Measurement Techniques

Measurement Techniques
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Measurement Techniques
INTRODUCTION
VS = 80 V
( > VR max.)
The characteristics of optoelectronics devices given in
datasheets are verified either by 100 % production tests
followed by statistic evaluation or by sample tests on typical
specimens. These tests can be divided into following
categories:
I = 10 µA
100 µA
constant
• Dark measurements
VR
• Light measurements
• Measurements of switching
frequency and capacitance
characteristics,
V
cut-off
Ri > 10 MΩ
• Angular distribution measurements
94 8206
• Spectral distribution measurements
Fig. 2
• Thermal measurements
Dark and light measurements limits are 100 %
measurements. All other values are typical. The basic
circuits used for these measurements are shown in the
following sections. The circuits may be modified slightly to
accommodate special measurement requirements.
For most devices, VR is specified at 10 μA reverse current.
In this case either a high impedance voltmeter has to be
used, or current consumption of DVM has to be calculated
and added to the specified current. A second measurement
step will then give correct readings.
Most of the test circuits may be simplified by use of a source
measure unit (SMU), which allows either to source voltage
and measure current or to source current and measure
voltage.
In case of IR diodes, total radiant output power, Φe, is
usually measured. This is done with a calibrated large-area
photovoltaic cell fitted in a conical reflector with a bore
which accepts the test item - see figure 3. An alternative test
set uses a silicon photodiode attached to an integrating
sphere. A constant DC or pulsating forward current of
specified magnitude is passed through the IR diode. The
advantage of pulse-current measurements at room
temperature (25 °C) is that results can be reproduced
exactly.
DARK AND LIGHT MEASUREMENTS
EMITTER DEVICES
IR Diodes
Forward voltage, VF, is measured either on a curve tracer or
statically using the circuit shown in figure 1. A specified
forward current (from a constant current source) is passed
through the device and the voltage developed across it is
measured on a high-impedance voltmeter.
Photo Voltaic Cell, Calibrated
VS = 5 V
I = 50 mA
100 mA
constant
IF
VF
V
94 8155
R i > 10 kΩ
Reflector
948205
Fig. 3
Fig. 1
To measure reverse voltage, VR, a 10 μA or 100 μA reverse
current from a constant current source is impressed through
the diode (figure 2) and the voltage developed across is
measured on a voltmeter of high input impedance (≥ 10 MΩ).
Rev. 1.4, 31-Jul-12
If, for reasons of measurement economy, only DC
measurements (figure 4) are to be made, then the energizing
time should be kept short (below 1 s) and of uniform
duration, to minimize any fall-off in light output due to
internal heating.
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DETECTOR DEVICES
Photovoltaic cells, photodiodes
VS = 5 V
• Dark measurements
I = 100 mA
constant
The reverse voltage characteristic, VR, is measured either on
a curve tracer or statically using the circuit shown in figure
6. A high-impedance voltmeter, which draws only an
insignificant fraction of device’s reverse current, must be
used.
Ik
RL
V
VS > VR
Ri ≥ 10 kΩ
RL = 1 Ω to 10 Ω
IR = 100 µA
constant
94 8207
Fig. 4
E=0
To ensure that the relationship between irradiance and
photocurrent is linear, the photodiode should operate near
the short-circuit configuration. This can be achieved by
using a low resistance load (≤ 10 Ω) of such a value that the
voltage dropped across is very much lower than the open
circuit voltage produced under identical illumination
conditions (Rmeas << Ri). The voltage across the load should
be measured with a sensitive DVM.
A knowledge of radiant intensity, Ie, produced by an IR
emitter enables customers to assess the range of IR light
barriers. The measurement procedure for this is more or less
the same as the one used for measuring radiant power. The
only difference is that in this case the photodiode is used
without a reflector and is mounted at a specified distance
from, and on the optical axis of, the IR diode (figure 5). This
way, only the radiant power of a narrow axial beam is
considered.
The radiant power within a solid angle of Ω = 0.01 steradian
(sr) is measured at a distance of 100 mm. Radiant intensity
is then obtained by using this measured value for calculating
the radiant intensity for a solid angle of Ω = 1 sr.
VF
V
Ri ≥ 10 MΩ
94 8209
Fig. 6
Dark reverse current measurements, Iro, must be carried out
in complete darkness - reverse currents of silicon
photodiodes are in the range of nanoamperes only, and an
illumination of a few lx is quite sufficient to falsify the test
result. If a highly sensitive DVM is to be used, then a current
sampling resistor of such a value that voltage dropped
across it is small in comparison with supply voltage must be
connected in series with the test item (figure 7). Under these
conditions, any reverse voltage variations of the test
samples can be ignored. Shunt resistance (dark resistance)
is determined by applying a very slight voltage to the
photodiode and then measuring dark current. In case of
10 mV or less, forward and reverse polarity will result in
similar readings.
VS = 20 V
Photo Voltaic Cell with Filter
(Calibrated), 1 cm 2
E=0
Ω = 0.01 sr
Iro
10 kΩ
mV
Ri ≥ 1 MΩ
a = 100 mm
94 8210
Position of the Emitting Area
Fig. 5
Fig. 7
94 8156
• Light measurements
The same circuit as used in dark measurement can be used
to carry out light reverse current, Ira, measurements on
photodiodes. The only difference is the diode is now
irradiated and a current sampling resistor of lower value
must be used (figure 8), because of the higher currents
involved.
Rev. 1.4, 31-Jul-12
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the same lux and color temperature calibration) may result
in readings that differ up to 20 %.
VS = 20 V
EA = 1 klx or
Ee = 1 mW/cm2
The simplest way to overcome this problem is to calibrate
(measure the light current) some items of a photodetector
type with a standard lamp (OSRAM WI 41/G) and then use
these devices for adjustment of the lamp used for field
measurements.
Ira
10 Ω
An IR diode is used as a radiation source (instead of a
Tungsten incandescent lamp), to measure detector devices
being used mainly in IR transmission systems together with
IR emitters (e.g., IR remote control, IR headphone).
Operation is possible both with DC or pulsed current.
mV
Ri = 10 kΩ
The adjustment of irradiance, Ee, is similar to the above
mentioned adjustment of illuminance, Ev. To achieve a high
stability similar to filament lamps, consideration should be
given to the following two points:
94 8211
Fig. 8
The open circuit voltage, VO, and short circuit current, Ik, of
photovoltaic cells and photodiodes are measured by means
of the test circuit shown in figure 9. The value of the load
resistor used for the Ik measurement should be chosen so
that the voltage dropped across it is low in comparison with
the open circuit voltage produced under conditions of
identical irradiation.
• The IR emitter should be connected to a good heat sink to
provide sufficient temperature stability.
• DC or pulse-current levels as well as pulse duration have
great influence on self-heating of IR diodes and should be
chosen carefully.
• The radiant intensity, Ie, of the device is permanently
controlled by a calibrated detector.
Phototransistors
The collector emitter voltage, VCEO, is measured either on a
transistor curve tracer or statically using the circuit shown in
figure 10. Normal bench illumination does not change the
measured result.
1 Ω to 10 Ω
EA = 1 klx or
Ee = 1 mW/cm2
VO
Ik
mV
Ri ≥ 10 MΩ
VS = 80 V
( < VCEO )
94 8212
Fig. 9
The light source used for the light measurements is a
calibrated incandescent tungsten lamp with no filters.
The filament current is adjusted for a color temperature of
2856 K (standard illuminant A to DIN 5033 sheet 7). A
specified illumination, Ev, (usually 100 lx or 1000 lx) is
produced by adjusting the distance, a, between the lamp
and a detector on an optical bench. Ev can be measured on
a V(λ)-corrected luxmeter, or, if luminous intensity, Iv, of the
lamp is known, Ev can be calculated using the formula:
Ev = Iv/a2.
It should be noted that this inverse square law is only strictly
accurate for point light sources, that is for sources where the
dimensions of the source (the filament) are small (≤ 10 %) in
comparison with the distance between the source and
detector.
Since lux is a measure for visible light only, near-infrared
radiation (800 nm to 1100 nm) where silicon detectors have
their peak sensitivity is not taken into account.
Unfortunately, the near-infrared emission of filament lamps
of various construction varies widely. As a result, light
current measurements carried out with different lamps (but
Rev. 1.4, 31-Jul-12
IC = 1 mA
constant
E < 100 lx
VCEO
V
Ri ≥ 1 MΩ
94 8213
Fig. 10
In contrast, however, the collector dark current, ICEO or ICO,
must be measured in complete darkness (figure 11). Even
ordinary daylight illumination of the wire fed-through glass
seals would falsify the measurement result.
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VS = 5 V
VS = 20 V
ICO
IC = constant
E=0
Ee = 1 mW/cm2 or
EA = 1 klx
VCEsat
mV
10 kΩ
V
Ri = 1 MΩ
Ri ≥ 1 MΩ
94 8214
94 8216
Fig. 11
Fig. 13
The same circuit is used for collector light current, Ica,
measurements (figure 12). The optical axis of the device is
aligned to an incandescent tungsten lamp with no filters,
producing a CIE illuminance A of 100 lx or 1000 lx with a
color temperature of Tf = 2856 K. Alternatively an IR
irradiance by a GaAs diode can be used (refer to the
photovoltaic cells and photodiodes section). Note that a
lower sampling resistor is used, in keeping with the higher
current involved.
SWITCHING CHARACTERISTICS
Definition
Each electronic device generates a certain delay between
input and output signals as well as a certain amount of
amplitude distortion. A simplified circuit (figure 14) shows
how input and output signals of optoelectronic devices can
be displayed on a dual-trace oscilloscope.
VS = 5 V
VS
VS
Channel II
Channel II
IF
Channel II
Ica
Ee = 1 mW/cm2 or
EA = 1 klx
GaAs-Diode
Channel I
94 8219
1 Ω to 10 Ω
mV
Ri ≥ 10 kΩ
94 8215
Fig. 12
Fig. 14
The switching characteristics can be determined by
comparing the timing of output current waveform with the
input current waveform (figure 15).
To measure collector emitter saturation voltage, VCEsat, the
device is illuminated and a constant collector current is
passed through. The magnitude of this current is adjusted
below the level of the minimum light current, Ica min, for the
same illuminance (figure 13). The saturation voltage of the
phototransistor (approximately 100 mV) is then measured
on a high impedance voltmeter.
Rev. 1.4, 31-Jul-12
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TECHNICAL DESCTIPTION - ASSEMBLY
IF
Emitter
0
tp
IC
t
100 %
90 %
10 %
0
tr
td
ts
t on
tp
td
tr
t on (= td + tr)
Pulse duration
Delay time
Rise time
Turn-on time
tf
t off
ts
tf
t off (= ts + tf)
t
Storage time
Fall time
Turn-off time
96 11698
Emitters are manufactured using the most modern liquid
phase epitaxy (LPE) process. By using this technology, the
number of undesirable flaws in the crystal is reduced. This
results in a higher quantum efficiency and thus higher
radiation power. Distortions in the crystal are prevented by
using mesa technology which leads to lower degradation. A
further advantage of the mesa technology is that each
individual chip can be tested optically and electrically, even
on the wafer.
DETECTOR
Vishay Semiconductor detectors have been developed to
match perfectly to emitters. They have low capacitance,
high photosensitivity, and extremely low saturation voltage.
Silicon nitride passivation protects surface against possible
impurities.
Fig. 15
Assembly
These time parameters also include the delay existing in a
luminescence diode between forward current (IF) and
radiant power Φe).
Notes Concerning the Test Set-up
Circuits used for testing IR emitting, emitting sensitive and
optically coupled isolator devices are basically the same
(figure 14). The only difference is the way in which test
device is connected to the circuit.
It is assumed that rise and fall times associated with
the signal source (pulse generator) and dual trace
oscilloscope are insignificant, and that the switching
characteristics of any radiant sensitive device used in set-up
are considerably shorter than those of the test item. The
switching characteristics of IR emitters, for example
(tr ≈ 10 ns to 1000 ns), are measured with aid of a PIN
Photodiode detector (tr ≈ 1 ns).
Photo- and darlington transistors and photo- and solar cells
(tr ≈ 0.5 μs to 50 μs) are, as a rule, measured by use of fast
IR diodes (tr < 30 ns) as emitters.
Red light-emitting diodes are used as light sources only for
devices which cannot be measured with IR diodes because
of their spectral sensitivity (e.g. BPW21R). These diodes
emit only 1/10 of radiant power of IR diodes and
consequently generate only very low signal levels.
Components are fitted onto lead frames by fully automatic
equipment using conductive epoxy adhesive. Contacts are
established automatically with digital pattern recognition
using well-proven thermosonic techniques. All component
are measured according to the parameter limits given in the
datasheet.
Applications
Silicon photodetectors are used in manifold applications,
such as sensors for radiation from near UV over visible to
near infrared. There are numerous applications in
measurement of light, such as dosimetry in UV, photometry,
and radiometry. A well known application is shutter control
in cameras.
Another large application area for detector diodes, and
especially phototransistors, is position sensing.
Examples are differential diodes, optical sensors, and reflex
sensors.
Other types of silicon detectors are built-in as parts of
optocouplers.
One of the largest application areas is remote control of TV
sets and other home entertainment appliances.
Different applications require specialized detectors and also
special circuits to enable optimized functioning.
Switching Characteristic Improvements on
Phototransistors and Darlington Phototransistors
As in any ordinary transistor, switching times are reduced if
drive signal level, and hence collector current, is increased.
Another time reduction (especially in fall time tf) can be
achieved by use of a suitable base resistor, assuming there
is an external base connection, although this can only be
done at the expense of sensitivity.
Rev. 1.4, 31-Jul-12
Document Number: 80085
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Equivalent circuit
Photodetector diodes can be described by the electrical
equivalent circuit shown in figure 16.
I sh
Photodiodes are often operated in photovoltaic mode,
especially in light meters. This is depicted in figure 17, where
a strong logarithmic dependence of the open circuit voltage
on the input signal is used.
I0
Rs
R1
Id
I ph
VD
V0
V0
R sh
RL
R2
94 8606
94 8607
Fig. 16
Fig. 17 - Photodiode in the Photovoltaic Mode Operating with a
Voltage Amplifier
I O = I ph - ID - I sh
qV D
- - 1 - I sh
I O = I ph - Is  exp ---------

kT
S
(
λ
)
x
φ
e - I sh
V OC = V T x ln  -------------------------------------- + 1


Is
(1)
(2)
As described in the chapter “I-V Characteristics of
illuminated pn junction”, the incident radiation generates a
photocurrent loaded by a diode characteristic and load
resistor, RL. Other parts of the equivalent circuit (parallel
capacitance, C, combined from junction, Cj, and stray
capacitances, serial resistance, RS, and shunt resistance,
Rsh, representing an additional leakage) can be neglected in
most standard applications, and are not expressed in
equations 5 and 7 (see “Physics and Technology”).
However, in applications with high frequencies or extreme
irradiation levels, these parts must be regarded as limiting
elements.
Searching for the right detector diode type
The BPW 20 RF photodiode is based on rather highly doped
n-silicon, while BPW34 is a PIN photodiode based on very
lightly doped n-silicon. Both diodes have the same active
area and spectral response as a function of wavelength is
very similar. These diodes differ in their junction capacitance
and shunt resistance. Both can influence the performance of
an application.
Detecting very small signals is the domain of photodiodes
with their very small dark currents and dark/shunt
resistances.
With a specialized detector technology, these parameters
are very well controlled in all Vishay photodetectors.
R1
V O ≈ V OC x 1 + ------ with
R2
(3)
S ( λ ) x φ e - I sh
V OC = V T x ln  -------------------------------------- + 1


Is
(2)
It should be noted that extremely high shunt/dark resistance
(more than 15 GΩ) combined with a high-impedance
operational amplifier input and a junction capacitance of
about 1 nF can result in slow switch-off time constants of
some seconds. Some instruments therefore have a reset
button for shortening the diode before starting a
measurement.
The photovoltaic mode of operation for precise
measurements should be limited to the range of low ambient
temperatures, or a temperature control of the diode
(e.g., using a Peltier cooler) should be applied. At high
temperatures, dark current is increased (see figure 18)
leading to a non-logarithmic and temperature dependent
output characteristic (see figure 19). The curves shown in
figure 18 represent typical behavior of these diodes.
Guaranteed leakage (dark reverse current) is specified with
Iro = 30 nA for standard types. This value is far from that one
which is typically measured. Tighter customer specifications
are available on request. The curve shown in figure 19 show
the open circuit voltage as a function of irradiance with dark
reverse current, IS, as a parameter (in a first approximation
increasing IS and Ish have the same effect). The parameter
shown covers the possible spread of dark current. In
combination with figure 18 one can project the extreme
dependence of the open circuit voltage at high temperatures
(figure 20).
The very small leakage currents of photodiodes are offset by
higher capacitances and smaller bandwidths in comparison
to PIN photodiodes.
Rev. 1.4, 31-Jul-12
Document Number: 80085
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Operating modes and circuits
The advantages and disadvantages of operating a
photodiode in open circuit mode have been discussed.
107
Reverse Bias
Voltage Vr = 20 V
Current (nA)
105
BPW24R
103
BPW20RF
101
10-1
- 20
0
20
60
100
140
Temperature (°C)
94 8608
Open Circuit Voltage (mV)
Fig. 18 - Reverse Dark Current vs. Temperature
Between these wavelength boundaries the output is almost
independent of temperature. By using this mode of
operation, the reverse biased or unbiased (short circuit
conditions), output voltage, VO, will be directly proportional
to incident radiation, φe (see equation in figure 21).
400
10 pA
R
300
100 pA
V0
200
1 nA
5 nA
100
0
0.01
94 8611
Fig. 21 - Transimpedance Amplifier, Current to Voltage Converter,
Short Circuit Mode
30 nA
0.1
1
10
100
1000
Irradiance (µW)
94 8609
Fig. 19 - Open Circuit Voltage vs. Irradiance, Parameter: Dark
Reverse Current, BPW20RF
Open Circuit Voltage (mV)
For operation in short circuit mode (see figure 21) or
photoconductive mode (see figure 22), current-to-voltage
converters are typically used. In comparison with
photovoltaic mode, the temperature dependence of the
output signal is much lower. Generally, the temperature
coefficient of the light reverse current is positive for
irradiation with wavelengths > 900 nm, rising with increasing
wavelength. For wavelengths < 600 nm, a negative
temperature coefficient is found, likewise with increasing
absolute value to shorter wavelengths.
VO = - R x Φe x S ( λ )
(4)
V OC = - I sc x R
(5)
Vb
400
R
300
200
V0
100
94 8612
0
0
94 8610
20
40
60
80
100
Temperature (°C)
Fig. 20 - Open Circuit Voltage vs. Temperature, BPW46
Rev. 1.4, 31-Jul-12
Fig. 22 - Transimpedance Amplifier, Current to Voltage Converter,
Reverse Biased Photodiode
The circuit in figure 21 minimizes the effect of reverse dark
current while the circuit in figure 22 improves the speed of
the detector diode due to a wider space charge region with
decreased junction capacitance and field increased velocity
of the charge carrier transport.
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30
Rise Time tr, Fall Time tf (µs)
Vb
V0
R1
C
R3
R2
25
tr = 0 V
tf = 0 V
tr = - 0 V
tf = - 10 V
20
15
10
5
0
550 600 650 700 750 800 850 900 950
94 8613
Wavelength (nm)
948615
Fig. 23 - RC-Loaded Photodiode with Voltage Amplifier
Fig. 25 - Switching Times vs. Wavelength for Photodiode
BPW20RF
Figure 23 shows photocurrent flowing into an RC load,
where C represents junction and stray capacity while R3 can
be a real or complex load, such as a resonant circuit for the
operating frequency.
A drastic increase in rise and fall times is observed at
wavelengths > 850 nm. Differences between unbiased and
biased operation result from the widening of the space
charge region.
However, for PIN photodiodes (BPW34/TEMD5000 family)
similar results with shifted time scales are found. An
example of such behavior, in this case in the frequency
domain, is presented in figure 26 for a wavelength of 820 nm
and figure 27 for 950 nm.
Vb
C2
12
V0
RL = 100 kΩ
Reverse Bias (V)
10
R1
C1
R3
R2
94 8614
Fig. 24 - AC-Coupled Amplifier Circuit
10 kΩ
8
1 kΩ
6
4
50 Ω
2
0
(6)
The circuit in figure 24 is equivalent to figure 23 with a
change to AC coupling. In this case, the influence of
background illumination can be separated from a
modulated signal. The relation between input signal
(irradiation, φe) and output voltage is given by the equation
in figure 24.
Frequency response
The limitations of switching times in photodiodes are
determined by carrier lifetime. Due to the absorption
properties of silicon, especially in photodiodes, most of
incident radiation at longer wavelengths is absorbed outside
the space charge region. Therefore, a strong wavelength
dependence of the switching times can be observed
(figure 25).
104
94 8616
105
106
107
108
- 3 dB - Bandwidth (Hz)
Fig. 26 - BPW34, TEMD5010X01, Bandwidth vs.
Reverse Bias Voltage, Parameter: Load Resistance, λ = 820 nm
12
10
Reverse Bias (V)
R1
V O ≈ φ e x S ( λ ) x R 3 x 1 + -----R2
8
RL = 1 MΩ
1 kΩ
100 kΩ 10 kΩ
6
50 Ω
4
2
0
103
94 8617
104
105
106
107
- 3 dB - Bandwidth (Hz)
Fig. 27 - BPW41, TEMD5110X01, Bandwidth vs.
Reverse Bias Voltage, Parameter: Load Resistance λ = 950 nm
Rev. 1.4, 31-Jul-12
Document Number: 80085
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Below about 870 nm, only slight wavelength dependence
can be recognized, while a steep change of cut-off
frequency takes place from 870 nm to 950 nm (different time
scales in figure 26 and figure 27). Additionally, the influence
of load resistances and reverse bias voltages can be taken
from these diagrams.
For cut-off frequencies greater than 10 MHz to 20 MHz,
depending on the supply voltage available for biasing the
detector diode, PIN photodiodes are also used. However,
for this frequency range, and especially when operating with
low bias voltages, thin epitaxially grown intrinsic (i) layers are
incorporated into PIN photodiodes.
As a result, these diodes (e.g., Vishay’s TESP5700) can
operate with low bias voltages (3 V to 4 V) with cut-off
frequencies of 300 MHz at a wavelength of 790 nm. With
application-specific optimized designs, PIN photodiodes
with cut-off frequencies up to 1 GHz at only a 3 V bias
voltage with only an insignificant loss of responsivity can be
generated.
The main applications for these photodiodes are found in
optical local area networks operating in the first optical
window at wavelengths of 770 nm to 880 nm.
WHICH TYPE FOR WHICH APPLICATION?
In table 1, selected diode types are assigned to different
applications. For more precise selection according to chip
sizes and packages, refer to the tables in introductory pages
of this data book.
TABLE 1 - PHOTODIODE REFERENCE TABLE
DETECTOR APPLICATION
PIN PHOTODIODE
PHOTODIODE
TEMD5010X01, BPW34, BPW24R, ...
BPW20RF
Photometry, light meter
Radiometry
BPW21R
Light barriers
Remote control, IR filter included, λ > 900 nm
IR Data Transmission fc < 10 MHz
IR filter included, λ > 820 nm
IR Data Transmission, fc > 10 MHz, no IR filter
Densitometry
Smoke detector
BPV10NF, BPW24R
BPV20F, BPV23F, BPW41N, S186P, TEMD5100X01
BPV23NF, BPW82, BPW83, BPV10NF,
TEMD1020, TEMD5110X01
BPW34, BPW46, BPV10, TEMD5010X01
BPW34, BPV10, TEMD5010X01
BPW20RF, BPW21R
BPV22NF, BPW34, TEMD5010X01
PHOTOTRANSISTOR CIRCUITS
A phototransistor typically operates in a circuit shown in
figure 28. Resistor RB can be omitted in most applications.
In some phototransistors, the base terminal is not
connected. RB can be used to suppress background
radiation by setting a threshold level (see equation 7 and 8)
VO = VS - B x φe x S ( λ ) x RL
(7)
0.6
V OC ≈ V S -  B x φ e x S ( λ ) - -------- x R L

RB 
(8)
For the dependence of rise and fall times on load resistance
and collector-base capacitance, see the chapter
“Properties of Silicon Phototransistors”.
Rev. 1.4, 31-Jul-12
Vs
94 8618
RL
V0
RB
Fig. 28 - Phototransistor with Load Resistor and
Optional Base Resistor
Document Number: 80085
9
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