Field Dependent Dopant Deactivation in Bipolar Devices at Elevated irradiation Temperatures Page: 4 of 10
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B. Charge Separation Technique
Normalized C-V characteristics corresponding to irradia-
tion biases of -11, 0 and +11 V are shown in Fig. 1. A pre-
irradiation C-V curve is included for comparison. Flatband,
midgap and inversion capacitances, computed as a function of
the net doping concentration , are indicated for clarity.
Following irradiation under any bias (especially under posi-
tive bias), the C-V characteristics undergo a parallel shift due
to oxide trapped charge and stretchout due to interface traps.
In addition, the capacitance measured in depletion and inver-
sion decreases measurably following the 0 V irradiation. This
reduction in capacitance is a direct indication of the neutrali-
zation of substrate acceptors by hydrogen , [211, -
, , -. In contrast, there is a negligible decrease
in capacitance following irradiation under either the positive
bias or the negative bias, implying that relatively little dopant
neutralization has occurred.
Concentrations of deactivated substrate dopants and radia-
tion-induced densities of net positive oxide trapped charge
and interface traps were determined from the C-V character-
istics using a recently introduced charge separation technique
, . Because the Si surface potential is a function of the
concentration of electrically active dopants, dopant neutrali-
zation leads directly to shifts in the midgap and inversion
voltages. The principal advantage of this technique is that it
accounts for the effect of net doping changes on the electro-
static potential. Once dopant-related shifts in the inversion
and midgap voltages are resolved, radiation-induced densities
of oxide trapped charge and interface traps are determined by
assuming charge neutrality of the interface traps at midgap
. In the limit of no radiation-induced dopant deactivation,
this approach reduces to the standard midgap charge separa-
tion technique  used widely for the analysis of room-
Fig. 2 shows the concentration of acceptors neutralized
near the Si surface as a function of irradiation bias. Vertical
dashed lines define regions of bias corresponding to accumu-
-8 -6 -4 -2 0 2 4 6 8 10 12 14
Gate-to-Body Bias (V)
Fig. 1. Effect of irradiation bias on the C-V characteristics for irradiation at
10 rad(SiO2)/s and 100 *C. The capacitance decrease in depletion and inver-
sion for 0 V irradiation is a direct indication of the neutralization of Si ac-
ceptors by hydrogen.
accumulation depletion: inversion
v ; 0
y0.3 0 0 8
z S 0
-12 -8 -4 0 4 8 12 16 20
Irradiation Bias (V)
Fig. 2. Effect of irradiation bias on the concentration of acceptors neutral-
ized near the Si surface. The acceptor neutralization is most severe for irra-
diation biases in depletion.
lation, depletion and inversion prior to irradiation. Each re-
gion exhibits a distinct behavior with respect to dopant neu-
tralization. The neutralization is most significant for irradia-
tion biases in depletion. Within this range, the acceptor neu-
tralization is a strong function of bias, peaking near 1 V. At
peak efficiency, approximately 25% of the electrically active
acceptors are neutralized by radiation exposure. By compari-
son, the concentration of neutralized acceptors is small for
irradiation biases in accumulation and inversion and increases
moderately with bias in inversion.
Densities of radiation-induced oxide trapped charge, ANot,
and interface traps, ANit, are plotted as a function of irradia-
tion bias in Figs. 3(a) and 3(b), respectively. The bias de-
pendencies of ANot and ANit are consistent with well-known
hydrogen - and trapped hole - models for
radiation-induced interface trap formation, where a moder-
ately positive E-field aids the transport of H+ ions and holes
to the Si-SiO2 interface. Above - 11 V, ANot and ANit de-
crease with bias due to a reduction in capture cross-sections
for hole traps near the Si-SiO2 interface . This reduction
in the buildup of oxide defects coincides with a moderate
increase in the neutralized acceptor concentration over the
same range of biases. The correlation of the two suggests that
the increase in acceptor neutralization at large positive biases
is related to an increase in the number of H+ ions and/or holes
reaching the Si depletion region.
III. MODEL FOR DOPANT DEACTIVATION
Hydrogen is known to neutralize many types of shallow ac-
ceptors (especially B) in Si , , -, , -
. Considerable evidence exists to suggest that the neu-
tralization occurs primarily through two mechanisms. In the
case of acceptor passivation , , -, H0 can de-
activate B- through the reaction
B- + H + h+ - (BH) ,
where h+ represents a free hole. In the passivating state, the
* - flatband
0 V midgap
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WITCZAK,STEVEN C.; LACOE,RONALD C.; SHANEYFELT,MARTY R.; MAYER,DONALD C.; SCHWANK,JAMES R. & WINOKUR,PETER S. Field Dependent Dopant Deactivation in Bipolar Devices at Elevated irradiation Temperatures, article, August 15, 2000; Albuquerque, New Mexico. (https://digital.library.unt.edu/ark:/67531/metadc723495/m1/4/: accessed April 18, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.