Dirk Fahle

The effect of the inversion channel at the AlN/Si interface on the vertical breakdown characteristics of GaN-based devices

GaN-on-Si transistors attract increasing interest for power applications. However, the breakdown behavior of such devices remains below theoretical expectations, for which the Si substrate is typically made responsible. In this work, the effect of the thickness of an aluminum nitride buffer layer on the vertical breakdown voltage, measured relative to a grounded silicon substrate, has been investigated. A voltage-polarity-dependent breakdown mechanism has been observed. It has been found that the breakdown in the positive bias voltage regime is initiated by carrier injection, for which the carriers originate from an inversion channel formed between the epitaxial layers and the p-silicon substrate. TCAD simulations have confirmed the proposed explanations, and suggest that appropriate modification of the electronic structure at the AlN/silicon interface could significantly improve the vertical breakdown voltage.

Effect of stress voltage on the dynamic buffer response of GaN-on-silicon transistors

Back-gated measurements on conductive silicon substrates have been performed to investigate the effect of stress voltage on the dynamic behaviour of GaN-on-silicon (GaN-on-Si) transistors. Two comparable samples were studied with the only difference being the vertical dislocation density. Results show a clear correlation between dislocation density and the ability of the GaN buffer to dynamically discharge under high stress conditions.

Effect of Different Carbon Doping Techniques on the Dynamic Properties of GaN-on-Si Buffers

The effect of different carbon doping techniques on the dynamic behavior of GaN-on-Si buffer was investigated. Intentional doping using a hydrocarbon precursor was compared with the more common autodoping technique. Breakdown and dynamic behavior of processed devices indicate that extrinsic carbon doping delivers better dynamic properties for the same blocking voltage capabilities. Modeling and simulations have revealed that charge transport across the GaN buffer is the main limiting factor during the buffer discharge process.

The effect of AlN nucleation growth conditions on the inversion channel formation at the AlN/silicon interface

In recent years, GaN grown on silicon substrates (GaN-on-Si) managed to offer a commercial solution for harvesting the advantages of GaN-based devices whilst benefiting from the well-established silicon-based infrastructure. Although having an upper hand when it comes to cost, GaN-on-Si suffers from a vertical breakdown voltage which was found to be limited by the silicon substrate [1]. Earlier investigations showed that this breakdown is related to the formation of an inversion channel at the interface between the nucleation layer and the silicon substrate [2, 3]. This inversion layer acts as a source for electron injection during the application of high positive voltages relative to the substrate, thus leading to a premature breakdown. In this work we report on the characterization of this inversion channel and demonstrate that it can be suppressed by optimizing the growth conditions of the AlN nucleation layer. To obtain a deeper understanding of the inversion layer properties, two MOCVD AlN nucleation samples were grown on highly p-doped Silicon substrates, growth temperatures between 800 °C and 950 °C, while the desorption temperature was held at 950 °C. Vertical structures were then etched, with annealed Ti/Al/Ni/Au as ohmic contact to the silicon and Ni/Au Schottky contact to the AlN.

AlGaN/GaN heterostructure field-effect transistors regrown on nitrogen implanted templates

We demonstrate the application of nitrogen (N) implantation in GaN as a current-blocking layer. In a first step, vertical current-blocking behavior was confirmed by processing quasi-vertical Schottky diodes with full-area N-implantation. The leakage current was only 10−6 A cm−2 in forward and reverse directions. Also, the regrowth of AlGaN/GaN heterostructure field-effect transistors on N-implanted and, for reference, non-implanted GaN templates is demonstrated. Even though a decrease in the mobility and sheet carrier density of the two-dimensional electron gas was observed, excellent off-state properties were achieved. Regrown devices exhibited leakage currents as low as 10−7 mA mm−1, showing very good quality of the regrowth interface. However, a detailed analysis with pulsed I–V and C–V measurements suggest an increased presence of traps due to regrowth, especially on N-implanted templates.

CMOS-compatible Ti/Al ohmic contacts (R c ° C)

Recently the development of CMOS-compatible fabrication technologies for GaN HEMTs has attracted increasing levels of interest [1]-[4]. A low temperature ohmic contact technology is required for gate-first device fabrication and CMOS-first GaN-Si integration process, however, typical ohmic contacts need annealing at > 800°C [1], [2]. In the past, we have reported an approach to realize low contact resistance (R C ) using CMOS-compatible metal schemes annealed at 500°C through an n + -GaN/n-AlGaN/GaN structure [4]. This method has a drawback that the n-doped AlGaN barrier increases the gate leakage current. In this work, we present the first low temperature (<;450°C) CMOS-compatible Ti/Al ohmic contact technology for conventional unintentionally-doped AlGaN/AlN/GaN HEMT structures.