The effect of band-tails on the design of GaAs/AlGaAs bipolar transistor carrier-injected optical modulator/switch

The variations in the absorption coefficient and the refractive index in the waveguide region of a double-heterostructure N-AlGaAs/p-GaAs/N-AlGaAs bipolar transistor carrier-injected optical modulator/switch are calculated theoretically using a band-tail model. The refractive index change is calculated by a Kramers-Kronig analysis of the absorption curve, which changes in the vicinity of the absorption edge primarily because of the effect of band-filling, band-gap shrinkage, and plasma dispersion associated with the presence of free carriers. The performance of bipolar transistor carrier-injected optical modulator/switch is analyzed on the basis of the calculated results, including the alpha-parameter and the maximum modulation depth. It is shown that small frequency chirping and high modulation depth with low absorption loss can be achieved by properly selecting the optimum operating conditions