Graded AlGaN Channel Transistors for Improved Current and Power Gain Linearity

We report on the small-signal high frequency characteristics of highly scaled graded AlGaN channel polarization-doped field-effect transistors (PolFETs) that show constant current gain cutoff frequency (f
_T
) and maximum oscillation frequency (f
_max
) profiles as a function of current density or gate bias. The device design includes upward and downward Al composition grading to induce a distributed 3-D charge profile, and eliminate abrupt heterojunction band offsets to achieve nonalloyed ohmic contacts with low resistance. The highest extrinsic f
_T
of 52 GHz and f
_ma
x of 67 GHz were measured at V
_GS
= -1.5 V and V
_DS
= 9 V, and constant f
_T
and f
_max
over wide input voltage (V
_GS
) and output current range (IDS) were achieved.