Joff Derluyn

Improvement of AlGaN∕GaN high electron mobility transistor structures by in situ deposition of a Si3N4 surface layer

We have made AlGaN∕GaN high electron mobility transistors with a Si3N4 passivation layer that was deposited in situ in our metal-organic chemical-vapor deposition reactor in the same growth sequence as the rest of the layer stack. The Si3N4 is shown to be of high quality and stoichiometric in composition. It reduces the relaxation, cracking, and surface roughness of the AlGaN layer. It also neutralizes the charges at the top AlGaN interface, which leads to a higher two-dimensional electron-gas density. Moreover, it protects the surface during processing and improves the Ohmic source and drain contacts. This leads to devices with greatly improved characteristics.

Low On-Resistance High-Breakdown Normally Off AlN/GaN/AlGaN DHFET on Si Substrate

Ultrathin-barrier normally off AlN/GaN/AlGaN double-heterostructure field-effect transistors using an in situ SiN cap layer have been fabricated on 100-mm Si substrates for the first time. The high 2DEG density in combination with an extremely thin barrier layer leads to enhancement-mode devices with state-of-the-art combination of specific on-resistance that is as low as 1.25 m¿·cm2 and breakdown voltage of 580 V at V GS = 0 V . Despite the 2-¿m gate length used, the transconductance peaks above 300 mS/mm. Furthermore, pulsed measurements show that the devices are dispersion free up to high drain voltage V DS = 50 V. More than 200 devices have been characterized in order to confirm the reproducibility of the results.

Silicon Substrate Removal of GaN DHFETs for Enhanced (<1100 V) Breakdown Voltage

In this letter, we present a novel approach to enhance the breakdown voltage (<i>V</i>_BD) for AlGaN/GaN/AlGaN double-heterostructure FETs (DHFETs), grown by metal-organic chemical vapor deposition on Si (111) substrates through a silicon-substrate-removal and a layer-transfer process. Before removing the Si substrate, both buffer isolation test structures and DHFET devices showed a saturation of <i>V</i>_BD due to the electrical breakdown through the Si substrate. We observed a <i>V</i>_BD saturation of 500 V for isolation gaps larger than 6 μm . After Si removal, we measured a <i>V</i>_BD enhancement of the AlGaN buffer to 1100 V for buffer isolation structures with an isolation gap of 12 μm. The DHFET devices with a gate-drain (<i>L</i>_GD) distance of 15 μm have a V_BD > 1100 V compared with ~300 V for devices with Si substrate. Moreover, from Hall measurements, we conclude that the substrate-removal and layer-transfer processes have no impact on the 2-D electron gas channel properties.

AlGaN/GaN/AlGaN Double Heterostructures on Silicon Substrates for High Breakdown Voltage Field-Effect Transistors with low On-Resistance

AlGaN/GaN/AlGaN double heterostructure field-effect transistors (DHFET) with high breakdown voltage and low on-resistance were fabricated on silicon substrates. A linear dependency of the breakdown voltage on the buffer thickness and on the buffer Aluminium concentration was found. A breakdown voltage as high as 830 V and an on-resistance as low as 6.2 Ωmm were obtained in devices processed on 3.7 µm buffer thickness. The gate–drain spacing was 8 µm and the devices did not have any field plates.

AlGaN/GaN High Electron Mobility Transistors Grown on 150 mm Si(111) Substrates with High Uniformity

AlGaN/GaN high electron mobility transistors (HEMTs) grown on 150 mm Si(111) substrates are reported in this work. The sheet resistance of the AlGaN/GaN HEMT structure is as low as 260 ±3.4 Ω/. The electron mobility is in the range of 1560–1650 cm2 V-1 s-1. The crack-free mirror-like wafers were obtained by using a simple AlGaN/AlN buffer. The mechanism for dislocation reduction in GaN above the AlGaN/AlN buffer is presented in this work. The dislocation density is around (1.5–2.5)×109/cm2. Some of the wafers were processed and a current density close to 1 A/mm was achieved. The maximum transconductance was 270 mS/mm and the on-state resistance was as low as 2.6 Ω mm.

The role of Al on Ohmic contact formation on n-type GaN and AlGaN/GaN

A standard metallization scheme for the formation of Ohmic contacts on n-type GaN does exist. It has the following multilayer structure: Ti∕Al∕metal∕Au. Ti is known to extract N out of the GaN. This leaves a high density of N vacancies (donors) near the interface pinning the Fermi level. The created tunnel junction is responsible for an Ohmic contact behavior. Au is deposited as the final metal layer to exclude oxidation of the contact and the metal should limit the diffusion of Au into the layers below and vice versa. Al in the metallization scheme is known to improve the contact resistance, but the reason why has not been reported yet. We studied Ti and Ti∕Al contacts on GaN and AlGaN∕GaN as a function of annealing temperature by transmission electron microscopy. The role of Al in the metal multilayer, and of Al in the AlGaN on the Ohmic contact formation, has been determined. The latter result indicates that the standard metallization scheme for GaN cannot be simply transferred to AlGaN∕GaN structures.

Improved Thermal Performance of AlGaN/GaN HEMTs by an Optimized Flip-Chip Design

AlGaN/GaN high electron mobility transistors (HEMT) on sapphire substrates have been studied for their potential application in RF power applications; however, the low thermal conductivity of the sapphire substrate is a major drawback. Aiming at RF system-in-a-package, the authors propose a flip-chip-integration approach, where the generated heat is dissipated to an AlN carrier substrate. Different flip-chip-bump designs are compared, using thermal simulations, electrical measurements, micro-Raman spectroscopy, and infrared thermography. The authors show that a novel bump design, where bumps are placed directly onto both source and drain ohmic contacts, improves the thermal performance of the HEMT

Experimental and simulation study of breakdown voltage enhancement of AlGaN/GaN heterostructures by Si substrate removal

The breakdown mechanism in GaN-based heterostructures (HFETs) grown on silicon substrate is investigated in detail by TCAD simulations and silicon substrate removal technique. High-voltage electrical measurements show that the breakdown voltage saturates for larger gate-drain distances. This failure mechanism is dominated by the avalanche breakdown in the Si substrate. High-voltage TCAD simulations of AlGaN/GaN/Si substrate structures show higher impact ionization factor and electron density at the Si interface indicating a leakage current path where avalanche breakdown occurs. Experimentally, by etching off the Si substrate the breakdown voltage no longer saturates and linearly increases for all gate-drain gaps. We propose the silicon removal technique as a viable way to enhance the breakdown voltage of AlGaN/GaN devices grown on Si substrate.

Effect of Surface Passivation on Two-Dimensional Electron Gas Carrier Density in AlGaN/GaN Structures

The effect of surface passivation on two-dimensional electron gas (2DEG) carrier density in undoped AlGaN/GaN heterostructures is investigated by capacitance–voltage (C–V) measurements. All oxides used (SiO2, Al2O3, and Ta2O5) experienced a decrease in 2DEG carrier concentration with increasing thickness of the respective oxide layers between the gate and the AlGaN layer. In contrast, the 2DEG carrier concentration increased markedly with increasing Si3N4 layer thickness. An elementary polarization model was used to fit the behavior for all materials and thicknesses leading to quantitative results. In combination with bowing measurements, the fitting suggests that the effect of the Si3N4 on the 2DEG carrier concentration can be traced back mainly to an increase in piezoelectric polarization charge due to strain, whereby changes in the 2DEG carrier concentration caused by oxides are explained by 2DEG charge accumulation due to fixed interface charges at the AlGaN/oxide interface and in the bulk of the oxides.

Low leakage high breakdown e-mode GaN DHFET on Si by selective removal of in-situ grown Si 3 N 4

We describe the fabrication and characteristics of high voltage enhancement mode SiN/AlGaN/GaN/AlGaN double heterostructure FET devices. The Si3N4 not only acts as a passivation layer but is crucial in the device concept as it acts as an electron donating layer (1). By selective removal under the gate of the in-situ SiN, we realize e-mode operation with a very narrow threshold voltage distribution with an average value of +475 mV and a standard deviation of only 15 mV. Compared to the reference depletion mode devices, we see no impact of the e-mode architecture on the breakdown behaviour. The devices maintain very low leakage currents even at drain biases up to 80% of the breakdown voltage.