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_g and drain-to-source voltage V_ds as the acceptor doping varies from 10¹³ to 10¹⁵ cm^-3. For these acceptor doping densities, the V_g, for a given V_ds, needed to maintain a constant subthreshold current varies by only approximately 0.1 V. If the acceptor density is increased to 10¹⁷ cm^-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.