PHOTO-HALL STUDIES OF MODULATION-DOPED FIELD-EFFECT TRANSISTORS WITH SHORT-PERIOD SUPERLATTICE CHANNELS RATHER THAN ALLOY CHANNELS

We studied the photo-Hall mobility and the photo-Hall density of modul ation-doped field-effect transistor structures using either an InGaAs alloy channel or a short-period superlattice channel. In defining the short-period superlattice channel we changed the thicknesses of the In As and GaAs layers and the number of InAs/CaAs interfaces, The thickne sses of the InAs layers varied from 0.5 to 1.6 monolayers, those of Ga As layers from 4 to 20 monolayers, and the number of InAs/GaAs interfa ces varied from 3 to 7. As a consequence the total short-period superl attice channel thicknesses varied too, but they were kept below the cr itical layer thickness to relax the pseudomorphic structure. The persi stent photoconductivity effect at 77 K and the use of a red light emit ting diode as the illumination source were employed to induce variatio n of the Hall mobility and the Hall density in the conduction channel. For the short-period superlattice samples our results indicate that t he Hall mobility increases (a) with a reduction in the number of InAs/ GaAs interfaces; (b) by employing an integral number of InAs monolayer s; and (c) by increasing the thickness of the GaAs layers. These resul ts suggest that the three effects are effective in producing surface s moothing. When the short-period superlattice samples are compared with samples having an InGaAs channel, we have shown that for an indium co ntent around 8% the electrical properties are basically the same. On t he other hand, for an indium content around 25% the mobility increases up to 35% proving that use of the short-period superlattice channel i s useful in reducing the atomic disorder. The scattering associated wi th the alloy disorder increases with the indium content, and the delet erious effect of disorder can be reduced by replacing the alloy channe l by a short-period superlattice channel. (C) 1996 American Vacuum Soc iety.