Novel 4H-SiC bipolar junction transistor (BJT) with improved current gain

Silicon Carbide (SiC) is becoming increasingly of interest to the power electronics industry due to its superior characteristics such as high critical electric field, larger bandgap and higher thermal conductivity in comparison to Silicon (Si). Taking advantage of these material properties, high voltage devices could be fabricated with lower power losses and high temperature and frequency operability, which could surpass the limits enforced by Si power devices. SiC BJTs in particular are advantageous for the medium to high-voltage application range (e.g. HVDC multi-level converters) as they exhibit lower on-state losses while maintaining superior switching performance. The only drawback is being a current driven device in contrast to voltage-controlled SiC MOSFETs. Therefore, devices with a large current gain (β) are highly desirable for simplifying the base drive. In this paper, a novel circular structure with improved β has been demonstrated. Although the material quality of epilayers and surface passivation layer are the most significant factors limiting β, the geometrical design also plays a vital role in achieving a higher β. Therefore, the influence of the emitter geometry on β and the Rittner effect have been discussed by comparing rectangular and circular cell structures, with the aid of the software Sentaurus TCAD.