Forward Active and Blocking Performance of 4H-SiC Bipolar Junction Transistors
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Forward Active and Blocking Performance of 4H-SiC Bipolar Junction Transistors S. Balachandran, T. P. Chow and A. Agarwal1 Center for Integrated Electronics, Department of Electrical, Computer and Systems Engineering, Rensselaer Polytechnic Institute, Troy, NY 12180 USA. 1 Cree, Inc., Durham, NC 27703 ABSTRACT We evaluate the performance capabilities and limitations of high voltage 4H-SiC based Bipolar Junction Transistors (BJTs). Experimental forward characteristics of a 4kV BJT are studied and simulations are employed to determine the factors behind the higher than expected specific on-resistance (Ron,sp) for the device. The impact of material (minority carrier lifetimes), processing (surface recombination velocity) and design (p contact spacing from the emitter mesa) parameters on the forward active performance of this device are discussed and ways to lower Ron,sp, below the unipolar level, and increase the gain (β) are examined. A correlation between the open base blocking behavior (forward blocking) and the current gain (forward active) for 4H-SiC based high-voltage BJTs with lightly doped collector regions is presented and experimental device characteristics are utilized to verify our numerical analysis. INTRODUCTION SiC BJTs have demonstrated high blocking voltage and high current handling capability, taking full advantage of the material properties of SiC. The BJT has an even number of junctions and hence its on-state voltage can be minimized through cancellation of junction voltages and by conductivity modulation of the thick, lightly doped drift region when it is in saturation. However, all the 4H-SiC based epitaxial emitter BJTs previously demonstrated [1-3], exhibit a specific onresistance (Ron,sp) value higher than the unipolar value for their drift regions, as shown in figure 1 and the exact reason for this has not been discussed previously. 10
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CREE, ‘95 RPI, ’04 RPI, ’00
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Figure 1. Specific on-resistance vs. breakdown voltage for 4H-SiC BJTs
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Another disadvantage of SiC BJTs is the large base drive required during their forward operation due to their lower current gain (β). For effective utilization of the device we need to formulate ways to decrease the specific on-resistance, by increasing the conductivity modulation of the lightly doped drift region, and increase the gain of the device by increasing the emitter injection efficiency (γ) and by reducing the recombination in the base region. In the following section, we will consider some critical factors that limit the device performance and discuss methods that could lead to a potential improvement in the forward active characteristics. Power BJTs, in the forward blocking mode, are normally operated in the open base configuration. The open base blocking voltage (BVCEO) of a B
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