AlGaN/GaN doped channel heterostructure field effect transistors

Abstract

AlGaN/GaN Heterostructure Field Effect Transistors (HFETs) are expected to have a high transconductance and a high saturation current because of the relatively high electron mobility and high saturation velocity of the two dimensional electron gas (2DEG) in AlGaN/GaN modulation doped structures. Our experimental results revealed that, in the fabricated devices, the transconductance is limited by a large source series resistance, Rs, and by a small sheet carrier concentration, ns, of the 2DEG in the device channel. In order to increase ns, we recently proposed to use a Doped Channel HFET (DC-HFET) design, similar to that used in AlGaAs/GaAs DC-HFETs. In AlGaN/GaN structures, an additional impurity scattering caused by the channel doping has relatively less of an effect on the electron mobility than in AlGaAs/GaAs devices, due to a much higher electron effective mass in GaN. One micron gate AlGaN/GaN Doped Channel HFETs reached the cutoff frequency fT of 18.3 GHz. Short channel AlGaN/GaN Doped HFETs with 0.25 micron gates demonstrated the cutoff frequency fT up to 36.1 GHz and the maximum oscillation frequency, fmax ≈ 97.3 GHz. These values of fT and fmax are the highest value reported for any wide band gap semiconductor device. Our simulations of AlGaN/GaN Doped Channel HFETs show that transconductances in access of 1000 mS/mm may be obtained for very thin AlGaN layers, a large gate voltage swing, and for a large surface sheet density of the 2D electron gas.