Novel Structure And Characteristics Study For 4H-SiC BJT Power Devices

Silicon carbide (SiC) is an attractive wide band-gap semiconductor material with the relatively mature material growth and device fabrication technology for high-temperature, high-frequency, anti-irradiation and high-power applications. In the field of power semiconductor device, 4H-SiC bipolar junction transistors (BJTs) are emerging as a promising technology for military and civilian applications in the power electronics system retaining the advantages of high power and stability in high temperature. However, although these performances are very promising, three limitations have direct confinement on the sustained growth of 4H-SiC BJT, that is low common emitter current gain to increase the power dissipation, low breakdown voltage to restrict the applied range, and trapping effects degenerating the performance to affect the working stability.The dissertation begins with improving the performance of 4H-SiC BJTs. Two new structures are proposed, which are n-type buried layer structure in the base to improve the common emitter current gain, and epitaxial junction termination structure to improve the breakdown voltage. And the interface states distribution model to study the trapping effect has been developed. And the electro-thermal properties of 4H-SiC BJT have been studied in high and low temperature. Meanwhile, the experimental research has been proposed, which are the filed plate of 4H-SiC SBD and BJT-BSIT on the silicon substrate with good thermal stability of current gain. The detail contributions of the dissertation are listed as followings:1. Novel 4H-SiC BJTs with n-type buried layer in the base: The proposed structure is attributed to the fact that the creation of built-in electric field to be strengthened by p-n junction has decreased the minority carriers transport time in the base to reduce the base recombination and thereby the collector current is increased, the same as the current gain. Furthermore, the charges in the buried layer modulate the electric-field to increase the breakdown voltage. Numerical simulations have indicated that the current gain shows a 108% increase compared to the conventional structure with the properties of high voltage and process compatibility. 2. Interface states distribution model of 4H-SiC BJTs: Two-dimensional analysis of the surface states effects modeled by the exponential function distribution in the forbidden band for 4H-SiC BJT (Bipolar Junction Transistor) is studied. The experimental results are well-matched with the simulation, which is modeled by exponential distributions of the interface state density replacing the single interface state trap. Simulation result indicates the mechanism of current gain degradation, which is surface Fermi level pinning leading to a strong downward bending of the energy bands to form the channel of surface electron recombination current. Meanwile, acceptor-type traps effect has strengthened the surface charge scattering to reduce the electron mobility and thus increase on resistance.3. Based on the surface charge modulation effect, two kinds of novel juntion termination structures are proposed:(1) Novel 4H-SiC BJTs with the epitaxial junction termination structure: The proposed structure has accomplished of the epitaxial junction termination structure to substitue fabircation with the the JTE (Junction Termination Extension) by the method of the ion implantation to eliminate the curvature effect. Furthermore, FFLRs (Floating Field Limiting Rings) have been implanted in the different zones of epitaxial JTE to achieve the different impurity level at the edge of main junction and JTE to provide a uniform distribution of surface electric field. The breakdown voltage of the proposed structure can be achieved 1570V compared to the conventional FFLRs increase 39%, to the conventional JTE increase 20%. Furthermore, the novel structure is not only compatible with the conventional process of 4H-SiC BJT but also saving the common junction termination process to realize the process steps simplified and fabrication cost reduced.(2) Novel 4H-SiC SBDs with step field plate(FP) structure: Based on the fabrication of conventional FP 4H-SiC SBDs ,novel step FP, combination of step FP and JTE, combination of groove step FP and JTE are proposed. The three novel structures have been etched to form different oxide thickness at the edge of main junction and field plate. The later two structures make the uniform surface peak field by the method of assisted JTE. Numerical simulation results optimizated show that three structures can increase breakdown voltage by 15%,23% and 92% , compared to the common FP structure which highest breakdown voltage is 1300V in the experiment. Based on simulation analysis of three structures, process design, device layout and process experiment have been carried out.4. Electro-thermal properities and experiment study for 4H-SiC BJT and Si BJT-BSIT(Bipolar Static Induction Transistor): This dissertation studies the electro-thermal properities (static and switching characteristics) of 4H-SiC BJT on the high and low temperature work conditions. Numerical simulation results show that the ionization rate of acceptor impurity at the base region enhanced to reduce the emitter injection efficiency and the current gain, the degenerated electron mobility can improve the on-resistance, and the degenerated impact ionization rate can increased the device breakdown voltage. This issue shows that 4H-SiC BJTs switching time and switching loss significantly increases in high temperature.Finally, this dissertation also proposes an electro-thermal analytical model and makes an experimental study for the BJT-BSIT compound device in the low current operation firstly. A best thermal compensating factor that indicates the relationship between the thermal variation of current gain and device structure is proposed. This is important for the design of compound device to be optimized. Furthermore, the model is found to be in good agreement with numerical simulation and experimental results. The test results demonstrate that thermal variation of current gain is below 1367ppm/℃in high temperature and below 2013ppm/℃in low temperature, which is less than 7000ppm/℃of silicon BJT to realize the effective temperature compensation.