Graduate Thesis Or Dissertation
 

Analysis and modeling of electrically long MESFETS and coupled schottky lines

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  • The properties of linear active coupled lines consisting of electrically long GaAs MESFET structures are investigated for possible applications as traveling-wave broad-band amplifiers. In addition, the nonlinear coupled Schottky lines are studied for applications as voltage tunable circuit elements. The analysis of the general active asymmetric coupled transmission lines in an inhomogeneous medium is presented. The scattering parameters of a four-port network based on the general normal mode analysis of coupled lines are derived in terms of the equivalent circuit elements associated with the long MESFET structure. These are then used to study the properties of the four-port device for applications as traveling-wave amplifiers. One of the objectives of this study is to model the device by calculating the electrical parameters of the structure from its physical (geometrical and material) parameters. An accurate computer program based on up to date theoretical formulas and experimental results for calculating these circuit elements is developed. A method based on the conformal mapping is developed to calculate the exact values of interelectrode capacitances for the two and three asymmetric coplanar strips. Empirical closed-form expressions are also obtained for these capacitances by using simple curve fitting procedures. In order to help optimize the performance of the device in terms of its gain and bandwidth, a CAD program "TOUCHSTONE" is used. Various techniques to compensate for the difference in the capacitive and inductive loading of the gate and drain lines and to synchronize the phase velocity of the waves on both lines are investigated. It is found that a distributed inductance has to be added to the drain line to equalize the velocities and optimize the device's performance, i.e., the gain-bandwidth product. The termination impedances are also optimized. Typical devices with the optimized termination impedances and synchronized phase velocities give more than 3 dB power gain for frequencies up to 34 GHz. The dependence of the Schottky junction capacitance under the gate on the applied voltage gives rise to the nonlinear behavior in the transmission lines. The coupled Schottky lines are analyzed by using a linearized and a nonlinear quasi-harmonic model. It is shown that such structures can be used as voltage tunable devices including directional couplers and electronic switches consisting of a long dual-gate type coplanar structure. Examples of voltage tunable couplers and a 15 GHz switch are included.
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