Using the steady-state model based on the coupled-mode and carrier rate equations, in this paper, we analyze the effects of trapezoidal tapered gratings on the dispersive bistable characteristics of a quarter wavelength phase-shifted distributed feedback semiconductor laser amplifier (QWS-DFB-SLA). We also compare the results with those of a purely QWS-DFB-SLA. For laser amplifiers driven to yield the same unsaturated peak amplifier gain, trapezoidal taper widens the spectral range of low-threshold bistable switching and increases the on-off switching contrast. The transfer-matrix method (TMM) is utilized for the numerical simulations. (c) 2007 Elsevier Ltd. All rights reserved.