Microwave Power Amplifiers Fabricated from Wide Bandgap Semiconductor Transistors
There is a need for solid state microwave sources that can operate at high RF output power with high power-added efficiency and at high ambient for applications such as mobile communications base station transmitters, phased array radars, satellite transponders, etc. To date, solid state microwave amplifiers fabricated from conventional semiconductor transistors have been limited to the 10‰s to 100‰s of watt levels. Although kilowatt amplifiers have been reported they have required combining the outputs of a large number of devices and have proved difficult to fabricate. Generally, high power microwave sources use vacuum tube technology such as TWT‰s or klystrons. Recent advances in wide bandgap semiconductor technology, particularly the III-Nitrides and the 6H and 4H polytypes of SiC, offer the possibility of fabricating improved solid state electronic devices that can operate at power levels significantly in excess of that available from standard devices. Microwave MESFET‰s have been produced in SiC, GaN, and GaN-based heterostructures. Microwave amplifiers with RF output power on the order of 3 W/mm and PAE as high as 67% at 850 MHz have been fabricated from 4H-SiC and amplifiers with about 2 W/mm and 45% PAE at 6 GHz have been fabricated from 6H-SiC. These results are nearing the performance theoretically predicted for these devices. Both HFET‰s and HEMT‰s can be fabricated using the AlGaN/GaN heterojunction. The heterointerface yields a two-dimensional electron gas with sheet charge density on the order of 1013 cm-2 and low field mobility in the range of 5000-8000 cm2/V-sec. This permits both HFET‰s and HEMT‰s with excellent high frequency performance to be fabricated. In addition, the critical electric field for breakdown is a factor of 3-5 greater than in Si and GaAs, and this permits electronic devices to support high bias voltages without experiencing breakdown. Static Induction Transistor (SIT) amplifiers with very good microwave performance have also been demonstrated. In this work the microwave performance of various devices fabricated from SiC and GaN-based materials is described and compared to similar devices fabricated from GaAs. Theoretical large-signal simulations of semiconductor device and amplifier performance are presented and used to explain the physical operation of the components. The operating principles of various transistors operating in class A and class B amplifier circuits will be discussed and design considerations for obtaining optimized performance presented. Elevated temperature operation is discussed. It is demonstrated that transistor amplifiers can yield RF output power on the order of 4-5 W/mm for SiC transistors and 10-12 W/mm for GaN-based transistors with power-added efficiency approaching the ideal values for class A and B operation.