The thesis is composed of tow topics: a fully bidirectional mixed- voltage-tolerant I/O cell using a new output stage circuit and a sub-3¡ÑVDD wide range fully bidirectional mixed-voltage-tolerant I/O cell. The first topic discloses a mixed-voltage-tolerant I/O cell implemented using 2P4M 0.35 £gm CMOS process, which uses a low static power dynamic gate bias generator providing three different logic voltage levels to the output stage to avoid gate oxide reliability and leakage current. The design also reveals a new output stage circuit, which enhances the output current to resolve the poor driving capability caused by the slow mobility and body effect of the stacked PMOS. The second topic shows a sub-3¡ÑVDD wide range fully bidirectional mixed-voltage-tolerant I/O cell using 1P6M 0.18 £gm CMOS process, which employs a new dynamic gate bias generator and a PAD voltage detector to provide appropriate gate biases. The design includes a new gate tracking circuit and a floating N-well circuit to avoid gate oxide reliability and leakage current, which relaxes the body effect at the output PMOS.