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 ( V _t) assignment in a dual- V _t process for subthreshold leakage ( I _sub) reduction. Second, for the minimization of gate oxide leakage current ( I _gate) which has become comparable to I _sub in 90nm technologies, we extend the above method to a combined sleep-state, V _t and gate oxide thickness ( T _ox) assignments approach in a dual- V _t and dual- T _ox process to minimize both I _sub and I _gate. By combining V _t or V _t/T _ox 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- V _t or thick- T _ox assignment. A significant improvement in the leakage/performance trade-off is therefore achievable using such combined methods. We formulate the optimization problem for simultaneous state/ V _t and state/ V _t/T _ox assignments under delay constraints and propose both an exact method for its optimal solution...