Analog Integr Circ Sig Process (2012) 70:1–13
DOI 10.1007/s10470-011-9641-6
CMOS silicon avalanche photodiodes for NIR light detection:
a survey
Afrin Sultana • Ehsan Kamrani • Mohamad Sawan
Received: 27 October 2010 / Revised: 15 January 2011 / Accepted: 5 April 2011 / Published online: 17 April 2011
Ó Springer Science+Business Media, LLC 2011
Abstract This paper surveys recent research on CMOS
silicon avalanche photodiodes (SiAPD) and presents the
design of a SiAPD based photoreceiver dedicated to nearinfrared spectroscopy (NIRS) application. Near-infrared
spectroscopy provides an inexpensive, non-invasive, and
portable means to image brain function, and is one of the
most efficient diagnostic techniques of different neurological diseases. In NIRS system, brain tissue is penetrated by
near-infrared (NIR) radiation and the reflected signal is
captured by a photodiode. Since the reflected NIR signal
has very low amplitude, SiAPD is a better choice than
regular photodiode for NIR signal detection due to SiAPD‘s ability to amplify the photo generated signal by
avalanche multiplication. Design requirements of using
CMOS SiAPDs for NIR light detection are discussed, and
the challenges of fabricating SiAPDs using standard
CMOS process are addressed. Performances of state-ofthe-art CMOS SiAPDs with different device structures are
summarized and compared. The efficacy of the proposed
SiAPD based photoreceiver is confirmed by post layout
simulation. Finally, the SiAPD and its associated circuits
has been implemented in one chip using 0.35 lm standard
CMOS technology for an integrated NIRS system.
Keywords Silicon avalanche photodiode (SiAPD) Near-infrared spectroscopy (NIRS) Transimpedance
amplifier (TIA) Clinical imaging Guard ring structure
A. Sultana (&) E. Kamrani M. Sawan
Polystim Neurotechnologies Laboratory,
Department of Electrical Engineering, Ecole Polytechnique de
Montreal, Quebec H3T 1J4, Canada
e-mail: afrin.sultana@polymtl.ca
1 Introduction
Silicon avalanche photodiode (SiAPD) is a solid state
photoconductor device where a p-n junction is operated
under high-reverse bias voltage. Incident light photons are
absorbed in the depletion region of the reverse biased
p-n junction and converted into electron–hole pairs (EHPs).
These primary EHPs drift along the applied high electric
field (E [ 105 V/cm) and they gain enough kinetic energy
to create secondary EHPs along their paths by impact
ionization. The impact ionization process leads to an
exponential increase in the number of EHPs with traversed
distance, that is, in avalanche multiplication gain. Compared to a p-i-n photodiode, SiAPDs have a bias dependent
internal gain which makes them compatible for low-level
light detection in the visible and near-infrared (NIR)
regions. Silicon avalanche photodiodes are more advantageous than photomultiplier tubes (PMTs) which are the
most commonly used photo detectors due to the drawbacks
of their vacuum tube technology. The PMTs are bulky,
subtle, sensitive to magnetic fields, and require high voltage supply [1]. As such, SiAPDs become popular for
several applications including light detection and ranging
(lidar), photon counting, and fiber optic communication
[1]. They are potential candidate for applications like
quantum cryptography, profilometry of remote objects,
fluorescence spectroscopy, and biomedical imaging systems such as positron emission tomography (PET), single
photon emission computed tomography (SPECT), and
near-infrared spectroscopy (NIRS) [2].
Near-infrared spectroscopy is an inexpensive, noninvasive and portable imaging technique to monitor the
brain function and biological tissues. In this technique
brain tissue is penetrated by near-infrared (NIR) radiation
and the reflected signal is observed to investigate the
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2.1 Near-infrared spectroscopy system for clinical
imaging
To analyze brain function NIR light is emitted on the
surface of the head which is penetrated by the brain tissue
and passed through the skull and brain. Brain function can
be studied by monitoring two major types of signals:
neuronal signal and hemodynamic signal [7]. Neuronal
signal is fast and arises due to optical changes directly
associated with the neuronal activity of brain. Neuronal
signal describes changes in light scattering properties of the
brain tissues or neurons within 200 ms after the onset of
the brain stimulation by NIR light [8, 9]. On the other hand,
slow hemodynamic signal arises due to the brain activity
which leads to an increase in the local oxygen consumption
followed by an increase in blood flow of activated neurons
due to neurovascular coupling [7]. The increase in blood
flow changes the haemoglobin concentrations and oxygenation of blood. As a result of increase in blood flow in
the brain, less light will pass through brain tissues. These
changes occur within a few seconds after brain stimulation
[9, 10].
Near-infrared spectroscopy is the only neuroimaging
system capable of detecting both fast neuronal signal and
slow hemodynamic signal [7]. The other brain monitoring
methods such as EEG and MEG can only measure the fast
neuronal signal, and PET and fMRI can measure only the
slow hemodynamic signal. The other advantages of NIRS
system are: painless, no need to use ionizing radiation,
immune to electromagnetic interference, and capable of
real-time, long term bedside monitoring [11]. All these
advantages have made NIRS the system of choice for
neuroimaging and a hot topic of research. Work is going on
in different research groups to improve the performance of
NIRS system. A simplified block diagram of such device is
shown in Fig. 1.
The NIRS system consists of NIR light source, sensor or
photodetector, data acquisition and control unit, and a
processing unit. The light source is placed on the surface of
the head (scalp) and it generates light in the NIR range
(wavelength is from 650 to 950 nm). Generally, the light
sources used for NIRS system are either LEDs or laser
diodes that emit NIR light with optical power around
Clinical imaging by monitoring human brain function is
considered as the most efficient technique of diagnosis and
investigation of different neurological diseases, such as,
stroke and epilepsy. The commonly used non-invasive
neuroimaging techniques are electro-encephalography
(EEG), magneto-encephalography (MEG), PET, functional
magnetic resonance imaging (fMRI), and near-infrared
spectroscopy (NIRS), which is the subject of the present
paper [5]. Near-infrared spectroscopy is a technique where
brain tissue is penetrated by NIR radiation and the resultant
absorption and scattering effects are observed to investigate the brain’s function [6].
Fig. 1 Block diagram of an NIRS system
brain’s function. In NIR range, water has relatively low
absorption while oxy- and deoxy-haemoglobin have high
absorption. Due to these properties, NIR light can penetrate
biological tissues in the range of 0.5–3 cm allowing investigation of deep brain tissues, and ability to differentiate
between healthy and diseased tissues [3]. Current commercially available NIRS devices are too bulky to be wearable or
portable for monitoring brain function [4]. Our goal is to
build a novel highly sensitive fully integrated multi-channel
wireless front-end receiver for a NIRS system. One of the
most important topics of research for NIRS front-end
receiver is to design a sensitive photodetector to ensure
maximum detection of the reflected NIR light. The fraction
of the incident NIR light photons survive to return to the
photodetector are strongly attenuated (7–9 orders of magnitude) by the biological tissues. Accordingly, the photodetector requires to be highly sensitive, enabling the reliable
conversion of the ultra-low amplitude light signal into a
detectable electric signal. SiAPD is a potential candidate for
low-level light detection due to its ability to amplify the
photogenerated signal by avalanche multiplication.
In this paper, first we briefly describe the NIRS system
for neuroimaging, and discuss the basic operation and
associated circuits of a SiAPD. In the next section we
define the design parameters of SiAPDs for NIRS application. Then we discuss the pros and cons of the available
SiAPD fabrication technologies and also compare the
structure of existing SiAPDs. We propose a device structure as well as a transimpedance amplifier (TIA) circuit
suitable for NIRS application and demonstrate the potential
performance of the proposed SiAPD by both device and
circuit level simulation. Finally we implement the SiAPD
and its associated circuits on the same chip using 0.35 lm
standard CMOS technology.
2 Background
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5 mW. The sensor is a photodetector that monitors the
intensity of the reflected NIR signal. The data acquisition
and control unit manages all sensors and light source,
synchronizes their operation, and acquires corresponding
real-time data. The processing unit records, visualizes, and
analyzes the acquired data. An integrated NIRS system
offers excellent performance and high sensitivity to obtain
required information from the brain tissues.
2.2 Operations and circuits of SiAPD
The structure of SiAPD along with its depletion region is
shown in Fig. 2. Depending on the magnitude of the
reverse bias voltage across the p-n junction, SiAPDs can
work in two different modes: Linear or proportional mode,
and Geiger or single photon counting mode.
In linear mode, the reverse bias voltage is kept below the
breakdown voltage (VBR) of the junction and the photogenerated charges are amplified with a finite multiplication
gain. Here, the statistical variations of the finite multiplication gain produce a noise contribution known as excess noise
preventing linear SiAPDs from single photon detection.
For linear mode operation, SiAPD requires a TIA to
convert the input photocurrent into a voltage signal.
Although TIA offers large gain, signal produced by TIA
needs to go through LA which boosts the voltage swings
and matches the output impedance to drive the output
(usually DMUX) [12]. The LA circuit is implemented by
cascading two stages of resistive load differential amplifiers and one stage of buffer [12]. The OTA preamplifier is
used to selectively amplify the low amplitude signal before
it is being filtered and then demodulated. Here we have
used OTA proposed in [3]. The OTA differential input
pairs are in a class AB configuration, and uses DTMOS
devices for input common-mode range enhancement. There
are many topologies of TIA reported in literature which has
high transimpedance gain and high bandwidth, accompanying with low input noise and low-power consumption
[12, 13]. Among them three widely used topologies are:
Fig. 2 General structure of SiAPD with its depletion region
3
common-gate TIA, resistive feedback TIA, and capacitive
feedback TIA. Figure 3 shows the schematic of three
configurations while the detail description of them can be
found in [13]. A common-gate configuration is typically
chosen as it can tolerate a wide range of SiAPD capacitance. However, resistive feedback architecture has better
noise performance and is more attractive when SiAPD
models are readily available. Phang et al. proposed a TIA
combining a sub 1-V current mirror and a common-gate
TIA based on a current-gain amplifier for optical communication [13, 14]. Achigui et al. modified this TIA by
adding an OTA with dynamic threshold transistor
(DTMOS) for NIRS front-end photo receiver [3].
On the other hand, in Geiger mode, the reverse bias
voltage is kept above VBR and SiAPDs work as trigger
devices rather than amplifying devices. In this later mode,
the electric field is so high that a single photo-generated
carrier can trigger a self-sustaining avalanche process that
swiftly builds up a macroscopic current in the mA range.
The current keeps flowing until the avalanche is quenched
by lowering the bias voltage below VBR by dedicated
quenching circuits [15, 16]. Avalanche photodiodes operated in Geiger mode are known as silicon single photon
avalanche diode (silicon SPAD). Unlike linear mode SiAPDs, silicon SPADs are not concerned with gain fluctuations since here the gain is virtually infinite. Additionally, in
Geiger mode, the signal amplitude does not provide intensity information since all the current pulses have the same
amplitude. Intensity information is obtained by counting the
pulses during a certain time frame or by measuring the
average time interval between successive pulses.
For operating the SiAPD in Geiger mode (single photon
counting mode) quenching and reset circuits are necessary
[17, 18]. Generally three types of quenching circuits have
been used for Geiger mode operation: passive, active and
mixed quenching circuits (MQCs).
Passive quenching circuit (PQC) uses the simplest
method of quenching an avalanche. It has two possible
configurations: voltage-mode output and current-mode
output. The voltage-mode provides longer pulses, which
might be convenient to visualize them in the oscilloscope
but might hinder high speed detection. On the other hand,
the current-mode output configuration allows high detection rates [38, 39]. The schematic of a typical PQC circuit
is shown in Fig. 4. Figure 4(a) is configuration with current-mode output and Fig. 4(b) is configuration with voltage-mode output [38, 39]. Here RL is the load resistor
(typically between 50 and 500 KX) and RS provides matched termination for a coaxial cable (*50 X). When no
current flows in the circuit, the SPAD is reversed biased at
Vdd. The difference between this voltage and the photodiode’s breakdown voltage, VBR, is called excess bias, VE,
and is given by:
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Fig. 3 Different TIA
structures: a common-gate,
b resistive feedback, and
c capacitive feedback
VE ¼ Vdd VBR :
ð1Þ
As soon as a light photon is absorbed in the photodiode,
the avalanche multiplication process begins and the avalanche current quickly rises to few mA, generating a
voltage drop across the resistance that eventually reduces
the reverse bias voltage below VBR, thus quenching the
multiplication process. Passive quenching circuit is easy to
implement and an effective method of quenching for small
area (\50 lm) photodiodes since they have very low
photodiode capacitance. Due to low photodiode capacitance, small area photodiodes facilitate high speed operation. PQC can be used for large area photodiodes if high
speed operation is not a requirement.
In order to achieve faster quenching, active quenching
circuit (AQC) is used since it forces the SPAD to drop the
reverse bias voltage much quicker. Faster quenching results
in lower power loss and hence less heating of the SPAD.
However, AQC involves complex circuitry and stringent
circuit requirements.
In order to relax the circuit design requirements, a
hybrid approach, known as MQC, can be employed [18].
Here, passive quenching is used as the first stage to limit
the avalanche current to a low value, followed by the
application of a quench pulse during the quench delay time
and a reset pulse to recharge the SPAD back to the reverse
bias voltage higher than VBR. This method adopts a simpler
design. In MQC, performance limitations are largely
dependent on switching delays, which are directly related
to the parasitic capacitances of the circuit [2, 18].
The circuit diagram of a typical AQC mixed with PQC
is shown in Fig. 5. This scheme has been originally proposed in [40, 41]. In quiescence condition, the cathode of
SPAD is biased to Vdd through R1 and is ready to detect a
photon. The onset of the avalanche current starts a passive
quenching action and the voltage drop across R1 reduces
the voltage at the SPAD cathode. As such, Ssense goes in
deeper conduction and the voltage drop caused by R3, turns
the quench transistors (Squench1 and Squench2) ON via Sfeedback. This starts the active quenching action by quickly
pulling the SPADs cathode down to ground. This brings the
reverse bias of the SPAD below breakdown and the avalanche current quickly dissipates. The quench transistors
(Squench1 and Squench2) are then turned OFF and the three
parallel reset transistors (Sreset1, Sreset2, and Sreset3) are
turned ON. The reset transistors are activated by an output
pulse from the reset monostable which triggers with the
end of the hold-off period. These reset transistors are
equivalent of the three low resistance transistors, which
resets the quiescent bias of the SPAD and brings the
SPAD’s cathode voltage back to detect the next photon.
Short duration of the reset time decreases the dead time
between photon counts [42].
3 Design parameters of SiAPD for NIRS application
(a)
(b)
Fig. 4 Schematic of a passive quenching circuit (PQC) with two
possible configurations [38, 39]
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The figures of merit to assess a NIRS system are its size,
portability and cost. The main advantages of using SiAPD
based detector are small size, ruggedness, low operating
voltages, and low cost. With the recent advances in CMOS
technology, it is now possible to develop SiAPD with the
necessary peripheral circuits on the same chip in order to
realize an integrated, ultra-sensitive, portable and inexpensive NIRS system. The design requirements of the
SiAPDs for NIRS system such as, area, depletion region
Analog Integr Circ Sig Process (2012) 70:1–13
5
3.2 Depletion region thickness of the SiAPD
The thickness of depletion region of SiAPD, W, is given by
[23]:
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
2eS 1
1
þ
ðui Va Þ:
ð2Þ
W¼
q Na Nd
Fig. 5 Circuit diagram of a typical active quenching circuit (AQC)
thickness, and signal to noise ratio (SNR) are discussed in
next paragraphs.
3.1 Area of the SiAPD
According to American National Standard Institute’s Laser
Safety Standard (ANSI Z136), the brain tissue exposition
limit is typically 200 mW/cm2 at NIR light to prevent any
biological hazards [19]. The amount of reflected NIR light
photons detected by the photodetector depends on the
power of the NIR light source, the attenuation due to
biological tissue, and the light source-to-photodetector
distance. The level of attenuation with a light source-tophotodetector distance of 4 cm, for a five-layer (scalp,
skull, cerebrospinal fluid (CSF) layer, and gray and white
matters) head model, can be approximated by
4.12 9 10-4 cm-2 [20]. If we consider an NIR light source
of 10 mW, the optical power seen by the photodetector is
only 4.12 lW/cm2. Thus, the area of the photodetector
should be large enough so that it can capture enough
optical photon to generate detectable electric signal.
Earlier silicon p-i-n photodiodes were successfully used
for NIRS systems with an active area of *7.5 mm2 [7, 20].
Silicon avalanche photodiodes are also commercially
available with comparable active area and are being used in
several NIRS systems, such as, Hamamatsu C5460-01
device has a large active area of 7 mm2 [5, 6]. However,
these commercial SiAPDs necessitate a dedicated fabrication process, and cannot be fabricated on the same chip
with the rest of the NIRS device. As such, fibre optic
bundles are used to guide the NIR light photons from the
scalp to the photodetector which results in huge loss (up to
40%) of optical signal. On the other hand, the performance
of SiAPDs fabricated using standard CMOS process
degrades with increase of its area which limits its active
area to 0.003 mm2 [21]. Recent advancements in the
standard CMOS fabrication process allow producing SiAPDs of active area up to 0.3 mm2 (*200 lm diameter)
while maintaining excellent performance [22].
Here, es (= 1.06 9 10-12 F/cm) is the dielectric constant of
silicon, q (= 1.6 9 10-19) is the unit charge, Na and Nd are
doping concentration for p and n type silicon respectively, ui
(=0.7 V) is the built in potential of silicon and Va is the
applied bias voltage. W can be varied either by changing the
reverse bias voltage across the photodiode or by changing the
doping concentration of p and n type silicon. Figure 6 shows
the variation of depletion region thickness of a p ? n photodiode (Na = 5 9 1020 cm-3, Nd = 1.28 9 1017 cm-3) as
a function of reverse bias voltage. The wavelengths of
interest for NIRS system is from 650 nm to 950 nm range.
For a NIR light source with optical power Pin at a wavelength
k, the input light photon flux, Gin, is given by [1]:
Gin ðkÞ ¼
Pin
k;
hc
ð3Þ
where h is Planck’s constant, and c is the speed of light in
vacuum. The input NIR light photons, Gin(k), are
attenuated by the brain tissues.
Gatt ðkÞ ¼ Gin ðkÞ K;
ð4Þ
where K is the attenuation constant of the brain tissues. The
attenuated light photons, Gatt(k) are incident on the SiAPD
and absorbed in the depletion region of the photodiode.
The fraction of Gatt(k) that are absorbed in the depletion
region of SiAPD depends on its photon absorption
efficiency, g(k), which is calculated as [24]:
gðkÞ ¼ 1 eaðkÞW ;
ð5Þ
where W is the thickness of depletion region of SiAPD and
a(k) is the absorption coefficient of silicon which is a
strong function of wavelength of light photons [25]. A very
small percentage of light is absorbed in the neutral region
of p?. The variation of a(k) of silicon within the NIR
wavelength range is shown in Fig. 7.
The photon absorption efficiency, g(k), of silicon for
different thickness of depletion region can be calculated
using Eq. 5 and plotted in Fig. 8. It is apparent from Eq. 5
and Fig. 8 that for a definite depletion region thickness, the
photon absorption efficiency is better at shorter wavelength. Again, the higher the thickness of depletion region
of SiAPD, the better is its photon absorption efficiency.
However, a thick depletion region increases the noise of
the SiAPD [24]. To ensure reasonable amount of photon
absorption (*70%) in NIR range, it is of primary
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16
4.51
4.01
3
Depletion Region
Thickness (µm)
14
Absorp. coeff,α (λ) (10 /cm)
6
12
10
8
6
4
2
0
0
5
10
15
20
3.3 Signal to noise ratio of the SiAPD
2.51
2.01
1.51
1.01
0.51
700
800
900
1000
Wavelength, λ (nm)
Fig. 7 Absorption coefficient, a(k), of silicon as function of wavelength [25]
Photon Absorp. Effic., (λ)
importance to design SiAPDs with at least 10 lm thick
depletion region.
3.01
0.01
600
Reverse Bias Voltage (V)
Fig. 6 Variation of depletion region thickness of SiAPD as a
function of reverse bias voltage
3.51
1.2
W_1_um
W_10_um
1
W_20_um
W_50_um
0.8
For NIRS system, the input optical signal is strongly
attenuated by the brain tissues, consequently the photodetector have to detect a low-level light signal. For this
purpose, SiAPDs offer substantial advantages over normal
photodiodes since the SNR of SiAPD for low level signal is
determined by thermal or amplifier noise and not by the
shot noise in the photodiode current [26]. However, since
avalanche multiplication is caused by carrier impact ionization events which occur with statistically distributed
probability, it is intrinsically noisy. As such, the performance of SiAPD is degraded by a factor known as excess
noise factor, F. Both excess noise and photocurrent of
SiAPD increase as the avalanche gain increase, so the best
SNR occurs at a certain gain. A SNR of *40 dB is needed
for NIRS application [20]. Silicon avalanche photodiodes
with dark current in nA range, and the generated photocurrent in hundreds of lA range confirms SNR of much
higher than 40 dB.
This special doping scheme hinders SiAPD’s integration in
standard CMOS technologies. It will be compact and costeffective to integrate SiAPDs and other electronic circuits
of an NIRS front-end receiver on the same chip using
standard CMOS process.
4 Design issues of SiAPD
4.1 Comparison between dedicated and standard
technology
Silicon avalanche photodiodes are commercially available
with reasonable characteristics and are being used in several NIRS systems [5–7]. Silicon avalanche photodiodes
work in avalanche mode which necessitates application of
high voltage across the device. As such, there is a risk of
premature breakdown of the device, particularly at the
peripheral junction since higher electric field exists at the
periphery. To circumvent the risk of device breakdown,
generally a guard ring structure is implemented where the
peripheral junction has a lower doped region than the
active junction because a slightly lower doped region has
lower electric field compared to a heavily doped one [27].
There are two approaches of design and fabrication of
SiAPDs: one is to use highly optimized dedicated processes
to achieve excellent device performance and the other is to
adapt SiAPD design to existing standard CMOS processes
to reduce cost and to maximize miniaturization [28, 29].
In the sixties and seventies, many active research groups
developed various structures of SiAPDs such as the Reachthrough structures and the Beveled-edge structures and
fabricated them using dedicated processes with outstanding
characteristics [28]. Silicon avalanche photodiodes fabricated using dedicated process can have low-doped p and
n layer resulting in wide depletion region extending from
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0.6
0.4
0.2
0
600
700
800
900
1000
Wavelength, λ (nm)
Fig. 8 Photon absorption efficiency of silicon calculated using Eq. 5
for different thickness of depletion region (W) as function of
wavelength
Analog Integr Circ Sig Process (2012) 70:1–13
Fig. 9 Schematic of the CMOS SiAPD cross section (not to scale)
showing p-well implantation acts as a guard ring, preventing
premature edge breakdown [31]
the cathode to the anode [28]. Due to the availability of
wide depletion region, they are efficient for absorption of
red and NIR photons. Nevertheless, the SiAPDs fabricated
in dedicated process have two major disadvantages: the
production cost is very high due to the specialized fabrication process, and the impossibility to integrate electronic
circuits on the same chip.
Later several dedicated SiAPD fabrication technologies
were proposed which are compatible with the fabrication of
CMOS circuits, and therefore, monolithic integration of
SPAD devices and CMOS circuits became possible [22].
However, optimizing the performance of both the CMOS
devices and the SiAPD is a non-trivial job. To overcome
these problems, researchers have investigated the design
and fabrication of SiAPDs in a standard CMOS process [16,
30, 31]. The advantages of standard CMOS fabrication
process are: the availability of a fully supported, mature and
reliable technology at reasonably low cost, and the possibility of developing a complete system on chip with a high
degree of complexity [22]. The mandatory requirement for
SiAPD fabrication in standard CMOS process is that a
suitable subset of CMOS fabrication process flow should be
able to build a planar p-n junction without device breakdown at the photodiode periphery [31, 32]. Silicon avalanche photodiodes fabricated using standard CMOS
process involves high doped p or n layer resulting in shallow or medium depth depletion region. As a consequence,
CMOS SiAPDs are inefficient to detect red and NIR photons, and are not suitable for NIR signal detection in neuroimaging. However, to increase the use of SiAPD based
front-end receivers for biomedical applications, integration
of the SiAPD and peripheral circuitry on the same chip
using standard CMOS technology is highly desired.
7
necessary peripheral circuits on the same chip for an
integrated system. However, it is challenging to make
SiAPDs in CMOS technology due to lack of special fabrication steps. In a CMOS process, the only available layers
are the n? and p? source/drain regions, the n-well and
p-substrate (or p-well and n-substrate). In a twin-tub
CMOS process, both p-well and n-well regions are available. Figure 9 shows the most commonly used SiAPD
structure fabricated using a twin-tub CMOS process. The
device consists of a shallow p?/deep n-well junction
surrounded by a shallow p-well acting as a guard ring to
prevent edge breakdown [31]. By relying on the p-well
guard ring, the breakdown voltage of the active area can be
above 20–25 V [31].
In a single-tub CMOS process, the guard ring cannot be
realized by inserting a narrow p-well at the peripheral
junction since p-well layer is not hand-drawn and is present
everywhere that there is no n-well. The solution is to split
the n-well into two n-tubs separated by a small interval
d constituting the guard ring [27, 30]. The guard ring is
formed by a weakly doped n-ring due to the n-wells’ lateral
diffusion that decreases the breakdown voltage of the
peripheral junction (see Fig. 10).
Another approach is to use shallow trench isolation
(STI) as a guard ring to withstand the high electric fields
between the anode and cathode well (see Fig. 11) [33, 34].
Here the edges of the drain implant are confined by the
oxide trench and formation of the curved edges is prevented. As a result, a uniform field is achieved more
compactly than with a diffused p-well ring. However, STI
dramatically increases the density of deep-level carrier
generation centers at its interface [34]. Since the active
region of the SiAPD is in direct contact with the STI, the
injection of free carriers into the sensitive region of the
detector results in a very high dark count rate (DCR) and
degrades the performance of SiAPD (Fig. 12).
4.3 Comparison of the CMOS SiAPDs performance
Several research groups have fabricated SiAPDs using
standard CMOS technology. The area and design of the
SiAPDs for different CMOS technologies are different
4.2 Existing CMOS SiAPD structures
The fabrication of SiAPDs in standard CMOS technology
permits fabrication of both the photodetector and the
Fig. 10 Schematic of the CMOS SiAPD cross section (not to scale)
with guard ring developed by lateral diffusion n-well [27]
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Analog Integr Circ Sig Process (2012) 70:1–13
Fig. 11 STI-bounded SPAD
[33]
Fig. 12 Block diagram of
proposed front-end
photoreceiver working in linear
and Geiger mode
which results in a wide range of performance matrix of
SiAPD as depicted in Table 1. It is evident from the Table
that photon detection efficiency is better for larger area
SiAPDs. Silicon avalanche photodiodes with larger area
can more suitably be designed using older CMOS technology. However, use of older CMOS technology will
increase area and power consumption for rest of the electronic circuits of the NIRS front-end receiver. On the other
hand, doping concentration levels in CMOS increase as the
technology advances, causing an increase in the peak
electric field in the depletion region and decrease in the
breakdown voltage of the diodes. It is clear from the Table
that advanced CMOS technology offers SiAPDs with low
VBR ensuring safer operating condition for biomedical
applications.
Therefore, we proposed following front-end photo receiver
structure consists of SiAPD and its associated circuits for
both modes. We can vary the bias voltage (Vbias) across the
SiAPD and operate in either of the modes.
5.1 Proposed device structure
CMOS SiAPDs with wide depletion region ([10 lm) are
appropriate for NIR light absorption which necessitates
designing the SiAPDs with low-doped layers. In this paper,
we proposed a new SiAPD structure using relatively lowdoped layers available in standard 0.35 lm CMOS technology. The design specifications of SiAPD for NIRS
application is described in Sect. 3 and the design parameters for the proposed SiAPD are summarized in Table 2.
5.1.1 Simulation results of the proposed siapd device
5 Implementation of the proposed front-end
Our aim is to design a highly sensitive SiAPD for NIRS
front-end receiver to ensure maximum detection of the
reflected NIR light. We would like to investigate the performance of the SiAPD in both Linear and Geiger mode.
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Optimization of the performance of SiAPD is done by
device level simulation using Sentaurus TCAD software.
The active junction of the photodiode exists between p?
(Na = 5 9 1019 cm-3) and deep n-well (Nd = 1.28 9
1017 cm-3). The doping concentrations for these layers are
Analog Integr Circ Sig Process (2012) 70:1–13
9
Table 1 Comparison of different CMOS SiAPDs
Technology
Operating Mode
Area
Photon detection efficiency
Breakdown voltage, VBR
0.13 lm [35]
Geiger
Octagonal
5% at 800 nm
10 V
0.18 lm [2]
Geiger
Octagonal
1% at 700 nm
10.2 V
Circular
10% at 750 nm
35 V
5 lm diameter
5% at 840 nm
Circular
For 400 lm SiAPD
25 to 800 lm diameter
30% at 650 nm
Circular
0.5% at 950 nm
10 lm diameter
0.35 lm high voltage (HV) [36]
0.7 lm HV [28]
Geiger
Linear
12 V Gain: 10–40
20% at 800 nm
0.8 lm HV [37]
Geiger
55 V
12 lm diameter
Table 2 Design specification of the proposed SiAPD
Parameter
Value
Fabrication technology
CMOS 35 lm
Supply voltage
3–5 V
Gain in linear mode
[100
Wavelength
700–950 nm
Photon absorption efficiency
85% at 700 nm
30% at 900 nm
Area
*200 9 200 lm2
Depletion layer thickness
[10 lm
fixed for 0.35 lm CMOS technology. We created the
masks for the SiAPD structure using Ligament Layout
Editor and created an input command file for Ligament
Flow Editor. The input command file emulates the fabrication process and creates the structure and its doping data.
The output from Ligament Flow Editor serves as an input
for Sentaurus Process, which produces doping profile and
electric field distribution of the diode. Figure 13 shows the
doping profile of the device. Figure 14 shows the electric
field distribution of the device under reverse bias. It also
shows that the maximum electric field (*105 V/cm)
appeared in the active p?-deep n-well junction and the
device is able to withstand the electric field without
breakdown.
5.1.2 Layout of the SiAPD device
We have submitted the layout of p?/n-well avalanche
photodiode with guard ring for fabrication in a 0.35 lm
CMOS technology. We have designed SiAPDs with two
different shapes: square and octagonal. For the octagonal
SiAPD, guard ring is realized by low-doped p-well around
p? active area. For the square shape SiAPD, guard ring is
realized by low-doped n-ring due to n-wells’ lateral
diffusion.
Fig. 13 Doping profile of the SiAPD using the Sentaurus TCAD
software
5.1.3 Proposed transimpedance amplifier
For linear mode operation, the first circuit element is a TIA.
We have proposed a new TIA with combination of common-gate and resistive feedback techniques, depicted in
Fig. 15 to achieve a low-noise, low-power and high-gain
front-end. This circuit is a modified version of the previously developed TIA in our lab [3]. The design consist of a
current amplifier implemented in a transimpedance configuration [43]. In this circuit, we have simply used the
combination of two transistors (M6 and M7) to work as a
feedback resistor to minimize the output ripple and omit
the extra drawn current.
The dc transimpedance gain is given by:
Vout
gM5 Rf 1
¼
;
gM4 þ gM5
Iin
ð6Þ
where gM4 and gM5 are the transconductance of transistors
M4, M5, and Rf is the feedback resistance implemented by
M6 and M7 biased in the linear region. The frequency
where the loop gain of the TIA is unity:
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10
Analog Integr Circ Sig Process (2012) 70:1–13
Fig. 14 Device simulation of the SiAPD under reverse bias using the Sentaurus TCAD software
Input Noise (A/ Hz)
7E-10
6E-10
5E-10
4E-10
3E-10
2E-10
1E-10
0E+00
1E+00
1E+02
1E+04
1E+06
1E+08
Frequency (Hz)
Fig. 16 Input noise of the proposed TIA as a function of frequency
Fig. 15 Schematic of the proposed TIA
xt A
gm1 R1
;
Rf C D
Rf CD
ð7Þ
where CD is the photodiode capacitance. The closed-loop
bandwidth of the TIA is approximately equal to the unitygain frequency:
BW 1 þ A gm1 R1
xt :
Rf C D
Rf CD
6 Conclusion
ð8Þ
Bandwidth of TIA increases by decreasing the CD. We
have used the CD = 1 pF in our simulations as this is the
commonly reported value [13, 18, 24].
5.1.4 Simulation results of the proposed TIA
We have used CADENCE schematic editor and Virtuoso
layout editor to design and simulate our proposed TIA in
0.35 lm CMOS technology. In order to optimize the performance of the amplifier, we have analyzed the sensitivity
123
of each circuit component on the output. Figure 16 shows
the input noise of the proposed circuit as a function of input
current’s (Iin) frequency. Figure 17 shows the transimpedance gain of the proposed circuit as a function of its
frequency. The input noise is very low (&0.13 nA/HHz at
100 Hz and 1 fA/HHz at 1 kHz) compared to existing
TIAs. Measured output noise is also about 15 lv/HHz at
1 kHz. The power consumption of the front-end circuit is
also very low (0.8 mW). The TIA has high transimpedance
gain (*250 MV/A) and high bandwidth (*10 kHz).
Silicon avalanche photodiodes are promising photodetector
for low-level light detection. In this paper, we present a
review of constraints and challenges to implement CMOS
SiAPDs for an integrated NIRS system. We proposed a
novel SiAPD along with its peripheral circuitry for NIRS
application. Our designed SiAPD has large area
(*200 9 200 lm2) to capture enough NIR signal and
wide depletion region ([10 lm) to increase the absorption
of NIR light. Device level simulations result shows that the
device can withstand the high electric field (*105 V/cm)
for avalanche without device breakdown. We also designed
Analog Integr Circ Sig Process (2012) 70:1–13
11
3.0E+08
Gain (V/A)
2.5E+08
2.0E+08
1.5E+08
1.0E+08
5.0E+07
0.0E+00
1E+00
1E+02
1E+04
1E+06
1E+08
Frequency (Hz)
Fig. 17 Transimpedance gain of the proposed TIA as a function of
frequency
a novel TIA appropriate for amplifying and filtering the
signal coming from the SiAPD. The main advantages of the
TIA are low-power consumption (0.8 mW), high transimpedance gain (up to 250 MV/A), and very low input noise
(1 fA/HHz). We confirmed the efficacy of our proposed
TIA by circuit level post layout simulation. We implemented SiAPD and its associated circuits using 0.35 lm
standard CMOS technology. Such excellent performance
of SiAPD substantiates its feasibility to serve as a highly
sensitive photodetector of an integrated NIRS device for
portable biomedical applications.
Acknowledgments We gratefully acknowledge financial support
from the Heart and Stroke Foundation of Canada (HSFC) and the
Canadian Institutes of Health Research (CIHR).
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Afrin Sultana received her
B.Sc. degree (with honors) in
Electrical and Electronic Engineering from Bangladesh University of Engineering &
Technology, Dhaka, Bangladesh, and M.A.Sc. and PhD
degrees in Electrical and Computer Engineering from the
University of Waterloo, Waterloo, Canada in 2002, 2004, and
2009, respectively. Her PhD
dissertation focused on amorphous silicon based large area
detector for protein crystallography. She has been working as a postdoctoral fellow with Professor
Mohamad Sawan in Polystim Lab at Ecole Polytechnique since
123
November 2009. She is currently working to develop highly sensitive
photodetector for near-infrared spectroscopy (NIRS) system for
monitoring brain function. She has received a number of government
and university scholarships including Ontario Graduate Scholarship,
University of Waterloo President’s Scholarship, and the Best Student
Paper Award in the Symposium on Circuits, Devices and Systems of
IEEE Canadian Conference on Electrical and Computer Engineering,
2008.
Ehsan Kamrani received his
B.Sc. degree in Biomedical
engineering from Shahid Beheshti Medical Science University,
Iran, and Masters degree in
Electrical and Control Engineering from Tarbiat Modares University, Iran, in 2002 and 2005
respectively. From 2005 to 2009
he has been an Academic Member-Instructor in the Department of Electrical and Electronics Engineering, University of
Lorestan, Iran. His expertises are
on wireless networked sensors,
web-based control systems and biomedical signal/image processing.
Since 2009 he has been doing his PhD on Biomedical Engineering under
supervision of Prof. Sawan at Polystim neurotechnologies Laboratory,
Ecole Polytechnique, Montreal, Canada. He is working on design and
implementation of an fNIRS photo receiver for real-time brain
monitoring.
Mohamad Sawan received the
B.Sc. degree in 1984 from Laval
University and the Ph.D. degree
in 1990 in electrical engineering,
from Sherbrooke University,
Canada. He joined Polytechnique
Montréal in 1991, where he is
currently a Professor of Microelectronics and Biomedical
Engineering. His scientific interests are the design and test of
mixed-signal (analog, digital,
RF, MEMS and optic) circuits
and Microsystems: integration,
assembly and validations. These
topics are oriented toward the biomedical and telecommunications
applications. Dr. Sawan is a holder of a Canada Research Chair in Smart
Medical Devices. He is leading the Microsystems Strategic Alliance of
Quebec (ReSMiQ) receiving membership support from 11 Universities.
Dr. Sawan is founder of the Eastern Canadian IEEE-Solid State Circuits
Society Chapter, the International IEEE NEWCAS Conference, and the
Polystim Neurotechnologies Laboratory at Polytechnique Montréal. He
is cofounder of the International Functional Electrical Stimulation
Society, the International IEEE conference on Biomedical Circuits and
Systems (BioCAS), and the BioCAS committee in the IEEE circuits and
systems society. He is also cofounder, Associate Editor and member of
the steering committee of the IEEE Transactions on BioCAS (TBCAS).
He is Deputy Editor-in Chief of the IEEE Trans. on circuits and systems II
(TCAS-II), Editor and Associate Editor of several other International
Journals. Dr. Sawan is member of the board, editor, guest editor and
associate editor of several other prestigious scientific Journals. Dr. Sawan
published more than 500 papers in peer reviewed journals and conference
proceedings, two books and more than ten book chapters, and he offered
more than 100 invited talks. He was awarded nine patents pertaining to
the field of biomedical sensors and actuators. Dr. Sawan received several
Analog Integr Circ Sig Process (2012) 70:1–13
prestigious awards; the most important of them are the Medal of Honor
from the President of Lebanon, the Bombardier Award for technology
transfer, the Barbara Turnbull Award for medical research in Canada,
and the achievement Award from the American University of Science
13
and Technology. Dr. Sawan is Fellow of the IEEE, Fellow of the
Canadian Academy of Engineering, Fellow of the Engineering Institute
of Canada, and Officer of the Quebec’s National Order.
123