Charge Collection Studies on Integrated Circuit Test Structures using Heavy-Ion Microbeams and MEDICI Simulation Calculations Page: 31
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possible due to the saturation of the drift velocity. Depending on the electric field,
MEDICI includes low field mobility models and high field dependent (transverse and
parallel field) models. Detailed discussions of these mobility models are given in the
MEDICI manual [111]. In this work, the CCSMOB semi-empirical mobility model is
chosen to account for the carrier-carrier scattering (CCS) effects due to the high
concentration of induced electron-hole pairs, the effects of lattice scattering and
ionization impurity scattering, and the effects of doping and temperature on mobility.
Diffusion coefficients For non-degenerately doped semiconductors where
Boltzmann statistics are applicable, the Einstein relationship bridges the carrier mobilities
and diffusion coefficients. The Einstein relations are derived from the assumption that the
doping density is light enough so that the Boltzmann statistics still apply. There are some
uncertainties about whether Einstein relations still apply for highly concentrated induced
electron-hole pairs along the ion track. A theoretical analysis predicts that the Einstein
relations should be modified so that CCS does not affect the ambipolar diffusion
coefficients due to the formation of electron-hole plasma along the ion track, even though
mobilities are affected [112]. However, experimental investigation indicates a
contradiction that CCS does affect the ambipolar diffusion coefficients, and the Einstein
relations still apply [113]. MEDICI assumes that the Einstein relations are still valid even
with the existence of transient high concentrations of electron-hole pairs along the ion
track.
Carrier Recombination and Generation (R-G) The dominated carrier R-G
mechanisms are photon transitions, phonon transitions, three particle (Auger) transitions,
and impact ionization. 1) Photogeneration occurs when sufficiently energetic photons are31
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Guo, Baonian. Charge Collection Studies on Integrated Circuit Test Structures using Heavy-Ion Microbeams and MEDICI Simulation Calculations, dissertation, May 2000; Denton, Texas. (https://digital.library.unt.edu/ark:/67531/metadc2469/m1/44/: accessed April 23, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; .