Wide Bandgap Semiconductor Electronics and Devices

The following sections are included:

• Preface
• Contents

The influence of the semiconducting Si substrate on the performance of GaN-on-Si RF technology is reviewed. Firstly, the formation of a parasitic conduction channel at the substrate-epitaxy interface is discussed in terms of its physical mechanism and its influence on RF loss, followed by schemes to minimize this effect. Secondly, it is shown that the presence of the parallel channel serves to back-bias the III-nitride epitaxial stack and lead to current collapse even on the highly-resistive Si substrates used for RF device fabrication, analogous to GaN-on-doped Si power devices. Strategies to mitigate this issue are also presented and critically compared. Thirdly, thermal generation of carriers in Si at elevated operating temperatures leading to increased substrate loss is quantified, also followed by a discussion of possible techniques to reduce its influence on RF loss.

Ga₂O₃ has gained worldwide interests for next-generation power electronics because its large critical field strength stemming from an ultra-wide bandgap promises miniaturized circuits and systems with high conversion efficiency. Intense pursuit of Ga₂O₃ power devices stimulated by early demonstrations of high-voltage Ga₂O₃ field effect transistors (FETs) has brought about tremendous advancements in this new technology, whose strong radiation tolerance and high thermal stability also befit harsh-environment applications. In this paper, we review the various types of depletion-and enhancement-mode Ga₂O₃ FETs — which have predominantly been lateral devices — for power switching and radiation-hard electronics. The development of vertical Ga₂O₃ transistors based on a low-cost, highly-manufacturable ion implantation doping process will also be discussed.

In this paper, we demonstrate Q-band power performance of carbon doped AlN/GaN high electron mobility transistors (HEMTs) using a deep sub-micrometer gate length (120 nm). With a maximum drain current density I_D of 1.5 A/mm associated to a high electron confinement and an extrinsic transconductance g_m of 500 mS/mm, this structure shows excellent electrical characteristics. A maximum oscillation frequency f_max of 242 GHz has been observed As a result, a state-of-the-art combination at 40 GHz of output power density (P_OUT = 7 W/mm) and power added efficiency (PAE) of 52% up to V_DS = 25V has been obtained. The achievement of such outstanding performance is attributed to the reduced thermal resistance (R_TH) as compared to the commonly used double heterostructure by means of Raman thermography.

In recent years, the emergence of the ultrawide‐bandgap (UWBG) semiconductor materials that have an extremely large bandgap, exceeding 5eV including AlGaN/AlN, diamond, β-Ga₂O₃, and cubic BN, provides a new opportunity in myriad applications in electronic, optoelectronic and photonics with superior performance matrix than conventional WBG materials. In this review paper, we will focus on high power and high frequency devices based on two most promising UWBG semiconductors, β-Ga₂O₃ and diamond among various UWBG semiconductor devices. These two UWBG semiconductors have gained substantial attention in recent years due to breakthroughs in their growth technique as well as various device engineering efforts. Therefore, we will review recent advances in high power and high frequency devices based on β-Ga₂O₃ and diamond in terms of device performance metrics such as breakdown voltage, power gain, cut off frequency and maximum operating frequency.

Atom Probe Tomography (APT) has emerged as a stand-alone technique for material characterization at the sub-nanometer level, with unrivaled spatial resolution, coupled with three-dimensional atomic mapping, and equal sensitivity of all elements. Over the past decade, APT has proven to be a valuable tool in advancing the understanding and design of GaN-based semiconductor technology, by revealing correlations between atomic-level structure chemistry and device performance. The uniqueness of APT is exemplified by its ability to directly analyze nanoscale features within a commercial device. In this review, the quantitative requirements for advancement in GaN-based device metrology are defined as accurate measurement of composition, structural inhomogeneities, elemental incorporation within thin interlayers, and quantification of dopants and impurities. These are bolstered by a review of recent advances in GaN-based devices, realized through APT. The rich compositional and spatial data provided by APT is necessary for the continued advancement of III-V semiconductor technology.

Gallium Oxide has attracted a great deal of attention for power electronics due to its large bandgap (~ 4.8 eV) as well as availability of cost effective, large area, high quality substrates. In this article, we will discuss advancements in homoepitaxial and heteroepitaxial growth of β-(Al,Ga)₂O₃ and their heterostructures via different growth techniques with the emphasis on metal organic chemical vapor deposition, molecular beam epitaxy, and halide vapor phase epitaxy. We will also review various device structures demonstrated for high power applications.

Recent breakthroughs in bulk crystal growth of β-Ga₂O₃ by the edge-defined film-fed technique has led to the commercialization of large-area β-Ga₂O₃ substrates. Standard epitaxy approaches are being utilized to develop various thin-film β-Ga₂O₃ based devices including lateral transistors. This article will discuss the challenges for metal organic chemical vapor deposition (MOCVD) of β-Ga₂O₃ and the design criteria for use of this material system in power electronic device structures.

We present a comprehensive review of high-field transport properties in an emerging and trending ultra-widebandgap semiconductor β-Ga₂O₃. The focus is on the theoretical understanding of the microscopic mechanisms that control the dynamics of far-from-equilibrium electrons. A manifold of density functional calculations and Boltzmann theory based transport formalism unravels the behavior of the electron distribution under a varied range of external electric fields. The key high-field transport properties that govern electronic device performance, like velocity and ionization co-efficients, are enlightened in detail with physical insights. Anisotropies in the above transport co-efficients are probed from the microscopic investigation of bandstructure, electron-phonon interactions, and electron-electron interactions.

High Al-composition Al_xGa_1-xN, an emerging class of materials, is gaining significant traction due to its high critical breakdown electric field exceeding that of GaN and high electron saturation velocity that is comparable to GaN. High Al-composition Al_xGa_1-xN holds promise for applications such as highly scaled next generation RF devices and power devices. However, significant strides remain to be made before Al_xGa_1-xN can take a share of the limelight. Encouraging progress has been made in recent years, including multiple reports of RF operation of Al_xGa_1-xN channel transistors with reported unity current gain cutoff frequency as high as 40 GHz, high current density devices with the maximum reported current density exceeding 600 mA/mm and reports of breakdown fields in lateral transistors that are almost 3× the reported breakdown field for GaN channel devices of similar dimensions. This paper focuses on the lateral devices demonstrated thus far and discusses breakthrough performance results achieved for Al_xGa_1-xN channel transistors together with the key challenges that are yet to be properly addressed. The paper starts with a discussion of contact formation to Al_xGa_1-xN films – the key difficulty of fabricating Al_xGa_1-xN transistors. This is followed by a discussion of the types of devices that are typically used. This is followed up with a discussion of the approach required to make high quality devices with Al_xGa_1-xN as a template together with some recent result highlights.

Silicon technology enabled most of the electronics we witness today, including power electronics. However, wide bandgap semiconductors are capable of addressing high-power electronics more efficiently compared to Silicon, where higher power density is a key driver. Among the wide bandgap semiconductors, silicon carbide (SiC) and gallium nitride (GaN) are in the forefront in power electronics. GaN is promising in its vertical device topology. From CAVETs to MOSFETs, GaN has addressed voltage requirements over a wide range. Our current research in GaN offers a promising view of GaN that forms the theme of this article. CAVETs and OGFETs (a type of MOSFET) in GaN are picked to sketch the key achievements made in GaN vertical device over the last decade.

This paper presents a self-consistent analytic model to describe the current-voltage (I-V) and charge-voltage (Q-V) behavior of quasi-ballistic III-nitride transistors. We focus on two types of transistor geometries: (i) high electron mobility transistors (HEMTs) suitable for radio frequency (RF) applications and (ii) nanowire field-effect transistors (FETs) for digital applications. Our core model is based on Landauer transport theory which is combined with the calculation of charge density and velocity of charges at the top-of-the-barrier in the transistor. The effect of extrinsic device features, such as the nonlinearity of access regions and Joule heating at high currents, are included in the static I-V model. In the case of the dynamic Q-V model, we calculate intrinsic terminal charges by approximating the solution of the 2D Poisson equation in the channel over a broad bias range. The effect of fringing capacitances, prominently inner-fringing capacitance that varies nonlinearly with the gate bias, is included in our Q-V model. We amend the model electrostatics and the description of source/drain rectifying contacts in our core model to represent the I-V characteristics of III-nitride nanowire FETs. The model shows excellent match against experimentally and numerically measured characteristics of GaN transistors with gate lengths ranging from 42 nm to 274 nm. With only 38 input parameters, most of which are extracted based on straightforward device characterization, our model can be used for device-circuit co-design and optimization using a standard hierarchical circuit simulator.

Tunnel junctions have garnered much interest from the III-Nitride optoelectronic research community within recent years. Tunnel junctions have seen applications in several material systems with relatively narrow bandgaps as compared to the III-Nitrides. Although they were initially dismissed as ineffective for commercial device applications due to high voltage penalty and on resistance owed to the wide bandgap nature of the III-Nitride material systems, recent development in the field has warranted further study of such tunnel junction enabled devices. They are of particular interest for applications in III-Nitride optoelectronic devices in which they can be used to enable novel device designs which could potentially address some of the most challenging physical obstacles presented with this unique material system. In this work we review the recent progress made on the study of III-Nitride tunnel junction-based optoelectronic devices and the challenges which are still faced in the field of study today.