Subthreshold I-V characteristics of AlGaAs/GaAs MODFETs: the role of unintentional acceptors

A strong-inversion depletion-layer model of threshold has been extended to describe subthreshold I-V characteristics in MODFETs. The results of this calculation yield the MODFET equivalent of the MOSFET charge sheet subthreshold model. For a typical molecular-beam-epitaxy-grown structure, the subthreshold current may differ by two orders of magnitude for a given gate voltage V/sub g/ and drain-to-source voltage V/sub ds/ as the acceptor doping varies from 10/sup 13/ to 10/sup 15/ cm/sup -3/. For these acceptor doping densities, the V/sub g/, for a given V/sub ds/, needed to maintain a constant subthreshold current varies by only approximately 0.1 V. If the acceptor density is increased to 10/sup 17/ cm/sup -3/, a large increase of approximately 0.8 V in the gate voltage is required to maintain a constant subthreshold current. These changes in subthreshold current with acceptor concentration in the bulk GaAs are significant and need to be included in an accurate MODFET model.<
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