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 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

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 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 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.

R = 16IphPopt">                                                     (2

where “ 16Iph"> 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. 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 = 16ANEP">                                                     (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:

16PN =∆fS(f)df">                                     (4)

where, PN is the noise power over a bandwidth range, S (f) is the power spectral density, and Δ_f 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 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 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.

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.

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.

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