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 times10
¹⁷
cm
^-3
) was measured with linear transmission line method structures to rho
_C
= 1.9 times 10
^-3
Omegacm
²
, whereas the contact resistivity with the base contact implantation was rho
_C
= 1.3 times 10
^-4
Omegacm
²
, both after rapid thermal processing annealing at 800degC.