Low-Forward-Voltage-Drop 4H-SiC BJTs Without Base Contact Implantation

Bipolar junction transistors (BJTs) of 4H-SiC, with a low collector-emitter forward voltage drop V_CE have been fabricated without base contact implantation. A comparison of BJTs on the same wafer with and without base contact implantation shows less than 10% higher V_CE for the BJTs without base contact implantation. Omitting the base contact implantation eliminates high concentrations of implantation-induced defects that act as recombination centers. This is advantageous because it allows a shorter distance W_p+ between the emitter edge and the base contact, without affecting the current gain when no base contact implantation is used. The BJTs without contact implantation show a constant current gain as W_p+ was reduced from 3 to 1 mum, whereas the gain decreased by 45% for the BJTs with base contact implantation for the same reduction of W_p+. A key to the successful fabrication of low-forward-voltage-drop SiC BJTs without base contact implantation is the formation of low-resistivity Ni/Ti/Al ohmic contacts to the base. The contact resistivity on the base region (N_A ap 4 times10¹⁷ cm^-3) was measured with linear transmission line method structures to rho_C = 1.9 times 10^-3 Omegacm², whereas the contact resistivity with the base contact implantation was rho_C = 1.3 times 10^-4 Omegacm², both after rapid thermal processing annealing at 800degC.