Investigation On Breakdown Characteristics And New Structures Of AlGaN/GaN HFET

Comparing with the conventional semiconductors, e.g. Si or GaAs, the wide-bandsemiconductor material gallium nitride （GaN） show the advantages in critical electricalfield, electron saturation velocity, thermal stability and anti-radiation capability.Moreover, a two-dimensional electron gas （2DEG） can be induced at the interface ofGaN and AlGaN by polarization effect. The2DEG has high electron sheet density andhigh electron mobility, thus is perfect for the application as transistor channel. Becauseof the above merits of GaN, the heterojunction field effect transistors （HFETs） t hatutilizing AlGaN/GaN heterostructure can achieve high breakdown voltage and lowon-state resistance at the same time. Therefore, the GaN-based HFETs are considered asone of the most promising candidates in power electron devices. However, severaltechnical issues that related to breakdown voltage, on-state resistance and output powerof AlGaN/GaN HFETs are still remaining to be solved. In order to address theseproblems, the electrical field distribution of AlGaN/GaN HFETs with plate field wasmodeled, and the device breakdown characteristics at various temperatures and highelectrical stress were studied. Moreover, several novel structures of AlGaN/GaN HFETswere proposed and optimizations were applied accordingly. The outline of researchtopics and main conclusions of the dissertation are listed as follows.（1） The relationship between lateral electrical field distribution along the channeland device parameters of an AlGaN/GaN HFET with field plate was studied. By solvingPoisson equation, a model for channel electrical field distribution is established. Themodel is applicable for both source-and gate-connected field plate structures. Themodel shows good agreements with numerical simulation and experimental data. Basedon the results from the model, optimization guidelines were put forward to maximizebreakdown voltage of the devices. Analytical expressions for the optimal devicestructure parameters were obtained.（2） The electrical characteristics of AlGaN/GaN transmission line measurementstructures and AlGaN/GaN HFETs were measured at various temperatures. The resultsindicate that the device performance degradation is mainly due to the decrease of2DEG mobility. The2DEG mobility at200°C is only34%of that at room temperature.The breakdown characteristics of passivated AlGaN/GaN HFETs were also studiedat various temperatures and high-field stress. It was found that the breakdown voltageincreases monotonically with the temperature. Moreover, the breakdown voltageincreases after high-field stress is applied, indicating that surface traps still take effectafter surface passivation. The experiment results indicate that surface traps cannot becharged unless the stress time is larger than300s. A critical electrical field is alsorequired to charge surface traps. The value of the critical electrical is0.12MV/cm atroom temperature, and it decreases to0.09MV/cm when200°C is applied.（3） To improve the electrical field uniformity in the RESURF AlGaN/GaN HFET, anovel RESURF AlGaN/GaN HFET with back electrode was proposed. When the deviceis reverse biased, negative charges are induced by the back electrode and thus thechannel electrical field distribution can be modified. This leads to better uniformity ofchannel electrical field and, as a result, higher breakdown voltage. The simulationresults show that the breakdown voltage of RESURF AlGaN/GaN HFET with backelectrode can be as high as1701V, comparing with the1118V of the conventionaldevices. Meanwhile, the as-proposed device structure has negligible negative impact onthe on-state resistance.（4） Vertical AlGaN/GaN HFETs （AlGaN/GaN VHFETs） structure was studied bysimulation and the parameter optimization was performed. It is found that the currentblocking layer （CBL） is critical to determine the breakdown mechanism and breakdownvoltage. The optimal thickness of CBL （TCBL-opt） has a relationship with CBL dopingconcentration （NCBL）, which is TCBL-opt=1/（0.18+0.69NCBL）, where the unit of NCBLis1017cm^-3. While CBL thickness is larger than TCBL-opt, the device breakdown is due toavalanche ionization; otherwise the device breakdown is caused by leakage current.Leakage current induced breakdown should be avoided in device structure design. Thestructure parameters were optimized according to the above mechanisms and theoptimal device parameters were obtained for maximized figure of merit.（5） To solve the CBL leakage problem due to low activation rate of p-type dopantin GaN, an AlGaN/GaN VHFET structure with polarization-doped current blockinglayer （PD-CBL） was proposed. In the AlGaN PD-CBL, the p-type dopant activation ratecan be improved by the polarization electrical field induced by graded Al fraction. Thus, the leakage current suppression effect by PD-CBL is enhanced and the devicebreakdown voltage increases. The simulation result shows that the breakdown voltageof AlGaN/GaN VHFET with0.5-μm-thick PD-CBL （the Al content increases linearlyfrom0to0.5） increases by35%compared with that of the conventional device. Inaddition, because the2DEG formed at the buffer/PD-CBL interface can preventdepletion region from extending to the buffer, the device on-state resistance does notdegraded.（6） To alleviate the electrical field lowering effect in the buffer regarding to thedistance from the buffer/CBL interface, an AlGaN/GaN VHFET structure with p-typeburied layers was proposed. As the p-type buried layer is introduced in the n-type GaNbuffer, peak electrical field is formed in each of the reverse biased p-n junction interface.This improves the electrical field uniformity and thus the breakdown strength of thebuffer layer effectively. When the buffer layer thickness is14.5μm and the dopantconcentration is1×10¹⁶cm^-3, the device breakdown voltage can be boosted to3100V,much higher than the1846V of the conventional device.（7） To further improve the breakdown voltage of AlGaN/GaN VHFET, asuper-junction AlGaN/GaN VHFET structure was proposed. In the as-proposedstructure, the p-n-p super-junction is along the lateral direction of the buffer. When thedevice is reserve biased, the electrical fields in the p-type regions and n-type regions arecompensated by each other, and a uniformly distributed electrical field is obtained in thebuffer. As a result, higher breakdown voltage is expected. For the device with9.5-μm-thick buffer, the figure of merit is6.8GW/cm², which is much higher than thatof conventional AlGaN/GaN VHFETs.