Isolation And Tunnel Injection RC-IGBT And Breakdown Voltage Design

Reverse conducting insulated-gate bipolar transistor (IGBT)(RC-IGBT) has beenattracting increasing attention since it integrates the unidirectional IGBT and theexternally anti-paralleled fast recovery diode (FRD) into single chip. As the samesilicon is utilized in both forward and reverse conductions, the chip size, the testing cost,and the package cost are reduced. Moreover, the reliability is improved due to theabsence of the temperature oscillations and the bonding wires between the IGBT andFRD chips. However, the conventional RC-IGBT is prone to snapback in the forwardconducting state, even when the dimensions of p-type collectors are enlarged to theorder of millimeters. Another challenge of conventional RC-IGBT is complex andcostly backside patterning process which is prone to low yield, especially for lowvoltage IGBTs (e.g.,600-1200V) thicknesses of which are about or lower than100μm.In this dissertation, two solutions are proposed aiming at overcoming these problems.As power semiconductor device, the edge termination of RC-IGBT is important andsingle-mask multi-zone junction termination extension (SM-MZJTE) is attractive sinceit brings lower cost especially in high-voltage devices. However, SM-MZJTE is verysensitive to the shape of the doping profile which is controlled by the geometricalparameters of the implantation mask and designed according to experiences. The detailinnovations of this dissertation are listed below:(1) Considering that the snapback originates from the collector short, to suppressthe snapback, the most direct solution is increase the collector short resistance. Onesimple method is inserting an oxide trench with depth larger than that of the bufferbetween the N-collector and the P-collector. The snapback can be completely eliminatedif the dimension of collector cell is no less than240μm. A more interesting structure,named TPRC-IGBT, features a floating P-layer between the N-collector and the driftregion. Compared to the former structure, TPRC-IGBT can suppress snapbackcompletely with collector cell dimension as small as40μm. When the current is large,the floating P-layer, combined with N-collector and drift region, acts as a low resistance path in both forward and reverse conduction states, and thus it shows better tradeoffbetween the turnoff loss and forward voltage drop. However, both of the two structuresneed oxide trench on the backside of the wafer, which is costly and prone to low yield.To reduce cost and increase yield, a diffusion formed floating P-region instead of oxidetrench is introduced between the N-collector and the P-collector. Furthermore,compared with TPRC-IGBT in which two electron extraction paths contribute to the fastturnoff, there is another, more efficient electron extraction path during turnoff.(2) However, backside patterning processes of all the three structures above arecomplex and costly and the current flows in a non-uniform distribution both in theforward and reverse conducting state. An RC-IGBT, named band-to-band tunnelinginjection IGBT (TIGT), aiming at overcoming the disadvantages encountered bybackside patterning by utilizing band-to-band tunnel effect is proposed. Compared withthe three structures above as well as the conventional RC-IGBT, TIGT features auniform tunnel collector p-n junction which needs no backside patterning, and thusconducts current uniformly in both forward and reverse conducting states which arefavorable to the increase of conducting capability and reduction of the reverse recoverypeak current. Moreover, during the reverse recovery, the electrons extracted from thedrift region to the collector induce hole injection into drift region and thus leads to softreverse recovery of the built-in diode.(3) To help design the doping profile shapes, a simple analytical model assumingthat the p-type SM-MZJTE region is completely depleted and the equipotential lines arecircular arcs for simplicity is developed and experimentally demonstrated. As theblocking voltage is sensitive to the ion distribution, the Boron segregation at Si-SiO2interface and oxide charge in SiO2are taken into consideration in this model. The modelhas been experimentally verified, and the normalized breakdown voltage is about0.92.