Brian Romanczyk

Benchmarking current density in staggered gap In0.53Ga0.47As/GaAs0.5Sb0.5 heterojunction Esaki tunnel diodes

The impact of dopant concentration on the current densities of In0.53Ga0.47As/GaAs0.5Sb0.5 heterojunction Esaki tunnel diodes is investigated. Increased doping density results in increased peak and Zener current densities. Two different structures were fabricated demonstrating peak current densities of 92 kA/cm2 and 572 kA/cm2, Zener current densities of 994 kA/cm2 and 5.1 MA/cm2 at a −0.5 V bias, and peak-to-valley current ratios of 6.0 and 5.4, respectively. The peak current scaled linearly with area down to a 70 nm diameter. The peak current densities were benchmarked against Esaki diodes from other material systems based on doping density and tunnel barrier height.

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.

High electron mobility recovery in AlGaN/GaN 2DEG channels regrown on etched surfaces

This paper reports high two-dimensional electron gas mobility attained from the regrowth of the AlGaN gating layer on ex situ GaN surfaces. To repair etch-damaged GaN surfaces, various pretreatments were conducted via metalorganic chemical vapor deposition, followed by a regrown AlGaN/GaN mobility test structure to evaluate the extent of recovery. The developed treatment process that was shown to significantly improve the electron mobility consisted of a N2 + NH3 pre-anneal plus an insertion of a 4 nm or thicker GaN interlayer prior to deposition of the AlGaN gating layer. Using the optimized process, a high electron mobility transistor (HEMT) device was fabricated which exhibited a high mobility of 1450 cm2 V−1 s−1 (R sh = 574 ohm/sq) and low dispersion characteristics. The additional inclusion of an in situ Al2O3 dielectric into the regrowth process for MOS-HEMTs still preserved the transport properties near etch-impacted areas.

Mapping Defect Density in MBE Grown

Growing good quality III-V epitaxial layers on Si substrate is of utmost importance to produce next generation high-performance devices in a cost effective way. In this paper, using physical analysis and electrical measurements of Esaki diodes, fabricated using molecular beam epitaxy grown In0.53Ga0.47As layers on Si substrate, we show that the valley current density is strongly correlated with the underlying epi defect density. Such a strong correlation indicates that the valley characteristics can be used to monitor the epi quality. A model is proposed to explain the experimental observations and is validated using multiple temperature diode I-V data. An excess defect density is introduced within the device using electrical and mechanical stress, both of which are found to have a direct impact on the valley current with a negligible change in the peak current characteristics, qualitatively supporting the model predictions.

{\rm In}_{0.53}{\rm Ga}_{0.47}{\rm As}

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.

Epitaxial Layers on Si Substrate Using Esaki Diode Valley Characteristics

In this study, the growth of high quality N-polar InGaN films by metalorganic chemical vapor deposition is presented with a focus on growth process optimization for high indium compositions and the structural and tunneling properties of such films. Uniform InGaN/GaN multiple quantum well stacks with indium compositions up to 0.46 were grown with local compositional analysis performed by energy-dispersive X-ray spectroscopy within a scanning transmission electron microscope. Bright room-temperature photoluminescence up to 600 nm was observed for films with indium compositions up to 0.35. To study the tunneling behavior of the InGaN layers, N-polar GaN/In0.35Ga0.65N/GaN tunnel diodes were fabricated which reached a maximum current density of 1.7 kA/cm2 at 5 V reverse bias. Temperature-dependent measurements are presented and confirm tunneling behavior under reverse bias.