The fabrication of a back-gated high electron mobility transistor-a novel approach using MBE regrowth on an in situ ion beam patterned epilayer

A new technique for the fabrication of GaAs/AlGaAs back-gated high electron mobility transistors (HEMTs) is described. First the authors demonstrate that a dose of >2*10 13 cm -2 Ga ions at an energy of 10 keV can be used to damage a 67 nm n + GaAs layer, rendering the implanted regions non-conducting. After implantation the epilayer has a 4 K sheet resistivity which is increased by a factor of approximately=10 7 when compared with the original unimplanted value. This isolation procedure is then used to form a patterned back-gated HEMT by MBE regrowth on top of an in situ ion-implanted n + GaAs layer. The resulting structure is designed so that the back gate is rendered highly resistive under the regions where the ohmic contacts to the two-dimensional electron gas (2DEG) are formed, thus making shallow ohmic contacts unnecessary. Using this fabrication technique, the authors obtain a high yield of working devices in which it is possible to alter the carrier concentration (n) in the 2DEG from 0 to 3.6*10 11 cm -2 using the back gate. Typical leakage currents at 1.2 K are <0.5 nA over the whole of this carrier concentration range-an improvement over devices of a similar design which have been fabricated using shallow ohmic contacts. At zero back-gate voltage the 1.2 K mobility ( mu ) of the 2DEG is approximately=400000 cm 2 V -1 s -1 with n approximately=2.8*10 11 cm -2 , whilst on altering the back-gate voltage it is found that mu varies as n x where x approximately=1.5. These results are characteristic of a high-quality 2DEG with mobility limited by remote ionized impurity scattering. This technique can therefore be used as a means of controlling the 2DEG carrier concentration, whilst leaving the surface of the HEMT structure free for conventional lithographic processing. Further, it is possible to pattern the back gate beneath the conducting channel. This would allow the shape of the 2DEG to be defined, and its width altered, using an in-grown patterned back gate a feature which is impossible using conventional techniques.