*Corresponding author: Waqas Ahmad, Email: waqas@szu.edu.cn, Tel: 0086-15602998010

Asian Journal of Nanoscience and Materials, 2018, 1(3), 122-134.

Simulation and Characterization of PIN Photodiode for Photonic

Applications

Waqas Ahmada,b,*, Muhammad Umair Alic, Vijay Laxmid, Ahmed Shuja Syed b aSZU-NUS Collaborative Innovation Centre for Optoelectronic Science & Technology, College of

Optoelectronic Engineering, Shenzhen University, Shenzhen 518060, P.R. China. bCentre for Advanced Electronics & Photovoltaic Engineering, International Islamic University, Islamabad

44000, Pakistan. cDepartment of Materials Science and Engineering, College of Engineering, Peking University, Beijing 100871,

P.R. China. dTHz Technical Research Center of Shenzhen University, College of Electronic Science and Technology,

Shenzhen University, Shenzhen 518060, P.R. China.

Received: 28 March 2018, Revised: 05 May 2018 and Accepted: 09 May 2018.

ABSTRACT: Research conducted on silicon based photodetector technology has recently shown

rapidly growing momentum to develop the robust silicon based detectors for photonic applications.

The thrust is to manufacture low cost and high efficiency detectors with CMOS process compatibility.

In this study, a new design and characterization of PIN photodiode is envisaged. The simulation tool,

Silvaco TCAD (and its variants), was used to design and simulate the processes of the device.

Electrical and optical measurements such as I-V characteristics (dark current), and internal/external

quantum efficiencies were analysed to evaluate the designed and processed device structure for its

potential applications in photonics and other detection mechanisms.

KEYWORDS: PIN Photodiode, CMOS, I-V Characteristics, Quantum Efficiency.

GRAPHICAL ABSTRACT:

1. Introduction Over the last five decades, photodiodes have

been used for extensive range of

applications including commercial use, and

military purposes [1]. Photodetectors are

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optoelectronic devices which convert light

energy into electrical energy and are

generally used in optical communication

systems. Typically, there are two types of

photodetectors which are commonly used,

named as p-i-n photodiodes and avalanche

photodiodes (APDs). These devices have a

wide range of applications in various fields

of science and technology. For instance,

they are used in light sensors, diagnostic

tools, digital radiography, and nuclear

medicines etc. The basic operation of a

silicon photodetector is to convert light

energy into electrical energy in which

electron-hole pairs are generated via reverse

bias voltage to increase the depletion region.

Due to carrier drift in opposite directions, a

signal is generated towards the collecting

electrodes. No signal will be generated even

if there is presence of electric field in the

depletion region but the electron-hole pairs

are produced outside. Photodetector

performance depends upon the interaction

of photons and charge carriers in the

depletion region. By increasing the bias

voltage, time performance of photodetectors

can also be increased [2, 3].

PIN photodiode is based on the P-N

junction which has highly doped regions. P,

I and N photodiode can be structured in

different ways as demonstrated in Fig. 1.

The planar PIN photodiode has lower cost

and better efficiency as compared to the

vertical PIN photodiodes [4]. In a PIN

photodiode, intrinsic region is increased to

gain higher efficiency and photons produce

more electron-hole pairs. PIN photodiode

has the capability to increase transient time

and decrease the junction capacitance. The

speed of a photodiode depends upon three

factors including diffusion, drift, and

junction capacitance. In diffusion, the

charge carriers are produced to the outer

sides of the depletion region which are close

enough to diffuse them. The drift time,

which is in the diffusion region, can be

reduced by increasing the intrinsic width of

the junction, while, the junction capacitance

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can be reduced by strong reverse bias [5].

Fig. 1. Basic structures of PIN Photodiodes.

Intrinsic silicon is used to fabricate the PIN

photodiode. In this structure, heavily doped

P and N regions are near the intrinsic

region, therefore it is referred as the P, I and

N diode. In a PIN photodiode, junction

capacitance is inversely proportional to the

depletion region, therefore, depletion region

absorbs more photons. To achieve high

frequency response of the PIN photodiode,

mobility of electrons should be greater than

the holes [6]. Compared to other optical

detectors, silicon PIN photodiode has

various advantages such as lower cost,

higher efficiency, smaller size, better

resolution, as well as room temperature

operation capability. It is used to measure

the position of light and its pixels are used

for image sensing. This type of device can

also be used to measure the short distance in

optical communications as well as optical

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storage. By reducing the surface reflectance,

the performance of a PIN photodiode can be

enhanced [7]. N. I. Shuhaimi et al. used the

Sentaurus TCAD tools to design the silicon

PIN diode. They investigated the I-V

characteristics as well as the effects in

variations of un-doped regions on the device

performance. They varied the width of the

PIN photodiode while the thickness was

kept constant at 40 µm. I-V characteristics

and device performance at different widths

(70 µm, 80 µm and 90 µm) were analysed

and it was observed that as the width is

increased, the current level is also increased,

suggesting the importance of width

optimization to realize high performance

photodiodes [9]. E Abiri et al. described

various applications of PIN diode by

demonstrating a device which is very

similar to the conventional PIN diode with

an extra layer at the centre of the layers in

PIN diode, realizing a device which possess

diverse applications such as modulators and

demodulators, amplifiers, high frequency

multipliers and resonators [9]. W. M. Jubadi

and co-workers used the Sentaurus TCAD

technology to simulate the four different

intrinsic layers of PIN diode with various

thickness (5 µm, 20 µm, 30 µm, and 50 µm)

and investigated the I-V characteristics of

PIN diode. They explored the “I” layer’s

thickness and its effect on PIN diode’s

performance. The results obtained from

simulation showed that the thinner is the “I”

layer, the more forward current flows in the

PIN diode, improving the overall device

performance [10]. A. W. Vergilio proposed

three terminal PIN diode structure which

works at forward conductive state,

demonstrating the accuracy in simulation

[11]. Z. Zainudin et al designed silicon on

insulator (SOI) PIN diode at Athena and

analysed its electrical characteristics using

Atlas Silvaco software. They investigated

the performance of SOI PIN diode and

simple PIN diode and realized that former

produces lower leakage current as compared

to the simple PIN diode. However, SOI

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photodiode shows poor performance in

temperature variations [12]. Here, in this

paper, we developed a simulation based PIN

photodiode using the commercial software

TCAD Silvaco i.e. Athena is employed for

designing the structure of the photodiode

and Atlas is used for the evaluation of

electrical and optical characteristics of the

proposed device.

2. Theory

Photodetection is one of the most emerging

research fields in optoelectronics. Silicon

photodiodes are low cost devices which

have the ability to detect light in different

ranges of wavelength. Silicon photodetector

has unique features such as 90 % surface

reflection, response time approaching to 30

ps and wavelength approaching to >850 nm

[13]. The quantum efficiency of a

photodetector is defined as the number of

electron-hole pairs generated per incident

photon and is given by:

η = (1-R) [1-exp (-αw)] (1)

where, “R” is the surface reflectivity, “α” is

the absorption coefficient and “w” is the

width of the absorption layer. The

photodetection performance can be

determined if quantum efficiency of the

device is known. The sensitivity of a

photodetector, its noise equivalent power

(NEP) and responsivity depend upon the

quantum efficiency [4]. Surface reflectivity

is a unique property of a photodetector

which is defined as output current divided

by the incident light i.e.

)2( = R

where “ ” is the output current and

“ ” is the optical power. It is linearly

independent of the quantum efficiency [5].

A relatively small electric current is present

in the absence of light which flows through

the photo-sensitive devices. Due to dark

current, photodetector can operate under an

applied bias and generates electrical signals

in the absence of light. The performance of

the dark current will be increased if a bias

voltage is applied across the photodetector.

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It has the capability to detect small signals

from the photodetector [5, 15]. In a photo-

detecting device, the noise power is defined

as the collection of light resulting in signal

to noise ratio of 1 or in other words, root

mean square (RMS) noise current divided

by the responsivity is known as noise power

and its unit is . it depends on the

frequency of light and normally

photodetectors are characterized by the

single value of NEP. It is used to evaluate

photodetector’s performance as well as to

compare the performance of various

photodetectors [15]. Detectivity is another

Fig. of merit for a photodetector to evaluate

its performance which is reciprocal of NEP

and is given by the following formula:

D = (3)

Detectivity is also used for comparing the

performance of photodetectors, however, it

is not suitable to find the response of a

photodetector to an intensity spectrum [15].

Noise spectrum of a photodiode depends

upon the electrical frequency. It is used to

find the magnitude of the noise if the

electrical frequency of a photodetector is

known. To find the noise power of the

system, a term, power spectral density is

used which is described as the noise content

as a function of frequency. Mathematically

it is expressed as:

(4)

where, PN is the noise power over a

bandwidth range, S (f) is the power spectral

density, and Δf is the bandwidth [5].

3. Experiment Details

We employed Silvaco TCAD tool to design

the PIN photodiode and investigated its

electrical and optical characteristics. Silvaco

is a simulator tool of TCAD process for

device simulation and consists of Athena

and Atlas [16]. Athena is used for the design

of physical structure and Atlas is used for its

electrical and optical characterizations [17].

The dimension of the designed two-

dimensional device in this study is 2500×22

µm, having silicon with phosphorous

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concentration of 1000 ohm/cm2. 0.6 µm

oxide layer was deposited on the silicon

wafer for the masking purpose as well as a

passivation layer. For making the P well and

N well, it is necessary to make the

geometrical etching at silicon based wafer

to form the P well and N-well. First, we

fabricated P-well for which geometrical

etching was done by diffusing Boron atoms

with a concentration of 8.09 ×1016 on the

left side of the silicon wafer at 1200 °C for

120 min. For N- Well, the right side of the

silicon wafer was diffused with phosphorus

with a concentration of the 8.02 × 1018 at

1150 °C for 70 min. Aluminium is mostly

used for the purpose of metallization to

make electric contacts between circuits of

the semiconductor materials. In the next

step, aluminium was annealed at 500 °C for

30 min. The final simulated structure of the

PIN photodiode is presented in Fig. 2. All

the simulation codes used for the device

design and characterization in this work are

provided in Supplementary File.

Fig. 2. Simulated structure of PIN photodiode.

4. Electrical and Optical

Characterizations.

Atlas is the module of the Silvaco simulator

which is used to obtain the electrical

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characteristics of the device. It is a two or

three dimensional simulator, used to get

electrical characterization of the device.

Atlas simulates the electrical, optical and

thermal behaviour of semiconductors with a

special reference to the output efficiency of

the fabricated device [18].

Fig. 3. Graph between optical wavelength and photocurrent (internal quantum efficiency).

The internal quantum efficiency of the PIN

photodiode is defined as the number of the

charge carriers composed by the device to

the number of photons of a given energy

incident on the device, which are then

absorbed by the device [19, 20]. It is

influenced by the absorption coefficient as

shown in Fig. 3. The internal quantum

efficiency varies with the wavelength of the

incident light.

Fig. 4 reveals the relationship between the

optical wavelength and the current

dynamics. When the light is incident on the

device, every single photon is supposed to

yield electrons-hole pairs. These electron-

hole pairs contribute to the photodetector’s

output current. It can be seen that after

certain rise, it is independent of the energy

of the photons striking at the surface of the

device [20]. High quantum efficiency can be

seen in Fig. 4, which is ensured by the prop-

osed device structure.

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Fig. 4. Graph between optical wavelength and normalized anode current.

Fig. 5. Graph between cathode current and cathode voltage.

The relationship between the cathode

current and the cathode voltage is

demonstrated in Fig. 5. It is referred as the

dark current of photodetection process. The

reverse bias is applied to achieve a very

small reverse saturation current [21, 22].

The device current is also observed to be

decreased as the voltage increases on the

cathode terminal of the photodetector.

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Fig. 6. Graph between light intensity and cathode current.

Fig. 6 exhibits the relation between the light

intensity and the current on the cathode.

When the light has no impact on the device,

negligible current will be observed, while,

with an increase in the light intensity,

cathode current will start rising, resulting in

a linear relationship, as evident by the graph

[23].

Fig. 7. Graph between photocurrent and cathode current.

Relationship between the photocurrent and

cathode current is shown in Fig. 7, which

reveals high detection capability of the

photodiode, when light is incident on it. The

graph clearly presents the capability to

detect even at very narrow scales [24]. It is

evident from the graph that when the

photocurrent increases, the cathode current

increases linearly.

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Fig. 8. Graph between source photocurrent and cathode current.

The variation in cathode current as a

function of source photocurrent is depicted

in Fig. 8 for the case when light is incident

on the PIN photodiode. The photocurrent

increases when the light is incident on the

detector, which in turn increases the cathode

current in a linear fashion [24]. These

optical and electrical characterizations

confirm standard device performance of the

simulated device.

5. Conclusion

A rigorous design approach was maintained

in this work to provide an industrial driven

process recipe in order to effectively design

a “planar” structure of quadrant

photodetector compatible with concurrent

CMOS technology. The trade-off designed

parameters between the quadrant geometry

and PIN photodiode processing yields a

significantly cost effective and efficient

solution to manufacture such

photodetectors. Less number of processing

steps with better uniformity was observed as

an advantage of the proposed design

strategy. The applicability of the process

and the device scheme was testified and

presented as a proficient design for the

consequent fabrication.

Vertical geometry of a photodetector based

on PIN diode structure may be investigated

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in order to evaluate the impact of geometry

on the efficiency of detection and sensing.

Detailed analysis and possible tailoring of

underlining physical model of the

simulation tool may yield a better

understanding on the fine tuning of process

and device parameters for photonic

applications. One may look for other

competing choices of starting material

(substrate), for example SiGe or III-V

semiconductors, to design this device in

“beyond CMOS” approach for variability of

applications.

Acknowledgment

The authors would like to appreciate M. Ali

from the Advanced Electronics

Laboratories, International Islamic

University Islamabad, Pakistan for his

precious support and guidance.

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How to cite this manuscript: Waqas Ahmad*, Muhammad Umair Ali, Vijay Laxmi,

Shuja Ahmed Syed. Simulation and Characterization of PIN Photodiode for Photonic

Applications. Asian Journal of Nanoscience and Materials, 2018, 1, 122-134.