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Standby leakage current minimization is a pressing concern for mobile applications that rely on standby modes to extend battery life. In this paper, we propose new leakage current reduction methods in standby mode. First, we propose a combined approach of sleep-state assignment and threshold voltage (Vt) assignment in a dual-Vt process for subthreshold leakage (Isub) reduction. Second, for the minimization of gate oxide leakage current (Igate) which has become comparable to Isub in 90-nm technologies, we extend the above method to a combined sleep-state, Vt and gate oxide thickness (Tox) assignments approach in a dual-Vt and dual-Tox process to minimize both Isub and Igate. By combining Vt or Vt/Tox assignment with sleep-state assignment, leakage current can be dramatically reduced since the circuit is in a known state in standby mode and only certain transistors are responsible for leakage current and need to be considered for high-Vt or thick-Tox assignment. A significant improvement in the leakage/performance tradeoff is therefore achievable using such combined methods. We formulate the optimization problem for simultaneous state/Vt and state/Vt/Tox assignments under delay constraints and propose both an exact method for its optimal solution as well as two practical heuristics with reasonable run time. We implemented and tested the proposed methods on a set of synthesized benchmark circuits and show substantial leakage current reduction compared to the previous approaches using only state assignment or Vt assignment alone.