SiC Varactors for Dynamic Load Modulation of Microwave Power Amplifiers

Licentiate thesis, 2011

The rapid consumer adoption of mobile services is leading to an exponential growth in wireless data traffic. In order to accommodate more concurrent high data-rate users, the required complexity of transmitting radio base station (RBS) power amplifiers (PAs) is increasing. There is a drive to reduce costs and environmental footprint by developing novel PA architectures. Dynamic load modulation (DLM) is a proven method to improve the PA energy efficiency when transmitting modern communication signals (3G and beyond). In DLM a tunable network, employing one or several varactors, is used to realize optimal PA load impedances versus output power. In this work wide bandgap silicon carbide (SiC) Schottky diode varactors, specifically for use in the DLM of PAs, have been designed, fabricated, and evaluated. A tailored doping profile in the epitaxial layers is used to distribute the capacitive tuning over a large bias voltage range. This enables good tunability even at large voltage swings, resulting in good power handling capabilities. Devices have been realized with a small-signal tuning range of 6:1, while capable of supporting a 3:1 tuning range at 36V swing. Breakdown voltages are measured in the −150 to −200V range. The high critical electric field in SiC allows for incorporation of high doping concentrations, resulting in low varactor series resistance, enabling microwave frequency operation. To reduce the parasitic series resistance a self-aligned interdigitated layout is used. A highest zero-bias Q-factor of 44 has been extracted at 2GHz, with a corresponding Q-factor exceeding 800 at the −50V punch-through. Advanced microwave characterization, using a multi-harmonic active source and load-pull measurement setup, has been used to study device operation versus bias, available power, and second harmonic load impedance. The results emphasize that varactors are inherently nonlinear devices, and not linear tunable capacitances, leading to conclusions on their optimal use in tunable networks. Nonlinear device models for harmonic balance (HB) simulations in circuit design have been developed and validated both on-wafer and in hybrid circuits. The fabricated varactors have been evaluated in several DLM PAs with good efficiency enhancement results. The measured drain efficiency of an RF pulse-width modulated (RFPWM) class-E PA is improved from 40% to 75% at 5 dB power back-off upon inclusion of an integrated SiC varactor-tuned DLM network.