AlGaN/GaN HEMTs on Si by Self-Aligned TaN Contact Ledge for Low-Voltage RF Applications

We propose a self-aligned contact ledge structure to achieve the small ohmic contact resistance ( $R_{\text {c}}$ ) of GaN-based high electron mobility transistors (HEMTs) on Si for low-voltage RF applications. Without additional photolithography processes, the ledge structure is achieved by magnetron sputtering with good particle filling on the sidewall. Due to the good coverage of sputtered Ta, the self-aligned TaN contact ledge not only has a clear boundary after high-temperature annealing but also introduces an additional current path to reduce on-resistance ( ${R} _{\text {on}}$ ), which is important for fabricating devices for low-voltage RF applications. Benefiting from the contact ledge structure and the low sheet resistance high Al component Al_0.6Ga_0.4N/GaN epitaxy, a low ${R} _{\text {on}}$ of $1.12~\Omega \cdot $ mm is obtained and the peak transconductance increased by 13%. At 3.5 GHz and operating voltage 5 V, the power-added efficiency (PAE) is 67.0% and the output power density ( ${P} _{\text {out}}$ ) is 0.75 W/mm. The PAE is 8.5% higher than that of noncontact ledge.