is applied at gate area. (a) (b) Fig. 2: Transfer characteristics of PE-FET device without and with different forces applied at gate and different piezoelectric layer thickness i.e. (a) 100nm and (b) 50nm. Page 114

4 Figure 2 shows transfer characteristics of PE-FET device with two different piezoelectric layer thickness, 100nm and 50nm. For these simulations, doping concentrations have also been changed resulting in increased drain current in comparision with previous simulation. The characteristics shows increase in drain cureent with applied force and shift in threshold voltage which enables use of this device for low voltage operations. IV. SIMULATION OF MULTI GATE PE-FET DEVICE To evade the changes in material properties because of applied gate voltage, multi gate PE-FET structures have been proposed. Asymmetric gate strategy has been employed to simulate double gate and triple gate structures. In DG PE-FET, Contact force to be measured has been applied at one gate and voltage for amplification of charge at another. Figure 3 (a & b) shows geometrical illustration of DG PE-FET using two different structures, one having lightly doped channel between drain and source regions with top and bottom gate electrodes and other using silicon on insulator technology to reduce parasitic capacitance. The device performance would be improved by using double gate approach, as the gate voltage applied at back gate will not affect dielectric properties of piezoelectric layer and will also incorporate advantages of double gate MOSFET to enhance device performance. (a) (b) (c) Fig. 3: (a) Geometrical illustration of DG PE-FET device. (b) DG PE-FET using SOI technology. (c) Cross-sectional view of TG PE-FET device Fig. 3(c) shows cross-sectional view of triple gate PE-FET. Triple gate MOSFET structures have already been reported in literature [16]. These structures have excellent shortchannel effect reduction, high transconductance and ideal sub threshold slope. Similar structures have been simulated to be used a pressure sensor. The force is applied at top gate terminal and voltage at side gates in symmetric mode for local amplification of channel charge. Transfer characteristics of TG PE-FET devices (ᴨ model and SOI TG PE-FET) and their comparison with DG PE-FET devices are shown in figure 4. Page 115

5 (a) (b) Fig. 4: Transfer characteristics and comparision of (a) DG PE-FET & TG PE-FET device without and with force applied at gate, (b) SOI DG PE-FET & TG PE-FET Devices Better control ability of the gate on the threshold voltage in case of multi gate PE-FET makes them good candidates to replace single gate device in terms of improved current driving capabilities and sub threshold slope. V. CONCLUSIONS Simulation of pressure sensor device based on double gate and triple gate structures have been performed using Sentaurus TCAD tool. The transfer characteristics of proposed structures have been analysed by measuring the change in drain current when different forces are applied. In single gate device there is approximately 7% increment in drain current when force of 1N is applied at gate area while comparing with drain current with no force applied. This increment can be further used by signal processing unit to extract contact force information. In case of DG PE-FET and TG PE-FET devices, this increment is approximately 7.69% and 10% respectively. In case of SOI double gate and triple gate devices, increase in drain current is 6.12% and 6.52% but simultaneously it provides reduced parasitic capacitances and better control on threshold voltage variability. The multi gate PE-FET devices provide improved current driving capabilities and sub threshold slope. REFERENCES [1] Sunita Malik, Manoj Kumar Duhan, Simulation of Piezoelectric Transducer Based Pressure Sensor, International Journal of Computer Application, Issue 8, Vol. 1, 2018 [2] Sunita Malik, Manoj Kumar Duhan, Analytical Modeling of Piezoelectric-FET Device based Pressure Sensor International Journal of Computational Engineering & Management. (In Press) [3] Chandrashekhar Bendigeri et al., Detailed Formulation and Programming Method for Piezoelectric Finite Element, International Journal of Pure Appl. Sci. Technol., 7(1), pp. 1-21, [4] Jayant Sirohi and Inderjit Chopra, Fundamental understanding of Piezoelectric Strain Sensors, Journal of Intelligent Material Systems and Structures, Vol.11, pp ,April [5] Antonio L. Gama et al., Proposal of new Strain Transducer based on Piezoelectric Sensors, IEEE Sensors Journal, vol. 15, No. 11, Nov [6] M.R. Cutkesky et al., Force & Tectile Sensors, Springer Handbook of Robotics, Springer, pp , Page 116

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