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A quantum-well design for controlling carrier dynamics was investigated for high-speed direct-modulation semiconductor lasers. The proposed well-in-well (WWell) structure decreases the carrier relaxation time into an active well by designing the wave function that is related to longitudinal optical phonon scattering rate. The electron relaxation time was analyzed numerically and was estimated to become 1/2–1/10 times smaller than that of a conventional quantum-well structure. This reduction of the relaxation time is effective for reducing the optical gain damping. The advantage of an asymmetrical WWell structure is also shown.

A large kink in the light output versus current characteristics was observed in semiconductor lasers with a tunnel injection quantum well structure. The output power was changed from 13 to 1.9 mW under a small current increase of 0.06I th (30 mA). The peak-to-valley (P/V) ratio of the output power was as high as 6.8. The kink current and the P/V ratio changed greatly within a monolayer thickness variation of the tunnel injection well. The kink mechanism that can be assumed was presented on the basis of the threshold current and the threshold voltage characteristics of the experimental results.