J. Y. Tsao,* S. Chowdhury, M. A. Hollis,* D. Jena, N. M. Johnson, K. A. Jones, R. J. Kaplar,* S. Rajan, C. G. Van de Walle, E. Bellotti, C. L. Chua, R. Collazo, M. E. Coltrin, J. A. Cooper, K. R. Evans, S. Graham, T. A. Grotjohn, E. R. Heller, M. Higashiwaki, M. S. Islam, P. W. Juodawlkis, M. A. Khan, A. D. Koehler, J. H. Leach, U. K. Mishra, R. J. Nemanich, R. C. N. Pilawa-Podgurski, J. B. Shealy, Z. Sitar, M. J. Tadjer, A. F. Witulski, M. Wraback, and J. A. Simmons

/pdf/ultrawide-bandgap-semiconductors-research-opportunities-and-49ev1zu2ym.pdf

Ultrawide-Bandgap Semiconductors: Research Opportunities and Challenges

Wide bandgap semiconductor aluminum gallium nitride (AlGaN) alloys have established themselves as the key materials for building ultraviolet (UV) optoelectronic and power electronic devices. However, further improvements of device performance are lagging behind largely due to the difficulties in precisely controlling the carrier behavior, both carrier generation and carrier transport within the AlGaN-based devices. Fortunately, it has been discovered that instead of using AlGaN layers with fixed Al content, by grading the Al composition along the growth direction, we are capable of (1) generating high-density electrons and holes via polarization-induced doping; (2) manipulating carrier transport behavior via energy band modulation, or so-called "band engineering". Consequently, such compositionally graded AlGaN alloys have attracted extensive interest as the promising building blocks for efficient AlGaN-based UV light emitters and power electronic devices. In this review, we focus on the unique physical properties of graded AlGaN alloys and highlight the key roles that such graded structures play in device exploration. Firstly, we elaborate on the underlying mechanisms of efficient carrier generation and transport manipulation enabled by the graded AlGaN alloys. Thereafter, recent progress of UV light emitters and power electronic devices incorporating the graded AlGaN structures are comprehensively summarized and discussed. Finally, we outline the prospects associated with the implementation of graded AlGaN alloys in the pursuit of high-performance optoelectronic and power electronic devices.

Compositionally Graded III-Nitride Alloys: Building Blocks for Efficient Ultraviolet Optoelectronics and Power Electronics.

Gallium nitride (GaN) is one of the front-runner materials among the so-called wide bandgap semiconductors that can provide devices having high breakdown voltages and are capable of performing efficiently even at high temperatures. The wide bandgap, however, naturally leads to a high density of surface states on bare GaN-based devices or interface states along insulator/semiconductor interfaces distributed over a wide energy range. These electronic states can lead to instabilities and other problems when not appropriately managed. In this Tutorial, we intend to provide a pedagogical presentation of the models of electronic states, their effects on device performance, and the presently accepted approaches to minimize their effects such as surface passivation and insulated gate technologies. We also re-evaluate standard characterization methods and discuss their possible pitfalls and current limitations in probing electronic states located deep within the bandgap. We then introduce our own photo-assisted capacitance–voltage (C–V) technique, which is capable of identifying and examining near mid-gap interface states. Finally, we attempt to propose some directions to which some audience can venture for future development.

/pdf/controlling-surface-interface-states-in-gan-based-o8aryl8nl8.pdf

Controlling surface/interface states in GaN-based transistors: Surface model, insulated gate, and surface passivation

The piezoelectric and spontaneous polarization of wurtzite ScxAl1−xN, GaxAl1−xN, and InxAl1−xN ternary compounds dramatically affects the electrical properties of pseudomorphic MexAl1−xN/GaN, MexAl1−xN/AlN, and MexAl1−xN/InN heterostructures and devices (Me: = Sc, Ga, In), due to bound interface charges caused by gradients in polarization at surfaces and heterointerfaces. We have calculated the piezoelectric and spontaneous polarization of undoped, metal polar ScxAl1−xN barrier layers (0 ≤ x ≤ 0.5) pseudomorphically grown on InN, GaN, and AlN buffer layers, in order to compare the polarization induced surface and interface charges determined to the ones predicted and measured in heterostructures with GaxAl1−xN and InxAl1−xN barriers (0 ≤ x ≤ 1.0). To facilitate the inclusion of the predicted polarization in future simulations, we give explicit prescriptions to calculate polarization induced bound interface charges for arbitrary x and barrier thicknesses up to 50 nm in each of the ternary III-N alloy heterostructures. In addition, we predict the electron sheet charges confined in heterostructures with positive polarization induced interface charges taking limitations for the epitaxial growth by strain and critical barrier thicknesses into account. Based on these results, we provide a detailed comparison of the sheet resistances and current-carrying capabilities of the heterostructures investigated, pointing to a superior potential of ScAlN/GaN based heterostructures for processing improved high electron mobility transistors for high-frequency and power electronic applications.

Polarization induced interface and electron sheet charges of pseudomorphic ScAlN/GaN, GaAlN/GaN, InAlN/GaN, and InAlN/InN heterostructures

The interplay of self-heating and polarization affecting resistance is studied in AlGaN/GaN transmission line model (TLM) heterostructures with a scaled source-to-drain distance. This paper is based on meticulously calibrated TCAD simulations against I-V experimental data using an electro-thermal model. The electro-thermal simulations show hot-spots (with peak temperature in a range of ~566 K–373 K) at the edge of the drain contact due to a large electric field. The electrical stress on Ohmic contacts reduces the total polarization, leading to the inverse/converse piezoelectric effect. This inverse effect decreases the polarization by 7%, 10%, and 17% during a scaling of the source-to-drain distance in the 12 $\mu \text{m}$ , 8 $\mu \text{m}$ , and 4 $\mu \text{m}$ TLM heterostructures, respectively, when compared with the largest 18- $\mu \text{m}$ heterostructure.

/pdf/role-of-self-heating-and-polarization-in-algan-gan-based-43wa9gkt6r.pdf

Role of Self-Heating and Polarization in AlGaN/GaN-Based Heterostructures

We report on the direct measurement of two-dimensional sheet charge density dependence of electron transport in AlGaN/GaN high electron mobility transistors. Pulsed IV measurements established increasing electron velocities with decreasing sheet charge densities, resulting in saturation velocity of 1.9 x 10^7 cm/s at a low sheet charge density of 7.8 x 10^11 cm-2. A new optical phonon emission-based electron velocity model for GaN is also presented. It accommodates stimulated LO phonon emission which clamps the electron velocity with strong electron-phonon interaction and long LO phonon lifetime in GaN. A comparison with the measured density-dependent saturation velocity shows that it captures the dependence rather well. Finally, the experimental result is applied in TCAD-based device simulator to predict DC and small signal characteristics of a reported GaN HEMT. Good agreement between the simulated and reported experimental results validated the measurement presented in this report and established accurate modeling of GaN HEMTs.

Density-Dependent Electron Transport and Precise Modeling of GaN HEMTs

A monolithic microwave integrated circuit structure having a semiconductor substrate structure with a plurality of active devices and a microwave transmission line having an input section, an output section and a interconnecting section electrically interconnecting the active devices on one surface and a metal layer as well as a heat sink on an opposite surface overlaying the interconnection section and absent from overlaying the input section and the output section.

Monolithic microwave integrated circuit

A semiconductor structure having: a Group III-N channel layer, a Group III-N top-barrier polarization-generating layer forming a heterojunction with an upper surface of the channel layer; and a Group III-N back-barrier polarization-generating layer forming a heterojunction with a lower surface of the channel layer. The channel layer has disposed therein a predetermined n-type conductive dopant.

Double heterojunction group III-nitride structures

A monolithic microwave integrated circuit structure having: a semiconductor substrate structure (12) having a plurality of active devices (16) and a microwave transmission line (18) having an input section (24), an output section (26) and a interconnecting section (28) electrically interconnecting the active devices on one surface of the substrate; a thermally conductive, electrically non-conductive heat sink (40); and a thermally conductive bonding layer (39) for bonding the heat sink to the substrate, the thermally conductive bonding layer having an electrically conductive portion (39a) and an electrically non-conductive portion (39b), the electrically conductive portion being disposed between the heat sink and an opposite surface of a portion of the substrate having the active devices and the electrically non-conductive portion being disposed on the opposite surface portion overlaying portion of the microwave transmission line section.

Patterned conductive epoxy heat-sink attachment in a monolithic microwave integrated circuit (MMIC)

A heterojunction semiconductor field effect transistor HFET having a pair of layers of different semiconductor materials forming a quantum well within the structure to support the 2DEG. Source, drain and gate electrodes are disposed above the channel. The HFET has a predetermined transconductance. A transconductance control electrode varies an electric field within the structure under the channel to vary the shape of the quantum well and thereby the transconductance of the FET in accordance with a variable control signal fed to the transconductance control electrode.

Heterojunction field effect transistor (HFET) variable gain amplifier having variable transconductance

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