In a high-frequency amplifying semiconductor device in which a MOS field effect transistor and a bipolar transistor are formed within the same wafer and a source electrode of the MOS field effect transistor is connected to a lead frame by a bonding wire, use is made of a wafer for fabricating the MOS field effect transistor and the bipolar transistor, in which on a semiconductor substrate of P++ type is formed a first epitaxial layer of P or P- type, a buried layer of N+ type is formed in the first epitaxial layer of the first conductivity type and a second epitaxial layer of P type is formed on the buried layer and the first epitaxial layer. The use of such a wafer, which has no P- type Si substrate, allows the source resistance of the MOS field effect transistor to be decreased. The high-frequency amplifying semiconductor device is improved in high-frequency gain and NF.