D. Regan

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 fT exceeding 450 GHz (with a simultaneous fmax of 440 GHz) and a fmax close to 600 GHz (with a simultaneous fT 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 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.

Deeply-scaled self-aligned-gate GaN DH-HEMTs with ultrahigh cutoff frequency

We report record DC and RF performance in deeply-scaled self-aligned gate (SAG) GaN-HEMTs operating in both depletion-mode (D-mode) and enhancement-mode (E-mode). Through aggressive lateral scaling of the gate length (L<inf>g</inf>) and the source-drain distance (L<inf>sd</inf>) using a novel self-aligned gate technology and engineering of a thin top barrier layer, 20-nm gate AlN/GaN/AlGaN double-heterojunction (DH) HEMTs operating in D-mode (and E-mode) exhibited record DC and RF characteristics with high yield and uniformity; R<inf>on</inf> = 0.29 (0.33) Ω·mm, I<inf>dmax</inf> = 2.7 (2.6) A/mm, a peak extrinsic g<inf>m</inf> = 1.04 (1.63) S/mm, threshold voltage uniformity σ (V<inf>th</inf>) = 44 (63) mV over a 3-inch wafer area, and a simultaneous f<inf>T</inf>/f<inf>max</inf> = 310/364 (343/236) GHz. Delay time analysis clarified that an unique dependence of f<inf>T</inf> on V<inf>ds</inf> resulted from suppressed drain delay and enhanced electron velocity due to the lateral source-drain (S-D) scaling.

Electron Velocity Enhancement in Laterally Scaled GaN DH-HEMTs With

In this letter, we report the first experimental observation of electron velocity enhancement by aggressive lateral scaling of GaN HEMTs. Through reduction of the source-drain distance down to 170 nm using <i>n</i>⁺-GaN ohmic regrowth, 45-nm gate AlN/GaN/Al_0.08Ga_0.92N HEMTs exhibited an extremely small on resistance of 0.44 Ω·mm , a high maximum drain current density of 2.3 A/mm, a high peak extrinsic transconductance of 905 mS/mm, and a record <i>fT</i>/<i>f</i>_max of 260/394 GHz. Delay time analysis showed that the outstanding <i>fT</i> was mainly due to significantly reduced electron transit time at higher drain-source voltages resulting from suppressed drain delay and enhanced electron velocity in the laterally scaled GaN HEMTs.

f_{T}

This letter reports record RF performance of deeply scaled depletion-mode GaN-high-electron-mobility transistors (GaN-HEMTs). Based on double heterojunction AlN/GaN/AlGaN epitaxial structure, fully passivated devices were fabricated by self-aligned-gate technology featuring recessed n+-GaN ohmic contact regrown by molecular beam epitaxy. Record-high fT of 454 GHz and simultaneous fmax of 444 GHz were achieved on a 20-nm gate HEMT with 50-nm-wide gate- source and gate-drain separation. With an OFF-state breakdown voltage of 10 V, the Johnson figure of merit of this device reaches 4.5 THz-V, representing the state-of-the-art performance of GaN transistor technology to-date. Compared with previous E-mode GaN-HEMTs of similar device structure, significantly reduced extrinsic gate capacitance and enhanced average electron velocity are the key reasons for improved frequency characteristic.

of 260 GHz

We report record DC and RF performance obtained in deeply-scaled self-aligned-gate GaN-HEMTs with heavily-doped n⁺-GaN ohmic contacts to two-dimensional electron-gas (2DEG). High density-of-states of three-dimensional (3D) n⁺-GaN source near the gate mitigates “source-starvation,” resulting in a dramatic increase in a maximum drain current (I_dmax) and a transconductance (g_m). 20-nm-gate D-mode HEMTs with a 40-nm gate-source (and gate-drain) distance exhibited a record-low R_on of 0.23 Ω·mm, a record-high I_dmax of >4 A/mm, and a broad g_m curve of >1 S/mm over a wide range of I_ds from 0.5 to 3.5 A/mm. Furthermore, 20-nm-gate E-mode HEMTs with an increased L_sw of 70 nm demonstrated a simultaneous f_T/f_max of 342/518 GHz with an off-state breakdown voltage of 14V.

Ultrahigh-Speed GaN High-Electron-Mobility Transistors With

An enhancement-mode (E-mode) AlN/GaN/AlGaN double-heterojunction field-effect transistor (DHFET) with record high-frequency performance is reported. E-mode operation was achieved through vertical scaling of the AlN barrier layer. Parasitic resistances were reduced through ohmic contact recess etching followed by regrowth of n+ GaN by molecular-beam epitaxy and SiN deposition to increase the sheet charge density in the access regions of the device, resulting in an extremely low on-resistance of 1.06 Ω · mm. A DHFET with an 80-nm gate length had a threshold voltage of 0.21 V, an extrinsic transconductance (gm) of 0.70 S/mm, a current-gain cutoff frequency (fT) of 112 GHz, and a maximum oscillation frequency (fmax) of 215 GHz. To our knowledge, these are the highest gm , fT, and fmax values reported to date for an E-mode GaN HFET.

f_{T}/f_{\mathrm {max}}

Highly scaled GaN T-gate technology offers devices with high ft/fMAX, and low minimum noise figure while still maintaining high breakdown voltage and high linearity typical for GaN technology. In this paper we report an E-band GaN power amplifier (PA) with output power (Pout) of 1.3 W at power added efficiency (PAE) of 27% and a 65-110 GHz ultra-wideband low noise amplifier (LNA). We also report the first G-band GaN amplifier capable of producing output power density of 296mW/mm at 180 GHz. All these components were realized with a 40 nm T-gate process (ft= 200 GHz, fMAX= 400 GHz, Vbrk > 40V) which can enable the next generation of transmitter and receiver components that meet or exceed performance reported by competing device technologies while maintaining > 5x higher breakdown voltage, higher linearity, dynamic range and RF survivability.

of 454/444 GHz

We have achieved the monolithic integration of two Ill-nitride device structures through the use of etching and re growth by molecular beam epitaxy (MBE). Using this regrowth technique, we integrated enhancement-mode (E-mode) and depletion-mode (D-mode) AIN/GaN/AlGaN double-heterojunction field-effect transistors (DHFETs) on a single SiC substrate, wherein the E-mode devices had a 2-nm-thick AlN barrier layer and the D-mode devices had a 3.5-nm-thick AlN barrier layer. The direct-current and radio-frequency (RF) performance of the resulting DHFETs was equivalent to devices fabricated using our baseline process with a normal MBE growth. D-mode devices with a gate length of 150 nm had a threshold voltage Vth of -0.10 V, a peak transconductance gm value of 640 mS/mm, and current gain and power-gain cutoff frequencies fT and fmax of 82 and 210 GHz, respectively. E-mode devices on the same wafer with the same dimensions had a Vth value of +0.24 V, a peak gm value of 525 mS/mm, and fT and fmax values of 50 and 150 GHz, respectively. The application of this regrowth technique is not, in any way, limited to the integration of E- and D-mode devices, and this method greatly expands the design possibilities of RF and power switching circuits in the nitride material system.

Self-aligned-gate GaN-HEMTs with heavily-doped n + -GaN ohmic contacts to 2DEG

Highly scaled AlN/GaN metal-oxide-semiconductor heterojunction field-effect transistors (MOS-HFETs) with Al₂O₃ gate dielectrics of varying thicknesses deposited by atomic layer deposition (ALD) were fabricated, and their performance was compared with Schottky-barrier HFETs (SB-HFETs). MOS-HFETs with an ultrathin 2-nm-thick Al₂O₃ dielectric and a gate length of 40 nm had direct-current (dc) and radio-frequency (RF) performances similar to the SB-HFETs, with a high extrinsic transconductance of 415 mS/mm, <i>f</i>_T of 134 GHz, and <i>f</i>_max of 261 GHz. In contrast, the dc and RF performances of a MOS-HFET with a 4-nm-thick Al₂O₃ dielectric were degraded by short-channel effects. The 2-nm-thick Al₂O₃ gate insulator reduced the forward-bias gate current by more than two orders of magnitude. The data suggest the promise of ultrathin ALD Al₂O₃ gate dielectrics for next-generation high-speed GaN HFETs.

Enhancement-Mode AlN/GaN/AlGaN DHFET With 700-mS/mm

We report record RF performance in 40nm-gate GaN-HEMT technology. Through vertical scaling in an AlN/GaN/AlGaN double heterojunction (DH) HEMT structure and reduction of access resistance using MBE re-growth of n⁺-GaN ohmic contacts, fully-passivated 40-nm devices exhibited excellent DC characteristics, such as an R<inf>on</inf> of 0.81Ω·mm, an I<inf>dmax</inf> of 1.61A/mm, a BV<inf>off</inf> of 42V, and a peak extrinsic g<inf>m</inf> of 723mS/mm, resulting in a peak f<inf>T</inf> of 220GHz and a peak f<inf>max</inf> of 400GHz. The measured f<inf>T</inf> and f<inf>max</inf> are the highest ever reported in a GaN-HEMT technology. Small signal model and delay time analysis showed that the parasitic charging time was only 10% of total delay time and the gate transit time scaled with the gate length (L<inf>g</inf>) down to 40nm, demonstrating high scalability of the new technology.