A detailed analysis of the electrical and optical properties of GaSb-AlSb quantum well infrared photodetector structures is presented. Hall effect measurements were performed, and a two-band conduction model was employed to extract the electron concentration in the L subband, indicating that at typical detector operating temperatures T=80 K there are at least 10(12) cm-2 carriers in the L subband. Temperature-dependent dark current measurements indicate that the dark current mechanism in detector structures consists of a tunneling regime for T<100 K, and a thermionic regime for T>100 K. An activation energy of 170 meV is extracted from the temperature dependence of the thermionic dark current. Photoluminescence and absorption measurements were also used to obtain information on the subband levels relative to the bottom of the well, and to investigate the effect of doping on the quality of the GaSb quantum wells. We also report on a method to push the intersubband transition to longer wavelengths by growing center-loaded structures in which a thin layer of AlSb is deposited in the center of the well in order to raise the energy of only the ground state in the well.