We present numerical calculations of the electron and hole impact ionization coefficients in bulk Ga0.47In0.53As, Al0.48In0.52As and related multiquantum well structures. It is found that significant enhancement of the electron impact ionization rate over the hole impact ionization rate can be achieved by more than an order of magnitude in simple multiquantum well APD's at low applied electric fields, but only at the expense of severe carrier trapping effects. At large applied fields, ∼ 200 kV/cm, trapping becomes insignificant but the hole ionization rate increases dramatically. An alternative device that has a graded region at the end of the well, grown using a SLAM superlattice structure, is examined. In this structure electron trapping effects are eliminated, but hole trapping can still occur. It is determined that the graded structure shows an improvement over the simple multiquantum well device. The improvement may not be substantial enough to warrant the more complicated development of the graded superlattice device.