Nitrogen-polar deep-recess GaN HEMTs have demonstrated great promise for mm-wave power amplification applications. In this work, the structure is further explored with a focus on improving mm-wave efficiency and linearity. A brief overview of N-polar GaN is provided, along with a summary of the modern N-polar deep-recess HEMT structure with a delineation of areas of potential improvement. Then, various experiments are reported which attack each of these areas.
First, the structure of the recessed gate was addressed in an attempt to improve small-signal RF performance. Two changes to the structure were implemented: etching of the GaN channel and tailoring of the gate dielectric profile. Etching of the GaN channel increased both the RF transconductance (gm,RF) and the parasitic gate fringe capacitance (Cg,fringe) originating from the recessed sidewalls, resulting in no significant change in the current gain cutoff frequency times gate length (ftxLG). Tailoring of the gate dielectric profile via plasma etching de-coupled gm,RF and Cg,fringe, resulting in an increase in ftxLG.
Second, a Schottky-gate HEMT structure was investigated with the previously reported ALD Ru Schottky contact replaced with ALD TiN/Ru. Diodes incorporating the TiN layer demonstrated a marked reduction in reverse-bias leakage current compared to Ru-only counterparts. Also, the inclusion of TiN increased the breakdown voltage of highly-scaled Schottky-gate HEMTs by more than 20 V. The TiN/Ru Schottky gate HEMT demonstrated a peak ft/fmax of 193/362 GHz, both records for the N-polar deep-recess structure. Large signal load-pull measurements at 94 GHz revealed a record 53.4% power added efficiency (PAE) with an associated output power of 3.7 W/mm.
Third, an N-polar deep-recess HEMT structure with high-magnitude channel undulations (deemed corrugations) was investigated for low-power high-linearity applications. The output-referred third-order intercept point over DC power (OIP3/PDC) was evaluated at 30 GHz. The Corrugated MISHHEMT demonstrated a peak OIP3/PDC of 12.0 dB. Bias-sensitivity of OIP3/PDC was also investigated, with the Corrugated MISHEMT significantly outperforming a standard N-polar deep-recess MISHEMT. This bias-insensitivity was achieved via transconductance derivative superposition afforded by the channel corrugations.
Fourth, different gate geometries were investigated for Schottky-gate HEMTs. A mini field plate (MFP) structure sought to increase breakdown voltage (VBD) and thus increase mm-wave output power. A modest 4 V increase in VBD was achieved, but with little effect on small or large signal performance, motivating future work with more aggressive field-plating. Several gate periphery variations – number of gate fingers, finger width, gate pitch – were fabricated and their small-signal performance compared. The reduction in RG granted by the inclusion of TiN in the Schottky-gate HEMT structure resulted in gate widths of 150 um (both 4 x 37.5 um and 6 x 25 um) with an fmax of over 300 GHz, paving the way for higher single-HEMT output powers and efficiencies at W-band frequencies.
Finally, N-polar GaN electrochemical (EC) etching for porous pseudo-substrate fabrication was pursued. Optimization of EC etching and subsequent MOCVD regrowth conditions resulted in smooth GaN surfaces atop a porous underlayer. Such as substrate can serve as a template for structures which take advantage of strain-relaxation enabled by porous GaN for electronic or optoelectronic applications.
