Research On AlGaN/GaN-on-Si Power Diodes

Compound semiconductor material GaN features the characteristics of wide bandgap,high critical breakdown electric field strength and stable chemical properties.It is one of the ideal materials for the fabrication of the next generation power semiconductor devices.Two-dimensional Electron Gas(2DEG)formed at the AlGaN/GaN heterointerface has the characteristics of high concentration and high mobility,therefore the heterostructure becomes a basic structure for GaN power semiconductor devices.At present,silicon-based GaN(GaN-on-Si)technology is the mainstream technology for GaN power devices,which has the dual advantages of low material cost and compatibility with silicon-based CMOS process.As a kind of critical power semiconductor devices,GaN power diodes are expected to achieve new breakthroughs on silicon-based GaN technology platform.In the thesis,silicon-based AlGaN/GaN Schottky barrier diodes(SBDs)and current regulating diodes(CRDs)are studied,and the the monolithic integration of them are explored.Traditional AlGaN/GaN SBDs have relatively high turn-on voltage(i.e.,>1 V)and low breakdown voltage(i.e.,<1 kV).To address these drawbacks,the device structure and etching process are studied.On the other hand,GaN CRDs have not been reported so far.The device is studied in the thesis.The main research contents and achievements are as follows:(1)A low temperature inductively coupled plasma etching technique for GaN was proposed to significantly suppress lateral etching.The technique is based on the enhancement of the etching resistance of electron beam photoresist at low temperature,which greatly reduces the lateral etching rate and significantly suppress the lateral etching occurring at room temperature.The lateral etching width decreases from 80 nm at room temperature to 5 nm at 5 ~oC.Combining with the electron beam lithography technique,the electron beam photoresist with thickness of only 200 nm can be directly used as the etching mask.The high quality variable nanotrench structure of GaN has been successfully realized by the single-step etching,which lays the foundation for the device preparation.(2)A novel variable nanotrench anode AlGaN/GaN SBD has been proposed and high performance devices have been successfully fabricated based on the developed low temperature etching technique.The results measured by conductive atomic force microscopy(C-AFM)show that the electron density in the variable nanotrench region presents a lateral gradient distribution.Under forward bias,the variable nanotrench anode structure can make the current distribution more uniform.At the same time,under reverse bias,the variable nanotrench anode structure can reduce the peak surface electric field in the anode region.Compared with the traditional single trench anode AlGaN/GaN SBD,the heavy current conducting capability and reverse blocking voltage of the device are increased by 10%and 50%,respectively.The turn-on voltage of the fabricated diode is0.61 V,the on-resistance is 7.8??mm,and the maximum breakdown voltage is 1317 V.Pulse measurement results show that the current collapse of the AlGaN/GaN SBD can be mitigated by the variable nanotrench anode structure without surface passivation.(3)The pioneering work on AlGaN/GaN CRD has been conducted.A hybrid cathode structure was designed,the constant current mechanism of the device was revealed,and a series-resistance model for the device was proposed.The AlGaN/GaN CRD with better performance than silicon-based countpart has been demonstrated.The maximum forward operating voltage is beyond 400 V,the temperature coefficient of the output current is as low as-0.3%/~oC,and the dynamic response time is less than 100 ns.(4)An AlGaN/GaN reverse blocking CRD(RB-CRD)is proposed,which extends the type of CRD.Based on the integration of the aforementioned AlGaN/GaN SBD and CRD,the RB-CRD not only has the forward constant current characteristic of the traditional CRD,but also has the reverse blocking capability,which is hopeful to open up a new application field of CRD.