E5 Defects and Deep Levels

Topic

A crystal lattice defect in this semiconductor material, or a transition metal impurity within that same lattice, introduces a deep level within the material's bandgap—distinct from the shallow donor and acceptor impurities (described by another model in this set) located near the band edges. This deep level lies much closer to the middle of the bandgap than to either edge; consequently, ionizing it requires significantly more energy than the low ionization energy needed for those shallow donor or acceptor impurities. A more severe crystal lattice defect or a higher concentration of the transition metal impurity results in a correspondingly higher density of these deep levels within the bandgap.

The occupancy of this deep level—whether or not a charge carrier is trapped in it at a given moment—depends on both the semiconductor's Fermi level (defined by another model in this set) and the thermal velocity of the surrounding free charge carriers. The closer the Fermi level is to the deep level, the longer the level remains occupied by a trapped charge carrier; conversely, a higher thermal velocity among free charge carriers increases the frequency with which they collide with the deep level—either to become trapped or to escape if already trapped. Thus, as the Fermi level moves closer to the deep level, the occupancy of that deep level increases. Thus, this model—defined by the deep level and the specific nature of its introduction—allows that same deep level to function in one of two ways depending on the scenario: either as a recombination center (described in greater detail by another model in this set) or as a charge trap (characterized experimentally via capacitive relaxation following a filling pulse). As a recombination center, the deep level captures an electron followed by a hole (or vice versa), enabling them to recombine via the level itself rather than directly across the full bandgap; as a charge trap, the same deep level simply captures and releases charge carriers without any recombination occurring. The fact that the density of these deep levels—introduced here by a crystal lattice defect or a transition metal impurity—is identical to the defect density that degrades carrier lifetime in this semiconductor material (according to the Messenger-Spratt relationship) when that same lattice defect is induced by the displacement damage described by another model in this set, implies that both models ultimately describe the same physical consequence of the deep level acting as a recombination center.

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