ISSN 10637397, Russian Microelectronics, 2010, Vol. 39, No. 2, pp. 91–99. © Pleiades Publishing, Ltd., 2010.
Original Russian Text © V.S. Pershenkov, D.V. Savchenkov, A.S. Bakerenkov, V.N. Ulimo, 2010, published in Mikroelektronika, 2010, Vol. 39, No. 2, pp. 102–112.
When irradiated to a given total absorbed dose,
bipolar junction transistors (BJTs) of whatever type
show an anomalous decrease in current gain (by one
order or more of magnitude) if the dose rate is low
enough. This phenomenon, known as enhanced low
doserate sensitivity (ELDRS), was traced to the iso
lation oxide . Affecting both bipolar and BiCMOS
integrated circuits (ICs), it can seriously reduce their
lifetime in space . In fact, ELDRS of the same
nature is also observed in the field oxides of power n
MOSFETs and advanced verylargescale ICs. Yet a
physical model of ELDRS suitable for making precise
and accurate predictions is still lacking; creating such
a model is particularly desirable for ICs designed to
work ten years in orbit, given that laboratory tests for
ELDRS will require dose rates three or four orders of
magnitude greater than those found in space if the tests
are to be conducted within hours.
A number of physical models have been proposed to
explain ELDRS. Fleetwood et al.  attribute the weaker
degradation of current gain at higher dose rates to the
presence of space charge inside the isolation oxide, which
should prevent charges from accumulating at the oxide–
semiconductor interface. Belyakov et al.  and Zebrev
 relate the weaker gain degradation to the capture of
radiationgenerated carriers by shallowlevel traps in the
oxide. Higher dose rates imply shorter periods of irradia
tion, fully occupied traps, and a lower yield of positive
charge due to recombination. At lower dose rates, carriers
should have enough time to leave the traps, making no dif
ference to the oxidetrapped charge. Rashkeev et al. 
extend the model of Fleetwood et al.  to include inter
facetrap buildup, emphasizing the considerable differ
ence in mobility between hydrogen ions and holes. Boch
et al.  and Hjalmarson et al.  investigate a bimolecu
lar mechanism of interfacetrap buildup at low dose rates.
The above models employ a large number of fitting
parameters to characterize interfacetrap buildup and
carrier dynamics (capture cross sections, capture
rates, recombinationcenter density, etc.). The trouble
with these quantities is that they cannot be deduced
from measurements, and have to be set arbitrarily.
Consequently, predictions made by the models are
bound to be qualitative, and are not well suited to use
This paper aims to construct a mathematical model
of ELDRS from the conversion model of interface
2. PREMISES AND GENERAL POINTS
2.1 The degradation of BJT current gain is associ
ated with an increase in the peripheral surface compo
nent of the base recombination current. The surface
recombination current is in general a function of the
interfacetrap density and surface potential of the
oxidesemiconductor interface .
An increase in the recombination loss is related to
the accumulation of a positive charge inside the isola
tion oxide and the formation of traps at the interface.
The model assumes that ELDRS is governed by the
relation of the interfacetrap buildup rate to dose rate.
The influence of the oxidetrapped charge on the
interface potential is assumed to be insignificant, given
that the charge is distributed throughout the oxide
(due to a weak electric field) in such a way that its cen
troid is located a considerable distance from the inter
face. This means that excess base current should be a
linear function of excess interfacetrap density only.
Conversion Model of Enhanced LowDoseRate Sensitivity
for Bipolar ICs
V. S. Pershenkov, D. V. Savchenkov, A. S. Bakerenkov, and V. N. Ulimov
Moscow Engineering Physics Institute (State University), Moscow, Russia
Received August 14, 2009
—A physical model is proposed for enhanced lowdoserate sensitivity (ELDRS) in bipolar junction
transistors. It is based on the conception of a shallow and a deep set of radiationinduced oxide traps that are
converted into interface traps under irradiation at high or low dose rates, respectively. Excess base current is
calculated by introducing an exponential or a powerlaw representation of the impulse response used in a
convolution integral. The former representation makes it possible to define time constants of conversion at
high or low dose rates, respectively. Values of fitting model parameters are determined that lead to close agree
ment with previously reported experimental results.
AND SIMULATION IN SILICON MICROELECTRONICS