The present invention relates to a semiconductor device structure that includes at least one SRAM cell formed in a substrate. Such SRAM cell comprises two pull-up transistors, two pull-down transistors, and two pass-gate transistors. The pull-down transistors and the pass-gate transistors are substantially similar in channel widths and have substantially similar source-drain doping concentrations, while the SRAM cell has a beta ratio of at least 1.5. The substrate preferably comprises a hybrid substrate with at two isolated sets of regions, while carrier mobility in these two sets of regions differentiates by a factor of at least about 1.5. More preferably, the pull-down transistors of the SRAM cell are formed in one set of regions, and the pass-gate transistors are formed in the other set of regions, so that current flow in the pull-down transistors is larger than that in the pass-gate transistors.