Quantum cascade laser gain medium modeling using a second-nearest-neighbor sp3sâ‹Š tight-binding model
The current turn-on and turn-off in a resonant tunneling diode (RTD) is determined by the crossing of the central resonance subband with the Fermi level in the emitter, the subbands of quasibound states in the emitter and the conduction band edge in the emitter. In a typical RTD the subbands in the central well and the emitter are similar, resulting in a simple resonant current flow for almost all transverse momenta. Since most of the electrons have zero transverse momentum, one therefore observes that most of the carriers travel straight through the structure. This paper presents a mechanism that can generate off-zone-center current flow in electron resonant tunneling diodes, where most of the carriers travel through the structure at an angle for a certain bias range. The basic idea is that if the effective mass in the RTD well is much smaller than the effective mass in the emitter, subband crossings will occur outside the zone center, resulting in this unintuitive distribution of the current as a function of transverse momentum. This mechanism is shown to increase the valley current within a single band approximation without non-parabolicity.