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Publications (13)16.32 Total impact

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    ABSTRACT: A multilayer, low-parasitic interconnection scheme for highly scaled GaN high electron mobility transistors is reported. The fabrication process offers three Au interconnects embedded in benzocyclobutene (BCB) dielectric, with an integrated air-box in the active area in order to minimise the gate parasitic capacitances. With the addition of the air-box, it is demonstrated that the performance of the BCB encapsulated device is similar to that of a non-encapsulated device. Furthermore, by utilising the multilayer interconnection scheme a low-loss (0.75 dB) 3 dB tandem coupler operating from 140 to 220 GHz is demonstrated.
    Electronics Letters 01/2014; 50(4):302-303. · 1.04 Impact Factor
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    ABSTRACT: Direct-coupled field-effect transistor (FET) logic inverters and 501-stage ring oscillators (ROs) are fabricated using highly scaled GaN heterojunction FET with gate lengths of 20 and 40 nm. A 40-nm gate-length E/D inverter has logic-low and logic-high noise margins of 0.465 and 1.59 V, respectively, and a logic voltage swing of 2.38 V measured at Vdd = 2.5 V. The corresponding 40-nm 501-stage RO frequency and stage delay are 0.067 GHz and 15 ps, whereas the frequency and stage delay of a 20-nm RO are 0.133 GHz and 7.5 ps. The yield of the 20-nm 501-stage RO circuits is 52% across a 3-in diameter wafer. With 1006 transistors, the 501-stage ROs represent the highest level of transistor integration to date for a GaN circuit, whereas the stage delay is the shortest reported for a GaN digital circuit.
    IEEE Electron Device Letters 01/2013; 34(7):846-848. · 2.79 Impact Factor
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    ABSTRACT: 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.
    IEEE Transactions on Electron Devices 01/2013; 60(10):2982-2996. · 2.06 Impact Factor
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    ABSTRACT: In this paper, we report state-of-the-art high-frequency performance of GaN-based HEMTs achieved through innovative device scaling technologies such as vertically-scaled AlN/GaN/AlGaN double-heterojunction (DH) HEMT epitaxial structure, low-resistance n+-GaN ohmic contacts regrown by MBE, and manufacturable 20-nm self-aligned sidewall gate process. Engineering top barrier layer structure enabled both enhancement- and depletion-mode (E/D) device operations with record cutoff frequencies while maintaining Johnson figure of merit (JFoM) breakdown performance. Furthermore, E/D-mode devices were monolithically integrated using a full epitaxial regrowth technique with a successful demonstration of DCFL ring oscillator circuits. Deeply-scaled E/D-mode GaN-HEMTs with an unprecedented combination of high-frequency and high-breakdown characteristics offer practical advantages in circuit applications such as sub-millimeter-wave power amplifiers, ultra-linear mixers, and increased output power digital-to-analog converters.
    Compound Semiconductor Integrated Circuit Symposium (CSICS), 2012 IEEE; 01/2012
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    ABSTRACT: 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 (Idmax) and a transconductance (gm). 20-nm-gate D-mode HEMTs with a 40-nm gate-source (and gate-drain) distance exhibited a record-low Ron of 0.23 Ω·mm, a record-high Idmax of >4 A/mm, and a broad gm curve of >1 S/mm over a wide range of Ids from 0.5 to 3.5 A/mm. Furthermore, 20-nm-gate E-mode HEMTs with an increased Lsw of 70 nm demonstrated a simultaneous fT/fmax of 342/518 GHz with an off-state breakdown voltage of 14V.
    Electron Devices Meeting (IEDM), 2012 IEEE International; 01/2012
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    ABSTRACT: Highly scaled AlN/GaN metal-oxide-semiconductor heterojunction field-effect transistors (MOS-HFETs) with Al<sub>2</sub>O<sub>3</sub> 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 Al<sub>2</sub>O<sub>3</sub> 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, f <sub>T</sub> of 134 GHz, and f <sub>max</sub> of 261 GHz. In contrast, the dc and RF performances of a MOS-HFET with a 4-nm-thick Al<sub>2</sub>O<sub>3</sub> dielectric were degraded by short-channel effects. The 2-nm-thick Al<sub>2</sub>O<sub>3</sub> gate insulator reduced the forward-bias gate current by more than two orders of magnitude. The data suggest the promise of ultrathin ALD Al<sub>2</sub>O<sub>3</sub> gate dielectrics for next-generation high-speed GaN HFETs.
    IEEE Electron Device Letters 09/2011; · 2.79 Impact Factor
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    ABSTRACT: 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 V<sub>th</sub> of -0.10 V, a peak transconductance g<sub>m</sub> value of 640 mS/mm, and current gain and power-gain cutoff frequencies f<sub>T</sub> and f<sub>max</sub> of 82 and 210 GHz, respectively. E-mode devices on the same wafer with the same dimensions had a V<sub>th</sub> value of +0.24 V, a peak g<sub>m</sub> value of 525 mS/mm, and f<sub>T</sub> and f<sub>max</sub> 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.
    IEEE Transactions on Electron Devices 05/2011; · 2.06 Impact Factor
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    ABSTRACT: 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 (Lg) and the source-drain distance (Lsd) 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; Ron = 0.29 (0.33) Ω·mm, Idmax = 2.7 (2.6) A/mm, a peak extrinsic gm = 1.04 (1.63) S/mm, threshold voltage uniformity σ (Vth) = 44 (63) mV over a 3-inch wafer area, and a simultaneous fT/fmax = 310/364 (343/236) GHz. Delay time analysis clarified that an unique dependence of fT on Vds resulted from suppressed drain delay and enhanced electron velocity due to the lateral source-drain (S-D) scaling.
    Electron Devices Meeting, 1988. IEDM '88. Technical Digest., International 01/2011;
  • IEEE Electron Device Letters 01/2011; 32(8):1074-1076. · 2.79 Impact Factor
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    ABSTRACT: The high frequency performance of GaN-based HEMTs has been significantly improved through innovative device scaling technologies such as AlGaN or InGaN back barriers, thin AlN top barriers, lattice-matched InAlN barriers, ultra-short gates, and self-aligned gates. In this paper, we review our scaling technologies for ultra-high-speed operation of GaN-HEMTs, which provide not only high yield and uniformity but also a large-scale integration of E/D-mode HEMTs for future RF and mixed-signal applications.
    01/2011;
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    ABSTRACT: 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 (g<sub>m</sub>) of 0.70 S/mm, a current-gain cutoff frequency (f<sub>T</sub>) of 112 GHz, and a maximum oscillation frequency (f<sub>max</sub>) of 215 GHz. To our knowledge, these are the highest g<sub>m</sub> , f<sub>T</sub>, and f<sub>max</sub> values reported to date for an E-mode GaN HFET.
    IEEE Electron Device Letters 11/2010; · 2.79 Impact Factor
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    ABSTRACT: In this paper, we report on vertically scaled GaN/AIN DH-HEMTs with regrown n+GaN ohmic contacts by MBE. Our conventional AlGaN barrier was replaced with an AlN barrier, greatly reducing the barrier thickness while maintaining a high carrier density. A selective-area MBE regrowth of an n+GaN ohmic contact layer significantly reduced access resistance. The 60 nm device exhibited a low R<sub>on</sub> of 0.81Ω·mm with a record transconductance (gm) of 850 mS/mm, an ft , of 156 GHz, and an fmax of 282 GHz.
    Device Research Conference (DRC), 2010; 07/2010
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    ABSTRACT: 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 Ron of 0.81Ω·mm, an Idmax of 1.61A/mm, a BVoff of 42V, and a peak extrinsic gm of 723mS/mm, resulting in a peak fT of 220GHz and a peak fmax of 400GHz. The measured fT and fmax 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 (Lg) down to 40nm, demonstrating high scalability of the new technology.
    Electron Devices Meeting, 1988. IEDM '88. Technical Digest., International 01/2010;