IMPACT OF SURFACE-LAYER ON IN0.52AL0.48AS/IN0.53GA0.47AS/INP HIGH ELECTRON-MOBILITY TRANSISTORS

The surface potential of FET's has shown a strong effect on the channel potential and charge control in the channel. We have investigated the role of undoped versus doped cap layers in Ino.52Alo.48As/Ino.s3Gao.47As/InP HEMT's. As the result of surface potential effect, direct comparison of 0.3 x 150-μ m2 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 SO mS/mm for the doped structure. The surface undoped HEMT achieved a very high maximum stable gain of 19.2 dB compared to 16.0 dB for surface doped HEMT at 18 GHz, largely due to the improved gm /g0 ratio. This study demonstrates that control of the surface potential in Ino.52Alo.48As/Ino.s3Gao.47As/InP HEMT's is consistent with the effect of a gate recess in MESFET's. This study also shows that, in achieving high-gain applications of HEMT's, the surface potential near the gate edge should be optimized through unconventional surface layer design. © 1990 IEEE