Acoustic charge transport in an (Al,Ga)As/GaAs heterojunction structure grown by molecular-beam epitaxy

Abstract

The demonstration of charge transport in GaAs epitaxial layers by surface acoustic waves in 1982 has led to a great deal of interest in using acoustic charge transport (ACT) technology for high-speed analog signal processors. Successful implementation of ACT effects requires confining the moving charge packets to a buried layer. This was originally accomplished by biasing a thick, lightly doped n-GaAs layer (about 4 μm low-1015/cm3) with electrodes at the epilayer surface and at the back of the semi-insulating substrate, leading to problems in device fabrication and reproducibility, as well as making monolithic integration with field effect transistors (FET’s) difficult. The logical solution to these problems seemed to be using the charge confining capability inherent in heterojunctions. We report here the first observation of acoustic charge transport in a heterojunction structure. The heterojunction acoustic charge transport (HACT) epitaxial structures were grown by molecular-beam epitaxy and are comprised of a GaAs quantum well with (Al,Ga)As barriers. The top (Al,Ga)As layer is doped to satisfy surface states and to facilitate formation of Ohmic contacts. These structures are similar to quantum well or double heterojunction high electron mobility transistor (HEMT) [modulation-doped FET (MODFET)] epilayers, but with somewhat different material requirements for good device operation. As examples, acoustic charge transport devices operate at low current levels (about 100 μA) and thus require Schottky contacts with very low leakage. But since they operate at relatively low frequencies (hundreds of MHz) they do not need extremely high mobility two-dimensional electron gases. Initial results indicate that these devices function best with a low sheet carrier concentration, also in contrast to HEMT’s. Further details of materials and device fabrication will be presented.