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Thermal Analysis of 100V GaN RF Transistors for CW High Power ISM Applications
Keywords: ISM, GaN, high power
This paper reports the first ever demonstration of an RF GaN transistor operating in CW conditions at 100V bias to achieve 200W output power and 80% drain efficiency. With just 15mm gate periphery the device exhibits a power density of 13W/mm. The die physical size is 40mil x 152mil and the GaN on SiC thickness is 3mil. The package flange is made of CPC with 220W/m-K thermal conductivity and it is 40mil thick. Operating frequency is 430MHz. The modeled thermal resistance of the packaged device is 2 C/W corresponding to a junction temperature of 160 C when the air cooled base plate reached a temperature of 60 C during measurements. The thermal model includes the temperature dependence of the SiC substrate and the GaN epitaxial layer. Typical field operation limits the base plate temperature at 80 C by air or liquid cooling in which case the model predicts a junction temperature of 186 C which is acceptable for long-term reliability. A methodology is presented to achieve a high efficiency 1kW CW output power device or module using this technology suitable for replacing vacuum tube devices in several ISM applications. The proposed approach does not rely on expensive GaN on diamond technology; nor is it based on diamond or aluminum nitride heat spreaders or copper heat sink with over-molded plastic packages. The presented approach is based on a combination of die design and circuit techniques that are enhanced by the increased bias. With 100V operation and higher power density the gate periphery of the die required for a target output power is reduced which allows further spreading of the active areas in the die layout and lowers the thermal resistance; lower gate periphery also translates into lower output capacitance CDS to facilitate 2nd and 3rd harmonic tuning as typically done in switch-mode class E and class F or inverse class F power amplifiers to achieve the reported 80% efficiency.
Gabriele Formicone, Director RF Transistor Design
Integra Technologies, Inc.
El Segundo, CA

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