Theoretical Model And Novel Structures For RC-IGBT

The Insulated Gate Bipolar Transistor( IGBT) has MOS( Metal Oide Semicondutor)gate control capability and Bipolar conductivity modulation effect. However, it is a uni-directional conduction device which needs an anti-parallel diode to conduct the reverse current, and to suppress the additional Voltage which is generated by the parasitic inductance in the circuit. In recent years, by incorporating partial N-Colllector to replace the P-Collector, the Reverse Conducting Insulated Gate Bipolar transistor(RC-IGBT)successfully integrates the Free-Wheeling Diode(FWD) and IGBT into a single silicon chip. It not only has the bi-directional conduction capability but also enhances the reliability between the IGBT chips and FWD chips due to the absence of bonds.However, due to the introduction of the N-Collector, the electron will flow directly to the N-Collector at low applied voltage. The P-Collector will need higher current density and applied voltage to inject holes to the drift. That will induce a transition process from the single electronic conduction mode to the bipolar conduction mode for the RC-IGBT, which leads to a negative resistance(Snapback) phenomenon in the output current-voltage curve, and ultimately induces the reliability problems. So an analytical model to analyze the physical mechanism is essential to address the problems. On the basis of the models new structure is a good method to suppress the Snapback phenomenon. At the same time, the breakdown voltage and the current distribution of the RC-IGBT will also be changed due to the incorporation of the N-Collector. So the breakdown leakage current model and the conduct current distribution mode will be important to enhance the comprehension of the physical mechanism of the RC-IGBT. In addition, the integrated FWD in the conventional RC-IGBT needs to be optimized to match the IGBT chip in the switching process, and to improve the performance especially the frequency of the integrated devices. Thus a low on-state voltage drop and superior dynamic property with short reverse recovery time trr, low reverse recovery current IRRM, and soft reverse recovery characteristics of the FWD are needed.So, in this dissertation, the physical mechanism of the RC-IGBT is studied in details. The mechanism and analytical model of the Snapback phenomenon in the forward conduction is mainly investigated, and the bareakdown leakage current model and the current distribution model are also proposed to describel the physical process of the RC-IGBT. Then the corresponding new structures such as the Double Anodes RC-IGBT、the Separated RC-IGBT and the Junction Isolated RC-IGBT are proposed to solve the Snapback phenomenon. The main content can be concluded into three parts as listed in the following:(1)The analytical model of the Snapback phenomenon for the RC-IGBT is proposed, it will ont only enhance the comprehension of the physical mechanism of the Snapback, but also it will indicate the way to eliminate the problem. From the VPN voltage model for the P-Collector/n-buffer, we can improve the VPN by increasing the resistance of the n-buffer layer and increase the length of the P-Collector, thus the RC-IGBT will enter the bipolar conduction mode from the beginning. From the △VSB model for the negative resistance effect, we can improve the collector resistance and reduce the drift region resistance to reduce the △VSB so as to eliminate the negative resistance effect. Guided by the model, then the Double Anodes RC-IGBT is proposed. By introducing the double anode structure the collector resistance is increased and the the Snapback phenomenon is effectively suppressed. Moreover, the simulations results show that the new structure also has a lower voltage drop V and a better trade off property compared with the conventional RC-IGBT.(2)According of the analytical model of the Snapback phenomenon, the Separated RC-IGBT is proposed. The collector reistance of the new RC-IGBT is integrated by changing the regions of the P-Collector and the N-Collector. Thus the Separated RC-IGBT can enter the bipolar conduction mode at low applied Voltage. Through simulation verification, the Separated RC-IGBT can completely eliminate the Snapback phenomenon and it can be achieved only through the layout design. Moreover, the breakdown process of the integrated IGBT in the Separated RC-IGBT chip is aslo discussed, with the simulation and experiment results, the leakage current of CS-IGBT is reduced when compared with the conventional IGBT, and the breakdown property is enhanced at both room and high temperature. So the CS-RC-IGBT could be more superior to the conventional RC-IGBT.(3)The Junction Isolated RC-IGBT is proposed. The introduction of the P float current plug and the NPN transistor in the buffer layer can block the electron current flowing directly to N-Collector, which separates the IGBT independently from the FWD. Meanwhile, the reverse recovery characteristics of the FCE diode integrated in the RC-IGBT are discussed, compared to the conventional PIN diode with current Snap-off phenomenon, the FCE type diode in the RC-IGBTs can eliminate the problem by the injected holes from the P-Collectors in the reverse recovery process, thus the abrupt extraction of the large amounts of excess carriers in the drift can be suppressed. Ultlimately, the FCE type diode has a lower IRRM and a smoother reverse recovery current curve.