High power bipolar junction transistors in silicon carbide
Sammanfattning: As a power device material, SiC has gained remarkable attention to its high thermal conductivity and high breakdown electric field. SiC bipolar junction transistors (BJTs) are interesting for applications as power switch for 600 V-1200 V applications. The SiC BJT has potential for very low specific on-resistances and this together with high temperature operation makes it very suitable for applications with high power densities. One disadvantage of the BJT compared with MOSFETs and Insulated Gate Bipolar Transistors (IGBTs) is that the BJT requires a more complex drive circuit with higher power capability. For the SiC BJT to become competitive with field effect transistors, it is important to achieve high current gains to reduce the power required by the drive circuit. Although much progress in SiC BJTs has been made, SiC BJTs still have low common emitter current gain typically in the range 10-50. In this work, a record high current gain exceeding 60 has been demonstrated for a SiC BJT with a breakdown voltage of 1100 V. This result is attributed to an optimized device design, a stable device process and state-of-the-art epitaxial base and emitter layers.A new technique to fabricate the extrinsic base using epitaxial regrowth of the extrinsic base layer was proposed. This technique allows fabrication of the highly doped region of the extrinsic base a few hundred nanometers from the intrinsic region. An important factor that made removal of the regrowth difficult was that epitaxial growth of very highly doped layers has a faster lateral than vertical growth rate and the thickness of the p+ layer therefore has a maximum close to the base-emitter sidewall. A remaining p+ regrowth spacer at the edge of the base-emitter junction is proposed to explain the low current gain.Under high power operation, the SiC BJTs were strongly influenced by self-heating, which significantly limits the performance of device. The DC I-V characteristics of 4H-SiC BJTs have also been studied in the temperature range 25 °C to 300 °C. The DC current gain at 300 °C decreased 56 % compared to its value at 25 °C. Selfheating effects were quantified by extracting the junction temperature from DC measurements.To form good ohmic contacts to both n-type and p-type SiC using the same metal is one important challenge for simplifying SiC Bipolar Junction Transistor (BJT) fabrication. Ohmic contact formation in the SiC BJT process was investigated using sputter deposition of titanium tungsten to both n-type and p-type followed by annealing at 950 oC. The contacts were characterized with linear transmission line method (LTLM) structures. The n+ emitter structure and the p+ base structure contact resistivity after 30 min annealing was 1.4 x 10-4 Ωcm2 and 3.7 x 10-4 Ωcm2, respectively. Results from high-resolution transmission electron microscopy (HRTEM), suggest that diffusion of Si and C atoms into the TiW layer and a reaction at the interface forming (Ti,W)C1-x are key factors for formation of ohmic contacts.
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