  Spontaneous pair production for spin-$1/2$ and spin-$0$ particles is explored
in a quantitative manner for a static $\tanh$-Sauter potential step (SS),
evaluating the imaginary part of the effective action. We provide finite-valued
per unit-surface results, including the exact sharp-edge Klein paradox (KP)
limit, which is the upper bound to pair production. At the vacuum instability
threshold the spin-$0$ particle production can surpass that for the spin-$1/2$
rate. Presenting the effect of two opposite sign Sauter potential steps
creating a well we show that spin-$0$ pair production, contrary to the case of
spin-$1/2$, requires a smoothly sloped wall.
