  In this paper, we have studied the Klein's paradox in the presence of both
scalar and vector potential barriers. From the corresponding Dirac equation we
have calculated the transmission and reflection coefficients. It is shown that
the presence of a scalar barrier the scalar potential wides the gap between
positive and negative energies and so the forbidden region. Accordingly, the
Klein's paradox disappears when the scalar barrier exceeds a critical value.
Considering the problem within the framework of quantum field theory, we have
calculated the related pair creation probability, the mean number of created
particles and the probability of a vacuum to remain a vacuum. Then it is shown
that the scalar potential cut down the Klein range and minimizes the creation
of particles; The particle creation decreases as the scalar potential increases
and ceases definitely when the scalar potential reaches the critical value.
