This article proposed the first fully integrated synchronous electric-charge extraction (SECE) interface, featuring: 1) no external passive components and the smallest normalized on-chip capacitance; 2) an increased energy-extraction efficiency; and 3) a maximized extracted power regardless of the input and output conditions. With the multi-step soft-charging (MSC) technique relaxing energy-extraction conduction loss, we can effectively minimize the on-chip capacitance while improving the energy-extraction efficiency. To further depress the control loss, the positive current selector (PCS) is proposed to realize digital rectification during charge extraction. With the SECE strategy facilitating output independence, we further investigate an automatic phase and amplitude tuning method to enable maximum-power-point tracking (MPPT) over an extended input excitation bandwidth. Fabricated in 0.18- $\mu$ m CMOS, the proposed rectifier achieves a maximum output power improving rate (MOPIR) of 8.14 $\times$ with $V_D$ $=$ 0.12 V and a 405% bandwidth improvement over the piezoelectric energy harvester (PEH) natural bandwidth.