Xun Zheng

Engineering the (In, Al, Ga)N back-barrier to achieve high channel-conductivity for extremely scaled channel-thicknesses in N-polar GaN high-electron-mobility-transistors

Scaling down the channel-thickness (tch) in GaN/(In, Al, Ga)N high-electron-mobility-transistors (HEMTs) is essential to eliminating short-channel effects in sub 100 nm gate length HEMTs. However, this scaling can degrade both charge density (ns) and mobility (μ), thereby reducing channel-conductivity. In this study, the back-barrier design in N-polar GaN/(In, Al, Ga)N was engineered to achieve highly conductive-channels with tch < 5-nm using metal organic chemical vapor deposition. Compositional-grading was found to be the most effective approach in reducing channel-conductivity for structures with tch ∼ 3-nm. For a HEMT with 3-nm-thick-channel, a sheet-resistance of 329 Ω/◻ and a peak-transconductance of 718 mS/mm were demonstrated.

Demonstration of β-(Al x Ga1− x )2O3/β-Ga2O3 modulation doped field-effect transistors with Ge as dopant grown via plasma-assisted molecular beam epitaxy

β-(Al x Ga1− x )2O3/β-Ga2O3 heterostructures were grown via plasma-assisted molecular beam epitaxy. The β-(Al x Ga1− x )2O3 barrier was partially doped by Ge to achieve a two-dimensional electron gas (2DEG) in Ga2O3. The formation of the 2DEG was confirmed by capacitance–voltage measurements. The impact of Ga-polishing on both the surface morphology and the reduction of the unintentionally incorporated Si at the growth interface was investigated using atomic force microscopy and secondary-ion mass spectrometry. Modulation doped field-effect transistors were fabricated. A maximum current density of 20 mA/mm with a pinch-off voltage of −6 V was achieved on a sample with a 2DEG sheet charge density of 1.2 × 1013 cm−2.

N-Polar GaN MIS-HEMTs on Sapphire With High Combination of Power Gain Cutoff Frequency and Three-Terminal Breakdown Voltage

Nitrogen polar SiN_x/AlGaN/GaN/AlGaN metal- insulator-semiconductor high-electron-mobility transistors (MIS-HEMTs) with 28.6-nm equivalent GaN channel thickness grown by metal-organic chemical vapor deposition on sapphire substrate with a high combination of current/power gain cutoff frequencies (f_T/f_max) and three-terminal breakdown voltage (BV_DS) are demonstrated. f_T/BV_DS of 103 GHz/114 V and f_max/BV_DS of 248 GHz/114 V were achieved in devices with the gate widths of 2 × 50 μm and 2 × 25 μm, respectively, comparing well with recent reports of fully passivated and vertically scaled Ga-polar GaN HEMTs. Devices with a gate width of 2 × 75 μm showed the peak output power densities of 5.74 W/mm at 4 GHz and 6.29 W/mm at 10 GHz obtained by load-pull measurements.

N-Polar GaN Cap MISHEMT With Record Power Density Exceeding 6.5 W/mm at 94 GHz

A novel N-Polar GaN cap (MIS)high electron mobility transistor demonstrating record 6.7-W/mm power density with an associated power-added efficiency of 14.4% at 94 GHz is presented. This state-of-the-art power performance is enabled by utilizing the inherent polarization fields of N-Polar GaN in combination with a 47.5-nm in situ GaN cap layer to simultaneously mitigate dispersion and improve access region conductivity. These excellent results build upon past work through the use of optimized device dimensions and a transition from a sapphire to a substrate for reduced self-heating.

N-Polar Deep Recess MISHEMTs With Record 2.9 W/mm at 94 GHz

W-band power performance is reported on an N-polar GaN HEMT for the first time, resulting in a record output power density for any GaN device on a sapphire substrate. This result is achieved using an N-polar GaN deep recess MISHEMT structure grown by metal-organic chemical vapor deposition on the sapphire substrates. The key component in this device design is the addition of an in situ unintentionally doped GaN epitaxial passivation layer in the access regions of the transistor. This GaN layer functions both to control DC-to-RF dispersion as well as to increase the conductivity in the access regions of the HEMT. Devices with very low dispersion and a simultaneous fmax/ft combination of 276/149 GHz are demonstrated. Load pull measurements at 94 GHz give a peak power added efficiency (PAE) of 20% with an associated output power density of 1.73 W/mm at VDS = 8 V. A record 2.9-W/mm maximum output power density with an associated 15.5% PAE at VDS = 10 V is achieved despite the low thermal conductivity of the samples sapphire substrate.

Small-signal model extraction of mm-wave N-polar GaN MISHEMT exhibiting record performance: Analysis of gain and validation by 94 GHz loadpull

In this paper we extract a small-signal model of a mm-wave deep-recess N-polar GaN MISHEMT exhibiting record 94 GHz power density. We show that certain existing methods for extrinsic parasitic extraction cannot be easily employed because of the device design but that an existing cold-bias method provides accurate extraction. The small-signal model with pad layout parasitics is then validated with the gain measured at low input powers by a 94 GHz loadpull system. The factors impacting the measured gain are analyzed to show their origins and relative impact, giving guidance and predictions for future improvement.

W-band passive load pull system for on-wafer characterization of high power density N-polar GaN devices based on output match and drive power requirements vs. gate width

A W-band on-wafer passive load pull system constructed for the characterization of high power density N-polar GaN devices is presented. N-Polar GaNs large RF voltage swing enables high power densities but also increases the power match impedance which must be synthesized with the limited on-wafer tuning range. Increasing test cell gate width to decrease impedance increases the systems drive power requirement. The tradeoff between these is analyzed, showing that a passive load pull system can characterize a wide range of devices. This is demonstrated with measured data from an N-polar GaN device exhibiting 4.1 W/mm power density at 94 GHz.

mm-Wave N-polar GaN MISHEMT with a self-aligned recessed gate exhibiting record 4.2 W/mm at 94 GHz on Sapphire

GaN based high electron mobility transistors have emerged as a leading technology for mm-wave solid state power amplification at W-band. To date, reports on W-band GaN HEMTs and MMICs have primarily featured devices grown in the Ga-polar orientation [1, 2]. In this work, the advantages of the N-polar orientation are exploited to produce a MISHEMT exhibiting record high 4.2 W/mm peak output power (Pout) at 94 GHz. The key enabling advantage of N-polar GaN devices are their inverted polarization fields. These fields create a natural, charge-inducing back-barrier that decouples the tradeoff between aspect ratio and channel electron density. Further, the reversed field in an AlGaN cap above the GaN channel opposes gate leakage and improves breakdown voltage [3]. Additionally, to mitigate surface-state induced dispersion and enhance the conductivity of the access regions, a GaN cap layer is added in the access regions through which the gate is recessed [4]. The fabrication process reported in this paper extends that of [4, 5] to have the foot gate metal deposited in a self-aligned fashion to the GaN cap recess etch.

W-band N-polar GaN MISHEMTs with high power and record 27.8% efficiency at 94 GHz

We report on the W-band power performance of N-polar GaN MISHEMTs demonstrating a record power-added efficiency (PAE) of 27.8% while maintaining an excellent output power density of 3 W/mm and 7.4 dB peak gain at 94 GHz. To enable this performance, a novel device technology is presented that utilizes the advantages of the inverted polarization of N-polar GaN to mitigate dispersion and improve access region conductivity through the addition of a 47.5 nm in-situ GaN cap layer. To obtain these results past work has been extended through pad layout optimization and reduction of lateral dimensions.

N-polar GaN Cap MISHEMT with record 6.7 W/mm at 94 GHz

In this work a record output power density for any III-N based transistor at W-band is demonstrated. The output power of this N-polar technology exceeds that of any reported Ga-polar device by a significant margin (Fig. 5) and scales exceptionally well with drain bias. Moreover, a large design space remains for further optimization of this N-polar MISHEMT technology. Significant performance enhancements in power density, efficiency, and gain are expected with further vertical and lateral scaling of the device dimensions. Additional improvement will be realized with improved layout of the pad design.