Semiconductor device developments are typically guided by simulations – what is the best epitaxial design, or how should field plates or edge terminations be designed. To assess how a particular device reflects what has been designed, comparing IV curves measured and simulated is typical, but for example different implantation or doping levels can lead to the same too low breakdown voltage, and there is then little guidance for example how to mitigate such a problem to optimize/enhance breakdown voltage. We illustrate the latest capability of a technique we developed, based on electricfield- induced optical second-harmonic generation (EFISHG), on vertical GaN-on-GaN pn junction examples, to directly access with submicron spatial resolution electric strength in a device, to aid improving device design and their implementation, and reducing product development cycles.

We demonstrate an ion-implanted triple-zone junction termination extension (JTE) for vertical GaN p-n diodes. Due to the spatial distribution of fixed charge in the triple-zone JTE structure, the peak electric fields at the contact metal edge and at the edge of the JTE are significantly reduced compared to conventional approaches. The forward and reverse characteristics of diodes with conventional single-zone JTE and the triple-zone JTE explored here have been studied and compared experimentally. GaN p-n diodes fabricated using the triple-zone JTE obtain an experimentally measured maximum breakdown voltage of 1.27 kV, appreciably higher than the 1.01 kV achieved using the single-zone JTE structure. The triple-zone JTE design also provides a wider window for fabrication processing and epitaxial wafer growth to achieve the high breakdown voltage compared to single-zone designs. The triple-zone JTE is promising for cost-effective fabrication of GaN power electronics.

Edge termination has emerged as an important area in the design and realization of vertical GaN power electronic devices. While the material properties of GaN are promising for high-performance devices, in practice the breakdown voltage can be compromised by inadequate edge termination (ET). While solutions in other materials (e.g. Si, SiC) are well-known, these are challenging to implement in GaN due to inherent difficulties in p-type doping GaN. In this work, we report a etch-free triple-zone graded junction termination extension (JTE) for vertical GaN diodes formed by Nitrogen ion implantation. The triple-zone design offers lower peak fields for effective field control. In addition, the proposed triple-zone JTE is beneficial for increasing the fabrication process window and allowing for more variability in epitaxial wafer growth in terms of p-GaN doping and thickness while maintaining high breakdown voltage. The fabricated GaN p-n diodes with triple-zone JTE obtain a maximum breakdown voltage of 1.73 kV with specific on-resistance Ron of 0.4 mΩcm2. Temperature-dependent reverse characteristics show that the devices have a positive temperature coefficient of the breakdown voltage indicating an avalanche breakdown mechanism. These results suggest that vertical GaN p–n diodes with N-implanted triple-zone JTE are promising for power applications.

GaN-based high-electron mobility transistors are widely recognized for their exceptional performance at RF and microwave frequencies, and are increasingly being explored for millimeter-wave amplifier applications. An additional application that is critical for future systems is signal switching and routing at millimeter-wave frequencies; this is essential for enabling millimeter-wave wireless communication systems (e.g. 6G and beyond) that require frequency agility and reconfigurability. For this type of RF and mm-wave switch applications, the high carrier concentration and high 2DEG mobility of III-N HEMTs leads to low on resistance and low insertion loss. However, the isolation is limited by off-state capacitance, and nonlinearity of the HEMT limits the power handling capabilities. We have observed that the inclusion of a ferroelectric gate dielectric (using ALD-deposited Hf0.5Zr0.5O2) in the device can significantly enhance performance. By combining polarization engineering of the III-N HEMT with the hysteretic and dispersive polarization characteristics of the ferroelectric gate stack, substantial improvements in the switch figure of merit (FOM=1/2π(RonCoff)) can be achieved. The reduced effective off-state capacitance enabled by ferroelectrics integrated with GaN-based transistors has led to switches with FOM of 2.5 THz. Combining this with advanced processing (e.g. regrowth of source and drain ohmic contacts, gate length scaling), further improvements in performance are expected.

GaN based high-electron-mobility transistors (HEMTs) have been widely investigated for RF applications because of their high electron mobility and high two-dimensional electron gas (2DEG) density. The combination of GaN based HEMTs and ferroelectric gate stacks can potentially improve device characteristics relevant to RF switch applications by utilizing the abrupt ferroelectric polarization switching. AlGaN/GaN HEMTs with Hf 0.5 Zr 0.5 O 2 ferroelectric gate stacks have been explored under DC and RF switching conditions. By combining these features, mm-wave switches with above 1 THz switch figure of merit have been achieved. This demonstrates that ferroelectric-enhanced GaN-based HEMTs are promising candidates for future RF/millimeter-wave switching applications.

We characterized the electric field distribution of GaN-on-GaN p–n diodes with partially compensated ion-implanted edge termination (ET) using an electric field induced second harmonic generation technique (EFISHG). The distributed electric field from the anode to the outer edge of the ET demonstrates the effectiveness of the ET structure. However, EFISHG also shows that its effectiveness is strongly dependent on the acceptor charge distribution in the ET's partially compensated layer (PC). A generally lower amount of acceptor charge can be inferred from the measured electric field distribution resulting from excessive ion implantation energy or dose during ET fabrication and causing lower than optimal breakdown voltage. Localized field crowding can be observed when the remaining acceptors uncompensated by the implant in the PC layer are nonuniformly distributed around the periphery of the devices. Important information can be obtained from these direct electric field measurements and used for optimizing the device design and fabrication process.

The temperature dependence of the electron and hole impact ionization coefficients in GaN has been investigated experimentally.Two types of p-i-n diodes grown on bulk GaN substrates have been fabricated and characterized, and the impact ionization coefficients for both electrons and holes have been extracted using the photomultiplication method.Both the electron and hole impact ionization coefficients decrease as the temperature increases.The Okuto-Crowell model was used to describe the temperature dependence of the electron and hole impact ionization coefficients.Based on the measured impact ionization coefficients, the temperature dependence of the breakdown voltage of GaN non-punch through p-n diodes can be predicted; good agreement with experimentally reported results is obtained.

GaN p–n junction diodes grown on native GaN substrates have been fabricated and characterized. The devices exhibit a positive temperature coefficient of breakdown obtained from variable temperature current–voltage measurements, confirming the impact ionization avalanche. The low‐frequency noise characteristics of these devices have been measured under forward and reverse bias conditions. The forward bias noise spectra are dominated by the 1/f noise, and the current spectral density is proportional to I1.6. Under reverse bias, the noise spectra show 1/f noise at reverse biases below the avalanche threshold. However, at reverse biases in the avalanche regime, the multiplication noise overwhelms the 1/f noise, resulting in a white noise spectrum. To further characterize the avalanche process, the excess noise factor, F, is obtained from the measured noise spectra of diodes biased in reverse avalanche. For the case of pure hole injection (achieved by incorporating a thin pseudomorphic In0.07Ga0.93N layer at the cathode of the device and illuminating with 390 nm UV light), a low excess noise factor is achieved. The impact ionization ratio α/β extracted from the multiplication noise ranges from 0.07 to 0.38 over the electric field ranging from 2.8 to 3.7 MV cm−1, consistent with the impact ionization coefficients reported previously using the photomultiplication method.

The unique material and transport properties of GaN and related III-N materials have led to their importance in both optoelectronic and RF/microwave power amplification applications, as well as to their promise for emerging applications including power control and conversion, harsh-environment and radiation-resistant sensing and signal processing, and biomedical applications. The use of epitaxial lift-off with III-N materials provides an approach that can improve the electrical and thermal performance of electronic and optoelectronic devices, while at the same time reducing cost and dramatically reducing device size and weight. However, in contrast to other III-V material systems, epitaxial lift-off of III-N materials is challenging due to the lack of high-selectivity, high-etch-rate wet etches in this material system. As an alternative, we have explored the use of band gap selective photoelectrochemical (PEC) wet etching to perform epitaxial lift-off of GaN-based materials and devices. By combining the use of a thin pseudomorphic InGaN release layer and KOH electrolyte with high-intensity filtered ultraviolet illumination tuned to generate electron-hole pairs only in the InGaN release layer (and not in the surrounding GaN material), sufficient selectivity and etch rate have been achieved to allow epitaxial lift-off of large-area films [1]. This process has been demonstrated to improve the electrical and thermal performance of Schottky diodes on non-native substrates [2, 3] and high-voltage, high-power pn junctions grown on low-dislocation-density bulk GaN substrates [4-6] without compromising material quality, while also resulting in ultra-thin devices with total thicknesses below 15 µm. The devices show no evidence of additional defects or recombination centers as a result of either the inclusion of the pseudomorphic InGaN release layer in the epitaxial layer structure or the epitaxial lift-off fabrication processing steps. In addition to device performance, device cost can be improved both through potential re-use of the substrate after lift-off [7] and reduction in die size due to improved thermal performance [8]. While demonstrated for power electronics applications, the ability to form high-quality, high-performance III-N based devices in an ultra-thin, flexible form factor may be an enabling technology for additional applications in flexible electronics, displays, wearable electronics, RF/microwave communication systems, and other fields. References: [1] C. Youtsey, R. McCarthy, R. Reddy, K. Forghani, A. Xie, E. Beam, J. Wang, P. Fay, T. Ciarkoski, E. Carlson, and L. Guido, “Wafer-scale epitaxial lift-off of GaN using bandgap-selective photoenhanced wet etching,” Phys. Stat. Solidi B, vol. 254, no. 8, pp. 1600774-1-6, DOI 10.1002/pssb.201600774, 2017. [2] J. Wang, C. Youtsey, R. McCarthy, R. Reddy, L. Guido, A. Xie, E. Beam, and P. Fay, “Demonstration of Thin-Film GaN Schottky Diodes Fabricated with Epitaxial Lift-Off,” Proc. Device Research Conf., p. 181-182, Newark, DE, 2016. [3] J. Wang, C. Youtsey, R. McCarthy, R. Reddy, N. Allen, L. Guido, J. Xie, E. Beam, and P. Fay, “Thin-film GaN Schottky diodes formed by epitaxial lift-off,” Appl. Phys. Lett., vol. 110, pp. 173403-1 -5, DOI: 10.1063/1.4982250, 2017. [4] J. Wang, R. McCarthy, C. Youtsey, R. Reddy, J. Xie, E. Beam, L. Guido, L. Cao, and P. Fay, “High Voltage Vertical GaN p-n Diodes by Epitaxial Lift-Off from Bulk GaN Substrates,” IEEE Electron Dev. Lett., vol. 39, no. 11, pp. 1716-1719, DOI: 10.1109/LED.2018.2868560, 2018. [5] J. Wang, R. McCarthy, C. Youtsey, R. Reddy, J. Xie, E. Beam, L. Guido, L. Cao, and P. Fay, “Ion-implant Isolated Vertical GaN p-n Diodes Fabricated with Epitaxial Lift-Off from GaN Substrates,” Physica Status Solidi A, DOI: 10.1002/pssa.201800652, 2018. [6] J. Wang, C. Youtsey, R. McCarthy, R. Reddy, N. Allen, L. Guido, A. Xie, E. Beam, and P. Fay, “Thin-Film GaN p-n Diodes and Epitaxial Lift-Off from GaN Substrates,” Int’l Symp. On Compound Semiconductors, paper B8.5, Berlin, May 2017. [7] H. Amano et al., “The 2018 GaN power electronics roadmap,” J. Physics D: Appl. Phys, vol. 51, no. 16, 163001 pp. 1-48, DOI 10.1088/1361-6463/aaaf9d, 2018. [8] P. Fay, J. Wang, L. Cao, R. McCarthy, R. Reddy, C. Youtsey, and J. Xie, “Epitaxial Lift-Off for Vertical GaN Power Devices,” Government Microcircuit Applications and Critical Technologies Conf. (Gomactech), Albuquerque, NM, March 2019.

The DC and large signal characteristics of a GaN IMPATT diode designed for operation at W-band are investigated using Sentaurus TCAD simulations. The impact ionization model for GaN is calibrated using measured impact ionization coefficients of both electrons and holes, and both field- and carrier-concentration dependent velocity-field relationships are considered for accurate simulation. Large signal simulation results show that the IMPATT diode design evaluated is capable of generating an output RF power larger than 1 MW/cm 2 from 80 GHz to 120 GHz with an efficiency larger than 18%.

GaN pn junction diodes grown on native GaN substrates have been fabricated and characterized. The temperature-dependent current-voltage characteristics exhibit a positive temperature coefficient of breakdown as expected from impact ionization avalanche. However, for IMPATT and avalanche photodiode applications, device noise is very important. The low frequency noise characteristics of these devices have been measured under forward and reverse bias conditions. The forward noise spectra is dominated by 1/f noise, and the current spectral density is proportional to I 1.6 . Under reverse bias, the noise spectra show 1/f noise at reverse biases below the avalanche threshold. However, at reverse biases in the avalanche regime, the multiplication noise overwhelms the 1/f noise, resulting in a white noise spectrum. The measured results are consistent with expectations from avalanche noise theory.

Ion‐implant isolated vertical GaN p‐n junction diodes fabricated with epitaxial lift‐off (ELO) from GaN substrates are demonstrated. For the ELO process, a band‐gap selective photoelectrochemical (PEC) wet etch with a pseudomorphic InGaN release layer is utilized. Compared with devices isolated using mesa etching, the ion‐implant isolated devices exhibit more ideal forward current–voltage characteristics and lower leakage currents. Devices are also compared with and without ELO processing. Devices measured after ELO processing and mounting to metallized carrier substrates show similar electrical performance to GaN‐on‐GaN control samples without ELO processing. No indication of material quality degradation is found on the ELO devices. The ELO devices exhibit turn‐on voltages of 3.15 V (at a current density of 100 A cm −2 ), with specific on resistance (R on ) of 0.52 mΩ cm 2 at 4.8 V and breakdown voltage (V br ) approximately of 750 V.

Carbon, a compensator in GaN, is an inherent part of the organometallic vapor phase epitaxy (OMVPE) environment due to the use of organometallic sources. In this study, the impact of growth conditions are explored on the incorporation of carbon in GaN prepared via OMVPE on pseudo-bulk GaN wafers (in several cases, identical growths were performed on GaN-on-Al2O3 templates for comparison purposes). Growth conditions with different growth efficiencies but identical ammonia molar flows, when normalized for growth rate, resulted in identical carbon incorporation. It is concluded that only trimethylgallium which contributes to growth of the GaN layer contributes to carbon incorporation. Carbon incorporation was found to decrease proportionally with increasing ammonia molar flow, when normalized for growth rate. Ammonia molar flow divided by growth rate is proposed as a reactor independent predictor of carbon incorporation as opposed to the often-reported input V/III ratio. A low carbon concentration of 7.3 × 1014 atoms/cm3 (prepared at a growth rate of 0.57 µm/h) was obtained by optimizing growth conditions for GaN grown on pseudo-bulk GaN substrates.

The gallium nitride (GaN) material system has become one of the most promising material systems for high efficiency power switching applications due to its unique material properties, including a wide band gap, high critical electric field, and large mobility and saturation electron velocity. For high voltage, high current applications, vertical device structures on bulk GaN substrates are especially promising [1]. Although the ability to scale the devices to large absolute currents is critical for power electronics, to date the current-carrying capability of many demonstrated devices has fallen short of theoretical expectations due to non-idealities in device area scaling. High performance vertical GaN-on-GaN p-n diodes using N ion-implantation edge termination incorporating a partially-compensated layer were demonstrated to achieve breakdown voltages of 1.2 kV, and both the reverse and forward current densities were found to scale well with area [2]. In this work, a device structure designed for higher breakdown operation was explored, and devices with a range of areas were fabricated and tested under high-current drive conditions. Peak forward currents approaching 10 A at a forward voltage of 5.3 V were obtained for typical 550 μm diameter GaN-on-GaN p-n diodes having a breakdown voltage of 1.6 kV.

Gallium nitride (GaN) is a compound semiconductor that has tremendous potential to facilitate economic growth in a semiconductor industry that is silicon-based and currently faced with diminishing returns of performance versus cost of investment. At a material level, its high electric field strength and electron mobility have already shown tremendous potential for high frequency communications and photonic applications. Advances in growth on commercially viable large area substrates are now at the point where power conversion applications of GaN are at the cusp of commercialisation. The future for building on the work described here in ways driven by specific challenges emerging from entirely new markets and applications is very exciting. This collection of GaN technology developments is therefore not itself a road map but a valuable collection of global state-of-the-art GaN research that will inform the next phase of the technology as market driven requirements evolve. First generation production devices are igniting large new markets and applications that can only be achieved using the advantages of higher speed, low specific resistivity and low saturation switching transistors. Major investments are being made by industrial companies in a wide variety of markets exploring the use of the technology in new circuit topologies, packaging solutions and system architectures that are required to achieve and optimise the system advantages offered by GaN transistors. It is this momentum that will drive priorities for the next stages of device research gathered here.

An approach to realizing high-voltage, high-current vertical GaN-on-GaN power diodes is reported. We show that by combining a partially compensated ion-implanted edge termination (ET) with sputtered SiNx passivation and optimized ohmic contacts, devices approaching the fundamental material limits of GaN can be achieved. Devices with breakdown voltages (Vbr) of 1.68 kV and differential specific on resistances (Ron) of 0.15 mΩ cm2, corresponding to a Baliga figure of merit of 18.8 GW/cm2, are demonstrated experimentally. The ion-implantation-based ET has been analyzed through numerical simulation and validated by experiment. The use of a partially compensated ET layer, with approximately 40 nm of the p-type anode layer remaining uncompensated by the implant, is found to be optimal for maximizing Vbr. The implant-based ET enhances the breakdown voltage without compromising the forward characteristics. Devices exhibit near-ideal scaling with area, enabling currents as high as 12 A for a 1 mm diameter device.

High-performance vertical GaN based p-n junction diodes fabricated using bandgap selective photoelectrochemical etching-based epitaxial liftoff (ELO) from bulk GaN substrates are demonstrated. The epitaxial GaN layers and pseudomorphic InGaN release layer were grown by MOCVD on bulk GaN substrates. A comparison study was performed between devices after liftoff processing (after transfer to a Cu substrate) and nominally identical control devices on GaN substrates without the buried release layer or ELO-related processing. ELO and bonded devices exhibit nearly identical electrical performance and improved thermal performance, compared with the control devices on full-thickness GaN substrates. The breakdown voltage, ideality factor, and forward turn-ON performance were found to be nearly identical, indicating that the transfer process does not degrade the quality of the p-n junctions. The devices exhibit turn-ON voltages of 3.1 V at a current density of 100 A/cm(2), with a specific ON-resistance (R-ON) of 0.2-0.5 m Omega. cm(2) at 5 V and a breakdown voltage (V-br) of 1.3 kV. Both optical and electrical characterization techniques show that the thermal resistance of ELO devices bonded to a Cu carrier is approximately 30% lower than that for control devices on GaN substrates.

We demonstrate an electrically-injected device that emits in the Reststrahlen band of GaAs. The device comprises a superlattice designed to generate longitudinal optical (LO) phonons and a grating with a mode at the energy of the phonons. Emission, peaking at the LO phonon, is observed.

Epitaxial p-i-n structures grown on native GaN substrates have been fabricated and used to extract the impact ionization coefficients in GaN. The photomultiplication method has been used to experimentally determine the impact ionization coefficients; avalanche dominated breakdown is confirmed by variable-temperature breakdown measurements. To facilitate photomultiplication measurements of both electrons and holes, the structures include a thin pseudomorphic In0.07Ga0.93N layer on the cathode side of the drift layer. Illumination with 193 nm and 390 nm UV light has been performed on diodes with different intrinsic layer thicknesses. From the measured multiplication characteristics, the impact ionization coefficients of electrons (α) and holes (β) were determined for GaN over the electric field range from 2 MV/cm to 3.7 MV/cm. The results show that for transport along the c-axis, holes dominate the impact ionization process at lower electric field strengths; the impact ionization coefficient of electrons becomes comparable to that of holes (β/α<5) for electric field strengths above 3.3 MV/cm.