We have implemented a self-consistent non-equilibrium Green's functions approach for vertical charge transport and
optical gain in terahertz quantum cascade lasers (THz-QCL) in the stationary limit. We present theoretical results of the
current-voltage characteristics and the temperature dependence of the optical gain for GaAs/Al.15Ga.85As THz-QCLs and
compare our results with experimental data. We find excellent quantitative agreement for the current-voltage
characteristics and the peak gain energy and identify non-radiative transitions between the laser levels as primary factor
that limits the maximum operation temperature. Furthermore, we find significant coherent leakage of electrons in the
upper laser level that increases the threshold current density. We propose a broadening of the collector well to efficiently
suppress this coherent leakage and to reduce the threshold current.
We present a novel charge self-consistent eight-band k•p envelope function method for the calculation of the
electronic structure of type-II broken-gap heterostructures. Standard multiband k•p approaches fail to yield
the correct occupation of electronic states in broken-gap heterostructures, because the strong hybridization of
conduction band and valence band states is incompatible with the separate occupation of electron and hole states
that is common to envelope function approaches. In our method, we occupy all included subbands with electrons
according to the Fermi statistics and subsequently subtract a positive background ionic charge that guarantees
charge neutrality. With this procedure, we have calculated local charge densities and subband dispersions of
periodically n and p doped GaAs layers as well as effective band gaps of intrinsic InAs/GaSb superlattices.
We present a theoretical study of hot-carrier induced light emission in III-V semiconductor devices. Carrier heating under the intense electric fields present under high bias conditions are studied via a selfconsistent Monte Carlo simulation. The carrier distribution functions obtained from the simulation are then incorporated into a pseudo-potential algorithm that describes the direct optical transitions and calculates the corresponding spectra. We show that the light emission due to hot carriers is dominated by direct radiative interband transitions within the conduction and valence bands. Good agreement between theory and experiment is obtained for GaAs MESFET and GaAs/AlGaAs HBTs.
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