Effective mobility in silicon surface channels as measured with the MOS transistor
Pierret, Robert Francis
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https://hdl.handle.net/2142/25063
Description
Title
Effective mobility in silicon surface channels as measured with the MOS transistor
Author(s)
Pierret, Robert Francis
Issue Date
1966
Doctoral Committee Chair(s)
Sah, C.T.
Department of Study
Physics
Discipline
Physics
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
silicon surface channels
MOS transistor
mobility theory
p-channel
n-channel
Language
en
Abstract
A detailed study, both theoretical and experimental, was made
of the effective mobility in silicon surface channels. Theoretical
pursuits included a) ,specialization of the existing effective mobility
theory to silicon and to the MOS transistor, and b) extension of the
existing effective mobility theory by the removal of various
simplifying assumptions. Mobility measurements were confined primarily
to inversion p-channel MOS transistors, although some built-in-channel
and inversion n-channel results were obtained.
A careful examination of effective mobility theory revealed that
the Schrieffer constant field approximation introduced a significant
error into the mobility predicted for the operational region of an
inversion-channel, MOST. Extensive numerical calculations using the
exact-field diffuse scattering theory exhibited a strong bulk doping
dependence which was open to direct experimental verification.
Theoretical extensions of the existing theory were accomplished through
the direct integration of the Boltzmann equation. An effective mobility
expression, valid for a non-constant 7: and for an arbitrary surface
scattering condition was developed employing this technique. Theoretical
consideration was also given to the problems of part-spectral scattering
and to that of a non-constant electric field parallel to the surface
of the semiconductor. Finally, a general solution of the Boltzmann
equation was obtained for the many valley band structure which is
appropriate for electrons in silicon. It was shown that the mobility
is isotropic in the plane parallel to the surface if the field
perpendicular to the surface is oriented in a.OOO) direction.
Mobility formulas and calculations were presented for this particular
orientation.
Experimentally, great care was taken to obtain accurate transistor.
parameters and thus eliminate the error due to parametric uncertainty.
This was accomplished, in part, through the fabrication of an MOS
capacitor adjacent to each transistor. The experimental mobility deduced
under the constant Qs s assumption, when compared with the exact-field
diffuse scattering theory, was still found to be high near the band
edge and low near the turn-on of inversion-channel devices. Further,
the doping dependence expected near turn-on was not observed. The
discrepancies between theory and experiment near turn-on were concluded
to be a direct result of a variation in the surface state concentration.
The disagreement near the band edge was attributed to the failure
of the existing diffuse scattering theory in this region. On the basis
of the data presently available, however, it was felt that no definite
conclusion could be made concerning the nature of the surface scattering.
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