High-electron-mobility transistor

A high-electron-mobility transistor (HEMT or HEM FET), also known as heterostructure FET (HFET) or modulation-doped FET (MODFET), is a field-effect transistor incorporating a junction between two materials with different band gaps (i.e. a heterojunction) as the channel instead of a doped region (as is generally the case for a MOSFET). A commonly used material combination is GaAs with AlGaAs, though there is wide variation, dependent on the application of the device. Devices incorporating more indium generally show better high-frequency performance, while in recent years, gallium nitride HEMTs have attracted attention due to their high-power performance. Like other FETs, HEMTs are used in integrated circuits as digital on-off switches. FETs can also be used as amplifiers for large amounts of current using a small voltage as a control signal. Both of these uses are made possible by the FET’s unique current–voltage characteristics. HEMT transistors are able to operate at higher frequ

Diamond on GaN integration for power semiconductor devices with efficient thermal management

Reza Soleiman Zadeh Ardebili

The international actions against global warming demands reductions in carbon emission and more efficient use of energy. Energy efficiency in the conversion and use of electricity, as an important form of energy in the modern life, has strong environmental and economical impacts. Power electronic devices determining roles in the overall system efficiency in many applications such as power converters and inverters. Gallium Nitride (GaN) devices have emerged as an superior alternative to the silicon devices and enabled unprecedented efficiencies. GaN-based high electron-mobility transistors (HEMTs) offer excellent characteristics such as high switching speed, low switching and conduction losses, small device size and high power density capabilities. However, such significant increases in the power density and the reduction of device size comes at the price of dramatic increases in the heat fluxes and appearance of hot spots in the device footprint that cannot be addressed using conventional methods. The integration of diamond and GaN has been highly pursued for thermal management purposes as well as combining their exceptional complementary properties for power electronics applications and novel semiconductor heterostructures. In this thesis, we discuss the key hindrances towards realisation of diamond on GaN substrates and devices, and we propose novel solutions for improving the feasibility of diamond-on-GaN integration for efficient thermal management of hot spots in GaN devices and development of diamond devices integrated with GaN devices in the same chip.The growth of diamond on GaN is challenging due to the high lattice and thermal expansion mismatches. The weak adhesion of diamond to GaN and high residual stresses after the deposition often result in the diamond film delamination or development of cracks, which prevents the subsequent device fabrication. In this thesis, a new seed dibbling method is presented for seeding and growing high-quality diamond films on cost-effective GaN-on-Si substrates, which result in significantly larger diamond grains and lower residual stresses (as low as 0.2 GPa) compared to conventional methods. Moreover, the excellent adhesion obtained by this method enabled a reliable polishing of the as-grown diamond films on GaN-on-Si without any delamination, resulting in smooth diamond-on-GaN substrates with sub-nanometer roughness. The high temperature hot spots in GaN devices are addressed by the near-junction heat spreaders using diamond films deposited on GaN. An analytical model for the heat spreading is presented to analyse the performance and optimize the heat spreaders, considering the geometry and thermal conductivity of the heat spreader and the substrate. Experimental demonstration of diamond heat spreaders on vertical GaN diodes together with the thermal characterization of their behaviour shows significant reduction of thermal spreading resistances, hot spot temperatures and temperature gradients from the device footprint, which highlights the impact of such heat spreaders for the thermal management of high power density GaN devices.The diamond films deposited on GaN were also used to demonstrate high-performance diamond transistors integrated on AlGaN/GaN-on-Si substrates with characteristics such as high breakdown voltage, high current density, and high on/off ratio, which opens many new possibilities for the development of power ICs with complementary logics, gate drivers and power

EPFL2021

Charge transport in proximitized graphene within van der Waals heterostructures

Katharina Polyudov

Since their discovery, graphene and other 2D materials have become a subject of intense research in condensed matter physics. Especially the vast possibilities of combining those materials into heterostructures are promising for the discovery of novel physical phenomena. The heterostructures accessible through state-of-the-art techniques have suitably clean interfaces between the layers. Graphene has gained a lot of attention due to its remarkable mechanical stability and extremely high electron mobility. However, the zero bandgap of graphene is a major bottleneck for implementing this 2D material into most electronic applications. The ability to tune the properties of graphene by proximity effects has shifted the focus of graphene research towards the combination of graphene with other van der Waals materials. The main objective of the present thesis was to explore for two different types of graphene-based van der Waals heterostructure whether fingerprints of electronic proximity effects can be traced in their low temperature magnetotransport properties. The first part of the thesis deals with heterostructures of graphene and Bi2Te2Se (BTS). The strong spin-orbit coupling of BTS makes it a three-dimensional topological insulator with topological surface states which are protected by time-reversal symmetry. At the same time, BTS is promising to exert a pronounced spin-orbit proximity effect on graphene, and thereby to open a band gap and/or introduce a spin texture in the latter. The graphene/BTS heterostructures were fabricated by the direct growth of BTS on graphene to ensure a clean interface between both materials and correspondingly a good electronic coupling between them. Analysis of the weak localization effect observed in the magnetoconductivity revealed the presence of enhanced SOC in the proximitized graphene. The second part of the thesis focuses on heterostructures wherein graphene is combined with a-RuCl3 which has recently gained a lot of attention as a potential quantum spin liquid system. Previous studies on graphene/a-RuCl3 heterostructures found an unusual temperature evolution of the quantum oscillation amplitude, whose origin remained unclear. The magnetotransport data collected in this thesis point toward two possible origins for this behavior, namely spin fluctuations associated with the magnetic transition into an antiferromagnetic phase at the Néel temperature of approximately 7 K, and the hybridization-induced formation of heavy (flat) bands, both of which are likely to depend on the a-RuCl3 layer thickness. In addition, heterostructures comprising an a-RuCl3 monolayer were found to display a unusual gate dependence of the quantum oscillations, further corroborating the importance of the a-RuCl3 thickness.

EPFL2020

Physical Properties of Al1-xInxN/(AIN)/GaN (0.07

Marcus Gonschorek

AlInN is a material which is known to be difficult to be grown among the III-nitride ternary compounds. It attracted much attention only recently and starts to be studied intensively mainly for electronic applications. The aim of this work is manifold. Various structural, electrical and optical measurements are brought together to underline the outstanding quality of AlInN epi-layers grown by metalorganic vapor phase epitaxy (MOVPE). Especially the electrons confined at the heterointerface of coherently grown AlInN on GaN buffer layers determine crucially the electronic properties. An appropriate model based on charge balance is proposed allowing the extraction of all significant electrical properties. An AlN interlayer has been introduced at the AlInN/GaN interface and was found to tremendously affect transport properties. This issue is studied intensively and results indicate that even slightest deviations from atomically perfect interfaces leads to the creation of huge piezoelectric fields disturbing carrier transport in the case of strongly mismatched epilayers. A similar debate was conducted concerning the localization of carriers in InGaN quantum wells. High electron mobility transistors (HEMT) processed from these heterostructures exhibits excellent device performance exceeding 2 A/mm even on sapphire substrates. This are the highest currents ever reported for nitride heterostructures. The same structures grown on Si(111) substrates allow reaching cut-off frequency in excess of 100 GHz . However, results are puzzling and on at first sight striking since the device saturation currents of structures grown on SiC are similar to results on sapphire despite its much larger thermal diffusivity. Therefore we studied the temperature arising in the device under applied voltages using a micro-photoluminescence (µPL) setup. To our knowledge this is the first study of this type. The main advantage is the use of the short wavelength for excitation in comparison with well-established Raman methods. However we find from this analysis a significant heat development up to 1200 K for devices with 5 µm drain-source distance. The heating can be understood in terms of excited optical phonons due to field accelerated electrons under applied voltages. The comparison of the available amount of phonons in the finite volume covered by the 2D electrons and the number of created phonons in the acceleration process allows formulating a simple relation for the saturation currents as a function of the transport properties. However, this mechanism also explains why the saturation currents are far below expected values from simple velocity saturated currents models and independent of the substrate. However, heat, i.e. the decomposition of local optical phonons into acoustical phonons with large group velocity, is dissipated differently depending on substrate influencing therefore mainly the reliability of the devices. Beside the electrical breakdown due to avalanche carriers a second thermally induced breakdown mechanisms is proposed. Finally, the capability of those heterostructures for sensing is discussed. Especially the electrically response on external stimulus such as UV radiation and the exposition of the surface to polar liquids is investigated.

EPFL2011