  We address two outstanding issues in the sequential accretion scenario for
gas giant planet formation, the retention of dust grains in the presence of gas
drag and that of cores despite type I migration. The efficiency of these
processes is determined by the disk structure. Theoretical models suggest that
planets form in protostellar disk regions with an inactive neutral ``dead
zone'' near the mid plane, sandwiched together by partially ionized surface
layers where magnetorotational instability is active. Due to a transition in
the abundance of dust grains, the active layer's thickness decreases abruptly
near the ice line. Over a range of modest accretion rates ($\sim
10^{-9}-10^{-8} M_\odot$ yr$^{-1}$), the change in the angular momentum
transfer rate leads to local surface density and pressure distribution maxima
near the ice line. The azimuthal velocity becomes super-Keplerian and the
grains accumulate in this transition zone. This barrier locally retains
protoplanetary cores and enhances the heavy element surface density to the
critical value needed to initiate efficient gas accretion. It leads to a
preferred location and epoch of gas giant formation. We simulate and reproduce
the observed frequency and mass-period distribution of gas giants around solar
type stars without having to greatly reduce the type I migration strength. The
mass function of the short-period planets can be utilized to calibrate the
efficiency of type I migration and to extrapolate the fraction of stars with
habitable terrestrial planets.
