The purpose of this paper is to present a review of the basic issues implicit in the design of confined state photodetectors. The basic device structure consists of repeated unit cells each comprised of a narrow gap semiconductor layer sandwiched between barrier layers of wider band gap material. Gain in these structures is derived through carrier multiplication via impact excitation of confined electrons out of the narrow gap semiconductor layer. Different device designs are considered in an attempt to maximize the device gain at minimum dark current. In some implementations, the barrier layers are chosen to be graded such that the leading edge discontinuity is at least twice that at the trailing edge of the well forming an asymmetric well design. It is found that an asymmetric well design offers a much higher impact excitation of electrons confined within the well at lower operating voltage than a symmetric well design however at the expense of increased dark current. Quantum versus classical confinement of the electrons within the well is also investigated.