Scaling of GaN HEMTs and Schottky Diodes for Submillimeter-Wave MMIC Applications

In this paper, we report state-of-the-art high frequency performance of GaN-based high electron mobility transistors (HEMTs) and Schottky diodes achieved through innovative device scaling technologies such as vertically scaled enhancement and depletion mode (E/D mode) AlN/GaN/AlGaN double-heterojunction HEMT epitaxial structures, a low-resistance
$n^{+}$
-GaN/2DEG ohmic contact regrown by MBE, a manufacturable 20-nm symmetric and asymmetric self-aligned-gate process, and a lateral metal/2DEG Schottky contact. As a result of proportional scaling of intrinsic and parasitic delays, an ultrahigh
$f_{T}$
exceeding 450 GHz (with a simultaneous
$f_{\rm max}$
of 440 GHz) and a
$f_{\rm max}$
close to 600 GHz (with a simultaneous
$f_{T}$
of 310 GHz) are obtained in deeply scaled GaN HEMTs while maintaining superior Johnson figure of merit. Because of their extremely low on-resistance and high gain at low drain voltages, the devices exhibited excellent noise performance at low power. 501-stage direct-coupled field-effect transistor logic ring oscillator circuits are successfully fabricated with high yield and high uniformity, demonstrating the feasibility of GaN-based E/D-mode integrated circuits with
${>}{1000}$
transistors. Furthermore, self-aligned GaN Schottky diodes with a lateral metal/2DEG Schottky contact and a 2DEG/
$n^{+}$
-GaN ohmic contact exhibited
$RC$
-limited cutoff frequencies of up to 2.0 THz.