1.5 kV Power AlGaN/GaN HFETs

Abstract

AlGaN/GaN power heterostructure field-effect transistors (HFETs) on sapphire substrate with up to 1600-V breakdown voltage (VBR) for power electronic applications have been fabricated. The devices have a low on-resistance (RON) of 3.4 mΩ.cm 2 , making the best VBR-RON relationship yet reported. The power device figure of merit VBR 2 / RON = 7.5x10 8 V 2 /(Ω cm -2 ) is among the best reported values for any AlGaN/GaN HEMT in their class. As silicon power semiconductor devices fail to meet the stringent frequency and temperature requirements imposed by new concepts and applications in the field of power electronics to produce the systems with increased efficiency, the SiC and GaN technologies emerge as the solutions for the future energy-conversion systems. SiC devices, although having the advantages of mature technology, possibility of vertical field effect transistor (FET) design, high breakdown voltages of several kilovolts, are limited by relatively high on-resistance and low switching frequencies. These are the key performance parameters for power conversion devices. On the other hand, AlGaN/GaN-based technology, thanks to a high-density two-dimensional electron gas (2DEG) (above 1x 10 13 cm -2 ) and high electron mobility (above 1500 cm 2 /V.s) allows for low on-resistance high-speed HFETs. For GaN- based high voltage lateral HFET the minimal value of ON-resistance, RON=VBR 2 /(qµnEC,GaN 2 ) while for vertical SiC devices it is given as RON=4VBR 2 /(erµEC,SiC 3 ). Substituting the material parameters for SiC and GaN one can see that the ratio RON,GaN/ RON,SiC ≈ 4.5 x 10 -2 . In this paper, we present a detailed study of AlGaN/GaN HFET breakdown voltage on device geometry and compare the achieved results with the theoretical limits and the best reported values. The HFETs devices were fabricated over sapphire substrate. The wafer sheet resistance was around 350 Ω, the threshold voltage, VT =-4.5V. The ohmic contacts were formed by Ti(200A)/Al(1000A)/Ti(500A)/Au(1500A) metal stacks. These were annealed at 850 °C for 1 min. in a forming gas ambient. The source-gate spacing was 2 µm, gate - drain spacing was varying from 2 to 20 µm. The Au/Ni gates with the gate length varying from 2µm to 12µm were formed using optical lithography. No field-plates or passivation layers have been deposited on the HFETs for this study. The breakdown voltage of AlGaN/GaN HFETs was found to depend linearly on the gate-drain spacing. This behavior is possible if (i) the breakdown is limited by the surface breakdown or (ii) the width of the 2DEG depletion region expands linearly with the gate- drain spacing. Detailed experiments using surface potential profiling and breakdown voltage - residual channel current dependencies have been carried out to confirm the breakdown mechanism. From these experiments, the breakdown of the HFETs was found to be surface limited. For devices with 20µm gate-drain spacing, the breakdown voltage VBD = 1600 V was measured with the corresponding value of RON = 3.4 mΩ.cm 2 . To the best of our knowledge, these are the highest reported results for the GaN based HFETs. Since the surface effects are dominating, an importance of the passivation or the encapsulation material used to cover the device will be discussed.