Abstract
We report on the noise figure, associated gain, and the current gain cutoff frequency for comparable 0.25-/spl mu/m gate GaAs MESFETs and GaAs pseudomorphic HEMTs (p-HEMTs) as a function of cryogenic temperature. Contrary to previously published results which suggest that p-HEMTs should have a higher electron velocity and a lower noise figure than MESFETs due to the effects of the two-dimension electron gas (2-DEG), we have experimentally verified that this is not the case. We show clear evidence that the transport properties of the 2-DEG in p-HEMTs do not make a significant contribution to the speed enhancement and noise reduction during high-frequency operation of these devices. It is the fundamental InGaAs material properties, specifically the /spl Gamma/-L valley separation in the conduction band and associated effective mass of the electron in either GaAs or InGaAs channel, which limits the high-field electron velocity and thus the speed and noise performance of the devices.
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