ASTM F996-11(2018)
(Test Method)Standard Test Method for Separating an Ionizing Radiation-Induced MOSFET Threshold Voltage Shift Into Components Due to Oxide Trapped Holes and Interface States Using the Subthreshold Current–Voltage Characteristics (Withdrawn 2023)
Standard Test Method for Separating an Ionizing Radiation-Induced MOSFET Threshold Voltage Shift Into Components Due to Oxide Trapped Holes and Interface States Using the Subthreshold Current–Voltage Characteristics (Withdrawn 2023)
SIGNIFICANCE AND USE
5.1 The electrical properties of gate and field oxides are altered by ionizing radiation. The method for determining the dose delivered by the source irradiation is discussed in Practices E666, E668, E1249, and Guide E1894. The time dependent and dose rate effects of the ionizing radiation can be determined by comparing pre- and post-irradiation voltage shifts, ΔVot and ΔVit. This test method provides a means for evaluation of the ionizing radiation response of MOSFETs and isolation parasitic MOSFETs.
5.2 The measured voltage shifts, ΔVot and ΔVit, can provide a measure of the effectiveness of processing variations on the ionizing radiation response.
5.3 This technique can be used to monitor the total-dose response of a process technology.
SCOPE
1.1 This test method covers the use of the subthreshold charge separation technique for analysis of ionizing radiation degradation of a gate dielectric in a metal-oxide-semiconductor-field-effect transistor (MOSFET) and an isolation dielectric in a parasitic MOSFET.2,3,4 The subthreshold technique is used to separate the ionizing radiation-induced inversion voltage shift, ΔVINV into voltage shifts due to oxide trapped charge, ΔVot and interface traps, ΔV it. This technique uses the pre- and post-irradiation drain to source current versus gate voltage characteristics in the MOSFET subthreshold region.
1.2 Procedures are given for measuring the MOSFET subthreshold current-voltage characteristics and for the calculation of results.
1.3 The application of this test method requires the MOSFET to have a substrate (body) contact.
1.4 Both pre- and post-irradiation MOSFET subthreshold source or drain curves must follow an exponential dependence on gate voltage for a minimum of two decades of current.
1.5 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
1.6 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.
1.7 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
WITHDRAWN RATIONALE
This test method covered the use of the subthreshold charge separation technique for analysis of ionizing radiation degradation of a gate dielectric in a metal-oxide-semiconductor-field-effect transistor (MOSFET) and an isolation dielectric in a parasitic MOSFET.
Formerly under the jurisdiction of F01 on Electronics, this test method was withdrawn in November 2023. This standard is being withdrawn without replacement because Committee F01 was disbanded.
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This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
Designation: F996 − 11 (Reapproved 2018)
Standard Test Method for
Separating an Ionizing Radiation-Induced MOSFET
Threshold Voltage Shift Into Components Due to Oxide
Trapped Holes and Interface States Using the Subthreshold
Current–Voltage Characteristics
ThisstandardisissuedunderthefixeddesignationF996;thenumberimmediatelyfollowingthedesignationindicatestheyearoforiginal
adoptionor,inthecaseofrevision,theyearoflastrevision.Anumberinparenthesesindicatestheyearoflastreapproval.Asuperscript
epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope 1.6 This standard does not purport to address all of the
safety concerns, if any, associated with its use. It is the
1.1 This test method covers the use of the subthreshold
responsibility of the user of this standard to establish appro-
charge separation technique for analysis of ionizing radiation
priate safety, health, and environmental practices and deter-
degradation of a gate dielectric in a metal-oxide-
mine the applicability of regulatory limitations prior to use.
semiconductor-field-effect transistor (MOSFET) and an isola-
2,3,4 1.7 This international standard was developed in accor-
tion dielectric in a parasitic MOSFET. The subthreshold
dance with internationally recognized principles on standard-
technique is used to separate the ionizing radiation-induced
ization established in the Decision on Principles for the
inversion voltage shift, ∆V into voltage shifts due to oxide
INV
Development of International Standards, Guides and Recom-
trapped charge, ∆V and interface traps, ∆V . This technique
ot it
mendations issued by the World Trade Organization Technical
usesthepre-andpost-irradiationdraintosourcecurrentversus
Barriers to Trade (TBT) Committee.
gate voltage characteristics in the MOSFET subthreshold
region.
2. Referenced Documents
1.2 Procedures are given for measuring the MOSFET sub-
2.1 ASTM Standards:
thresholdcurrent-voltagecharacteristicsandforthecalculation
E666Practice for CalculatingAbsorbed Dose From Gamma
of results.
or X Radiation
1.3 The application of this test method requires the MOS-
E668 Practice for Application of Thermoluminescence-
FET to have a substrate (body) contact.
Dosimetry (TLD) Systems for Determining Absorbed
DoseinRadiation-HardnessTestingofElectronicDevices
1.4 Both pre- and post-irradiation MOSFET subthreshold
source or drain curves must follow an exponential dependence E1249Practice for Minimizing Dosimetry Errors in Radia-
tionHardnessTestingofSiliconElectronicDevicesUsing
on gate voltage for a minimum of two decades of current.
Co-60 Sources
1.5 The values stated in SI units are to be regarded as
E1894Guide for Selecting Dosimetry Systems for Applica-
standard. No other units of measurement are included in this
tion in Pulsed X-Ray Sources
standard.
3. Terminology
3.1 Definitions of Terms Specific to This Standard:
This test method is under the jurisdiction of ASTM Committee F01 on
3.1.1 anneal conditions—thecurrentand/orvoltagebiasand
Electronics and is the direct responsibility of Subcommittee F01.11 on Nuclear and
Space Radiation Effects.
temperature of the MOSFET in the time period between
Current edition approved March 1, 2018. Published April 2018. Originally
irradiation and measurement.
approved in 1991. Last previous edition approved in 2011 as F996–11. DOI:
10.1520/F0996-11R18.
3.1.2 doping concentration— n-or p-type doping, is the
McWhorter, P. J. and P. S. Winokur, “Simple Technique for Separating the
concentration of the dopant in the MOSFET channel region
Effects of Interface Traps and Trapped Oxide Charge in MOS Transistors,” Applied
adjacent to the oxide/silicon interface.
Physics Letters, Vol 48, 1986, pp. 133–135.
DNA-TR-89-157, Subthreshold Technique for Fixed and Interface Trapped
Charge Separation in Irradiated MOSFETs, available from National Technical
Information Service, 5285 Port Royal Rd., Springfield, VA 22161.
4 5
Saks, N. S., and Anacona, M. G., “Generation of Interface States by Ionizing For referenced ASTM standards, visit the ASTM website, www.astm.org, or
Radiation at 80K Measured by Charge Pumping and Subthreshold Slope contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
Techniques,” IEEE Transactions on Nuclear Science, Vol NS–34 , No. 6, 1987, pp. Standards volume information, refer to the standard’s Document Summary page on
1348–1354. the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
F996 − 11 (2018)
3.1.3 Fermi level—this value describes the top of the
collection of electron energy levels at absolute zero tempera-
ture.
3.1.4 intrinsic Fermi level—the energy level that the Fermi
level has in the absence of any doping.
3.1.5 inversion current, I —the MOSFETchannel current
INV
at a gate-source voltage equal to the inversion voltage.
3.1.6 inversion voltage, V —the gate-source voltage cor-
INV
responding to a surface potential of 2φ .
B
3.1.7 irradiation biases—the biases on the gate, drain,
source, and substrate of the MOSFET during irradiation.
3.1.8 midgap current, I —theMOSFETchannelcurrentat
MG
a gate-source voltage equal to the midgap voltage.
3.1.9 midgap voltage, V —the gate-source voltage corre-
MG
FIG. 1 Determination of Radiation Induced Voltage Shift for
sponding to a surface potential of φ .
B p-Channel MOSFET
3.1.10 oxide thickness, t —thethicknessoftheoxideofthe
ox
MOSFET under test.
3.1.11 potential, φ —the potential difference between the
B
5.2 The measured voltage shifts, ∆V and ∆V , can provide
ot it
Fermi level and the intrinsic Fermi level.
a measure of the effectiveness of processing variations on the
3.1.12 subthreshold swing—the change in the gate-source
ionizing radiation response.
voltage per change in the log source or drain current of the
5.3 This technique can be used to monitor the total-dose
MOSFET channel current below the inversion current. The
response of a process technology.
value of the subthreshold swing is expressed in V/decade (of
current).
6. Interferences
3.1.13 surface potential, φ —the potential at the MOSFET
s
6.1 Temperature Effects—The subthreshold drain current
semiconductor surface measured with respect to the intrinsic
variesastheexponentialof qφ /kT,andothertermswhichvary
Fermi level. B
as a function of temperature. Therefore, the temperature of the
measurement should be controlled to within 6 2°C, since the
4. Summary of Test Method
technique requires a comparison of pre- and post-irradiation
4.1 The subthreshold charge separation technique is based
data. At cryogenic temperatures, this test method may give
on standard MOSFET subthreshold current-voltage character- 4
misleading results.
istics.The subthreshold drain or source current at a fixed drain
6.2 Floating Body (Kink) Effects—Floating body effects
to source voltage, V , is measured as a function of gate
DS
occur in MOSFETs without body (substrate) ties. This test
voltage from the leakage current (or limiting resolution of the
method should not be applied to a MOSFET without a
measurement apparatus) through inversion. The drain current
V
G substrate or substrate/source contact.
and gate voltage are related by I α 10 . When plotted as log
D
I versus V , the linear I-V characteristic can be extrapolated
6.3 Short Channel Effects—To minimize drain voltage de-
D G
toacalculatedmidgapcurrent, I .Bycomparingthepre-and
pendence on the subthreshold curve, a small drain measure-
MG
post-irradiationcharacteristics,themidgapvoltageshift,∆V ment voltage is recommended but not necessary.
MG
can be determined.The value of ∆V is equal to ∆V , which
MG ot
6.4 Leakage Current—Because the MOSFET midgap cur-
is the voltage shift due to oxide trapped charge.The difference
rent is below the capabilities of practical current-voltage
between the inversion voltage shift, ∆V , and ∆V is equal
INV MG
measurement instrumentation, extrapolation of the subthresh-
to ∆V , which is the voltage shift due to interface traps. This
it
old swing is required for the determination of a MOSFET
procedure is shown in Fig. 1 for a p-channel MOSFET.
midgap voltage. Extrapolation of ideal linear MOSFET sub-
threshold current-voltage characteristics is unambiguous, be-
5. Significance and Use
cause of the constant subthreshold swing.An example of near
5.1 The electrical properties of gate and field oxides are ideal subthreshold characteristics is given in Fig. 2, where the
−11
altered by ionizing radiation. The method for determining the subthresholdcurrentswingisrelativelyconstantbetween10
−6
dose delivered by the source irradiation is discussed in Prac- and 10 A. Nonideal subthreshold characteristics, that are
tices E666, E668, E1249, and Guide E1894. The time depen- aberrations from the theoretical linear subthreshold swing, can
dent and dose rate effects of the ionizing radiation can be complicate the subthreshold current swing extrapolation to the
determined by comparing pre- and post-irradiation voltage midgap voltage. For subthreshold characteristics that have
shifts, ∆V and ∆V . This test method provides a means for multiple subthreshold swings, the value of the midgap voltage
ot it
evaluation of the ionizing radiation response of MOSFETs and would be dependent on the values of the subthreshold current
isolation parasitic MOSFETs. from which the extrapolation is made. Nonideal subthreshold
F996 − 11 (2018)
usually be much smaller than a standard width MOSFET
layout. Thus, when the MOSFET channel is in strong
inversion, the channel current will typically dominate.
However, as the channel current is reduced, edge leakage can
go from a minimal fraction to dominating the measured drain
or source current if the parasitic MOSFETinversion voltage is
less than the intentional MOSFET.This effect can be observed
in the measured subthreshold characteristics as a deviation
fromtheideallinearsubthresholdcurvethatisafunctionofthe
gate-source voltage. Examples of parasitic MOSFET induced
deviationsfromtheideallinearsubthresholdswingaregivenin
Fig. 3 and Fig. 4.In Fig. 3, the subthreshold swing changes
from the initial swing near inversion to a much larger mV/
decade swing. In Fig. 4, a more pronounced deviation is
shown. The section of the subthreshold curve that should be
FIG. 2 Near Ideal Subthreshold Characteristics from an
used for extrapolation to the midgap voltage is shown in both
n-Channel Transistor
figures. The upper section of the subthreshold curve above the
lower current level deviations was used. Any lower current
characteristics are caused by MOSFET leakage currents that
change in the subthreshold swing from the initial subthreshold
can be either independent of, or a function of, gate-source
swing below strong inversion should be considered a parasitic
voltage.
MOSFETinduced deviation. Only the part of the subthreshold
6.4.1 Junction Leakage Current—This leakage current is
curve above this deviation should be used for extrapolation as
from the drain to the substrate and is independent of gate-
is shown in Fig. 3 and Fig. 4. Some n-channel MOSFETs may
source voltage. Junction leakage current masks the actual
have post-irradiation edge leakage sufficiently large to prevent
MOSFET channel subthreshold current below the leakage
any observation of a subthreshold swing. The subthreshold
current level. Junction leakage current is easily distinguished
charge separation technique cannot be applied to these
from the channel subthreshold current as is shown in Fig. 2 by
samples.Aminimumoftwodecadesofsourceordraincurrent
−11
the flat section of the drain current, I , below 10 A. This
D
above any subthreshold swing deviation is required for appli-
figure also shows the advantage of using the source current, I
cationofthistestmethod.Openandclosed(annular)geometry
, for extrapolation. The source current is not affected by
S
layouts can be used to separate edge leakage current from the
junction leakage so that a measure of the MOSFET channel
MOSFET channel current.
currentisobtainedtotheinstrumentationnoiselevel.However,
6.4.4 Backchannel and Sidewall Leakage in a SOI
if there is not a separate source and substrate contact (for
MOSFET—Inasilicon-on-insulator(SOI)MOSFET,theback-
example, power MOSFETs), the drain current must be used.
channelleakagearisesfromaparasiticMOSFETlocatedatthe
Only the part of the subthreshold curve above the junction
interface between the epitaxial silicon and the insulator. Side-
leakage or instrumentation noise level should be used for
wall leakages arise from the parasitic MOSFET formed at the
extrapolation. A minimum of two decades of source or drain
edges of the intentional MOSFET. These parasitics distort the
currentabovetheleakageornoiseisrequiredforapplicationof
subthreshold curve in the same manner as described in 6.4.3.
this test method.
6.4.2 Gate Leakage—Gate leakage can be any combination
of leakage from the gate to source, drain, or substrate.
Typically this leakage will be a function of the gate-source
voltage. If gate leakage is greater than 1.0 µA for any
gate-source voltage, the test method should not be applied.
Gate leakages less than 1.0 µA can still cause nonideal
subthreshold characteristics. The minimum value of the sub-
threshold source or drain current used for extrapolation to the
midgapvoltagemustbeaboveanychangesinthesubthreshold
swing that can be attributed to gate leakage. Plotting the log of
thegateleakagealongwithlogsourceanddraincurrentonthe
same graph, will aid in the determination of gate leakage
effects on the drain and source subthreshold swing.
6.4.3 Edge Leakage Current—Most microcircuit MOSFETs
useanopengeometrylayoutsothationizingradiationinduced
drain to source leakage can occur in n-channel devices outside
of the intentional MOSFET channel. The effect of this edge
leakage on the subthreshold swing is dependent on the aspect
FIG. 3 Example of a Parasitic MOSFET Induced Deviation From
ratios and threshold voltages of the intentional and parasitic
the Ideal Linear Subthreshold Swing
MOSFETs. The aspect ratio of the parasitic MOSFET would
F996 − 11 (2018)
so as to minimize static discharge or other voltage transients
that may damage (alter current-voltage characteristics) the
DUT.
7.8 For control of measurement and data recording, the
subthreshold current-voltage characteristics can be measured
with a programmable tester having the proper current and
voltage capabilities, or with computer control of independent
power supplies and ammeters. T
...
NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
Contact ASTM International (www.astm.org) for the latest information
Designation: F996 − 11 (Reapproved 2018)
Standard Test Method for
Separating an Ionizing Radiation-Induced MOSFET
Threshold Voltage Shift Into Components Due to Oxide
Trapped Holes and Interface States Using the Subthreshold
Current–Voltage Characteristics
This standard is issued under the fixed designation F996; the number immediately following the designation indicates the year of original
adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A superscript
epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope 1.6 This standard does not purport to address all of the
safety concerns, if any, associated with its use. It is the
1.1 This test method covers the use of the subthreshold
responsibility of the user of this standard to establish appro-
charge separation technique for analysis of ionizing radiation
priate safety, health, and environmental practices and deter-
degradation of a gate dielectric in a metal-oxide-
mine the applicability of regulatory limitations prior to use.
semiconductor-field-effect transistor (MOSFET) and an isola-
2,3,4
1.7 This international standard was developed in accor-
tion dielectric in a parasitic MOSFET. The subthreshold
dance with internationally recognized principles on standard-
technique is used to separate the ionizing radiation-induced
ization established in the Decision on Principles for the
inversion voltage shift, ΔV into voltage shifts due to oxide
INV
Development of International Standards, Guides and Recom-
trapped charge, ΔV and interface traps, ΔV . This technique
ot it
mendations issued by the World Trade Organization Technical
uses the pre- and post-irradiation drain to source current versus
Barriers to Trade (TBT) Committee.
gate voltage characteristics in the MOSFET subthreshold
region.
2. Referenced Documents
1.2 Procedures are given for measuring the MOSFET sub-
2.1 ASTM Standards:
threshold current-voltage characteristics and for the calculation
E666 Practice for Calculating Absorbed Dose From Gamma
of results.
or X Radiation
1.3 The application of this test method requires the MOS-
E668 Practice for Application of Thermoluminescence-
FET to have a substrate (body) contact.
Dosimetry (TLD) Systems for Determining Absorbed
1.4 Both pre- and post-irradiation MOSFET subthreshold Dose in Radiation-Hardness Testing of Electronic Devices
E1249 Practice for Minimizing Dosimetry Errors in Radia-
source or drain curves must follow an exponential dependence
tion Hardness Testing of Silicon Electronic Devices Using
on gate voltage for a minimum of two decades of current.
Co-60 Sources
1.5 The values stated in SI units are to be regarded as
E1894 Guide for Selecting Dosimetry Systems for Applica-
standard. No other units of measurement are included in this
tion in Pulsed X-Ray Sources
standard.
3. Terminology
3.1 Definitions of Terms Specific to This Standard:
This test method is under the jurisdiction of ASTM Committee F01 on
3.1.1 anneal conditions—the current and/or voltage bias and
Electronics and is the direct responsibility of Subcommittee F01.11 on Nuclear and
Space Radiation Effects.
temperature of the MOSFET in the time period between
Current edition approved March 1, 2018. Published April 2018. Originally
irradiation and measurement.
approved in 1991. Last previous edition approved in 2011 as F996 – 11. DOI:
10.1520/F0996-11R18.
3.1.2 doping concentration— n- or p-type doping, is the
McWhorter, P. J. and P. S. Winokur, “Simple Technique for Separating the
concentration of the dopant in the MOSFET channel region
Effects of Interface Traps and Trapped Oxide Charge in MOS Transistors,” Applied
adjacent to the oxide/silicon interface.
Physics Letters, Vol 48, 1986, pp. 133–135.
DNA-TR-89-157, Subthreshold Technique for Fixed and Interface Trapped
Charge Separation in Irradiated MOSFETs, available from National Technical
Information Service, 5285 Port Royal Rd., Springfield, VA 22161.
4 5
Saks, N. S., and Anacona, M. G., “Generation of Interface States by Ionizing For referenced ASTM standards, visit the ASTM website, www.astm.org, or
Radiation at 80K Measured by Charge Pumping and Subthreshold Slope contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
Techniques,” IEEE Transactions on Nuclear Science, Vol NS–34 , No. 6, 1987, pp. Standards volume information, refer to the standard’s Document Summary page on
1348–1354. the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
F996 − 11 (2018)
3.1.3 Fermi level—this value describes the top of the
collection of electron energy levels at absolute zero tempera-
ture.
3.1.4 intrinsic Fermi level—the energy level that the Fermi
level has in the absence of any doping.
3.1.5 inversion current, I —the MOSFET channel current
INV
at a gate-source voltage equal to the inversion voltage.
3.1.6 inversion voltage, V —the gate-source voltage cor-
INV
responding to a surface potential of 2φ .
B
3.1.7 irradiation biases—the biases on the gate, drain,
source, and substrate of the MOSFET during irradiation.
3.1.8 midgap current, I —the MOSFET channel current at
MG
a gate-source voltage equal to the midgap voltage.
3.1.9 midgap voltage, V —the gate-source voltage corre-
MG
FIG. 1 Determination of Radiation Induced Voltage Shift for
sponding to a surface potential of φ .
B
p-Channel MOSFET
3.1.10 oxide thickness, t —the thickness of the oxide of the
ox
MOSFET under test.
3.1.11 potential, φ —the potential difference between the
B
5.2 The measured voltage shifts, ΔV and ΔV , can provide
ot it
Fermi level and the intrinsic Fermi level.
a measure of the effectiveness of processing variations on the
3.1.12 subthreshold swing—the change in the gate-source
ionizing radiation response.
voltage per change in the log source or drain current of the
5.3 This technique can be used to monitor the total-dose
MOSFET channel current below the inversion current. The
response of a process technology.
value of the subthreshold swing is expressed in V/decade (of
current).
6. Interferences
3.1.13 surface potential, φ —the potential at the MOSFET
s
6.1 Temperature Effects—The subthreshold drain current
semiconductor surface measured with respect to the intrinsic
varies as the exponential of qφ /kT, and other terms which vary
Fermi level.
B
as a function of temperature. Therefore, the temperature of the
measurement should be controlled to within 6 2°C, since the
4. Summary of Test Method
technique requires a comparison of pre- and post-irradiation
4.1 The subthreshold charge separation technique is based
data. At cryogenic temperatures, this test method may give
on standard MOSFET subthreshold current-voltage character- 4
misleading results.
istics. The subthreshold drain or source current at a fixed drain
6.2 Floating Body (Kink) Effects—Floating body effects
to source voltage, V , is measured as a function of gate
DS
occur in MOSFETs without body (substrate) ties. This test
voltage from the leakage current (or limiting resolution of the
method should not be applied to a MOSFET without a
measurement apparatus) through inversion. The drain current
V
G substrate or substrate/source contact.
and gate voltage are related by I α 10 . When plotted as log
D
I versus V , the linear I-V characteristic can be extrapolated
6.3 Short Channel Effects—To minimize drain voltage de-
D G
to a calculated midgap current, I . By comparing the pre- and
pendence on the subthreshold curve, a small drain measure-
MG
post-irradiation characteristics, the midgap voltage shift, ΔV
ment voltage is recommended but not necessary.
MG
can be determined. The value of ΔV is equal to ΔV , which
MG ot
6.4 Leakage Current—Because the MOSFET midgap cur-
is the voltage shift due to oxide trapped charge. The difference
rent is below the capabilities of practical current-voltage
between the inversion voltage shift, ΔV , and ΔV is equal
INV MG
measurement instrumentation, extrapolation of the subthresh-
to ΔV , which is the voltage shift due to interface traps. This
it
old swing is required for the determination of a MOSFET
procedure is shown in Fig. 1 for a p-channel MOSFET.
midgap voltage. Extrapolation of ideal linear MOSFET sub-
threshold current-voltage characteristics is unambiguous, be-
5. Significance and Use
cause of the constant subthreshold swing. An example of near
5.1 The electrical properties of gate and field oxides are ideal subthreshold characteristics is given in Fig. 2, where the
−11
altered by ionizing radiation. The method for determining the subthreshold current swing is relatively constant between 10
−6
dose delivered by the source irradiation is discussed in Prac- and 10 A. Nonideal subthreshold characteristics, that are
tices E666, E668, E1249, and Guide E1894. The time depen- aberrations from the theoretical linear subthreshold swing, can
dent and dose rate effects of the ionizing radiation can be complicate the subthreshold current swing extrapolation to the
determined by comparing pre- and post-irradiation voltage midgap voltage. For subthreshold characteristics that have
shifts, ΔV and ΔV . This test method provides a means for multiple subthreshold swings, the value of the midgap voltage
ot it
evaluation of the ionizing radiation response of MOSFETs and would be dependent on the values of the subthreshold current
isolation parasitic MOSFETs. from which the extrapolation is made. Nonideal subthreshold
F996 − 11 (2018)
usually be much smaller than a standard width MOSFET
layout. Thus, when the MOSFET channel is in strong
inversion, the channel current will typically dominate.
However, as the channel current is reduced, edge leakage can
go from a minimal fraction to dominating the measured drain
or source current if the parasitic MOSFET inversion voltage is
less than the intentional MOSFET. This effect can be observed
in the measured subthreshold characteristics as a deviation
from the ideal linear subthreshold curve that is a function of the
gate-source voltage. Examples of parasitic MOSFET induced
deviations from the ideal linear subthreshold swing are given in
Fig. 3 and Fig. 4. In Fig. 3, the subthreshold swing changes
from the initial swing near inversion to a much larger mV/
decade swing. In Fig. 4, a more pronounced deviation is
shown. The section of the subthreshold curve that should be
FIG. 2 Near Ideal Subthreshold Characteristics from an
used for extrapolation to the midgap voltage is shown in both
n-Channel Transistor
figures. The upper section of the subthreshold curve above the
lower current level deviations was used. Any lower current
characteristics are caused by MOSFET leakage currents that
change in the subthreshold swing from the initial subthreshold
can be either independent of, or a function of, gate-source
swing below strong inversion should be considered a parasitic
voltage.
MOSFET induced deviation. Only the part of the subthreshold
6.4.1 Junction Leakage Current—This leakage current is
curve above this deviation should be used for extrapolation as
from the drain to the substrate and is independent of gate-
is shown in Fig. 3 and Fig. 4. Some n-channel MOSFETs may
source voltage. Junction leakage current masks the actual
have post-irradiation edge leakage sufficiently large to prevent
MOSFET channel subthreshold current below the leakage
any observation of a subthreshold swing. The subthreshold
current level. Junction leakage current is easily distinguished
charge separation technique cannot be applied to these
from the channel subthreshold current as is shown in Fig. 2 by
samples. A minimum of two decades of source or drain current
−11
the flat section of the drain current, I , below 10 A. This
D
above any subthreshold swing deviation is required for appli-
figure also shows the advantage of using the source current, I
cation of this test method. Open and closed (annular) geometry
, for extrapolation. The source current is not affected by
S
layouts can be used to separate edge leakage current from the
junction leakage so that a measure of the MOSFET channel
MOSFET channel current.
current is obtained to the instrumentation noise level. However,
6.4.4 Backchannel and Sidewall Leakage in a SOI
if there is not a separate source and substrate contact (for
MOSFET—In a silicon-on-insulator (SOI) MOSFET, the back-
example, power MOSFETs), the drain current must be used.
channel leakage arises from a parasitic MOSFET located at the
Only the part of the subthreshold curve above the junction
interface between the epitaxial silicon and the insulator. Side-
leakage or instrumentation noise level should be used for
wall leakages arise from the parasitic MOSFET formed at the
extrapolation. A minimum of two decades of source or drain
edges of the intentional MOSFET. These parasitics distort the
current above the leakage or noise is required for application of
subthreshold curve in the same manner as described in 6.4.3.
this test method.
6.4.2 Gate Leakage—Gate leakage can be any combination
of leakage from the gate to source, drain, or substrate.
Typically this leakage will be a function of the gate-source
voltage. If gate leakage is greater than 1.0 µA for any
gate-source voltage, the test method should not be applied.
Gate leakages less than 1.0 µA can still cause nonideal
subthreshold characteristics. The minimum value of the sub-
threshold source or drain current used for extrapolation to the
midgap voltage must be above any changes in the subthreshold
swing that can be attributed to gate leakage. Plotting the log of
the gate leakage along with log source and drain current on the
same graph, will aid in the determination of gate leakage
effects on the drain and source subthreshold swing.
6.4.3 Edge Leakage Current—Most microcircuit MOSFETs
use an open geometry layout so that ionizing radiation induced
drain to source leakage can occur in n-channel devices outside
of the intentional MOSFET channel. The effect of this edge
leakage on the subthreshold swing is dependent on the aspect
FIG. 3 Example of a Parasitic MOSFET Induced Deviation From
ratios and threshold voltages of the intentional and parasitic
the Ideal Linear Subthreshold Swing
MOSFETs. The aspect ratio of the parasitic MOSFET would
F996 − 11 (2018)
so as to minimize static discharge or other voltage transients
that may damage (alter current-voltage characteristics) the
DUT.
7.8 For control of measurement and data recording, the
subthreshold current-voltage characteristics can be measured
with a programmable tester having the proper current and
volt
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