Implanted emitter BJT

Figure 6.1 (a) Cross sections of an epitaxial emitter BJT. (b) Implanted emitter BJT in 4H-SiC. 

Figure 6.6 (a) Structure of an implanted emitter BJT. (b) I-V characteristics of an implanted emitter 4H-SIC. The device shows greater common emitter current gain in the reverse-active region. 

Figure 6.4 Carrier flow in implanted emitter 4H-SiC BJTs. Most of the electrons injected from the emitter are recombined at or near the SCR of the emitter-base pn junction.

Tang, Y., J. B. Fedison, and T. P. Chow, High-Voltage Implanted-Emitter 4H-SiC BJTs, IEEE Electron Device Letters, Vol. 23, No. 1, January 2002, pp. 16-18. 

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.

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.

From the previous discussion, it is gathered that there are two potential issues that need to be addressed in any future research on SiC BJTs (1) The current gain of the devices is low (10-15) at present and (2) the base resistance is rather high due to the high sheet resistance of the base layer and the necessity of keeping the base contact implant at least 5 micron away from the edge of the emitter mesa. The current gain... 

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