Browsing by Subject "High field transport"
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Item Open Access Computational modeling of quantum-confined impact ionization in Si nanocrystals embedded in SiO2(2007) Sevik, C.; Bulutay, C.Injected carriers from the contacts to delocalized bulk states of the oxide matrix via Fowler-Nordheim tunneling can give rise to quantum-confined impact ionization (QCII) of the nanocrystal (NC) valence electrons. This process is responsible for the creation of confined excitons in NCs, which is a key luminescence mechanism. For a realistic modeling of QCII in Si NCs, a number of tools are combined: ensemble Monte Carlo (EMC) charge transport, ab initio modeling for oxide matrix, pseudopotential NC electronic states together with the closed-form analytical expression for the Coulomb matrix element of the QCII. To characterize the transport properties of the embedding amorphous SiO2, ab initio band structure and density of states of the α-quartz phase of SiO2 are employed. The confined states of the Si NC are obtained by solving the atomistic pseudopotential Hamiltonian. With these ingredients, realistic modeling of the QCII process involving a SiO2 bulk state hot carrier and the NC valence electrons is provided.Item Open Access Harmonic enhancement of Gunn oscillations in GaN(American Institute of Physics, 2005) Sevik, Cem; Yılmaz, Dündar E.; Bulutay, CeyhunHigh field transport in wide bandgap semiconductors like GaN is of great technological importance. The negative differential mobility regime at high fields, under suitable conditions, can lead to millimeter-wave Gunn oscillations. Using extensive simulation based an ensemble Monte Carlo technique, the prospects of GaN Gunn diodes are theoretically investigated. The possibility of operating these Gunn diodes at their higher harmonic modes are explored. Main finding of this research is that the carrier dynamics in GaN can be tailored by an optimum choice of doping profile, temperature and bias conditions so that the efficiency of higher harmonic Gunn oscillations can be boosted.Item Open Access High field transport phenomena in wide bandgap semiconductors(Bilkent University, 2003) Sevik, CemThe Ensemble Monte Carlo (EMC) method is widely used in the field of computational electronics related to the simulation of the state of the art devices. Using this technique our specific intention is to scrutinize the high-field transport phenomena in wide bandgap semiconductors (Such as GaN, AlGaN and AlN). For this purpose, we have developed an EMC-based computer code. After a brief introduction to our methodology, we present detailed analysis of three different types of devices, operating under high-field conditions, namely, unipolar n-type structures, avalanche photodiodes (APD) and finally the Gunn diodes. As a testbed for understanding impact ionization and hot electron effects in sub-micron sized GaN, AlN and their ternary alloys, an n +−n−n + channel device is employed having a 0.1 µm-thick n region. The time evolution of the electron density along the device is seen to display oscillations in the unintentionally doped n-region, until steady state is established. The fermionic degeneracy effects are observed to be operational especially at high fields within the anode n +-region. For AlxGa1−xNbased systems, it can be noted that due to alloy scattering, carriers cannot acquire the velocities attained by the GaN and AlN counterparts. Next, multiplication and temporal response characteristics under a picosecond pulsed optical illumination of p +-n-n + GaN and n-type Schottky Al0.4Ga0.6N APDs are analyzed. For the GaN APD, our simulations can reasonably reproduce the available measured data without any fitting parameters. In the case of AlGaN, the choice of a Schottky contact APD is seen to improve drastically the field confinement resulting in satisfactory gain characteristics. Moreover, alloy scattering is seen to further slow down the temporal response while displacing the gain threshold to higher fields. Finally, the dynamics of large-amplitude Gunn domain oscillations from 120 GHz to 650 GHz are studied in detail by means of extensive EMC simulations. The basic operation is checked under both impressed single-tone sinusoidal bias and external tank circuit conditions. The width of the doping-notch is observed to enhance higher harmonic efficiency at the expense of the fundamental frequency up to a critical value, beyond which sustained Gunn oscillations are ceased. The degeneracy effects due to the Pauli Exclusion principle and the impact ionization are also considered but observed to have negligible effect within the realistic operational bounds. Finally, the effects of lattice temperature, channel doping and DC bias on the RF conversion efficiency are investigatedItem Open Access Simulation of millimeter-wave gunn oscillations in gallium nitride(2004) Sevik, C.; Bulutay, C.High field transport in wide bandgap semiconductors like GaN is of great technological importance. The negative differential mobility regime at high fields, under suitable conditions, can lead to millimeterwave Gunn oscillations. Using extensive simulation based an ensemble Monte Carlo technique, the prospects of GaN Gunn diodes are theoretically investigated. The possibility of operating these Gunn diodes at their higher harmonic modes are explored. The main finding of this research is that the carrier dynamics in GaN can be tailored by an optimum choice of doping profile, temperature and bias conditions so that the efficiency of higher harmonic Gunn oscillations can be boosted. Finally, the physical origin of these Gunn oscillations is sought exploring whether it is the intervalley scattering mechanism, the Γ valley nonparabolicity, or the effective mass discrepancy between the Γ and the lowest satellite valleys as the responsible mechanism.