Domenica Visalli

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.

Record Breakdown Voltage (2200 V) of GaN DHFETs on Si With 2-

In this letter, we present a local substrate removal technology (under the source-to-drain region), reminiscent of through-silicon vias and report on the highest ever achieved breakdown voltage (V_BD) of AlGaN/GaN/AlGaN double heterostructure FETs on a Si (111) substrate with only 2-μm-thick AlGaN buffer. Before local Si removal, V_BD saturates at ~700 V at a gate-drain distance (L_GD) ≥ 8 μm. However, after etching away the substrate locally, we measure a record V_BD of 2200 V for the devices with L_GD = 20 μm. Moreover, from Hall measurements, we conclude that the local substrate removal integration approach has no impact on the 2-D electron gas channel properties.

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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.

Buffer Thickness by Local Substrate Removal

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.

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

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.

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

III-Nitride materials are very promising to be used in next-generation high-frequency power switching applications. In this letter, we demonstrate the performance of normally off AlGaN/GaN/AlGaN double-heterostructure FETs (DHFETs) using a boost-converter circuit. The figures of merit of our large (57.6-mm gate width) GaN transistor are presented: RON * QG of 2.5 Ω·nC is obtained at VDS = 140 V. The switching performance of the GaN DHFET is studied in a dedicated high-frequency boost converter: both the switching times and power losses are characterized. We show converter efficiency values up to 96.1% at 500 kHz and 93.9% at 850 kHz at output power of 100 W.

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

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.

A 96% Efficient High-Frequency DC–DC Converter Using E-Mode GaN DHFETs on Si

We demonstrate a high-voltage low on-resistance AlN/GaN/AlGaN double heterostructure grown by metal–organic chemical vapor deposition on a silicon (111) substrate using a total buffer thickness of less than 2 µm. A fully scalable local substrate removal technique was developed to dramatically enhance the off-state breakdown voltage of the transistors. The three-terminal breakdown voltage of these devices using a gate–drain distance of 15 µm increased significantly, from 750 V to 1.9 kV, after local substrate removal. The high two-dimensional electron gas carrier density (2.3 × 1013 cm−2) associated with the low sheet resistance enables a record combination of a specific on-resistance (1.6 mΩ cm2) and high breakdown voltage for GaN-on-Si transistors.

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

We report on the first measurement results to obtain over 2 kV breakdown voltage (VBD) of GaN-DHFETs on Si substrates by etching a Si Trench Around Drain contacts (STAD). Similar devices without trenches show VBD of only 650 V. DHFETs fabricated with STAD technology show excellent thermal performance confirmed by electrical measurements and finite element thermal simulations. We observe lower buffer leakage at high temperature (100°C) after STAD compared to devices with Si substrate, enabling high temperature device operation.

1900 V, 1.6 mΩ cm2 AlN/GaN-on-Si power devices realized by local substrate removal

We investigated the limitations of the field plate (FP) effect on breakdown voltage <i>V</i>_BD that is due to the silicon substrate in AlGaN/GaN/AlGaN double heterostructures field-effect transistors. In our previous work, we showed that in devices with large gate-drain distance (L_GD > 8 μm), the breakdown voltage does not linearly increase with <i>L</i>_GD because of a double leakage path between the silicon substrate and the metal contacts, which makes the device break at the silicon interface. In this paper, we showed that the effect of the FP for such large <i>L</i>_GD is not significant because the breakdown is still dominated by the silicon substrate. The increase in <i>V</i>_BD due to the FP is significant only for devices with small gate-drain distances (<i>L</i>_GD <; 8 μm). Indeed we show that for such small <i>L</i>_GD the increase in the breakdown voltage is more than double, whereas for larger <i>L</i>_GD, this is only about 10%. Simulations of AlGaN/GaN/AlGaN devices for small <i>L</i>_GD are carried out with different FP lengths and passivation thickness in order to study the electric field distribution.