Semiconductor methods and device structures for suppressing latch-up in bulk CMOS devices. The method comprises forming a trench in the semiconductor material of the substrate with first sidewalls disposed between a pair of doped wells, also defined in the semiconductor material of the substrate. The method further comprises forming an etch mask in the trench to partially mask the base of the trench, followed by removing the semiconductor material of the substrate exposed across the partially masked base to define narrowed second sidewalls that deepen the trench. The deepened trench is filled with a dielectric material to define a trench isolation region for devices built in the doped wells. The dielectric material filling the deepened extension of the trench enhances latch-up suppression.