This paper demonstrates how sodium enhanced oxidation of Si face 4H-SiC results in removal of near-interface traps at the SiO2∕4H-SiC interface. These detrimental traps have energy levels close to the SiC conduction band edge and are responsible for low electron inversion channel mobilities (1–10cm2∕Vs) in Si face 4H-SiC metal-oxide-semiconductor field effect transistors. The presence of sodium during oxidation increases the oxidation rate and suppresses formation of these near-interface traps resulting in high inversion channel mobility of 150cm2∕Vs in such transistors. Sodium is incorporated by using carrier boats made of sintered alumina during oxidation or by deliberate sodium contamination of the oxide during the formation of the SiC∕SiO2 interface.
A new switch mode power amplifier (SMPA) topology particularly suitable for energy efficient amplification of radio frequency pulse width modulation (RF-PWM) signals is derived. It is analytically shown that high efficiency can be maintained over a wide power dynamic range if the imaginary part of the Class-E load impedance is varied along with the duty cycle (pulse width). Using the theory developed, an explicit design procedure is presented that allows practical realization of the proposed topology from the circuit and component specifications. Following the design procedure, and using in-house (Chalmers University) SiC varactor diodes to implement the tunable imaginary load impedance, a 2 GHz 10 W peak output power GaN HEMT circuit demonstrator is realized. RF-PWM input signals for characterization of the prototype PA is generated with a dedicated 65 nm CMOS modulator. The measurements show that a drain efficiency > 70% can be obtained over an 6.5 dB dynamic range, which verifies the theory presented and demonstrates the feasibility of the proposed PA topology.
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