Study On GaN Heterojunction Bipolar Transistor And Related Technology

The wide-bandgap GaN material system has attracted much attention for high power microwave and millimeter wave device applications due to superior electronic and material properties. In this paper, nitride-based heterojunction bipolar transistor (HBT) and related techniques are discussed. BecauseⅢ-Ⅴnitride HBT has better device linearity, high temperature and high power properties, it has been considered as an ideal device in the future. Although GaN material is an excellent candidate for high frequency and high power applications, there are a series of difficulties to be conquered in both aspects of theory and experiment. At present the study of GaN-based devices is in the primary phase, especially for GaN HBTs, many mechanisms of device operating haven't last word. Under the condition, these problems, which exists in the research of GaN HBT are discussed from the view of theory and experiment.First, the paper introduces the basic theories of heterojunction bipolar transistor. The bandgap structures, the distribution of potential and electric field of space-charge area and the transportation of currents are described in detail.Using the combination of Thermionic Emission Theory and Diffusion Theory, we calculate the discontinuity in quasi-fermi levels at the interface of the heterojunction and the Shockley Boundary Conditions which are for heterojunction. Then theⅠ-Ⅴcharacteristics of ideal abrupt heterojunctions and heterojunction bipolar transistors is given.The device simulation offers the direction for the device design, which is very important in the process of device development. So the GaN HBT is numerical simulated. The influences brought by the polarization effect and graded emitter are studied. The results show that polarization effect has a little affection to the characteristics of Ga-face terminated device, while the graded emitter can eliminate the peak of conduction band and improve the inject efficiency. Then we discuss the problems to which attention should be paid in the vertical design (emitter, base and collector design).GaN-based devices are different from Si'. To the former, the growth of material is a very important process since they are in-situ doped. The paper introduces the growth process flow of GaN HBT material in detail, and analyzes these materials, especially the p type GaN material. It has been resulted that the p-GaN has good crystal quality and electrical characteristic. The ratio of Ga(Mg) to N is 1.106. The defect density is about 3×1018cm-2.In the fabrication of GaN HBT, the non-ohmic contact to etched p-GaN is a big hurdle. To this point, the contact to etched p-GaN is discussed. Firstly, we optimize the etching conditions to reduce the etch damage as low as possible; and secondly, we also discuss the methods of repairing and reducing the etching damage.The paper discusses the kernel problems in the fabrication of GaN HBT. The device layouts, the fabrication and characteristic analysis of CB junction are also shown. The turn-on voltage is about 4V, and the breakdown voltage is above 50V. Besides, this paper gives the pprocess flow of GaN HBT. We trial-produce the GaN HBT device and test the characteristics. It is shown that the poor doping of base, too thin base and over-etching in the process, Mg memory effect and diffusion under high temperature lead to theⅠC-ⅠCE curves show resistance characteristic.Moreover, characteristic of SiCOI (SiC On Insulator) MESFET is also investigated using the device simulation platform set up in this paper. A new type of SiCOI MESFET device structure, SiCOI MESFET with dielectric groove isolation, is presented, and the device structure is optimized. We see SiCOI MESFET with multi-step dielectric groove isolation is a better structure, of which a large decrease of saturated drain current and transconductance doesn't occur while a large increase of the breakdown voltage can be achieved.