Device Process And Electrical Properties Of GaN MOSFETs Assisted By Dry-Etching Process

Gallium nitride (GaN) has excellent physic properties, such as its wide bandgap, high breakdown field, high electron saturation velocity and so on. These properties made GaN an ideal material for high power, high frequency and high temperature devices. Compared with AlGaN/GaN high electron mobility transistor (HEMT) device, GaN metal-oxide-semiconductor field-effect transistor (MOSFET) has advantages of lower gate leakage, wider dynamic gate voltage range etc.,which make GaN MOSFET become one of the most attractive research areas. However, to realize GaN MOSFET with the typical Si MOSFET structure is quite difficult for the low quality of the substrate p-GaN and narrower bandgap of its native oxide Ga2O3. Athough the GaN MOSFET on AlGaN/GaN heterostructure is an interesting candidate, the fabrication process should be further optimized and the device characterization should also be much careful due to problems caused by the dry etching process in this structure.To solve these problems, four parts of research were done to study the GaN MOSFET, they are 1) device and process design of GaN MOSFET on AlGaN/GaN heterostructure; 2) study of the accurate mobility characterization method of GaN MOSFET; 3) process optimization for GaN MOSFETs on AlGaN/GaN heterostructure; and 4) device and process design of self-aligned GaN MOSFETs and GaN HEMTs.(1) Device design of GaN MOSFET on AlGaN/GaN heterostructure is elaborately illustrated including the layout design and fabrication process design. In the end of this chapter, some preliminary experiments on the dry recess process are done including the etching gas flow rate, etching protection mask and the etching chamber pressure. Atomic force microscope (AFM) was used to investigate the etching profile, surface roughness etc. It is found that the etched surface would become much rougher at higher gas flow rate. Etched with gas flow of 6 sccm, the suface even showed granular hillock with height of more than 20 nm. PR masked samples showed stronger trenching effect and rougher surface. Also, the deposition effect is found in the condition of higher chamber pressure which is not beneficial to obtain a clean GaN surface. Finally, a relatively optimum dry recess condition with SiO2 etching protection mask, etching gas flow of 3 sccm, etching chamber pressure of 0.25 pa was confirmed.(2) The problems in mobility characterization of GaN MOSFETs were analyzed based on our experiments. It is found that even in the same sample, the extracted channel mobility will be quite different with different device pattern when using the traditional method. A phenomenon of parallel channel caused by worse field isolation at the gate pad outside the channel was found in bar-type GaN MOSFETs based on AlGaN/GaN heterostructure. It will result a phenomenon of two-piece mobility and finally lead an overestimation on the mobility extracted by the C-GM method. Also, the variation of channel length extracted by electrical measurement was found. It will lead an obvious underestimation on mobility, especially in the case of short channel MOSFETs. To characterize the channel mobility precisely, we have verified and analyzed these phenomena and presented several improved methods to characterize the mobility of MOSFETs. The mobility extracted by our method agreed quite well with that extracted from a long channel ring type MOSFET which was thought to be reasonable showing theses method are effective to obtain the correct value of the channel mobility. In the end of this chapter, the interface state density extraction method was given a brief introduction on both I-V method of MOSFET and C-Vmethod of MOS capacitor.(3) Process optimization including the etching gas, etching bias power, etching protection mask, oxide type, oxide thickness of the GaN MOSFETs are investigated and analyzed. The charges near the SiO2/GaN interface of the GaN MOSFETs with different etching conditions were evaluated. It is found that stronger bombard damages in dry process will bring more charges near the interface and finally make the threshold voltage of the device becoming more negative. The effects of nitrogen plasma treatment and ammonia water treatment were investigated. These treatments are effective and can recover or remove the dry etching damaged layer. An E-mode GaN MOSFET with the maximum field-effect mobility of 148.12 cm2/Vs was realized by ammonia water treatment. GaN MOS capacitors were also prepared to investigate the influence of these treatments on the interface state densities using Terman method. The corresponding interface state density for the ammonia water treated sample was around 3×1011 cm-2eV-1 in the EC-ET range from 0.2 to 0.6 eV.(4) The drawbacks of GaN MOSFET on AlGaN/GaN structure was analyzed. Then a series of non-annealing and low temperature ohmic process was developed on both GaN and AlGaN. Gate-first GaN MOSFET, Self-aligned GaN MOSFET and AlGaN/GaN HEMT were fabricated using these non-annealing and low temperature ohmic formation process assisted by ICP dry etching system. The gate-first GaN MOSFET showed good pinch-off characteristics with channel mobility of 163.8 cm2/Vs. The self-aligned GaN MOSFET and AlGaN/GaN HEMT showed output characteristics and also some non-ideal effect demonstrating that the fabrication process should be further improved and optimized.