Development of 4H-silicon carbide BJTs for power electronic applications
Recent research has shown the potential of silicon carbide (SIC) power devices in power electronic applications. They are intensely studied because their on-state power dissipation is lower than conventional silicon power devices due to its superior material properties. The critical electric field of SiC is 10 times higher than Si. As a result, the Baliga Figure of Merit for SiC is roughly 400 times better. Most of the research has focused on 4H-SiC because of its higher and more isotropic electron mobility than other polytypes. Among many device structures proposed, Bipolar Junction Transistor (BJT) is projected to benefit from the conductivity modulation in the drift region and has a lower forward voltage drop than the unipolar theoretical limit. In this dissertation, physics of 4H-SiC BJTs is discussed and simulations are performed to identify the key design issues such as finger length, JTE dose and width, and the gap between p+ implant and emitter finger. Large area (>1 mm2) 4H-SiC BJTs are fabricated and good I-V characteristics are observed (V CEO > 3,000V, β > 50). Because the BJT is a current drive device, additional power is dissipated in the drive current IB path. The control circuitry is also more complex than voltage drive devices such as MOSFETs. With increased current gain, these difficulties can be overcome. Boron base transistors (BBTs) are expected to have even higher current gain because boron in 4H-SiC forms deeper accpertor levels than aluminum and has a lower ionization percentage at room temperature. Prelimenary emperimental results show that BBTs are suffering from the low carrier lifetime in the base. They exhibit current gain as low as 3 and large turn on voltage. However, from simulation, it is expected that current gain will improve and turn on voltage will reduce if the carrier lifetime can be improved.
Cooper, Purdue University.
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