Integration of ferroelectrics with III-N transistors for high performance millimeter-wave applications

GaN-based high-electron mobility transistors are widely recognized for their exceptional performance at RF and microwave frequencies, and are increasingly being explored for millimeter-wave amplifier applications. An additional application that is critical for future systems is signal switching and routing at millimeter-wave frequencies; this is essential for enabling millimeter-wave wireless communication systems (e.g. 6G and beyond) that require frequency agility and reconfigurability. For this type of RF and mm-wave switch applications, the high carrier concentration and high 2DEG mobility of III-N HEMTs leads to low on resistance and low insertion loss. However, the isolation is limited by off-state capacitance, and nonlinearity of the HEMT limits the power handling capabilities. We have observed that the inclusion of a ferroelectric gate dielectric (using ALD-deposited Hf_0.5Zr_0.5O₂) in the device can significantly enhance performance. By combining polarization engineering of the III-N HEMT with the hysteretic and dispersive polarization characteristics of the ferroelectric gate stack, substantial improvements in the switch figure of merit (FOM=1/2π(R_onC_off)) can be achieved. The reduced effective off-state capacitance enabled by ferroelectrics integrated with GaN-based transistors has led to switches with FOM of 2.5 THz. Combining this with advanced processing (e.g. regrowth of source and drain ohmic contacts, gate length scaling), further improvements in performance are expected.