Bipolar transistors of high cut off frequency (f_{T}=9GHz) and small size have been fabricated on the same chip with a standard CMOS using the buried twin well structure. 1.3 µm LDD CMOS FETs were formed in the thin epitaxial layer(1-1.5µm) with the buried twin well, without degrading the device characteristics of the MOS FET. Ring oscillators of the BiCMOS gate have been fabricated. A 0.4ns gate delay time at 0.6pF and 3.5 times larger driveability than that of the same area CMOS gate were obtained.