Performance Enhancement in Bipolar Junction Transistors Using Uniaxial Stress on (100) Silicon

This paper demonstrates that collector current (
$I_{C}$
), dc current gain (
$\beta $
), cutoff frequency (
$f_{T}$
), and maximum oscillation frequency (
$f_{\max }$
) of bipolar transistors (BJTs) can be improved by uniaxial stress. A unified modeling approach that includes a piezoresistive model for changes in mobility and a deformation potential model for changes in intrinsic carrier concentration was used to analyze the stress effects. This approach provides clear insight into the dominant effects of stress on BJT parameters. Experimental results of uniaxial in-plane normal stress are compared with simulation results. Our analysis reveals that for a vertical npn, substantial enhancement in
$I_{C}$
,
$\beta $
,
$f_{T}$
and
$f_{\max }$
can be achieved using a uniaxial in-plane compressive or out-of-plane tensile stress. In a vertical pnp, considerable improvement in
$I_{C}$
can be achieved with an in-plane or out-of-plane compressive stress while the changes in
$\beta $
,
$f_{T}$
, and
$f_{\max }$
are minimal. These results show that stress analysis/prediction for different current-stress orientations can be performed with this model and the outcome can be used as a guide for strain engineering in BJTs.