High-efficiency 0.25- mu m gate-length pseudomorphic power heterostructure FETs at millimeter-wave frequencies

The authors compare the 35-GHz power performance of four different 0.25- mu m gate-length pseudomorphic AlGaAs/InGaAs/GaAs heterostructure FET (HFET) devices. The devices that are examined include the HEMT (high-electron-mobility transistor), double-heterojunction HEMT (DHHEMT), doped-channel HFET (DCHFET), and the doped-channel MODFET (DCHMODFET). A maximum 35-GHz power-added-efficiency and a power density of 49% and 0.94 W/mm were measured, for the DCHMODFET; 43% and 0.97 W/mm for the DHHEMT, 32% and 0.75 W/mm for the HEMT, and 31% and 0.77 W/mm for the DCHFET. The DC parameters that influence RF power performance were analyzed, and it was found that the I-V linearities of the DCHMODFET, DCHFET, and DHHEMT are much better than that of the HEMT and that the pinchoff characteristics of the DCHMODFET are superior to those of the DHHEMT. The first point explains why the efficiencies of the DCHMODFET and DHHEMT are significantly better than that of the HEMT. It is believed that the second point is responsible for the better efficiency in the DCHMODFET compared to the DHHEMT.