This article analysed and optimised horizontal Silicon insulator Silicon capacitor (H-SISCAP) phase shifter structures with insulator thickness tox up to 40 nm. The phase shifter has an effective capacitance (Ceff) around 0.5 fF/μm and a phase change efficiency 1.8 V·cm, of which the balance between capacitance and phase efficiency is comparable with silicon rib waveguide based depletion type phase shifters. Silicon Mach–Zehnder interferometer (MZI) modulators with 200 μm long H-SISCAP phase shifters optimised for TM polarised light have been fabricated and demonstrated with a 3 dB EO bandwidth above 35 GHz and intrinsic H-SISCAP RC bandwidth around 150 GHz. The modulator displays open eye-diagrams for data rates up to 60 Gbit/s without detection equalization and 72 Gbit/s with 2 taps equalization in NRZ-OOK operation. The demonstrated H-SISCAP phase shifter paves the way to build high bandwidth segmented, travelling wave and compact ring resonators modulators for applications of optical transmitters, microwave generations, and LIDAR applications, etc.