S‐type switching characteristics have been observed at room temperature in AlAs/GaAs multiquantum well diodes at low forward bias. The multiquantum well structure consisted of a set of 10 periods of undoped 50‐nm AlAs barriers with 50‐nmn+(1×1018/cm3) GaAs quantum wells inserted between ap+‐GaAs anode andn+‐GaAs cathode. A threshold voltage for switching from the low to high conductance state of 2.2 V was measured and a minimum holding voltage in the high conductance state of 0.9 V was observed. Switching to the high conductance state occurs due to impact ionization of electrons out of the heavily dopedn+wells forming positive space charge which enhances the cathode field and thereby electron injection giving rise to a positive feedback mechanism.