Improved microwave performance of InP based HEMT using a floating gate tetrode arrangement

A novel floating gate tetrode device concept is presented and evaluated for high speed performance. In this structure the second gate is DC and RF unconnected and internally slightly positively self-biased. Even so, the second FET as well as the first FET need to be operated in saturation for optimum high gain performance. To obtain a lower output conductance and feedback capacitance, a deeper recess of the second gate in respect to the first gate is required. Then, under optimum bias conditions of gate 1, the channel current is reduced to approximately 0.4 of the maximum current and the f_max cut-off frequency is increased by a factor of 1.75 from f_max,triode=205 GHz to f _{max,FG-tetrode}=360 GHz. Due to the cascode circuit arrangement, the f_T is reduced from f_T,triode=110 GHz to f_T,tetrode=90 GHz. These numbers are obtained at a low drain bias of V_D=1.2 V