Date of Award
Campus Access Dissertation
Grigory S Simin
Microwave switches are important components of most microwave systems such as radars, phase array antennas, cell phone and satellite communications etc. III-Nitride Heterojunction Field Effect Transistor (HFET) microwave switches have demonstrated significant advantages over most other switch types including pin- diodes GaAs high electron mobility transistors (HEMT) and MEMS due to high breakdown, very low insertion loss, higher isolation, high switching power and low power consumption. An important switch characteristic is the level of introduced nonlinear distortions. Intermodulation distortions occur due to device nonlinearity when multiple input frequencies produce undesired output signals. 3rd order intermodulation product is very important because it occurs in the frequency range close to that of fundamental signals and is impossible to filter out. The only solution to the problem is to minimize the level of this distortion. The IP3 (Third order intercept point), a measure of third order distortions level is an important figure of merit for microwave devices.
In this work, the 3rd order distortions in III-Nitride switches are studied by using direct two-tone measurements, extraction from current-voltage characteristics and by simulations. It has been shown that III-Nitride microwave switches exhibit high IP3 points of 64 -78 dBm, which compare favorably or exceed those of other switch types. Different mechanisms contributing the switch nonlinearity have been studied, such as mobility - field dependence, gate bias induced nonlinearity, contact nonlinearity. The contact contribution to the total device nonlinearity has not been studied before, to the best of our knowledge. Novel experimental technique allowing to independently extracting the channel and contact nonlinearity has been developed. It has been found that contact and channel 3rd order nonlinearity coefficients have opposite signs. Based on these findings a novel method of nonlinearity compensation is proposed.
Field Effect Transistor (FET) channel current-voltage (I-V) characteristic is sublinear and hence the related nonlinearity can be compensated by adding the component with superlinear I-V. First the criterion for nonlinearity compensation is derived. Next, as a proof of concept, a discrete prototype circuit containing metal oxide field effect transistor (MOSFET) and compensating circuit has been designed, built and tested. As confirmed by two-tone measurements, the reduction of the 3rd order distortions up to 26 dBm has been achieved by using nonlinearity compensation network. Furthermore the nonlinearity compensation feasibility is extended to III-Nitride MMIC level. The design of III-Nitride single pole single throw (SPST) switch was carried out resulting in reduction of 3rd order intermodulation distortions.
Khan, M. B.(2011). Third Order Nonlinearity In III-Nitride Microwave Switches. (Doctoral dissertation). Retrieved from http://scholarcommons.sc.edu/etd/2188