Philipp Döring

The impact of the sheet carrier density, the barrier design, the introduction of AlN interlayer, and the substrate temperature on the saturation velocity in ungated GaN-based heterostructures were measured over multiple process runs. In addition, gate-transit delays were extracted on 150-nm transistors to verify the extracted data on a transistor level. Empirical models were derived for the saturation and effective carrier velocity in dependence on all parameters to allow for a simple estimation of the current density for a wide majority of GaN-based heterostructures. In addition, an empirical model for the temperature dependence (up to 400 K substrate temperature) is provided, allowing for a direct estimation of the intrinsic saturation velocity decrease for substrate temperatures close to the operation condition of high-power amplifiers.

This work investigates the thermal characteristics of gallium nitride (GaN) high-electron mobility transistors (HEMTs) for power applications. Exemplarily, a 650-V class HEMT is analyzed by different thermal characterization methods such as transient $I$ – $V$ measurements, infrared (IR) microscopy, and simulations, presenting a comprehensive insight into the thermal behavior. A new electrothermal model function is derived for the extraction of thermal parameters by transient $I$ – $V$ measurements, and it is not limited to the linear region of the output characteristic, as a model presented in previous work, but also applicable in the saturation region. Fifth-order Foster model parameters are extracted for different operating points in linear and saturation regions. For verification and better understanding, the measurements are also performed by IR microscopy at same operating points enabling the characterization of the device’s surface temperature distribution. Two new analytical models allow the interpretation of the temperature distribution and are used as fit formulas for extracting the thermal parameters found by IR microscopy. Finally, the research includes an investigation using technology computer-aided design (TCAD) simulation, followed by a thermal 3-D finite element simulation. The results allow a deeper understanding of the electrothermal behavior of GaN power HEMTs at various operating points.

The electrical properties of SiNy/Al1−xScxN/GaN-based metal–insulator–semiconductor (MIS) structures, including electron sheet density, electron mobility, depletion behavior, and off-state leakage current, were systematically investigated as a function of Sc concentration (x = 0.06–0.17). A reduction in polarization-induced charge with increasing Sc concentration was observed, evidenced by lower electric fields needed to deplete the two-dimensional electron gas (2DEG) confined close to the Al1−xScxN/GaN interface and the corresponding decreases in electron sheet density from 2.5 × 1013 to 2.0 × 1013 cm−2. These measured values are at least two times lower than theoretical predictions, likely due to expected high carbon and oxygen impurity levels in the grown barrier layers. A pinch-off instability was found to occur after SiNy passivation removal. The electron mobility was found to be inversely correlated with Sc concentration declining from 950 to almost 750 cm2/Vs, attributed to interactions between the 2DEG and the barrier. Furthermore, the off-state leakage current investigations revealed a minimum in leakage current at x = 0.12. Accordingly, an optimal Sc concentration range between x = 0.10 and 0.12 was identified that balances depletion behavior, mobility, electron sheet density, and leakage current, potentially due to near lattice-matched conditions between Al1−xScxN barrier and GaN-channel layer. These findings provide crucial insights for the development of high-performance Al1−xScxN/GaN-based transistors for next-generation high-frequency and high-power electronic applications.

This work presents a method for extracting higher order thermal models of GaN power high-electron mobility transistors (HEMTs) from I–V measurements using a typical commercial power analyzer. The approach involves deriving and fitting an electrothermal model function to measure data using a nonlinear least squares solver, yielding the thermal parameters for a higher order thermal model. Measurements are conducted using a parameter analyzer and a controlled thermal chuck, with a transient drain current response signal. The method is employed to derive the thermal impedance parameters of a 5th-order thermal Foster model for a GaN power transistor. The Foster model parameters are presented in both time and frequency domains and are subsequently transformed into Cauer model parameters. The results demonstrate strong agreement with data obtained from an on-chip temperature sensor, confirming the method’s validity. This new extraction method can be executed using standard laboratory equipment typically available for the electrical characterization of GaN power transistors.

In this work, degenerate n-type GaN thin films prepared by co-sputtering from a liquid Ga-target were demonstrated and their low-field scattering mechanisms are described. Extremely high donor concentrations above 3 × 10²⁰ cm⁻³ at low process temperatures (<800 °C) with specific resistivities below 0.5 mΩcm were achieved. The degenerate nature of the sputtered films was verified via temperature-dependent Hall measurements (300–550 K) revealing negligible change in electron mobility and donor concentration. Scattering at ionized impurities was determined to be the major limiting factor with a minor contribution of polar optical-phonon scattering at high temperatures.

The allocated frequency bands of next-generation wireless communication networks will be shifting further up in the spectrum to be able to meet the stringent data rate requirements. Doing so, demands higher bandwidth capabilities from the electronic devices in the RF chain. Numerous cutting-edge systems are using AlGaN/GaN-HEMT based components, in particular in the transmit chain, due to their favorable combination of output power and bandwidth capability. Still, this semiconductor technology is approaching an upper bound in this regard, known as the Bode-Fano limit. Devices based on an AlScN/GaN heterointerface have been identified as candidates to push these boundaries and enable components that deliver higher power over a broader frequency range. We report on the status of MOCVD-grown AlScN/GaN HEMTs, their current capabilities but also on challenges that still hamper the exploitation of the material system's full potential. The first reported AlScN/GaN HEMTs were fabricated on epitaxial stacks grown by MBE. However, the majority of commercial GaN-based HEMT technologies is relying on MOCVD-grown epitaxy due to higher growth rate and lower cost of operation. We managed to grow AlScN/GaN heterostructures showing sheet carrier densities (n_S) of up to 3·10¹³cm^-2 and mobilities of 1100 cm²/Vs. These results were achieved with a MOCVD reactor that had been modified to cope with the low vapor pressure of the used precursors Cp₃Sc and (MCp)₂ScCl. Depending on the temperature, the obtained growth rates were as high as 0.015 nm/s. Currently, further precursors with allegedly higher vapor pressure are evaluated that should allow for even higher growth rates, further improving the quality of the fabricated heterostructures. Furthermore, HEMT devices were fabricated on top of the epitaxial wafers that showed power densities (P_out) of up to 8.4 W/mm and power-added efficiencies (PAE) of more than 48 % at a frequency of 30 GHz, even surpassing the current state-of-the-art in terms of combined P_out and PAE of Ga-polar devices. However, there are still a number of unsolved issues visible in most reported AlScN/GaN HEMT devices, regardless of the used growth method. One is concerning the level of leakage currents and the consequential premature soft breakdown. The root cause of this phenomena is not yet fully understood but is believed to originate from poor dielectric/isolating properties of the Sc-containing barrier. This hypothesis is supported by a comparably high level of impurites that is found in the AlScN layers. Minimizing theses residuals by purification of the precursors is, therefore, necessary to further improve the electrical device characteristics. Secondly, transport properties of AlScN/GaN heterointerfaces are not as well studied as more established junctions, e.g., AlGaN/GaN and AlN/GaN. An important parameter that has not been experimentally identified for 2DEGs induced by AlScN/GaN heterojunctions yet, is the saturation drift velocity (v_sat). It is of particular importance since the Bode-Fano limit, among other, depends on it. Since the extension of the latter is one of the great promises of AlScN/GaN devices, it is clear that the value of v_sat is essential to know in order to make quantitative predictions of the level of improvement that is achievable in terms of bandwidth capability but also with regard to power density. We will give an overview of recent advances of MOCVD-grown AlScN/GaN HEMTs with respect to their capability to extend on the achievable performance level of present GaN-based devices, in particular in the millimeter-wave frequency range. Further, we will benchmark the results against other competing technologies, i.e. GaAs pHEMT and Si-CMOS, to give an outlook of where optimized AlScN/GaN devices can takes us and where they cannot.

In this work, the simultaneously achievable matching bandwidth and output power of AlScN-based high electron mobility transistors (HEMTs) are derived and compared to conventional AlGaN, GaAs, and Si devices. Moll’s method is used to extract time delays resulting in carrier velocities close to 1 $\times$ 10 $^{\text{7}}$ cm/s for sheet carrier densities $\ge$ 1.5 $\times$ 10 $^{\text{13}}$ cm $^{-\text{2}}$ . Subsequently, theoretical current densities of 2.5–4 A/mm and a maximum transconductance higher than 600 mS/mm are derived for low barrier thicknesses of 5–10 nm and Sc-concentrations of 5%–20%. The matching bandwidth is estimated by the Bode–Fano criterion and connected to the output power of the transistor by the power-bandwidth product, which accounts for both parameters simultaneously. AlScN-based devices are found to exhibit a 4.5-times higher power-bandwidth product compared to conventional AlGaN-based HEMTs, quantifying the enormous, theoretical limits. Experimental data already show an improvement by a factor of 1.45 for AlScN-based devices, even in this early stage of development, which proves their superior properties, when aiming for wideband high-power millimeter-wave (mm-wave) devices.

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.

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.