Power BJTs

The blocking voltage of a power BJT is supported by the collector layer. In Si power BJTs, it is known that a very low specific on-resistance can be achieved due to the conductivity modulation in the lightly doped collector epilayer. However, none of... 

The blocking capability of a power BJT is generally determined by the open-base breakdown voltage (BVceo) of the device. The S usually much smaller... 

Figure 6.8 Simplified cross section of a unit cell of a 4H-SiC power BJT.

Figure 6.8 shows a simplified cross section of a unit cell of a 4H-SiC power BJT. The cell pitch, P, is given hy the sum of width of the emitter mesa, width of the p base implant, and the total base-to-emitter spacings. An example of the device layout is shown in Figure 6.9. The goal of unit cell design is to minimize the cell pitch, since most of the current flows along the sidewalls of emitter mesas. It is important to maximize the density of emitter mesa sidewall density without compromising the performance of the transistor.

Debiasing of emitter-base pn junction can be minimized by using a double metal process. A simplified cross section of a 4H-SiC power BJT with double metal process is shown in Figure 6.11. In this structure, the emitter electrode covers most of the active area and is connected to emitter fingers through vias, whereas the base electrode is placed outside of the active area. Use of this structure eliminates most of the resistive voltage drop in the emitter fingers at an increased cost of the fabrication process. 

Figure 6.12 Cross-sectional view of the 4H-SiC power BJT fabrication, (a) Starting epilayer structure. (b) Dry etching of emitter and base epilayers. (c) p implantation for guard rings and contacts to p-base. (d) Formation of ohmic contacts, (e) Over layer metal deposition, (f) Double metal process.

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