Wide-bandgap materials are poised to revolutionize power electronics, offering higher efficiency, reliability, and output power in a broad range of energy conversion and RF power amplifier (PA) devices. Gallium nitride (GaN) is emerging as the wide bandgap material of choice for both industrial and defense applications but thermal impediments present a significant bottleneck to realization of the full potential enabled by the GaN material properties. Traditional “remote cooling” solutions, which rely on thermal conduction and spreading through low thermal conductivity substrates and across multiple interfaces, are incapable of limiting device junction temperature rise. Recent “embedded cooling” efforts, largely funded by Defense Advanced Research Projects Agency Microsystems Technology Office (DARPA-MTO), have focused on reduction of the near-junction thermal resistance through the use of diamond substrates and efficient removal of the dissipated power with convective and evaporative microfluidics.
This paper will first motivate the need for advanced thermal management in GaN PAs. Attention will then turn to the accomplishments of the DARPA Near-Junction Thermal Transport (NJTT) program, including the thermal and electrical characteristics of GaN epitaxial layers bonded to bulk diamond, GaN with directly grown polycrystalline diamond as a replacement for the native substrate, and GaN on a native substrate with diamond-filled vias. The paper will then turn to on-going research in the DARPA ICECool program, which further enhances the performance of GaN PAs through intra-chip microfluidic cooling and the thermal and electrical co-design necessary to design high power GaN PAs.

The III-Nitride material system has the potential to create a new generation of highly efficient compact power electronics. In this work we will describe our work towards understanding the device physics of metal-insulator-semiconductor III-Nitride transistors with enhancement mode operation. Fundamental properties of III-Nitride materials, such as polarization and doping, make it challenging to achieve enhancement mode devices with good performance. We will first present our work on AlGaN/GaN metal-insulator high electron mobility field effect transistors, focusing on our theoretical and experimental work to understand the effect of interface states on electrostatics[1], transport[2], and threshold voltage stability in such devices. We will discuss our demonstration of interface charge density engineering [3] leading to the demonstration of normally off transistors [4] with high current density. We will then show that high threshold voltage and efficient device performance can be simultaneously achieved using ultra wide band gap AlGaN as the channel material [5]. A calculation of theoretical figures of merit for such devices taking into account the effects of electron scattering in the channel and breakdown field [5] will be presented to compare resistance and switching losses in ultra wide band gap AlGaN devices with those in GaN. Finally, we will discuss our work on experimentally realizing device structures based on ultra wide band gap materials, and our approach to improving electron transport and contacts in such devices.
[1] Esposto M, et al, Appl. Phys. Lett. , 99, 133503 (2011).
[2] Hung, TH, et al, Appl. Phys. Lett. 102, 072105 (2013)
[3] Hung TH, et al, Appl. Phys. Lett. , 162104 (2011).
[4] Hung, TH, et al, IEEE Elec. Dev. Lett., 35 (3), 99. 312-314 (2014)
[5] Bajaj, S, et al, Appl. Phys. Lett. 105.26, 263503, (2014)

Application of AlGaN/GaN high electron mobility transistor (HEMT) to power controlling devices is highly expected. AlGaN/GaN HEMT has excellent features for managing large electrical power: high breakdown voltage, high electron mobility, low on-state resistance, low switching losses and high thermal conductivity. It is now generally recognized that two dimensional electron gas (2DEG) is one of the key factor which determine the electrical property of AlGaN/GaN HEMT. The polarization in AlGaN/GaN heterostructure plays important roles in forming 2DEG. Thus, 2DEG and polarization are crucial factors of evaluation.
Techniques for device evaluation are important for effective development of high performance device at low cost. Scanning nonlinear dielectric microscopy (SNDM)[1] is one of candidates of evaluation techniques for analyzing carrier distribution and polarization. SNDM measures the variation of capacitance between sample and conductive tip responding to the applied ac voltage and can detect carrier polarity, density, and polarity of polarization. SNDM has such high capacitance variation sensitivity of 10^minus;22 F that the detailed profile of carrier and polarization distribution can be measured.
2DEG is observed by measuring the variation of depletion layer thickness of Schottky contact formed between tip and sample. Moreover, polarization can be observed by measuring the variation of capacitance due to nonlinear dielectric response related to the nonlinear dielectric constant whose sign is determined from polarization polarity[1].
AlGaN, GaN, and buffer layer thicknesses of the sample were 30nm, 1.6mu;m, and 2.4mu;m, respectively. The Al composition ratio in AlGaN was 30%. These layers were epitaxially grown on Si substrate. AlGaN layer and GaN layer have their own spontaneous polarization Psp. In addition, the lattice mismatch between GaN layer and AlGaN layer causes piezo-induced polarization in AlGaN layer [2]. The acquired SNDM data showed the direction of AlGaN polarization and that of the GaN polarization were same and the magnitude of polarization of AlGaN was larger than that of GaN. At the GaN layer, the signal value near the AlGaN/GaN interface went down from minus;50 Hz/V to minus;300 Hz/V. The difference of these signal values 250 Hz/V was caused by 2DEG. The half width of the 2DEG profile was about 10 nm, which means that the range of signal influenced by 2DEG was 10 nm. When the tip is near 2DEG, an electric field arrive at 2DEG from the tip because there are no carriers that block electric field in undoped AlGaN layer and undoped GaN layer. It is reasonable to assume that the half width is slightly thicker than real 2DEG thickness. Thus, we concluded that SNDM has a useful tool for evaluating the 2DEG and polarization distribution in AlGaN/GaN heterostructure.
[1] Y. Cho, A. Kirihara, and T. Saeki: Rev. Sci. Instrum., vol. 67, p. 2297, 1996.
[2] O. Ambacher et al.: J. Appl. Phys., vol. 85, p. 3222, 1999.

The emergence of wide band gap semiconductor devices has pushed the boundaries of power converter operation. The devices enable the user to increase the switching frequency of power converters while maintaining high efficiency. Potential operation at higher temperatures allows the user to further reduce the size and weight of such conversion systems. The talk will attempt to present the state of the art in wide band gap devices, the challenges associated with designing with such devices and its adoption in power conversion equipment.

AlGaN/GaN HEMTs attract lots of attention for high frequency and power applications owing to high mobility of two-dimensional electron gas (2DEG). However, one important issue concerning these devices is the formation of ohmic contacts with low contact resistance. Since contact metal layers are usually deposited on an insulating AlGaN layer, beneath which a 2DEG is induced, current pathways need to be formed through the AlGaN layer. On the other hand, thinning the AlGaN layer leads to a decrease in 2DEG concentration induced by the large polarization in the AlGaN layer. These properties result in an inherent tradeoff involving the AlGaN layer thickness. Recently, we proposed a new technique to reduce contact resistance overcoming the inherent tradeoff, in which uneven AlGaN layer structures were intentionally introduced [1]. In this technique, fringing effects at the edges of lateral patterns of the uneven structure are expected to play an important role. In this study, effects of scaling down of various lateral patterns on reduction of contact resistances are discussed.
An AlGaN/GaN heterostructure grown on a Si(111) wafer for HEMT applications was used as a substrate. The thickness and composition of the AlGaN layer was 30 nm and Al_0.25Ga_0.75N. Contact resistances were evaluated by the TLM method. The uneven structures were formed by laterally partial etching of the AlGaN layer, in which thin AlGaN regions (5 nm in thickness) and thick AlGaN regions (30 nm in thickness) coexist with particular periodic lateral patterns. The thin AlGaN regions were formed by Cl₂/BCl₃/Ar RIE. The uneven structures were formed under contact metal layers composed of Mo/Al/Ti (35/60/15nm). The lateral patterns, such as parallel line/space stripe configurations or dot matrix configurations, whose feature size was from a few hundreds nm to 5 µm were formed by electron beam lithography or photo lithography. Finally, annealing was carried out in N₂ ambient.
In the case of flat AlGaN structures (not uneven structures), the lowest contact resistance was obtained at an AlGaN thickness of around 10 nm. For the uneven structures with stripe configuration parallel to current flow, contact resistances were much smaller than the reference value for the 10 nm thick flat AlGaN structure. In particular pattern size regions, the resistance was fond to be inversely proportional to pattern density per unit area, indicating fringing effects. In the case of the stripe with 700 nm width, the contact resistance was reduced to 30% of the reference. The mechanism of contact resistance reduction is discussed along with the effects of pattern configuration and pattern size.
[1] Y. Takei et al., Physica Status Solidi A, DOI 10.1002/pssa.201431645, (2015).

Graphene has received lot of attention in the last few years due to its extraordinary electrical, optical, mechanical and thermal properties, opening the gate for potential applications in electronic and optoelectronic devices. Hence it is important to investigate the properties of graphene/semiconductor interfaces in order to understand their nature and functionality.
In the present work, electrical properties of graphene/AlGaN/GaN systems have been investigated in the temperature range 80-300 K. AlGaN/GaN heterostructures grown on Si by metal organic chemical vapor deposition (MOCVD) are used for the present study. 2DEG is formed at the interface of 24 nm Al_0.25.GaN_0.75 barrier layer and 500 nm GaN layer. Low surface roughness (RMS=0.5 nm) revealed good surface quality of the heterostructures. Sheet resistance and 2DEG sheet carrier concentration are found to be 384 ohm/sq and 1.1 × 10¹³ cm^-2, respectively. Four layer Ti/Al/Ti/Au contacts pads deposited onto this structure using e-beam evaporation served as Ohmic contacts. Another pad of SiO₂/Cr/Au (50/5/50 nm) is sputtered for graphene transfer, as the contact from graphene is taken via Au. Single layer grapheme (SLG) is transferred in such a way that one side of graphene is in contact with AlGaN/GaN surface while other side is in contact with Au pad. Self-adaptive contacts are established between graphene-AlGaN/GaN and graphene-Au. Raman measurements are performed on selected graphene layers prior to its transfer. The intensity ratio of G and 2D peaks revealed the presence of single layer graphene (SLG). Rectifying nature of I-V characteristics at each temperature indicates that graphene act as Schottky contact on AlGaN/GaN. Due to difference in work function of graphene (4.6 eV) and electron affinity of Al_XGaN_1-X/GaN (2.7 eV for x=0.25), a Schottky contact with barrier height equal to 1.9 eV should be formed theoretically as predicted by the Schottky-Mott model. However, experimental values of SBHs are always lower than the predicted values due to existence of interface states, barrier inhomogeneities, and surface modifications during device processing. In the present case, ideality factor (#414;) and Schottky barrier height (SBH) are calculated at 300 K using thermionic emission theory and found to be equal to 1.7 and 0.70 eV, respectively. On lowering the temperature to 80 K, ideality factor increases to 4.5 while SBH decreases to 0.2 eV. The increase in ideality factor and decrease in SBH on lowering the temperatures are attributed to the existence of barrier inhomogeneities as well as presence of other current transport mechanisms apart from the thermionic emission process. This kind of study can potentially be useful for electronic and optoelectronic devices for energy efficiency applications.

Introduction: Since 1990s, increasing the energy bandgaps of semiconductor materials from ~1eV in Si and GaAs to ~3.4 eV in GaN and SiC has created new revolutionary applications arenas in high-speed and high-power RF electronics and in solid-state lighting and lasers. Much of the current device technologies exploit the large energy bandgaps of GaN and SiC. We present a few preliminary structures that exploit electronic polarization to exceed the conventional power electronics figures of merit and initial experimental demonstrations of the new physics in action.
Prior and current work: The strong spontaneous and piezoelectric polarization fields in III-nitride semiconductor heterostructures is central to the realization of Al(Ga)N/GaN high-electron mobility transistors (HEMTs) on Silicon, which is currently being intensively investigated for ~600 Volt power electronics. Looking beyond, a pertinent question is - can one exceed the breakdown voltage - on resistance limits of GaN by combining heterostructures with polarization? To that end, we have investigated several vertical heterostructures on bulk GaN substrates with very low dislocation densities to exploit polarization effects^1,2 to exceed conventional figures of merit of breakdown and on-resistance. The results achieved are highly revealing not just about the effects of polarization, but of bulk GaN itself. For example, p-n junctions with ideality factors close to unity are realized for the first time in GaN, and leakage currents at ~ nA/cm² levels are measured due to the exponential dependence of leakage currents on the energy bandgap. P-i-n diodes with breakdown voltages in the 1-2 kV range are realized, and non-destructive avalanche breakdown is observed. When the traditional impurity-doped p-n junction is replaced by polarization-induced doping using graded AlGaN, enhanced performance with fundamental changes in transport mechanism is observed. The impurity and polarization doped high-voltage devices have distinct spectral emission, which helps probe the high-field transport mechanisms in these power devices, something that is difficult in Silicon and SiC due to their indirect bandgaps.

Recently, power conversion systems in vehicles are of increasing importance with the development of hybrid or electric vehicles (HV/EVs). Higher efficiency in these systems will contribute to energy-saving society in future. Wide-bandgap semiconductors such as GaN are expected as material of new-generation power devices for HV/EVs. There are mainly two major classes of power conversion system in vehicles, such as high-power modules and medium-power modules. A boost converter which is connected to a high voltage battery and a 3-phase inverter for motor driving are classified as high-power modules. GaN vertical power devices are strong candidates for these modules. There are following requirements of performance of the devices in this module: the breakdown voltage of 1.2 kV, the current capability of more than 200 A per device and the specific on resistance of less than 2 m#8486;middot;cm² beyond the performance of Si-IGBTs. To satisfy these demands, large diameter of high-quality GaN wafer and low carrier concentration control by epitaxial growth are desired. Recently, quality of GaN substrate is improving and pn-diodes over 3kV breakdown voltage and MOSFETs over 1.5kV breakdown voltage have been reported. Lower carrier concentration than 1×10¹⁶ cm^-3 is also required for 1kV breakdown voltage. For this requirement, reduction of residual carbons in the epitaxial layer which compensate Si donors and deteriorate electron mobility is a large issue of MOCVD growth. Other issues to be solved still remain in the fabrication process of the GaN vertical device. On the other hand, down converters for a low voltage source and a charging system are classified as medium-power modules. High-frequency operation over several hundred kHz and high current density are required as performance of devices though desired breakdown voltage is lower than 600V. GaN lateral power devices are suitable for this category. AlGaN/GaN HFETs on Si substrates are the main stream of the GaN lateral devices. Normally-off operation and current collapse free operation are achieved and they are applied to inverters or converters in industrial systems. For automotive applications, reliability of the device is strongly required, which is under investigation now. At the meeting, the GaN vertical device for high-power modules and issues in the fabrication process will be presented mainly.