ABSTRACT
Semiconductor lasers utilizing quantum wells as the active layers are called quantum well lasers. These lasers have superior characteristics, such as ultra-low threshold and less sensitivity to temperature. As the active layers are very thin in quantum well lasers, a slight lattice mismatch up to around 1% between the quantum well layers and the surrounding layers is allowed. These quantum well lasers are called the strained layer quantum well lasers. One of the main advantage of the strained quantum well lasers is that the limit of the choice of heterostructure combination is relaxed, by which the lasing wavelength which is impossible in the lattice matched case becomes possible. Lasers utilizing transitions only within the levels in the conduction band are also possible in quantum wells. These lasers are called the quantum cascade lasers or intersubband lasers. In these lasers, unipolar carriers are used.
