Fabian Radulescu

Power Amplifier Design Optimized for Envelope Tracking

In this paper, we present the design of an inverse Class F power amplifier GaN MMIC, specifically designed for an envelope-tracking application. Power transistors are not typically characterized for the drain modulation that is fundamental to envelope tracking, and the available device models are not usually validated over the required drain bias range. Here, we used fundamental load-pull to characterize a 6×100μm GaN HEMT device over the range of drain bias voltages that would be used in the envelope-tracking PA. This data was scaled to an 8×100μm device to achieve the target output power, and these empirical load-pull models were then used in the design of the power MMIC along with harmonic design in simulation. A total of eight 8×100 μm HEMTs were used in the final design, achieving a maximum power output of 32 W at 10 GHz with a drain efficiency of greater than 45% in back-off, on a die size of less than 4 × 4 mm2 under envelope tracking.

100 mm GaN-on-SiC RF MMIC technology

100 mm diameter 4H-SiC High Purity Semi-insulating substrates are now being manufactured in high volume. GaN HEMT layers grown on 100 mm SiC substrates have shown excellent sheet resistivity and AlGaN thickness uniformities (σ/mean) of 1.3 and 1.1%, respectively. The fabrication process for MMIC manufacture was adapted to the larger diameter substrates without requiring any change to the process design kits for the foundry. MIM capacitor processes were optimized, and resistor process, wafer thinning and slot via etching were all adapted to the larger platform. These 100 mm wafers are now being used in high volume production of both high power discrete GaN devices, as well as MMICs. Commercially available MMICs have been released to production using this 100 mm platform. A wide band 25 Watt power amplifier is discussed, along with a 3 watt driver capable of DC-4 GHz operation.