Abstract: A bipolar transistor comprising a first layer of a first semiconductor material having a first conductivity type, a second layer on the first layer, the second layer being of a second semiconductor material having a second conductivity type, the second semiconductor material having a bandgap less than that of the first semiconductor material, a third layer on the second layer, the third layer being of the second semiconductor material and having the first conductivity type, a first doped region in the first layer, the first doped region being of the first semiconductor material and being doped to be semi-insulating regions, and a second doped region in the third layer, the second doped region being of the second semiconductor material and being doped to the second conductivity type. The second layer has a portion forming an active base region and the third layer has a portion forming a collector region in contact with the active base region.