ISSN 1063-7397, Russian Microelectronics, 2006, Vol. 35, No. 3, pp. 177–184. © Pleiades Publishing, Inc., 2006.
Original Russian Text © G.I. Zebrev, 2006, published in Mikroelektronika, 2006, Vol. 35, No. 3, pp. 209–216.
The enhanced low-dose-rate sensitivity (ELDRS) of
bipolar integrated circuits (ICs) represents a serious
problem as it is found in many types of analog IC,
including operational ampliﬁers (op amps), compara-
tors, and voltage regulators . It is well known that the
radiation-induced degradation of metal–oxide–semi-
conductor ﬁeld-effect transistors (MOSFETs) grows in
magnitude with dose rate. In bipolar junction transis-
tors (BJTs), however, irradiation to a particular dose
may cause a larger amount of degradation if it occurs at
a lower dose rate, in which case the devices are said to
As a result, the radiation response of BJTs cannot be
predicted by the standard procedure designed for MOS-
FETs, which involves high-dose-rate irradiation fol-
lowed by annealing at an elevated temperature (typi-
C). With BJTs, the annealing tends to restore
the performance of devices under test.
2. CHARACTERISTICS OF ELDRS
A radiation-induced increase in BJT base current is
associated with recombination by radiation-induced
surface centers at the Si/SiO
interface [1, 2], which is
enhanced by p-base band bending due to positive-
charge buildup in the isolation oxide . The charge
buildup is mostly considered responsible for ELDRS,
but it is not yet clear why it proceeds more rapidly at a
lower dose rate.
ELDRS has the following main characteristics:
(i) It does not depend on the nature of the radiation,
occurring both in the laboratory (electron beams and
gamma and x rays) and in space .
(ii) It can affect any performance parameter. For
example, irradiation of LM124 op amps tends to
degrade the input current, so that ELDRS is commonly
associated with the base current of the input pnp BJT.
However, input-current degradation is also observed in
op amps with a MOSFET at the input, in which case
only the bias voltage is degraded.
(iii) It occurs in both npn and pnp BJTs.
(iv) It has not been observed in discrete devices,
which points to thick isolating layers as the source of
(v) It is stronger in relatively low ﬁelds (
as found in thick oxides.
(vi) It is accompanied by enhancement of both sur-
face-state generation and isolation-oxide charge
(vii) A higher temperature of irradiation leads to
much faster degradation; this effect allows one to pre-
dict low-dose-rate response by irradiation at 80–120
(viii) The degradation rate at a particular point of
time is determined by the dose rate at the same time
point; accordingly, the former can be varied by means
of the latter .
This last property is conclusive evidence for
ELDRS being a dose-rate effect, for which reason
ELDRS is often attributed to oxide ﬁeld reduction by
the space charge of radiation-induced mobile elec-
trons and holes [5–8]. The problem with this view is
that the net space charge is too small to affect the
oxide ﬁeld at a steady-state dose rate of any practical
magnitude, considering the low mobilities of elec-
trons and even holes,
) ~ 10 cm
) ~ 10
Some authors seek to relate ELDRS to the dose-rate
dependence of electron–hole recombination rate
[10, 11]. However, the extremely low densities of
mobile electrons and holes imply an insigniﬁcant
We propose that recombination can occur between
mobile carriers and ones trapped in the bulk of the
Modeling and Simulation of the Enhanced Low-Dose-Rate
Sensitivity of Thick Isolating Layers in Advanced ICs
G. I. Zebrev
Moscow Engineering Physics Institute (State University), Moscow, Russia
Received October 19, 2005
—A model is developed that yields a qualitative and a quantitative description of the enhanced low-
dose-rate sensitivity of BJTs. This effect is shown to be attributable to tail-state recombination increasing with
dose rate for thick isolating layers experiencing a relatively low electric ﬁeld.
AND SIMULATION IN SILICON MICROELECTRONICS