Semiconductor device

A semiconductor device whose insertion loss is reduced and isolation characteristics are improved in a high frequency band by reducing the capacitance component when an FET is off is provided. FETs ( 30 a, 30 b) having a gate electrode with a gate length of not more than 0.8 mum are formed on a semiconductor substrate in which a buffer layer ( 24 ) having an impurity concentration of at least 1010 cm-3 and at most 1014 cm-3 is formed on a semi-insulating semiconductor ( 25 ) having at least 1014 cm-3 and at most 1016 cm-3 p-type or n-type impurities and an active layer ( 23 ) having a p-type or n-type impurity concentration of at least 1015 cm-3 and at most 1017 cm-3 is formed on the buffer layer. N FETs are combined with each other, and when 1