High-switching frequency dc-dc converters are present in many applications, where wide regulation bandwidth and high efficiency are needed. A three-MOSFET cascode bridge is presented to get full benefit from digital CMOS technology advancements. Three-dimensional assembly with a capacitive interposer is proposed for high-frequency efficient decoupling. A test-chip is designed in 40-nm CMOS technology, validating the operation and the efficiency performance of the converter using the proposed cascode bridge. A conversion peak efficiency of 91.5% is demonstrated when converting 3.3 to 2.4 V with a 150-mA load current, while switching at 100 MHz.