Transient Photo-response Voltage Investigations of a Silicon

Journal of Computational Information Systems 7:1 (2011) 114-118
Available at http://www.Jofcis.com
Transient Photo-response Voltage Investigations of a Silicon Photodiode
Used in CCPD and SSPDA
Haoyang CUI1,2,3,†, Zhong TANG1, Yong FANG2, Jun LIU3,Chunxu YANG1
1
School of Computer Science and Information Engineering, Shanghai University of Electric Power, Shanghai 200090, China
2
School of Communication and Information Engineering, Shanghai University, Shanghai 200072, China
3
Hangzhou Qianjiang River Electic Group Co., Ltd., Hangzhou 311243, China
Abstract
In solid-state imaging devices, each pixel uses a silicon photodiode as a photosensitive unit, therefore, the performance of
silicon photodiode is a key factor affect the imaging device. In order to characterize the performance of the
photodiode under illumination, the transient photo-response voltage of a PIN silicon photodiode excited by pulsed
laser at 550 nm was reported in this paper. The transient photovoltaic profile consisted of two different decay components.
A bi-exponent model was adopted to fit the experimental data. The fitting result show that a fast decay component had a
characteristic of time constant 0.1 μs and the dominant slow decay component had a characteristic of time constant 5 μs.
This two decay component attributed to photo-generated carriers recombination in silicon photodiode involving surface
states and the carrier’s traps.
Keywords: Silicon Photodiode; Transient Photo-response; Surface State
1. Introduction
Solid-state imaging devices are often composed of photosensitive area, shift register and signal readout
circuits, the non-uniformity nature of imaging devices caused by the photosensitive area is one of the most
importance parameter to evaluate these devices. This non-uniformity nature will lead to a decline in image
quality and distortion. To improve the performance of imaging devices, the non-uniformity nature of device
must be reduced. It was recently reported that charge-coupled photodiode device (CCPD) and the
self-scanning photodiode array (SSPDA) which performance better than the charge-coupled device (CCD)
in the field of application of solid-state imaging device have been caused wide attention [1]. In these
devices, each pixel uses a silicon photodiode as a photosensitive unit, therefore, the performance of silicon
photodiode is a key factor affect the imaging device. As a matter of fact, better understanding of the
optoelectrical properties of silicon photodiode is not only of importance for their potential applications, but
also is essential to understanding of the transport mechanisms of carriers in silicon photodiode. As a
photosensitive element in the basic detection unit, the working mechanism of silicon photodiode is
depending on the built-in electric field to separate the photo-generated electron-hole pairs to transfer the
†
Corresponding author.
Email addresses: [email protected] (Haoyang CUI)
1553-9105/ Copyright © 2011 Binary Information Press
January, 2011
H. Cui et al. /Journal of Computational Information Systems 7:1 (2011) 114-118
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optical signal into electrical signals, and then enter to the readout circuit through the metal electrode. A
through understanding of the process is helpful in revealing the mechanism of light-electricity conversion
and related processes in the silicon photodiode and hence beneficial to their designs. Transient
photo-voltage is a useful tool for characterizing materials and understanding the basic physical processes
for light-electricity conversion [2-4]. The report of transient photovoltaic characteristics of silicon
photodiode used in imaging devices is rare; therefore, it has practical significance to study the transient
phenomena of silicon photodiode.
In this paper, we report transient photovoltaic investigations of a PIN-type silicon photodiode. It was
found that, at room temperature, the profile comprised two decay components, which can be attributed to
the recombination centers from the surface states and carrier traps respectively.
2. Experiment
The silicon photodiode was prepared on n-type (111) silicon substrate. A PIN photodiode was thus formed
by boron ion implantation into n-type Si layer, resulting in an abrupt n+-p structure between the Al and the
Au bottom electron. The acceptor and donor concentration are 3×1018cm-3 and 1×1014cm-3, respectively.
The width of p and n+ regions are about 0.7 and 420 μm respectively. The active area of MCT photodiode is
3×3 mm2. The sample was measure in room temperature.
We measured the photo-response voltage of Si photodiode induced by one laser beam. The excitation
laser pulses at 550 nm was provided by a commercial optical parametric oscillator and difference frequency
generator pumped with a picosecond Nd:YAG (yttrium aluminumgarnet) pulsed laser as source. The pulse
duration was 30 ps and the repetition rate was 10 Hz. A small portion of the laser beam was reflected by a
beam splitter and measured using an energy detector (COHERENT J4-09) in order to monitor the exciting
pulse energy. The pulsed photo-response of the silicon photodiode was measured from the voltage drop
across a 10 kΩ load resistor. Both signals from the energy detector and the silicon photodiode were input
into an Agilent Infiniium 54832B oscilloscope to monitor and record the pulse profiles. Based on the short
duration and low repeating rate of the pulse, the excited states could relax back to the initial state before a
next pulse irradiated on the photodiode. Also the cumulative energy problem was overcome. The intensity
fluctuation between the pulses was less than 1%; therefore, an average of 200 pulsed profiles was recorded
to eliminate the pulse-to-pulse fluctuation and to improve the signal-to-noise ratio [5,6].
3. Results and Discussion
Due to the phonon assistant in the electrons transition process, a portion of photo generated carriers in
silicon photodiode could be excited as excitons at 300 K. The excitons are electrically neutral and would
not contribute to the photo-response voltage. Since only the free carriers contributed to the photo-response,
the transient photovoltaic profiles could provide information about the decay processes of free carriers.
When the photodiode excited by 550 nm pulses laser, the photo-response voltage will show a rapid increase
and slow decay process. A typical photovoltaic profile of silicon photodiode is presented in figure 1, where
the open circles are the experimental data. This transient photo-response voltage phenomenon can be
explained as: under our experimental conditions, the transient photo-response voltage mainly comes from
the contributions of the excess photo-generated carries in the n base region and p emitter region. The large
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H. Cui et al. /Journal of Computational Information Systems 7:1 (2011) 114-118
number of excitons excited by the pulsed laser will be dissociated by the built-in electric field of pn
junction space charge region, in which photo-generated electronics will transfer to the n region, while the
photo-generated holes will transfer to the p region. These excess excitation carriers accumulate in p and n
regions respectively, forming a transient photo-response voltage. This process is consistent with the rapid
increase of transient photo-response voltage at t=0 shown in figure 1. The photo holes and photo electrons
annihilate through the circuit including loading resistance and junction equivalent capacitance. This process
corresponds with the slow decay of transient photo-response voltage shown in figure 1.
It can be seen that the transient photo-response voltage profile given in figure 1 is comprised of two
different decay components obviously. Thus a bi-exponential decay model was used to fit the experimental
data [7]:
V(t)=Aexp(-t/τ1)+Bexp(-t/τ2)+C
(1)
where coefficient A and B are the magnitudes of the two decay components, and C is the constant
introduced by a background illumination, and τ1 and τ2 are the characteristic decay times of the fast decay
component and slow decay component, respectively. It could be seen form figure 1 that the bi-exponential
decay model (shown in figure 1 with solid curve) fits the data very well when the fitting parameters τ1 was
taken to be 0.1 μs and τ2 to be 5 μs. Considering the response time of our experimental condition, the decay
time of the fast decay component should be less than 0.1 μs. The fact that coefficient B was nearly two
times lager than coefficient A suggests that the slow decay component dominated in the photovoltaic
response at room temperature.
The fast and slow decay component shown in figure1 could be attributed to two different recombination
mechanisms of the photo-generated carriers in silicon photodiode. As a number of electronic localization
states existed near the semiconductor surface, and distributed in the energy band gap. These energy states
can act as carrier recombination centers. The recombination of carrier is fast as the compound is located
near the surface layer. In this study, the fast decay correspond the carrier’s recombination of these centers.
Therefore, the fast decay process corresponds to the recombination of photo-generated carriers involving
surface states which has a characteristic decay time of several hundreds of nanoseconds. Inside the
semiconductor，there exist some other carrier localization states, they are far away from the surface layer
however, these carrier localization states will act as carrier traps. A port of photo-generated carriers will be
captured by these traps first, and then be released to recombination annihilation, so this process has led to
the carrier recombination slowly. In this study, the slow decay process correspond the carrier’s captured by
these traps.
The dependence of the photo-response voltage of the silicon photodiode on the incident intensity at room
temperature is presented in figure 2. The peak of the photo-response voltage of the photodiode increased
exponentially with increasing incident intensity and then reached saturation at intensity of 1.4 MW/cm-2.
The photo-response voltage saturation of the silicon photodiode could be attributed to the saturation of the
photo-generated excess carriers due to depletion of the carriers in the lower states in silicon photodiode.
The photo-response voltage measurement of the sample was also performed at the 335 nm, which was
produced from the third harmonic generation of the 1064 nm laser beam from the YAG laser. The
experimental results show the same as those obtained at 550 nm.
H. Cui et al. /Journal of Computational Information Systems 7:1 (2011) 114-118
117
Photo-response voltage (mV)
32
Experimental data
Fitting curve
silicon photodiode
λ=550 nm
T=300 K
24
16
8
0
-1
0
1
2
3
4
5
6
7
Time (μs)
Fig.1 The Transient Photovoltaic Profile of a Silicon Photodiode at Room Temperature. The Open Circle Points Represent the
Experimental Data and the Red Solid Line Represents the Fit Curve Obtained by using Bi-exponent Model.
Peak photo-response voltage (V)
0.6
0.5
silicon photodiode
λ=550 nm
T=300 K
0.4
0.3
Experiment data
Fitting curve
0.2
0.1
0.0
0.4
0.6
0.8
1.0
1.2
1.4
2
Incident intensity (MW/cm )
Fig.2 Peak Photo-response Voltage vs Incident Intensity at Room Temperature. The Circles Points are the Experimental Data and the
Solid Line is the Fitting Curve Using Exponent Model.
4. Conclusion
In summary, we have reported a transient photo-response in a silicon photodiode used in CCPD and
SSPDA. A fast decay process of the time constant 0.1 μs and a dominant slow decay process of time
constant 5 μs were observed when the silicon photodiode illuminated at room temperature. These two
processes were attributed to two different photo-generate carriers recombination process in the diode. In
addition, saturation of the photo-response voltage was observed at the incident intensity 1.4 MW/cm-2.
Acknowledgement
This work is supported by the Innovation Program of Shanghai Municipal Education Commission of China
under Grant No. 10YZ158; the Shanghai College Foundation for Excellent Young Teachers of China under
Grant No. sdl08025
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