Abstract :
Modulation-doped field effect transistors (MODFET´s) using Al,GaAs and GaAs have shown interesting but limited improvements in performance over those of GaAs metal semiconductors field effect transistors (MESFET´s). The use of indium alloys can be made to improve the properties of the active channel and the barrier confining the electrons. When indium is alloyed into GaAs the electron transport along the active channel is improved. An excellent example is that of Ga.47In.53As active channel grown lattice-matched on InP. The low-field mobility of electrons is raised by 50% at room temperature, and for .25 micron gate length, the electron average transit velocity is also raised by about this much at room temperature. Very high potential barriers of .5 eV can be achieved for electrons on this active layer by using Al.48In.52As, helping to confine energetic electrons. It is also possible to grow thin InxGa1-xAs layers with x < .20 on GaAs, and achieve part of these result even with the resulting lattice mismatch. The resulting reduction of bandgap when x rises above 0 also helps raise the potential barrier confining electrons. When AlyGa1-yAS is used to confine electrons to GaAs (or InxGa1-xAs), any necessary Si donors in the AlyGa1-yAs have a sharp increase in ionization energy for y ≥ .20 or so. The potential barrier is only .16-eV in the conduction band for Al.2Ga.8As. If (AlzGa1-z).51In.49P is grown lattice matched to GaAs, the donor ionization energy is low up to z = .3, where the conduction band potential barrier for electrons is .32 eV. Thus even GaAs MODFET´s will yield improved performance, due to better electron confinement, when indium alloy barriers are used. Predictions of MODFET fTvalues of over 100 GHz for active channel of lattice mismatched InxGa1-xAs on GaAs and over 120 GHz for In.53Ga.47As on InP are made for .25 µm gates at room temperature, compared to ≥ 80 GHz fTvalues achieved for Al, GaAs/GaAs MODFETs. Structures for impr- oved performance of the various MODFETs for switching and microwave operation are presented.
