New Structures And Field Optimization Techniques For High Voltage AlGaN/GaN HEMTs

For power electronics applications, to break through the material limits of siliconand to realize the drastic performance improvement, Wide Band Gap (WGB) compoundsuch as SiC and GaN have attracted much attention because of their superior physicalprosperities. AlGaN/GaN High Electron Mobility Transistors (HEMTs) are promisingcandidates to replace the existing silicon devices in High Voltage Integrated Circuit(HVIC) due to their AlGaN/GaN heterojunction, high breakdown field, high mobility2D electron gas (2-DEG), excellent thermally stability, high saturation velocity and easyto realize than that of SiC counterpart. As in case of the power semiconductor devices,blocking capability is one of the most important features. For lateral power devices, thechallenge of achieving high breakdown voltage with the minimum Ron_sp dictates anoptimal field distribution in the drift region. Major efforts are being carried out toimprove the OFF-state blocking capability. However, most of these works lacknumerical simulations and theoretical analysis.In this dissertation, to optimize two-dimensional electric field in AlGaN/GaNHEMTs, spontaneous-polarization and impact ionization model are proposed by usingCrosslight-APSYS TCAD tool. Surface and bulk electric field distributions are obtainedby solving2-dimensional and3-dimensional Poisson equations. In order to furtherenhance the breakdown voltage and suppress schottky gate premature breakdown, somekinds of novel structures are simulated and experimentally verified in detail, including:1.AlGaN/GaN HEMT with source-connected field plate: The proposed structurecan re-shape the surface electric field distribution to enhance the breakdown voltage.Simulation results show that breakdown voltage of150V was obtained with a driftregion length of6μm, a gate length of0.8μm, a GaN buffer thickness of2μm.Breakdown voltage of the conventional HEMT without field plate is60V. Experimentwas also carried out in order to calibrate with the simulation results. Experimentalresults show that the breakdown voltage of the optimized HEMT with conventionalsource-connected field plate is125V, which is237%better than the conventionalHEMTs without field plate (37V). Numerical analysis exhibits good agreement and the same trend as the fabrication results. Furthermore, breakdown mechanism is studied byusing temperature dependant measurement.2.Breakdown voltage enhancement for GaN High Electron Mobility Transistorswith localized Mg doping: a localized Mg buried layer is introduced underneath the2-DEG channel. The Mg layer can help deplete the2-DEG without introducing theforward ON resistance. The introduced Mg layer can modulate the electric fielddistribution. After optimization, the peak electric field shifted from the surface drainside gate edge to the bulk Mg layer edge. Simulation results indicated that900V of thebreakdown voltage was obtained with the gate length of1μm, a drift length of10μm, aGaN buffer layer of3μm. Breakdown voltage of the conventional HEMT is560V,which is61%lower than that of the Mg doped one. Mg doped HEMT with drain metalextension structure is also proposed to further optimize the surface electric fielddistribution. Simulation results showed that breakdown voltage of1390V was achievedwith drain metal length of3μm, which can be translated into55%breakdown voltageimprovement than the Mg doped only structure.3.Reduced Surface Electric Field was proposed for AlGaN/GaN HEMTs.Compared to the HEMT with conventional source-connected field plate and combinedwith source and gate field plate structures, drain side gate edge of surface electric fieldfor HEMT with Mg doped layer is reduced by5×and3×, respectively.4.A high voltage AlGaN/GaN HEMT with a source-connected Air-bridge FieldPlate: The device features a metal field plate that jumps from the source over the gateregion and lands between gate and drain. The Air-bridge Field Plate can minimize theparasitic gate to source input capacitance and channel resistance. The new HEMT alsoexhibits higher OFF-state breakdown voltage and one order of magnitude lower of drainleakage current. In a device with gate to drain distance of6μm, gate length of0.8μm,three times higher forward blocking voltage of375V was obtained at VGS=5V whencompared to that of an optimized conventional Field Plate device (125V). The specificON resistance for the device with the proposed air-bridge field plate is0.58m·cm~2atVGS=0V, which compares favorably with0.79m·cm~2for the device with aconventional Field Plate. Numerical analysis was also carried out to calibrate thefabrication results. Simulation results showed450V of the breakdown voltage for theHEMT with air-bridge field plate while the conventional HEMT with the breakdown voltage of only65V.