Comparison of 80-200 nm gate length Al0.25GaAs/GaAs/(GaAs:AlAs), Al0.3GaAs/In0.15 GaAs/GaAs, and In0.52AlAs/In0.65GaAs/InP HEMTs

In this paper we present a comparative study of the high frequency performance of 80-200 mm gate length Al_0.25GaAs/GaAs/(GaAs:AlAs) superlattice buffer quantum well (QW) HEMTs, Al_0.3GaAs/In_0.15GaAs/GaAs pseudomorphic HEMTs and In_0.52AlAs/In_0.65GaAs/InP pseudomorphic HEMTs. From an experimental determination of the delays associated with transiting both the intrinsic and parasitic regions of the devices, effective electron velocities in the intrinsic channel region under the gate of the HEMT´s were extracted. This analysis showed no evidence of any systematic increase in the effective channel velocity with reducing gate length in any of the devices. The effective electron velocity in the channel of the pseudomorphic In_0.65GaAs/InP HEMTs, determined to be at least 2.5×10⁵, was was around twice that of either the Al_0.25GaAs/GaAs quantum well or pseudomorphic In_0.15GaAs/GaAs HEMTs, resulting in 80 nm gate length devices with f_T´s of up to 275 GHz. We also show that device output conductance is strongly material dependent. A comparison of the different buffer layers showed that the (GaAs:AlAs) superlattice buffer was most effective in confining electrons to the channel of the Al_0.25GaAs/GaAs HEMTs, even for 80 nm gate length devices. We propose this may be partly due to the presence of minigaps in the superlattice which provide a barrier to electrons with energies of up to 0.6 eV. The output conductance of pseudomorphic In_0.65GaAs/InP HEMTs was found to be inferior to the GaAs based devices as carriers in the channel have greater energy due to their higher effective velocity and so are more difficult to confine to the 2DEG