A self-aligned "slant-field-plate" technology is presented as an improvement over the discrete multiple field plates for high breakdown voltage AlGaN/GaN HEMTs. Devices were tested in Fluorinert to eliminate the breakdown of air, which was identified to limit the breakdown voltage in AlGaN/GaN HEMTs. A single integrated field plate, which is self-aligned with the gate, is shown to support more than a kilovolt breakdown voltage (V br up to 1900 V was measured with Fluorinert). Devices made with this technology show a good large signal-frequency behavior. Various issues regarding breakdown measurements and interpretation of measurement results are presented.Index Terms-Breakdown voltage, field plate, GaN, high electron mobility transistor (HEMT).

Abstract-High-voltage Al 0 22 Ga 0 78 N-GaN high-electron mobility transistors have been fabricated using multiple field plates over dielectric passivation layers. The device breakdown voltage was found to increase with the addition of the field plates. With two field plates, the device showed a breakdown voltage as high as 900 V. This technique is easy to apply, based on the standard planar transistor fabrication, and especially attractive for the power switching applications.Index Terms-Breakdown voltage, field-effect transistors (FETs), field plates, high-electron mobility transistors (HEMTs), passivation, power electronics.

We investigated the suitability of ZrO2 as a high-k dielectric for GaN material systems. Thin Zr films (4nm) were deposited by electron-beam evaporation at room temperature on n-type GaN and Al0.22Ga0.78N(29nm)∕GaN high electron mobility transistor (HEMT) structures. The Zr-coated samples were subsequently oxidized at temperatures in the range of 200–400 °C in an ozone environment. Atomic force microscopy studies after oxidation show that the ZrO2 forms a conformal layer on the underlying GaN template. Cross-section transmission electron microscopy studies showed little intermixing of the ZrO2 with the AlGaN∕GaN. The relative dielectric constant of the ZrO2 was determined to be 23. In comparison with HEMTs with bare gates (no dielectric between the gate metal and AlGaN), the HEMTs with ZrO2 showed two to three order of magnitude reduction in gate leakage current. Optimization of the HEMT process on sapphire substrates with ZrO2 under the gates yielded devices with powers of 3.8W∕mm and 58% power-added efficiency at 4GHz.

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In this paper, the structure and processing of AlGaN/GaN high electron mobility transistors have been optimized for maximum small signal gain at high frequencies. The effect of the gate resistance, gate-to-drain capacitance and output conductance on the power gain cut-off frequency, f max , of the devices has been experimentally studied. The reduction of the gate width allowed a 4-fold decrease in gate resistance which resulted in almost a 100% increase in f max . To minimize C gd , Γ-shape gates have been processed instead of the conventional T-shape submicron gates. Finally, to reduce the output conductance, the confinement of the 2-dimensional electron gas was increased by using an ultra-thin InGaN layer below the GaN channel. This InGaN back-barrier caused a 2-fold improvement in output conductance which allowed a 20% increase in f max . By optimizing all these parameters, AlGaN/GaN transistors with a record f max of 230 GHz were obtained.