This new book studies the electrical, optical and structural properties of the Be-oped Ga In As/Al Ga As multiple quantum-well structures (MQWs) at different doping density in the Ga In As well, both designed and fabricated in house. The higher Be-doping density in the wells was found to enhance the compressive strain and increase barrier height of the Ga In As/Al Ga As MQWs through extensive optical characterization. It caused the shifts of the sub-energy levels in the valence band of the well material and the absorption wavelength resulting from the intersubband absorption. These observations were verified by theoretical calculation based on the six-band Luttinger-Kohn model by taking the Be-doping into account.