Miroslav Micovic

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.

GaN HFET for W-band Power Applications

In this paper we report high frequency GaN power device and measured power performance of the first W-band (75 GHz-110 GHz) MMIC fabricated in GaN material system. The first W-band GaN MMIC with 150 mum of output gate periphery produces 316 mW of continuous wave output power (power density =2.1 W/m) at a frequency of 80.5 GHz and has associated power gain of 17.5 dB. By comparison the reported state of the art for other solid state technologies in W-band is 427 mW measured in a pulsed mode on an InP HEMT MMIC with 1600 mum of output periphery (power density = 0.26 W/mm). The reported result demonstrates tremendous superiority of GaN device technology for power applications at frequencies greater than 75 GHz

GaN double heterojunction field effect transistor for microwave and millimeterwave power applications

We report development of a novel AlGaN/GaN/AlGaN double heterojunction field effect tansistor (DHFET) with improved device performance over the conventional single heterojunction GaN FET (SHFET). The GaN DHFETs with low Al content Al/sub 0.04/Ga/sub 0.96/N buffer layer exhibit three orders of magnitude lower subthreshold drain leakage current and almost three orders of magnitude higher buffer isolation than corresponding SHFET devices (600 M/spl Omega//sq. vs. 1 M/spl Omega//sq.). In GaN DHFETs with 0.15 /spl mu/m conventional T-gates we observed 30% improvement in saturated power density and 10% improvement in PAE at 10 GHz over a corresponding SHFET device.

AlGaN/GaN heterojunction field effect transistors grown by nitrogen plasma assisted molecular beam epitaxy

In this work, we demonstrate state of the art performance of GaN HFETs grown on SiC by rf Nitrogen plasma assisted molecular beam epitaxy (MBE) at 10 and 20 GHz and good power scalability of these devices at 10 GHz. A single stage power amplifier built by power combining four of our 1 mm devices exhibits continuous wave output power of 22.9 W with associated power added efficiency (PAE) of 37% at 9 GHz. This is to the best of our knowledge the highest CW power and the best combination of power and PAE demonstrated to date for a GaN based microwave integrated circuit at this frequency.

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.

92–96 GHz GaN power amplifiers

We report the test results of a family of 92-96 GHz GaN power amplifiers (PA) with increasing gate peripheries (150 µm to 1200 µm). The 1200 µm, 3-stage PA produces 2.138 W output power (Pout) with an associated PAE of 19% at 93.5 GHz (VD=14V). The amplifier offers Pout over 1.5W with associated PAE over 17.8% in the 92–96 GHz bandwidth. The measured data show that the maximum Pout scales linearly with increasing gate periphery at an almost constant PAE around 20%. This demonstrates the high efficiency of on-chip power combining and enables W-band high power single chip solid state power amplifiers.

W-Band GaN MMIC with 842 mW output power at 88 GHz

We report W-band GaN MMICs that produce 96% more power at a frequency of 88 GHz in continuous wave (CW) operation than the highest power reported in this frequency band for the best competing solid state technology[1], the InP HEMT. W-band power module containing a single three stage GaN MMIC chip with 600 µm wide output stage produced over 842 mW of output power in CW-mode, with associated PAE of 14.7% and associated power gain of 9.3 dB. This performance was measured at MMIC drain bias of 14 V.

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.

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In this paper we demonstrate the merits of GaN MMIC technology for high bandwidth millimeter-wave power applications and for microwave robust LNA receiver applications. We report the development of a broadband two-stage microstrip Ka-band GaN MMIC power amplifier, with 15dB of flat small signal gain over the 27.5GHz to 34.5GHz frequency range and 4W of saturated output power at 28GHz, with a power added efficiency of 23.8%. This is to the best of our knowledge the best combination of output power, bandwidth and efficiency reported for a GaN MMIC in Ka-band frequency range. We also report a robust two-stage wideband (0.5GHz-12GHz) GaN LNA MMIC, which can survive 4W of incident input RF power in CW mode without input power protective circuitry. The presented LNA MMIC has, to the best of our knowledge, the best combination of NF, bandwidth, survivability and low power consumption reported to date in GaN technology.

of 260 GHz

Significant investments and R&D efforts over the past two decades have established GaAs and InP electronic device technologies from substrate manufacturing to MMIC amplifier design and testing. Today, GaAs and InP HBTs and HFETs, as far as gain, efficiency, and power are concerned, dominate the whole spectrum from S- to W-band and beyond. In this paper we discuss recent advances in device technologies and survey the state of the art performance of GaAs and InP HFETs and HBTs.