AlGaN/GaN Heterojunction Bipolar Transistors With Record fᴛ/fₘₐₓ=21.6 /4.23 GHz

We report a novel GaN heterojunction bipolar transistor (HBT) featuring n-AlGaN/n+-GaN emitter layers, achieving a record ${f}_{T}/{f}_{\textit {MAX}} =21.6$ /4.23 GHz at the current density of 12.7 mA and V ${}_{\text {CE}}=10$ V. This is the first time that the frequency of GaN HBTs have been extended to the S-band. The exceptional performance is attributed to three advancements in material and device fabrication: 1) regrowth emitter technology is used to achieve base ohmic contact, thereby reducing the parasitic base resistance; 2) An n+-GaN emitter cap layer is used to achieve an ultra-low specific contact resistance of $1.35\times 10^{-{7}}\Omega \cdot $ cm2, effectively lowering the total emitter resistance, thereby reducing the collector charging time $\tau _{C}$ ; 3) down-scaling technologies, which reduce the parasitic base resistance and the base-collector (BC) junction capacitance, also helps to increasing the ${f}_{T}$ and ${f}_{\textit {max}}$ . The device demonstrates a power density of 1.31 MW/cm2, which is the highest for GaN HBTs on sapphire. These results underscore the potential of GaN HBTs for high-power, high-efficiency RF applications.