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Physics and Technology
EMITTERS
Materials
Infrared emitting diodes (IREDs) can be produced from a
range of different III-V compounds. Unlike the elemental
semiconductor silicon, compound III-V semiconductors
consist of two or more different elements of group three
(e.g., Al, Ga, In) and five (e.g., P, As) of periodic table. The
bandgap energies of these compounds vary between
0.18 eV and 3.4 eV. However, the IREDs considered here
emit in the near infrared spectral range between 800 nm and
1000 nm, and, therefore, the selection of materials is limited
to GaAs and mixed crystal Ga1-XAlXAs, 0  X  0.8, made
from pure compounds GaAs and AlAs.
Infrared radiation is produced by the radiative
recombination of electrons and holes from the conduction
and valence bands. Emitted photon energy, therefore,
corresponds closely to bandgap energy Eg. The emission
wavelength can be calculated according to the formula
(μm) = 1.240/Eg (eV). Internal efficiency depends on band
structure, doping material and doping level. Direct bandgap
materials offer high efficiencies, because no phonons are
needed for recombination of electrons and holes. GaAs is a
direct gap material and Ga1-XAlXAs is direct up to X = 0.44.
Doping species Si provides the best efficiencies and the
shifts emission wavelength below the bandgap energy into
the infrared spectral range by about 50 nm typically. Charge
carriers are injected into the material via pn junctions.
Junctions of high injection efficiency are readily formed in
GaAs and Ga1-XAlXAs. P-type conductivity can be obtained
with metals of valency two, such as Zn and Mg, and n-type
conductivity with elements of valency six, such as S, Se and
Te. However, silicon of valency four can occupy sites of
III-valence and V-valence atoms, and, therefore, acts as
donor and as acceptor. Conductivity type depends primarily
on material growth temperature. By employing exact
temperature control, pn junctions can be grown with the
same doping species Si on both sides of the junction. Ge,
on the other hand, also has a valency of four, but occupies
group V sites at high temperatures i.e., p-type.
Only mono crystalline material is used for IRED production.
In the mixed crystal system Ga1-XAlXAs, 0  X  0.8, lattice
constant varies only by about 1.5 x 10-3. Therefore, mono
crystalline layered structures of different Ga1-XAlXAs
compositions can be produced with extremely high
structural quality. These structures are useful because the
bandgap can be shifted from 1.40 eV (GaAs) to values
beyond 2.1 eV which enables transparent windows and
heterogeneous structures to be fabricated. Transparent
windows are another suitable means to increase efficiency,
and heterogeneous structures can provide shorter switching
times and higher efficiency. Such structures are termed
single hetero (SH) or double hetero structures (DH). DH
structures consist normally of two layers that confine a layer
with a much smaller bandgap.
Rev. 1.2, 06-Oct-14
The best production method for all materials needed is liquid
phase epitaxy (LPE). This method uses Ga-solutions
containing As, possibly Al, and a doping substance. The
solution is saturated at a high temperature, typically 900 °C,
and GaAs substrates are dipped into the liquid. The
solubility of As and Al decreases with decreasing
temperature. In this way epitaxial layers can be grown by
slow cooling of the solution. Several layers differing in
composition may be obtained using different solutions one
after another, as needed e.g. for DHs.
In liquid phase epitaxial reactors, production quantities of up
to 50 wafers, depending on type of structure required, can
be handled.
IRED CHIPS AND CHARACTERISTICS
In the past IRED chips are made only from GaAs. The
structure of the chip is displayed in figure 1.
Al
p - GaAs : Si
n - GaAs : Si
Au : Ge
n - GaAs Substrate
ca. 420 mm
94 8200
Fig. 1
On an n-type substrate, two Si-doped layers are grown by
liquid phase epitaxy from the same solution producing an
emission wavelength of 950 nm. Growth starts as n-type at
high temperature and becomes p-type below about 820 °C.
A structured Al-contact on p-side and a large area Au:Ge
contact on back side provide a very low series resistance.
The angular distribution of emitted radiation is displayed in
figure 2.
0°
30°
60°
94 8197
0
20
40
60
80
%
90°
Fig. 2
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The package of the chip has to provide good collection
efficiency of radiation emitted sideways, and has to diminish
the refractive index step between the chip (n = 3.6) and the
air (n = 1.0) with an epoxy of refractive index of 1.55. In this
way, the output power of chip is increased by a factor of
3.5 for the assembled device.
The chip described is the most cost-efficient one. Its
forward voltage at IF = 1.5 A has the lowest possible value.
Total series resistance is typically only 0.60 ; output power
and linearity (defined as optical output power increase,
divided by current increase between 0.1 A and 1.5 A) are
high. Relevant data on chip and a typical assembled device
are given in table 1.
The technology used for a chip emitting at 880 nm
eliminates the absorbing substrate and uses only a thick
epitaxial layer. The chip is shown in figure 3.
Au : Ge/Au
Au : Zn
n - GaAlAs : Si
0°
30°
60°
94 8198
0
20
40
60
80
%
90°
Fig. 4
Due to its shorter wavelength, Ga1-XAlXAs chip described
above offers specific advantages in combination with a Si
detector. Integrated opto ICs, like amplifiers or Schmitt
Triggers, have higher sensitivities at shorter wavelengths.
Similarly, phototransistors are also more sensitive. Finally,
the frequency bandwidth of pin diodes is higher at shorter
wavelengths. This chip also has the advantage of having
high linearity up to and beyond 1.5 A. The forward voltage,
however, is higher than the voltage of a GaAs chip. Table 2
(see “Symbols and Terminology”) provides more data on the
chip.
A technology combining some of the advantages of the two
technologies described above is summarized in figure 5.
p - GaAlAs : Si
ca. 370 mm
94 8201
Fig. 3
Al
Originally, the GaAs substrate was adjacent to the n-side.
Growth of Ga0.7Al0.3As started as n-type and became
p-type - as in the first case - through the specific properties
of the doping material Si. A characteristic feature of the
Ga-Al-As phase system causes the Al-content of growing
epitaxial layer to decrease. This causes the Al-concentration
at the junction to drop to 8 % (Ga0.92Al0.08As), producing an
emission wavelength of 880 nm. During further growth the
Al-content approaches zero. The gradient of the Al-content
and correlated gradient of bandgap energy produce an
emission band of a relatively large half width. The
transparency of the large bandgap material results in a high
external efficiency on this type of chip.
The chip is mounted n-side up, and the front side
metallization is Au:Ge/Au, whereas the reverse side
metallization is Au:Zn.
The angular distribution of the emitted radiation is displayed
in figure 4.
Rev. 1.2, 06-Oct-14
p - GaAlAs : Ge
p - GaAs : Si
n - GaAs : Si
Au : Ge
n - GaAs Substrate
ca. 420 mm
94 8202
Fig. 5
Starting an with n-type substrate, n- and p-type GaAs layers
are grown in a similar way to the epitaxy of a standard
GaAs:Si diode. After this, a highly transparent window layer
of Ga1-XAlXAs, doped p-type is grown. The upper contact to
the p-side is made of Al and the rear side contact is Au:Ge.
The angular distribution of emitted radiation is shown in
figure 6.
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0°
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30°
Anode
p - GaAlAs
p -GaAs
60°
n - GaAlAs
Cathode
94 8199
0
20
40
60
80
%
90°
ca. 420 mm
12781
Fig. 6
Fig. 7
This chip type combines a relatively low forward voltage
with a high electro-optical efficiency, offering an optimized
combination of the advantageous characteristics of the two
other chips. Refer again to table 2 (see “Symbols and
Terminology”) for more details.
The active layer is depicted as the thin layer between the
p- and n- type Ga1-XAlXAs confinement layers.
As mentioned in the previous section, double
heterostructures (DH) provide even higher efficiencies and
faster switching times. A schematic representation of such
a chip is shown in figure 7.
The contacts are dependent on the polarity of the chip. If p
is up, then the p-side contact is Al and the back side Au:Ge;
if n is up, then this side has an Au:Ge contact and the back
side Au:Zn.Two such chips that are also very suitable for
IrDA applications are given in table 1.
BULK AND SURFACE EMITTER TECHNOLOGY
A more recent technology, the surface emitter chip
technology, involves bonding the Infrared emitting diode
structure to a metalized conducting carrier substrate, after
which the substrate, which was originally used for the
epitaxial growth of the Infrared emitting crystal layers, is
chemically removed. The layer structure of these diodes is
extremely thin which has the favorable consequence that
side wall emission is minimized.
The layers of the surface emitter IRED structures are
deposited by metal-organic chemical vapor deposition
(MOVPE) on suitable substrates. The active region consists
of a multiple-quantum-well (MQW) or a DH structure. MQW
active regions for Infrared emitting diodes contain typically
one or more 5 nm thick InGaAs quantum wells which are
separated by 15 nm thick GaAlAsP barriers.
High electro-optical efficiencies are achieved by the
implementation of a metallic mirror on the back side of the
layer structure which redirects the incident radiation
effectively towards the top surface as well as a treatment of
the top surface to increase the extraction efficiency. As
sidewall emission is negligible radiance scales with chip
area and large devices can be realized without significant
increase of reabsorption losses.
The angular distribution of the emitted radiation
corresponds nearly perfectly to the lambertian emission
pattern of point sources I0 = I () x cos  and enables an
efficient coupling of the output power into optical systems.
Exemplary data on chip and assembled device are given in
table 1.
A further Infrared emitter chip technology, which makes use
of metal organic chemical vapor deposition, is the bulk
emitter chip technology. On an n-type GaAs substrate a
MQW structure similar to the one described above is grown
producing an emission wavelength of 940 nm. As the
substrate is transparent at this wavelength the bulk
emitter technology offers the high efficiencies of double
hetero structures in combination with exceptional low
forward voltages and very fast response times. With these
favorable characteristics bulk emitter chips can substitute
conventional GaAlAs/GaAs chips in many technical
applications. As electrode material for the top p-type
contact Al or Au are in use, whereas the back side contact
consists of an Au alloy. The angular distribution of the
emitted radiation resembles the one shown in figure 6.
Relevant chip and device data are given in table 1.
In order to provide a good current spreading and a uniform
current distribution surface emitter diodes are grown n-side
up. Both contacts, the structured top electrode to the n-side
and the large area back side contact, are made of gold.


Rev. 1.2, 06-Oct-14
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TABLE 1: CHARACTERISTICS DATA OF IRED CHIPS
TYPICAL CHIP DATA
TECHNOLOGY
e at 0.1 A
(mW)
p
(nm)
TYPICAL DEVICE DATA

(nm)
POLARITY
TYPICAL DEVICE
e at 0.1 A
(mW)
VF at 0.1 A
(V)
VF at 1.0 A
(V)
tr at 0.1 A
(ns)
GaAs
7.7
950
50
p up
TSUS540.
20
1.3
2.1
800
GaAlAs
12.8
875
80
n up
TSHA550.
27
1.5
3.4
600
GaAlAs (DDH)
20
890
40
p up
TSHF5410
45
1.5
2.3
30
GaAlAs (DDH)
26
870
40
p up
TSFF5410
50
1.5
2.3
15
Bulk Emitter
21
940
30
p up
VSLB3940
40
1.35
2.1
15
GaAlAs MQW
22
940
30
p up
TSAL6200
40
1.35
2.2
15
Surface Emitter
30
850
25
n up
VSLY5850
55
1.65
2.9
10
UV, VISIBLE, AND NEAR IR SILICON PHOTODETECTORS
(adapted from ”Sensors, Vol 6, Optical Sensors, Chapt. 8,
VCH - Verlag, Weinheim 1991”)
Silicon Photodiodes (PN and PIN Diodes)
The physics of silicon detector diodes
10-1
104
100
Si
103
101
102
102
101
103
Without external bias
h
Energy
105
Surface States
Penetration Depth (µm)
Absorption Coefficient (cm-1)
Absorption of radiation is caused by the interaction of
photons and charge carriers inside a material. The different
energy levels allowed and the band structure determine the
likelihood of interaction and, therefore, the absorption
characteristics of the semiconductors. The long wavelength
cutoff of the absorption is given by the bandgap energy. The
slope of the absorption curve depends on the physics of
interaction and is much weaker for silicon than for most
other semiconducting materials. This results in a strong
wavelength-dependent penetration depth which is shown in
figure 8. (The penetration depth is defined as that depth
where 1/e of the incident radiation is absorbed.)
operating modes of the pn diode. Incident radiation
generates mobile minority carriers - electrons on the
p-side, holes on the n-side. In the short circuit mode
shown in figure 9 (top), the carriers drift under the field of the
built-in potential of the pn junction. Other carriers diffuse
inside the field-free semiconductor along a concentration
gradient, which results in an electrical current through the
applied load, or without load, in an external voltage, open
circuit voltage, VOC, at contact terminals. Bending of
the energy bands near the surface is caused by surface
states. An equilibrium is established between generation,
recombination of carriers, and current flow through the load.
0
94 8596
Reverse bias applied
h
94 8595
0.6
0.8
1.0
104
1.2
Energy
100
0.4
Wavelength (nm)
Fig. 8 - Absorption and Penetration Depth of
Optical Radiation in Silicon
Depending on the wavelength, the penetration depth varies
from tenths of a micron at 400 nm (blue) to more than
100 μm at 1 μm (IR). For detectors to be effective, an
interaction length of at least twice the penetration depth
should be realized (equivalent to 1/e2 = 86 % absorbed
radiation). In the pn diode, generated carriers are collected
by the electrical field of the pn junction. Effects in the vicinity
of a pn junction are shown in figure 9 for various types and
Rev. 1.2, 06-Oct-14
0
94 8597
Fig. 9 - Generation-Recombination Effects in the Vicinity
of a PN Junction
Top: Short Circuit Mode, Bottom: Reverse Biased


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Recombination takes place inside the bulk material with
technology- and process-dependent time constants which
are very small near the contacts and surfaces of the device.
For short wavelengths with very small penetration depths,
carrier recombination is the efficiency limiting process. To
achieve high efficiencies, as many carriers as possible
should be separated by the electrical field inside the space
charge region. This is a very fast process, much faster than
typical recombination times (for data, see chapter
’Operating modes and circuits’).
The width, W, of the space charge is a function of doping the
concentration NB and applied voltage V:
Antireflection coating
p+
Space charge
1 to 10 Ωcm
n
n+
94 8598
Contact
W =
2 x  S x  o x  V bi + V 
----------------------------------------------------------q x NB
p+
1
(for a one-sided abrupt junction), where Vbi is built-in
voltage, s dielectric constant of Si, o vacuum dielectric
constant and q is electronic charge. The diode’s
capacitance (which can be speed limiting) is also a function
of the space charge width and applied voltage. It is given by
S x o x A
2
C = ----------------------------W
where A is the area of the diode. An externally applied bias
will increase the space charge width (see figure 8) with the
result that a larger number of carriers are generated inside
this zone which can be flushed out very fast with high
efficiency under the applied field. From equation (1), it is
evident that the space charge width is a function of the
doping concentration NB. Diodes with a so-called pin
structure show according to equation (1) a wide space
charge width where i stands for intrinsic, low doped. This
zone is also sometimes nominated as n or p rather than low
doped n, n- or p, p-zone indicating the very low doping. Per
equation (2), the junction capacitance C, is low due to the
large space charge region of PIN photodiodes. These
photodiodes are mostly used in applications requiring high
speed.
Figure 10 shows a cross section of PIN photodiodes
and PN diodes. The space charge width of the PIN
photodiodes (bottom) with a doping level (n = NB) as low as
NB = 5 x 1011 cm-3 is about 80 μm wide for a 2.5 V
bias in comparison with a pn diode with a doping (n) of
NB = 5 x 1015 cm-3 with only 0.8 mm.
Rev. 1.2, 06-Oct-14
Isolation layer
Contact
Space charge
i
ν: to 10 000 Ωcm
n+
94 8599
Fig. 10 - Comparison of PN Diode (Top)
and PIN Photodiode (Bottom)
PROPERTIES OF SILICON PHOTODIODES
I-V Characteristics of illuminated pn junction
The cross section and I-V-characteristics of a photodiode
are shown in figure 11 and 12. The characteristic of the
illuminated diode is identical to the characteristic of a
standard rectifier diode. The relationship between current, I,
and voltage, V, is given by
exp V
I = I S x  ----------------
 V T - 1
3
with VT = kT/q
k = 1.38 x 10-23 JK-1, Boltzmann constant
q = 1.6 x 10-19 As, electronic charge.
IS, the dark-reverse saturation current, is a material- and
technology-dependent quantity. The value is influenced by
the doping concentrations at pn junction, by carrier lifetime,
and especially by temperature. It shows a strongly
exponential temperature dependence and doubles every
8 °C.
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Forward Current (nA)
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8
6
2
- 80
40
- 40
-2
-4
94 8601
Rev. Current (nA)
- 120
Rsh = dV/dl
4
Reverse Voltage (mV)
- 160
flows into the load. When RL increases to infinity, the output
voltage of the diode rises to the open circuit voltage, VOC,
given by
S    x e
7
V OC = V T x  -------------------------
 IS + 1 
80
Forw. Volt. (mV)
Fig. 11 - Measured I-V-Characteristics of an Si Photodiode in the
Vicinity of the Origin
The typical dark currents of Si photodiodes are dependent
on size and technology and range from less than picoamps
up to tens of nanoamps at room temperature conditions. As
noise generators, dark current Ir0 and the resistance Rsh
(defined and measured at a voltage of 10 mV forward or
reverse, or peak-to-peak) are limiting quantities when
detecting very small signals.
The photodiode exposed to optical radiation generates a
photocurrent Ir exactly proportional to incident radiant
power e.
Because of this logarithmic behavior, the open circuit
voltage is sometimes used for optical light meters in
photographic applications. The open circuit voltage shows
a strong temperature dependence with a negative
temperature coefficient. The reason for this is the
exponential temperature coefficient of the dark reverse
saturation current IS. For precise light measurement, a
temperature control of the photodiode is employed. Precise
linear optical power measurements require small voltages at
the load, typically smaller than about 5 % of the
corresponding open circuit voltage. For less precise
measurements, an output voltage of half the open circuit
voltage can be allowed. The most important disadvantage of
operating in photovoltaic mode is the relatively large
response time. For faster response, it is necessary to
implement an additional voltage source reverse-biasing the
photodiode. This mode of operation is termed
photoconductive mode. In this mode, the lowest detectable
power is limited by the shot noise of the dark current, IS,
while in photovoltaic mode, the thermal (Johnson) noise of
shunt resistance, Rsh, is the limiting quantity.
SPECIAL RESPONSITIVITY
I
Efficiency of Si photodiodes:
Φe = 0
V OC
I Φ1
94 8600
Φe
The spectral responsivity, s, is given as the number of
generated charge carriers ( x N) per incident photons N of
energy h x  ( is percent efficiency, h is the Plancks
constant, and  is the radiation frequency). Each photon will
generate one charge carrier at the most. The photocurrent
Ire is then given as
I SC
Fig. 12 - I-V-Characteristics of an Si Photodiode under Illumination.
Parameter: Incident Radiant Flux
The quotient of both is spectral responsibility s(),
Ir
S    = -----------------------e  A  W 
I re =  x N x q
8
I re
xNxq
xq
S    =  ------ = ------------------------- = -------------  e
hxxN
hx
9
  μm 
S    = ----------------  A  W 
1.24
4
At fixed efficiency, a linear relationship between wavelength
and spectral responsivity is valid.
The characteristic of the irradiated photodiode is then given
by
Figure 8 shows that the semiconductors absorb radiation
similar to a cut-off filter. At wavelengths smaller than the
cut-off wavelength, the incident radiation is absorbed. At
larger wavelengths the radiation passes through the material
without interaction. The cut-off wavelength corresponds to
the bandgap of the material. As long as the energy of the
photon is larger than the bandgap, carriers can be
generated by absorption of photons, provided that the
material is thick enough to propagate photon-carrier
interaction. Bearing in mind that the energy of photons
decreases with increasing wavelength, we can see, that the
curve of the spectral responsivity vs. wavelength in ideal
exp V
I = I S x  ---------------- - S    x  e
 V T - 1
and in case V  0, zero or reverse bias we find,
I = - IS - S    x e
5
6
Dependent on load resistance, RL, and applied bias,
different operating modes can be distinguished. An
unbiased diode operates in photovoltaic mode. Under short
circuit conditions (load RL = 0 ), short circuit current, ISC
Rev. 1.2, 06-Oct-14
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case (100 % efficiency) will have a triangular shape (see
figure 13). For silicon photodetectors, the cut-off
wavelength is near 1100 nm.
spectral responsivity, s [A/(W/cm2)] is not a constant.
Rather, this relation is a function of wavelength and reverse
bias
In most applications, it is not necessary to detect radiation
with wavelengths larger than 1000 nm. Therefore, designers
use a typical chip thickness of 200 μm to 300 μm, which
results in reduced sensitivity at wavelengths larger than
950 nm. With a typical chip thickness of 250 μm, an
efficiency of about 35 % at 1060 nm is achieved. At shorter
wavelengths (blue-near UV, 500 nm to 300 nm) sensitivity is
limited by recombination effects near the surface of the
semiconductor. A reduction in efficiency starts near 500 nm
and increases as the wavelength decreases. Standard
detectors designed for visible and near IR radiation may
have poor UV/blue sensitivity and poor UV stability. Well
designed sensors for wavelengths of 300 nm to 400 nm can
operate with fairly high efficiencies. At shorter wavelengths
(< 300 nm), efficiency decreases strongly.
Stability of spectral responsivity
Surface effects and contamination are possible causes but
are technologically well controlled.
Angular dependence of responsivity
The angular response of Si photodiodes is given by the
optical laws of reflection. The angular response of a detector
is shown in figure 14.
0°
1.0
1.0
Spectral Responsitivity (A/W )
Si detectors for wavelengths between 500 nm and 800 nm
appear to be stable over very long periods of time. In
the literature concerned here, remarks can be found
on instabilities of detectors in blue, UV, and near IR
under certain conditions. Thermal cycling reversed the
degradation effects.
0.8
0.8
30°
A/W
60°
0.6
0.6
0.4
0.4
Acm2/W
0.2
0.2
0
- 90°
0
400
94 8602
600
800
1000
1200
Wavelength (nm)
Fig. 13 - Spectral Responsivity as a Function of Wavelength of a Si
Photodetector Diode, Ideal and Typical Values
Temperature dependence of spectral responsivity
The efficiency of carrier generation by absorption and the
loss of carriers by recombination are the factors which
influence spectral responsivity. The absorption coefficient
increases with temperature. The radiation of the long
wavelength is therefore more efficiently absorbed inside the
bulk and results in increased response. For shorter
wavelengths (< 600 nm), reduced efficiency is observed with
increasing temperature because of increased recombination
rates near the surface. These effects are strongly dependent
on technological parameters and therefore cannot be
generalized to the behavior at longer wavelengths.
Uniformity of spectral responsivity
Inside the technologically defined active area of
photodiodes, spectral responsivity shows a variation of
sensitivity on the order of < 1 %. Outside the defined active
area, and especially at lateral edges of the chips, local
spectral response is sensitive to applied reverse voltage.
Additionally, this effect depends on wavelength. Therefore,
the relation between power (W) related spectral
responsivity, s (A/W), and power density (W/cm2) related
Rev. 1.2, 06-Oct-14
94 8603
- 60°
- 30°
Angle
0
0.2
0.4
0.6
0.8
90°
Relative Responsitivity
Fig. 14 - Responsivity of Si Photodiodes as a Function of
the Angle of Incidence
Semiconductor surfaces are covered with quarter
wavelength anti-reflection coatings. Encapsulation is
performed with uncoated glass or sapphire windows.
The bare silicon response can be altered by optical imaging
devices such as lenses. In this way, nearly every arbitrary
angular response can be achieved.
Dynamic Properties of Si Photodiodes
Si photodiodes are available in many different variations.
The design of diodes can be tailored to meet special needs.
Si photodiodes may be designed for maximum efficiency at
given wavelengths, for very low leakage currents, or for high
speed. The design of a photodiode is nearly always a
compromise between various aspects of a specification.
Inside the absorbing material of the diode, photons can be
absorbed in different regions. For example at the top of a
p+n--diode there is a highly doped layer of p+ - Si. Radiation
of shorter wavelengths will be effectively absorbed, but for
larger wavelengths only a small amount is absorbed. In the
vicinity of the pn junction, there is the space charge region,
where most of the photons should generate carriers. An
electric field accelerates the generated carrier in this part of
the detector to a high drift velocity. Carriers which are not
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absorbed in these regions penetrate into field-free region
where the motion of the generated carriers fluctuates by a
slow diffusion process.
The dynamic response of the detector is composed of
different processes which transport carriers to contacts. The
dynamic response of photodiodes is influenced by three
fundamental effects:
• Drift of carriers in an electric field
• Diffusion of carriers
• Capacitance x load resistance
Vishay Semiconductors
PROPERTIES OF SILICON
PHOTOTRANSISTORS
The phototransistor is equivalent to a photodiode in
conjunction with a bipolar transistor amplifier (figure 15).
Typically, the current amplification, B, is between 100 and
1000 depending on type and application. The active area of
phototransistor is usually about 0.5 x 0.5 mm2.
The data of spectral responsivity are equivalent to those of
photodiodes, but must be multiplied by the factor current
amplification, B.
Carrier drift in the space charge region occurs rapidly with
very small time constants. Typically, transit times in an
electric field of 0.6 V/μm are on the order of 16 ps/μm and
50 ps/μm for electrons and holes, respectively. At
(maximum) saturation velocity, the transit time is on the
order of 10 ps/μm for electrons in p-material. With a 10 μm
drift region, traveling times of 100 ps can be expected.
Response time is a function of the distribution of the
generated carriers and is therefore dependent on
wavelength.
The diffusion of the carriers is a very slow process. Time
constants are on the order of some ms. The typical pulse
response of the detectors is dominated by these two
processes. Obviously, carriers should be absorbed in large
space charge regions with high internal electrical fields. This
requires material with an adequate low doping level.
Furthermore, a reverse bias of rather large voltage is useful.
Radiation of shorter wavelength is absorbed in smaller
penetration depths. At wavelengths shorter than 600 nm,
decreasing wavelength leads to an absorption in the
diffused top layer. The movement of carriers in this region is
also diffusion limited. Because of the small carrier lifetimes,
the time constants are not as large as in homogeneous
substrate material.
Finally, capacitive loading of output in combination with load
resistance limits frequency response.
Antireflection coating
Metallization
Isolation
Emitter
Base
Collector
n
p
n
EA
n+
94 8605
Backside contact
Fig. 15 - Phototransistor, Cross Section and Equivalent Circuit
The switching times of phototransistors are dependent on
current amplification and load resistance and are between
30 ms and 1 ms. The resulting cut-off frequencies are a few
hundred kHz.
The transit times, tr and tf, are given by
t r, f =
ft:
 1 2
2
 --------------2- + b x  RC B x V 
 2 x ft 
 10 
Transit frequency
R:
Load resistance, 1.6
CB:
Base-collector capacitance, b = 4 to 5
V:
Amplification
Phototransistors are most frequently applied in transmissive
and reflective optical sensors.
Rev. 1.2, 06-Oct-14
Document Number: 80086
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For technical questions, contact: [email protected]
THIS DOCUMENT IS SUBJECT TO CHANGE WITHOUT NOTICE. THE PRODUCTS DESCRIBED HEREIN AND THIS DOCUMENT
ARE SUBJECT TO SPECIFIC DISCLAIMERS, SET FORTH AT www.vishay.com/doc?91000