Browsing by Author "Robert J. Trew, Committee Chair"

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    Analytical Physics Based AlGaN/GaN HFET Large Signal Model and Nonlinearity Analysis with Nonlinear Source Resistance
    (2009-05-28) Liu, Yueying; Robert J. Trew, Committee Chair; Griff L. Bilbro, Committee Co-Chair; Doug Barlage, Committee Member; C.T.Kelley, Committee Member
    In this work the impact of nonlinear source resistance and RF channel breakdown on AlGaN/GaN HFETs RF and linearity performance were studied. AlGaN/GaN HFETs are well known strong candidates for high power devices due to its superior material properties, such as high electric field to achieve electron saturation velocity and high mobility. Practical amplifiers, however, do not demonstrate the good RF linearity performance predicted from fundamental semiconductor materials properties. In particular, it has been demonstrated that a nonlinear source resistance is generated in these devices due to the onset of space-charge limited (SCL) transport in the gate-source region. It has been demonstrated that the nonlinear source resistance modulation degrades the linearity of the device, even under modest RF drive conditions. Breakdown mechanism has been found to be a major factor that limits FET’s performance at large signal condition. For AlGaN/GaN HFETs, breakdown could happen both in the channel and on the surface. The lower critic value for channel breakdown determines that it dominates in the breakdown process and contributes most in device’s operation. A physics based large signal FET model was modified to include nonlinear source resistance effect and RF channel breakdown. The model was further modified to include multi-tone and wide-band signal simulation capability to study the device’s linearity performance. The goal of this work is to understand the physical mechanisms that determine device’s RF and linearity performance and provide the optimization options of improving these effects effectively and efficiently. The physical mechanisms were able to be quantified and determined based on the study using the analytical physics based model. It allows the development and optimization of power amplifier before the device is fabricated. The physical device model used in this work permits determination of the nonlinear distortion under large signal operation as a function of bias, device physical parameters such as structural dimensions, doping, etc, and circuit tuning conditions.
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    A Chopper Modulated Amplifier System Design for in vitro Neural Recording
    (2006-08-07) Dagtekin, Mustafa; Griff L. Bilbro, Committee Member; Robert J. Trew, Committee Chair; Robert M. Grossfeld , Committee Member; John M. Wilson, Committee Member; Veena Misra, Committee Member
    Neural recording systems measure very low-amplitude signals of less than 5 kHz bandwidth. Low-frequency noise processes such as flicker noise and DC offset can degrade the quality of recordings made by such systems. A chopper modulated amplifier system is described and shown to reduce the flicker noise in neural recording systems by 10 to 20 dB. The amplifier system was implemented using the MOSIS ABN 1.5 micron technology. While the amplifier system contained an imperfection that prevented it from working with actual tissue samples, it worked well enough to prove that chopper modulation does reduce flicker noise appreciably. All of the details are presented along with studies of gate-metal-free transistors and custom-made MOSIS-based recording electrodes.
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    TCAD simulation and modeling of AlGaN/GaN HFETs
    (2009-03-28) Kuang, Weiwei; Doug Barlage, Committee Member; Griff L. Bilbro, Committee Co-Chair; Zhilin Li, Committee Member; Robert J. Trew, Committee Chair
    This work focused on the TCAD simulation and modeling of AlGaN/GaN HFETs. AlGaN/GaN HFETs have demonstrated excellent RF performance, which benefits from the high sheet charge density in these hetero-structures, the high carrier mobility and saturation velocity in the channel, and the high breakdown voltage inherent in the GaN material. However, these devices experience physical phenomena that degrade their performance. In particular, AlGaN/GaN HFETs often demonstrate Cgs variations with input power drive that are opposite to classical FET behavior. The transconductance is also degraded at high current levels. The behavior affects the frequency performance and linearity of the device. It has been numerically confirmed in this work that the nonlinear source resistance due to the on-set of space charge limited current transport condition existing in AlGaN/GaN HFETs is the origin of the extrinsic gm and Cgs degradation at high drain current levels. AlGaN/GaN HFETs suffer from large gate leakage and reliability problems where dc drain current and RF output power are degraded as a function of stress time. In this work, TCAD simulations were performed to reproduce the measured bias-dependent and stress time-dependent drain current and gate current characteristics of AlGaN/GaN HFETs with excellent accuracy. A surface electron hopping model is proposed to explain the gate leakage and the reliability problem associated with the high voltage operation of AlGaN/GaN HFETs. According to the model, electrons that tunnel from the gate can accumulate at the gate edge on the drain side and/or travel along the AlGaN surface toward the drain through a trap-to-trap hopping mechanism. The extracted value for the activation energy of the surface traps is in the range of 0.25~0.35eV, which is consistent with the measured energy level associated with nitrogen vacancies and/or dangling bonds in the device. A discussion of the AlGaN/GaN HFET device design and optimization was also presented in this work. It is suggested that reduced gate-to-source spacing can be used to reduce the nonlinear source resistance and to improve RF performance and linearity of the device. Various techniques including processing methods (using passivation to reduce or immobilize the surface states), field engineering methods (using field plates) and polarization control methods (using a GaN cap layer above the AlGaN layer, or growing the device in M-plane, or using a lower AlN mole fraction in AlGaN) can be employed to mitigate the large gate leakage current and reliability problems.