Epitaxial growth of InAlN and InAlGaN barriers for advanced RF devices
Gallium nitride is used extensively in radio-frequency and microwave applications where its wide bandgap, high saturated electron velocity, and high critical field have enabled system performance up to mm-wave frequencies. Applications up to Ka-band typically rely on structures incorporating an AlGaN barrier with optional cap layers to fine-tune carrier injection and threshold voltage, as well as optional spacer layers improve electron confinement. However, 5G mobile networks are expected to require linear power up to 20 dBm for frequencies as high as 100 GHz, at which point the barrier scaling becomes unfavorable to AlGaN. HEMTs based on InAlN , and InAlGaN barriers enable deep scaling while maintaining high channel charge. Heterostructures based on the ternary alloy are typically lattice-matched (In0.17Al0.83N) and achieve channel charge greater than 1013 cm 2 due to the large spontaneous polarization difference relative to GaN, and good carrier confinement due to the large (~0.6 eV) conduction band offset. In combination with n+ ohmic contacts regrown by molecular beam epitaxy to reduce access resistance, , InAlN-based transistors have been demonstrated with cutoff frequencies fT / fmax up to 270 / 230 GHz. Large-signal operation up to 94 GHz has been demonstrated with power density of 1.35 W/mm and power-added efficiency of 12%. InAlGaN can be lattice-matched or strained in order to tailor channel charge and mobility and further reduce the barrier thickness, achieving 2x1013 cm-2 channel charge and electron mobility of 1500 cm2/Vs with a 4.8 nm-thick barrier. From the epitaxy perspective, the growth of InAl(Ga)N requires careful optimization due to the large difference in nominal melting point between the constituent binary materials. Characterization challenges also exist for thin layers with thickness in the single-digit nanometer range. The presentation will describe the primary material science considerations for this advanced barrier material and will provide a view of selected epitaxial growth and material characterization topics, linking material properties to device performance.