DC characteristics of In0.52Al0.48As/ In0.53(AlxGa1-x)0.47As NPN double heterojunction bipolar transistors

Wide band gap materials, InAlAs and InAlGaAs, were used in the collector of double heterojunction bipolar transistors (DHBTs). Base-collector designs and their effects on the DC characteristics of the DHBTs were investigated. It is found that in the InAlAs/InGaAs/InAlAs DHBT, the unusual large conduction band discontinuity results in a two-dimensional electron gas (2DEG) stored in base-collector abrupt junction. This 2DEG will push the heterojunction spike up and cause a reach-through effect with a InGaAs space layer inserted between the base-collector junction. Junction grading seems to be the only method to remove the effect. For the InAlAs/InGaAs/InAlGaAs DHBT, the smaller the conduction band discontinuity, the less significant is the 2DEG effect. When the discontinuity is smaller than 0.27 eV, the knee-shape and reach-through characteristics disappear