Current induced absorption modulation using quantum structures in the collector of heterojunction bipolar transistors

Heterojunction bipolar transistors (HBTs) can be operated as current sources with a bandwidth in excess of 100 GHz. In the work, various types of quantum structures were incorporated in the collector of InP based HBTs in order to study their current induced light modulation capability. Changes in the absorption spectrum due to electron capture in the quantum wells were observed and compared to the electric field induced quantum confined Stark effect. The results demonstrate the feasibility of a new type of light modulators, in which current rather than voltage modulates the absorption spectrum of the quantum wells. The physics underlying the carrier influence on the quantum well absorption was already demonstrated in the study of barrier reservoir and quantum well electron transfer structures. In the work, however, carrier accumulation is achieved by equilibrium between current induced carrier capture in the wells, and an electric field induced escape of carriers from the wells