Heavy impurity doping leads to bandgap narrowing (BGN) and this causes perturbations to the value of the band offsets at the hetero-interface. As a result, the form and height of energy barriers in abrupt HBT is disturbed, which changes the value of the current flowing through its interfaces. The analysis is based on the thermionic field-diffusion model which combines the drift-diffusion transport in the bulk of the transistor with the thermionic emission and tunneling at the base-emitter interface. The calculations reveal a more important role to the transport of carriers played by the modification of the built-in potential than that of the range of barrier energies available for tunneling because the impact of the built-in potential on the current is exponential. Therefore, it is important for a better description of the currents to use an accurate dopant-dependent BGN distribution model between bands.
In this paper, an InP-based mesa-structure uni-traveling-carrier photodetector is designed. By adopting Gaussian doping scheme in the absorption layer and incorporating an appropriate cliff layer, high speed and high saturation current characteristics are both achieved simultaneously. For the device with a 14 μm2 active area, the simulated results indicate that the bandwidth reaches 58 GHz and DC saturation current increases up to 158 mA at a reverse bias of 2 V. Under high optical injection, the bandwidth degradation and current saturation are studied, which are caused by energy band shift and electric field collapse.
The electronic structure tailoring of GaAs nanowires through surface modification was investigated by first-principles calculations. The effect of different surface-passivation materials (H, F, Cl, Br, and I) on the electronic structure of the GaAs nanowires was studied. The results show that for different atoms, the tailoring of the electronic structure is mainly determined by their passivation ability. The surface modification tunes the bandgap and also the bandgap types. The electronic structure of the GaAs nanowires was determined by the surface states and the quantum-confinement effect jointly. The amplitude of the bandgap variation on the diameter is different for the GaAs nanowires modified with different materials. Surface modification offers a new way to tailor the bandgap of GaAs nanowires without changing their diameter or crystal structure.
All-optical tunable slow light technology has promising applications in all-optical network and optical information process. Tunable slow light based on stimulated Brillouin scattering is experimentally investigated, using highly nonlinear microstructured fiber which is designed and drawn by ourselves. The experiment setup is composed of single pump and a single-stage delay. When the pump power reaches 162.6 mW, a maximum delay of 76 ns, equal to 0.76 pulse width, is achieved in the highly nonlinear microstructured fiber of 120 m in length. In addition, by adjusting the pump power, the tunable slow light can be realized. This scheme of slow light has advantages such as large delay, being all-optical tunable and compatible with optical communications systems.
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