Microwave wide bandgap GaN high electron mobility transistor development and its monolithic integrated circuits (Invited)

Wide bandgap gallium nitride (GaN) high electron mobility transistor (HEMT) has been extensively studied [1]. The material properties of GaN compared to competing materials are presented in Table I. The superior material properties high breakdown voltage, which allows large drain voltages to be used, leading to high output impedance per watt of RF power, and lower loss matching circuits. High current density of 2-D electron gas (2-DEG) leads to large sheet charge [2] and transistor area can be reduced resulting in high watts per millimeter of gate periphery. High saturated velocity leads to high saturation current densities and watts per unit gate periphery. These result GaN based HEMT are suitable for high-power and high-frequency monolithic microwave integrated circuit (MMIC) applications [3-5]. Due to these excellent material characteristics, the output power and efficiency of GaN power amplifier between L-band and Ka-band was not only more superior to the conventional LDMOSFETs and GaAs power amplifier, but also the die area can be reduced. The GaN HEMT can be operated at 42 V of VDS and even higher, while also demostrated the similar fT and fmax with GaAs pHEMT. The high powers from GaN HEMT transistors at a wide frequency range have been reported form a single die up to several hundred watts [6-7]. However, these high power densities also present extreme power dissipation on the layouts and the semiconductor substrates. Nevertheless, the SiC substrates with a high thermal conductivity (> 330 W/mK) allows high power densities to be efficiently dissipated for practical drain efficiencies, preventing the extreme channel that would result due to self-heating with other substrate technologies.