ASTM F448-99(2005)
(Test Method)Test Method for Measuring Steady-State Primary Photocurrent
Test Method for Measuring Steady-State Primary Photocurrent
SIGNIFICANCE AND USE
The steady-state photocurrent of a simple p-n junction diode is a directly measurable quantity that can be directly related to device response over a wide range of ionizing radiation. For more complex devices the junction photocurrent may not be directly related to device response.
Zener Diode— In this device, the effect of the photocurrent on the Zener voltage rather than the photocurrent itself is usually most important. The device is most appropriately tested while biased in the Zener region. In testing Zener diodes or precision voltage regulators, extra precaution must be taken to make certain the photocurrent generated in the device during irradiations does not cause the voltage across the device to change during the test.
Bipolar Transistor—As device geometries dictate that photocurrent from the base-collector junction be much greater than current from the base-emitter junction, measurements are usually made only on the collector-base junction with emitter open; however, sometimes, to obtain data for computer-aided circuit analysis, the emitter-base junction photocurrent is also measured.
Junction Field-Effect Device—A proper photocurrent measurement requires that the source be shorted (dc) to the drain during measurement of the gate-channel photocurrent. In tetrode-connected devices, the two gate-channel junctions should be monitored separately.
Insulated Gate Field-Effect Device—In this type of device, the true photocurrent is between the substrate and the channel, source, and drain regions. A current which can generate voltage that will turn on the device may be measured by the technique used here, but it is due to induced conductivity in the gate insulator and thus is not a junction photocurrent.
SCOPE
1.1 This test method covers the measurement of steady-state primary photocurrent, Ipp, generated in semiconductor devices when these devices are exposd to ionizing radiation. These procedures are intended for the measurement of photocurrents greater than 10-9 A-s/Gy(Si or Ge), in cases for which the relaxation time of the device being measured is less than 25% of the pulse width of the ionizing source. The validity of these procedures for ionizing dose rates as great as 108Gy(Si or Ge)/s has been established. The procedures may be used for measurements at dose rates as great as 1010Gy(Si or Ge)/s; however, extra care must be taken. Above 108Gy/s the package response may dominate the device response for technologies such as complementary metal-oxide semiconductor, (CMOS)/silicon-on sapphire (SOS). Additional precautions are also required when measuring photocurrents of 10-9 A-s/Gy(Si or Ge) or lower.
1.2 Setup, calibration, and test circuit evaluation procedures are also included in this test method.
1.3 Because of the variability between device types and in the requirements of different applications, the dose rate range over which any specific test is to be conducted is not given in this test method but must be specified separately.
1.4 The values state in International System of Units (SI) are to be regarded as the standard. No other units of measurement are included in this standard.
1.5 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 and health practices and determine the applicability of regulatory limitations prior to use.
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Designation:F448–99(Reapproved2005)
Standard Test Method for
Measuring Steady-State Primary Photocurrent
ThisstandardisissuedunderthefixeddesignationF448;thenumberimmediatelyfollowingthedesignationindicatestheyearoforiginal
adoptionor,inthecaseofrevision,theyearoflastrevision.Anumberinparenthesesindicatestheyearoflastreapproval.Asuperscript
epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope E668 Practice for Application of Thermoluminescence-
Dosimetry(TLD)SystemsforDeterminingAbsorbedDose
1.1 Thistestmethodcoversthemeasurementofsteady-state
in Radiation-Hardness Testing of Electronic Devices
primary photocurrent, I , generated in semiconductor devices
pp
F526 Test Method for Measuring Dose for Use in Linear
when these devices are exposed to ionizing radiation. These
Accelerator Pulsed Radiation Effects Tests
procedures are intended for the measurement of photocurrents
−9
greater than 10 A·s/Gy(Si or Ge), in cases for which the
3. Terminology
relaxation time of the device being measured is less than 25%
3.1 Definitions:
of the pulse width of the ionizing source. The validity of these
8 3.1.1 fall time, n—the time required for a signal pulse to
proceduresforionizingdoseratesasgreatas10 Gy(SiorGe)/s
drop from 90 to 10% of its steady-state value.
has been established. The procedures may be used for mea-
10 3.1.2 primary photocurrent, n—the flow of excess charge
surements at dose rates as great as 10 Gy(Si or Ge)/s;
carriers across a p-n junction due to ionizing radiation creating
however,extracaremustbetaken.Above10 Gy/sthepackage
electron-hole pairs throughout the device. The charges associ-
response may dominate the device response for technologies
ated with this current are only those produced in the junction
such as complementary metal-oxide semiconductor, (CMOS)/
depletion region and in the bulk semiconductor material
silicon-on sapphire (SOS). Additional precautions are also
−9
approximately one diffusion length on either side of the
required when measuring photocurrents of 10 A·s/Gy(Si or
depletion region (or to the end of the semiconductor material,
Ge) or lower.
whichever is shorter).
1.2 Setup,calibration,andtestcircuitevaluationprocedures
3.1.3 pulse width, n—the time a pulse-amplitude remains
are also included in this test method.
above 50% of its maximum value.
1.3 Because of the variability between device types and in
3.1.4 rise time, n—the time required for a signal pulse to
the requirements of different applications, the dose rate range
rise from 10 to 90% of its steady-state value.
over which any specific test is to be conducted is not given in
this test method but must be specified separately.
4. Summary of Test Method
1.4 The values stated in International System of Units (SI)
4.1 In this test method, the test device is irradiated in the
are to be regarded as standard. No other units of measurement
primary electron beam of a linear accelerator. Both the irradia-
are included in this standard.
tion pulse and junction current (Fig. 1) are displayed and
1.5 This standard does not purport to address all of the
recorded. Placement of a thin, low atomic number (Z#13)
safety concerns, if any, associated with its use. It is the
scattering plate in the beam is recommended to improve beam
responsibility of the user of this standard to establish appro-
uniformity; the consequences of the use of a scattering plate
priate safety and health practices and determine the applica-
relatingtointerferencefromsecondaryelectronsaredescribed.
bility of regulatory limitations prior to use.
The total dose is measured by an auxiliary dosimeter. The
steady-statevaluesofthedoserateandjunctioncurrentandthe
2. Referenced Documents
relaxation time of the junction current are determined from the
2.1 ASTM Standards:
data trace and total dose.
4.2 In special cases, these parameters may be measured at a
This test method is under the jurisdiction of ASTM Committee F01 on
single dose rate under one bias condition if the test is designed
Electronics and is the direct responsibility of Subcommittee F01.11 on Quality and
to generate information for such a narrow application. The
Hardness Assurance.
preferred approach, described in this test method, is to char-
Current edition approved Jan. 1, 2005. Published January 2005. Originally
approved in 1975 as F448–75T. Last previous edition approved in 1999 as
acterize the radiation response of a device in a way that is
F448–99. DOI: 10.1520/F0448-99R05.
useful to many different applications. For this purpose, the
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
response to pulses at a number of different dose rates is
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
required. Because of the bias dependence of the depletion
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
F448–99 (2005)
FIG. 1 Ionization Radiation Pulse and Typical Primary Photocurrent Response
volume, it is possible that more than one bias level will be tions to the observed signal are proportional to applied field,
required during the photocurrent measurements. whilethoseduetosecondaryemissioneffects(see6.2)arenot.
The effects of air ionization external to the device may be
5. Significance and Use
minimized by coating exposed leads with a thick layer of
paraffin, silicone rubber, or nonconductive enamel or by
5.1 The steady-state photocurrent of a simple p-n junction
making the measurement in vacuum.
diode is a directly measurable quantity that can be directly
related to device response over a wide range of ionizing
6.2 Secondary Emission —Anotherspuriouscomponentof
radiation. For more complex devices the junction photocurrent
the measured current can result from charge emission from, or
may not be directly related to device response.
charge injection into, the test device and test circuit. This may
5.2 Zener Diode— In this device, the effect of the photo-
be minimized by shielding the surrounding circuitry and
current on the Zener voltage rather than the photocurrent itself
irradiating only the minimum area necessary to ensure irradia-
is usually most important. The device is most appropriately
tion of the test device. Reasonable estimates of the magnitude
testedwhilebiasedintheZenerregion.IntestingZenerdiodes
to be expected of current resulting from secondary-emission
or precision voltage regulators, extra precaution must be taken
effects can be made based on the area of metallic target
−11
tomakecertainthephotocurrentgeneratedinthedeviceduring
materialsirradiated.Valuesgenerallyrangebetween10 and
−9 2
irradiations does not cause the voltage across the device to
10 A·s/cm ·Gy, but the use of a scatter plate with an intense
change during the test.
beam may increase this current.
5.3 Bipolar Transistor—As device geometries dictate that
6.3 Orientation— The effective dose to a semiconductor
photocurrent from the base-collector junction be much greater
junction can be altered by changing the orientation of the test
than current from the base-emitter junction, measurements are
unit with respect to the irradiating electron beam. Most
usually made only on the collector-base junction with emitter
transistors and diodes may be considered “thin samples’’ (in
open; however, sometimes, to obtain data for computer-aided
terms of the range of the irradiating electrons). However,
circuit analysis, the emitter-base junction photocurrent is also
high-power devices may have mounting studs or thick-walled
measured.
casesthatcanacttoscattertheincidentbeam,therebyreducing
5.4 Junction Field-Effect Device—A proper photocurrent
the dose received by the semiconductor chip. Care must be
measurement requires that the source be shorted (dc) to the
taken in the mounting of such devices.
drain during measurement of the gate-channel photocurrent. In
6.4 Bias—As the effective volume for the generation of
tetrode-connected devices, the two gate-channel junctions
photocurrent in p-n junction devices includes the space-charge
should be monitored separately.
region, I may be dependent on applied voltage. As applied
pp
5.5 Insulated Gate Field-Effect Device—In this type of
voltagesapproachthebreakdownvoltage, I increasessharply
pp
device, the true photocurrent is between the substrate and the
due to avalanche multiplication. If the application of the test
channel, source, and drain regions. A current which can
device is known, actual bias values should be used in the test.
generate voltage that will turn on the device may be measured
If the application is not known, follow the methods for
by the technique used here, but it is due to induced conduc-
checking the bias dependence given in Section 10.
tivity in the gate insulator and thus is not a junction photocur-
6.5 Nonlinearity— Nonlinearities in photocurrent response
rent.
result from saturation effects, injection level effects on life-
times, and, in the case of bipolar transistors, a lateral biasing
6. Interferences
6.1 Air Ionization— A spurious component of the current
measuredduringaphotocurrenttestcanresultfromconduction
throughairionizedbytheirradiationpulse.Althoughthisisnot
Sawyer, J.A., and van Lint, V.A. J., “Calculations of High-Energy Secondary
Electron Emission,” Journal of Applied Physics, JAPIA, Vol 35, No 6, June 1964,
likely to be a serious problem for photocurrents greater than
−9 pp. 1706–1711.
10 A·s/Gy(Si or Ge), the spurious contribution can easily be
checked by measuring the current while irradiating the test
fixture in the absence of a test device.Air ionization contribu-
F448–99 (2005)
effect which introduces a component of secondary photocur- 7.4 Cabling, to complete adequately the connection of the
rent into the primary photocurrent measurement. For these test circuit in the exposure area with the power supply and
reasons, photocurrent measurements must generally be made oscilloscopes in the data area.Any type of ungrounded wiring
over a wide range of dose rates. may be used to connect the power supply to the bias points of
6.6 Electrical Noise— Since linear accelerator facilities are the test circuit; however, coaxial cables properly terminated at
inherent sources of r-f electrical noise, good noise-minimizing the oscilloscope input are required for the signal leads.
techniquessuchassingle-pointground,filtereddcsupplylines, 7.5 Test Circuits— One of the following test circuits:
etc., must be used in photocurrent measurements. 7.5.1 Resistor-Sampling Circuit (Fig. 2)—For most tests,
6.7 Temperature— Device characteristics are dependent on the configuration of Fig. 2(a) is appropriate. The resistors R
junction temperature; hence, the temperature of the test should serve as high-frequency isolation and must be at least 20 V.
be controlled. Unless otherwise agreed upon by the parties to The capacitor C supplies the charge during the current tran-
the test, measurements will be made at room temperature (23 sient; its value must be large enough that the decrease in
6 5°C). voltage during a current pulse is less than 10%. Capacitor C
6.8 Beam Homogeneity and Pulse-to-Pulse Repeatability— should be paralleled by a small (approximately 0.01 µF)
The intensity of a beam from a linear accelerator is likely to low-inductance capacitor to ensure that possible inductive
vary across its cross section. Since the pulse-shape monitor is
effects of the large capacitor are offset. The resistor R is to
placed at a different location from the device under test, the provide the proper termination (within 62%) for the coaxial
measured dose rate may be different from the dose rate to
cable used for the signal lead. When the photocurrents are
which the device was exposed. The spatial distribution and large, it is necessary to use a small-value resistor, R , in the
intensity of the beam may also vary from pulse to pulse. The
configuration of Fig. 2(b) to keep the signal small so as to
beam homogeneity and pulse-to-pulse repeatability associated maintain the bias across the junction within 10% of its
with a particular linear accelerator should be established by a
nominal value during the test. The response characteristics of
thoroughcharacterizationofitselectronbeampriortoperform- thiscircuitmustbeadequatetoensurethatthecurrentsignalis
ing a photocurrent measurement. accurately displayed (see 9.4).
6.9 Ionizing Dose— Each pulse of the linear accelerator 7.5.2 Current Transformer Circuit (Fig. 3)—In this circuit,
impartsadoseofradiationtoboththedeviceundertestandthe
R and Chavethesamesignificanceasintheresistor-sampling
device used for dosimetry. The ionizing dose deposited in a circuit, but it may be required that the signal cable monitoring
semiconductor device can change its operating characteristics. thecurrenttransformerbematchedtothecharacteristicimped-
As a result, the photocurrent that is measured after several ance of the transformer, in which case R would have this
pulses may be different from the photocurrent that is charac- impedance (within 62%), which is specified by the manufac-
teristic of an unirradiated device. Care should be exercised to turer of the current transformer. The current transformer must
ensure that the ionizing dose delivered to the device under test have a bandwidth sufficient to ensure that the current signal is
is as low as possible consistent with the requirements for a accurately displayed. Rise time must be less than 10% of the
given dose rate and steady-state conditions. Generally, this is pulse width of the radiation pulse being used. The low
done by minimizing the number of pulses the device receives.
frequency cutoff of some commercial current transformers is
The dose must not exceed 10% of the failure dose for the such that significant droop may occur for pulse widths greater
device.
than 1 µs. Do not use a transformer for which this droop is
6.10 The test must be considered destructive if the photo- greater than 5% for the radiation pulse width used. When
current exceeds the manufacturer’s absolute limit.
monitoring large photocurrents, care must be taken that the
ampere-microsecond saturation of the current transformer is
7. Apparatus
not exceeded.
7.1 Regulated dc Power Supply, with floating output to
7.6 Irradiation Pulse-Shape Monitor—Oneofthefollowing
produce the voltages required to bias the junction.
todevelopasignalproportionaltothedoseratedeliveredtothe
7.2 Oscilloscopes— Either a single dual-beam, or two
test device:
single-beam oscilloscopes that have adequate bandwidth capa-
7.6.1 Fast Signal-Diode, in the circuit
...
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