Electron beam induced current (EBIC) investigations of GaAs-MESFET

Abstract

GaAs-MESFETs are investigated by measuring and simulating the electron beam induced alterations of both gate (ΔI g) and drain (ΔI ds) current. The measurements are carried out using a lock-in-amplifier and a square wave modulated electron beam with electron beam parameters W PE=4keV, W PE=20keV, and I PE=1–10pA as well as modulation frequencies f=0.5–100kHz. For understanding the physical processes within the MESFETs and for interpretation of the contrasts two-dimensional numerical simulations are carried out. The shape of linescans of the electron beam induced gate current measured perpendicularly to the gate depends strongly on the primary electron energy. These linescans are simulated by a program solving the two dimensional minority charge carrier continuity equation in consideration of the electrical field inside the MESFET and of experimental parameters like electron beam diameter and contact metallizations. The maximum height of the electron beam induced gate current is of the same magnitude as expected from excess charge carrier generation by the electron beam (current gain g=ΔI g/I PE≅870). On the other hand, the electron beam induced drain current is extraordinarily high and its amplitude is strongly frequency dependent. The measured current gain g is in the order of 10 6. Simulations with the semiconductor device simulation program MINIMOS [1], which solves the equation system consisting of the Poisson equation and the continuity equations for the charge carriers and which has been supplemented by the generation of excess charge carriers, result in a gain of merely g=3700. Therefore, the high electron beam induced drain current cannot be explained by considering the potential and the carrier concentrations only. Additional effects as e.g. the influence of traps have to be taken into consideration.