Impact of surface layer on In/sub 0.52/Al/sub 0.48/As/In/sub 0.53/Ga/sub 0.47/As/InP high electron mobility transistors

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Abstract
The surface potential of FETs has shown a strong effect on the channel potential and charge control in the channel. A study of the role of undoped versus doped cap layers in In/sub 0.52/Al/sub 0.48/As-In/sub 0.53/Ga/sub 0.47/As-InP high-electron-mobility transistors (HEMT) is discussed. As the result of surface potential effect, direct comparison of 0.3*150- mu m/sup 2/ gate devices yielded improved gate breakdown characteristics and a DC output conductance of less than 15 mS/mm for the surface undoped structure compared to 50 mS/mm for the doped structure. The surface undoped MEMT achieved a very high maximum stable gain of 19.2 dB compared to 16.0 dB for the surface doped HEMT at 18 GHz, largely due to the improved g/sub m//g/sub 0/ ratio. This study demonstrates that control of the surface potential in In/sub 0.52/Al/sub 0.48/As-In/sub 0.53/Ga/sub 0.47/As-InP HEMTs is consistent with the effect of a gate recess in MESFETs. This study also shows that, in achieving high-gain applications of HEMTs, the surface potential near the gate edge should be optimized through unconventional surface layer design.