PICOSECOND REAL-SPACE ELECTRON-TRANSFER IN GAAS-N-ALXGA1-XAS HETEROSTRUCTURES WITH GRADED BARRIERS - MONTE-CARLO SIMULATION

Using Monte Carlo simulation combined with iterative solution of the Poisson equation the real-space electron transfer (RSET) and negative differential conductivity (NDC) in GaAs-n-AlxGa1-xAs heterostructures with electric field applied parallel to the layer interfaces have been investigated. The original Hess RSET model has been modified by considering graded AlxGa1-xAs layers with proper spatial dependence of the Al composition. Simulation results confirm that this modification enables one to avoid undesirable effects due to space-charge fields: (1) Graded AlxGa1-xAs layers can be depleted in thermal equilibrium at higher donor concentrations than layers without compositional grading; (2) it is sufficient to use doped, but not compensated, AlxGa1-xAs layers because electron velocity in graded layers is low mainly due to electron transfer to L and X valleys; and (3) there is no confinement of cold electrons in graded AlxGa1-xAs layers due to space-charge fields when the RSET occurs. The RSET-induced NDC and characteristic time constants have been found close to 1 ps at 77 and 300 K. Similar results have been obtained in the RSET model without compositional grading; however, highly compensated AlxGa1-xAs material was needed and the total number of free electrons was ten times lower.