Showing papers by "Michael Mikulla published in 2023"

Effect of AlN and AlGaN Interlayers on AlScN/GaN Heterostructures Grown by Metal–Organic Chemical Vapor Deposition

Abstract: AlScN/GaN heterostructures with their high sheet carrier density (ns) in the two-dimensional electron gas (2DEG) have a high potential for high-frequency and high-power electronics. The abruptness of the heterointerface plays a key role in the 2DEG confinement, and the presence of interlayers (AlN, AlGaN) affects ns and electron mobility (μ) and determines the sheet resistance (Rsh). AlScN/GaN heterostructures suitable for high-electron mobility transistors (HEMT) with and without a nominal AlN interlayer were grown by metal–organic chemical vapor deposition (MOCVD) and characterized electrically and structurally to gain a systematic insight into the unintentional formation and control of graded AlGaN interlayers by diffusion of atoms at the heterointerface. The AlN interlayer increases ns from 2.52 × 1013 cm–2 to 3.25 × 1013 cm–2 and, as calculated by one-dimensional Schrödinger–Poisson simulations, improves the 2DEG confinement. The barrier growth temperature was varied from 900 °C to 1200 °C to investigate the effect of the thermal budget on diffusion. Growth at 900 °C reduces the thickness of the graded AlGaN interlayer and improves the 2DEG confinement, leading to Rsh of 211 Ω/sq, ns of 2.98 × 1013 cm–2, and μ of 998 cm2/(Vs).

A Study on the Performance of AlGaN/GaN HEMTs Regrown on Mg-Implanted GaN Layers With Low Channel Thickness

Abstract: In this work, a comprehensive study on the performance of AlGaN/GaN high-electron-mobility transistors (HEMTs) regrown on Mg-implanted layers is shown. A comparably sharp doping profile into regrown AlGaN/GaN-stacks was verified by secondary-ion mass spectrometry (SIMS) even at standard metal–organic chemical vapor deposition (MOCVD) temperatures above 1000 °C. Static and dynamic characterization by a full 100-mm wafer map exhibited neither an impact on the threshold voltage, transconductance nor on the saturation current even for channel thicknesses as low as 150 nm. Slight current collapse was observed at high OFF-state conditions with large recovery times above 5 s indicating rather slow traps from the nonconnected p-GaN. Within the leakage current, three different mechanisms were identified across the vertical epi-stack. While variable-range hopping (VRH) dominates below ${V}_{{\text {DS}}}$ = 35 V, the Pool–Frenkel emission (PFE) was identified for ${V}_{{\text {DS}}} >35$ V. At high electric fields ( ${E} >$ 1 MV/cm), the devices revealed either a direct change from PFE to drain-induced barrier lowering (DIBL) or a change from PFE to space-charge-limited currents (SCLCs) to DIBL was observed. The device results demonstrate the feasibility of the demonstrated process for reproducible device fabrication on large-scale wafers with low channel thicknesses for future device developments of CAVETs, SJ-HEMTs, p-GaN back gates, and intrinsic body diodes.

GaN Power Converter and High-Side IC Substrate Issues on Si, p-n Junction, or SOI

Abstract: The lateral GaN power semiconductor technology enables monolithic integration of complete power converter topologies such as half-bridges, multi-phase and multi-level converters. Fabrication on Si substrates enables low-cost and mass production. However, the operation of monolithic GaN power converters on a common conductive silicon (Si) substrate is limited compared to discrete GaN HEMTs, especially at high-voltage operation, due to substrate-biasing effects such as back-gated or trap-related static and dynamic on-resistance increase, and changed effective device capacitances. To circumvent the Si substrate related effects but still using a low-cost large-diameter Si substrate, this paper reviews isolation approaches for GaN ICs such as Si p-n junction isolation or floating Si substrates (GaN-on-Si) and buried oxide isolation using Silicon-on-Insulator substrates (GaN-on-SOI). Published GaN power converter ICs are reviewed. The effect of the isolation approaches on increased output and input transistor capacitances, influencing the switching behavior and switching loss, is calculated and compared for different voltage classes (below 100 V to 600 V-class). Finally, design guidelines for local substrate termination using the Si-based isolation approaches are discussed exemplary for a monolithic half-bridge with driver circuit. Monolithic GaN power converter integration combined with functional integration will result in a new class of advanced power converter ICs, and enable efficient, compact and low-cost power conversion applications.

GaN-HEMT with a Back-Gated Segment for High Voltage Cascodes

Abstract: This work presents the design, fabrication, and measurements of a GaN-HEMT with a back-gated segment and pull-down pin in a GaN-on-Si technology. The device is designed for the use in high voltage cascodes. The static and dynamic characteristics of the device is demonstrated in a three-stage hybrid cascode assembly. The cascode was measured with a blocking voltage up to 1250 V.

Advances in GaN Devices and Circuits at higher mm-Wave Frequencies invited paper

Abstract: In this paper, we outline the current state of the art and trends for future development in millimeter-wave (mmw) amplifiers based on gallium nitride (GaN) semiconductors. To that end, we give an overview of recent technological results of currently operational GaN foundries and classify them with respect to their maximum frequency of operation and the current-gain cutoff frequency. Furthermore, focusing on frequencies above 80 GHz, we develop a comprehensive survey of GaN high-power amplifiers (HPAs) and provide a comparison to competing technologies such as indium phosphide (InP) and silicon germanium (SiGe). We also introduce a newly developed 6-stage GaN amplifier that is targeted towards the upper D-band. It provides an output power of up to 22.0 dBm, which sets a new record for GaN HPAs in this band. From our survey, we find that while GaN exhibits an impressive power density, its gain and efficiency characteristics particularly beyond 120 GHz still warrant improvement. Therefore, we describe several areas of future improvement such as the gate module scaling, mitigation of trapping phenomena and recent trends in processing of ohmic contacts and alternative epitaxial stacks. With these further developments, more widespread adoption of GaN-based technologies at even higher frequencies seems feasible in the coming years.

Switching of GaN CAVET with Quasi-Monolithic Integrated HEMT Gate Driver

Abstract: In this letter, a vertical GaN power transistor driven by lateral devices fabricated in the same technology is demonstrated. The technology combines a co-integrated large area current aperture vertical electron transistor (CAVET) with high electron mobility transistors (HEMTs) for the realization of the driver on a GaN substrate. The quasi-monolithic integrated driver stage consists of two HEMT devices in a push-pull configuration. The CAVET and HEMTs are characterized, separated, packaged, and measured in a double pulse test setup with inductive load. The voltage signals of the HEMT driver with CAVET are shown in continuous operation up to 5 MHz and extreme duty-cycles. In pulsed operation, switching characteristics and waveforms under load up to 120 V and 4.1 A are shown with turn-on/-off switching times of 17.3/2.8 ns. Finally, this work demonstrates a GaN technology that combines the functional integration of a driver stage with a vertical power transistor and thus opens the pathway to continue lateral GaN power integration in vertical device concepts.

Voltage-margin limiting mechanisms of AlScN-based HEMTs

Abstract: In this work, the off-state characteristics of AlScN/GaN high electron mobility transistors (HEMTs) grown by metalorganic chemical vapor deposition (MOCVD) were studied and directly compared to an AlGaN- and an AlN-HEMT grown in the same MOCVD. Pinch-off instability and leaky capacitive measurements were observed for AlScN-based HEMTs, which was correlated with a higher ideality factor and lower effective potential barrier height than the AlGaN and AlN-HEMTs. However, the reverse bias characteristics exhibited a sudden drain-current increase without a significant increase in gate-leakage current. The drain-leakage current is assumed to be related to a parasitic channel across the AlScN-barrier as a result of trap-assisted carrier transport with a Poole–Frenkel characteristic. The demonstrated pinch-off instability led to significant gain expansion in load-pull measurements and early soft-breakdown, which, in turn, limits the achievable voltage-margin. The results demonstrate a key issue to reveal the full potential of AlScN-based HEMTs for mm-wave applications.