The Model And Experiment Research On Double-base-epilayer4H-SiC Bipolar Junction Transistor

Silicon Carbide(SiC), the third generation wide band gap semiconductor material,has outstanding properties such as wide bandgap, high critical breakdown field, higherthermal conductivity, and high electron saturation drift velocity, and it is particularlysuitable for fabrication of devices operated in high temperature, high voltage, highpower, and high radiation environment. Among the different polytypes,4H-SiC hasbeen extensive adopted due to its predominant characteristics. Among the high powerdevices,4H-SiC Bipolar Junction Transistors(BJTs) have been a subject of more andmore study, since power BJTs based on4H-SiC epilayer could eliminate the fataldrawbacks of Si based power BJTs and are capable of handling high power with muchgreater current gain. This dissertation presents a new type of double-base-epilayer4H-SiC BJTs, and the design, fabrication and testing of the typical BJTs has beendemonstrated. The main studies and contributions of this dissertation are as follows:1. The study on the model parameters of4H-SiC and the working principle of theBJTs. The basic characteristics of the4H-SiC material have been investigated based onestablishing the material models including bandgap, mobility, incomplete ionization,and the relationship between the model parameter and the temperature. At the same time,the basic working principle of the Bipolar Junction Transistors is analyzed, and thedevice parameter including common emitter current gain, breakdown voltage, and thebase transit time are calculated. Also several key factors that affect the direct currentcharacteristics of the BJTs are shown.2. The study on the base transit time of the double base epilayers of the BipolarJunction Transistors. On the purpose of improving the DC current gain, a noveldouble-base-epilayer BJTs is proposed creationarily based on its current transportprocess. According to the drift-diffusion theory, the analytical model of the base transittime τBof this structure has been established in this dissertation. And the baseconfiguration of this typical device has been investigated selectively based on thismodel. Followed by the orthogonal experimental results, the device parameters of thebase region including the doping concentration and the thickness have been optimized.The research results indicate that the build-in electric field formed in the base region canevidently improve the direct current performance of the double-base-player BJTs. Thecommon emitter current gain can be improved to over70.3. The research on the junction termination technology of the double-base-epilayer BJTs. In order to increase the breakdown voltage, two junction termination structureshave been introduced in this device configuration. The parameters including dopingconcentration, the depth of the ion implantation, the implantation dosage and energy arecalculated for different JTT structure. The research results show that the JTE canalleviate the strength of the electric field at the mesa corner obviously and increase theblocking voltage. The improved blocking voltage can be reached to2400V. At the sametime, several parameters that influence the device performance such as the distancebetween the base ohmic contact and the emitter mesa and the configuration parameter ofthe collector drift region are also investigated.4. Two other structures of the BJTs are also studied. These two new configurationsare gradely base BJTs and metal emitter BJTs. By calculating the base transit time at thecondition of the exponential base doping profiles and studying the schottky contacttheory, the common emitter current gain and the blocking voltage characteristics aresimulated particularly. Meanwhile, the device performances at different temperatureconditions are investigated.5. The experimental research on the4H-SiC double-base-epilayer BJTs. Based onthe theoretical achievements antecedent, the layout is designed and the fabrication isproceeded. An improved method that controls the precision of the ICP etching is putforward. Then the practical devices are fabricated on a four epilayers grown by CVD.At last the electrical characteristics including common-emitter current gain, ohmiccontact resistance and blocking voltage are tested. The measured results show that themax gain is16.8when IC=28.6mA(JC=183.4A/cm2) and it increases first and thendecreases with the collector current density increasing. The specific on-resistance(Rsp-on)is32.3m·cm2and the blocking voltage with open base is410V.