Theoretical calculations of charge confinement in a pn~np heterojunction acoustic charge transport device

An alternative structure for heterojunction acoustic charge transport (HACT) devices has been devised and analyzed. The GaAs/AlGaAs structure uses a pn^~np doping profile near the surface of the device to create a charge transport layer and provide top vertical confinement. This is contrary to previous n-type HACT structures which rely on residual surface states and a heterojunction discontinuity for the same functions. The use of the pn^~np doping as the channel depletion mechanism makes the device insensitive to the residual surface state density, thus providing a more robust design. In addition, the use of the back np junction enables widening of the transport layer thereby increasing the amount of charge that can be transported by the acoustic wave. As a result of the increased charge capacity it is expected that the pn^-np ACT device will exhibit a greater dynamic range and current than previous HACT designs. The analysis of the device structure is accomplished herein using a two dimensional hydrodynamic simulation code, Semiconductor Total Energy Balance Simulator in two Dimensions (STEBS-2D), which has been modified to account for the potential created by the surface acoustic wave. The calculated results indicate that an order of magnitude enhancement in charge capacity is possible using the new structure. Transfer efficiency calculations for several different lifetimes in the transport layer show high efficiency values with a Shockley Read Hall lifetime of 10 nsec.