than those of the small bandgap material in the collector in one embodiment collector layer 14 is 2 5 u03bcm in thickness and the insertion layer 34 is 0 5 u03bcm in thickness in the embodiment of fig3 the wide bandgap algaas layer 34 is located entirely in the collector structure as noted however to ensure the high field region that contributes to collector breakdown and therefor soa boundary ii at medium to high current levels appears entirely in the wide bandgap material the wide bandgap insertion layer can be extended into the heavily doped subcollector layer as illustrated in the section view of fig4 the insertion layer now includes two layers 36 38 with the n u2212 doped layer 36 abutting collector layer 14 in forming a part of the collector structure while an n insertion layer 38 abuts the subcollector layer and forms part of the subcollector structure again the transistor is an ingap gaas hbt with algaas insertion layers for example in addition to the discrete layers in the structures of fig3 and 4 step or continuous compositional grading can be applied in the transition from the small bandgap collector layer to the wide bandgap insertion layer or.