A novel Field Effect Photodiode to control the output photocurrent and fast optical switching

This paper presents a novel device called Field Effect Photodiode (FEPD) to overcome the inherent drawbacks of PIN Photodiode (PIN-PD) and control the output photocurrent. Also, the proposed PIN-PD can be applied as a fast optical switch that provides a desired ION/IOFF ratio for optical applications in the nanoscale regime. The proposed device combines a Metal Semiconductor Field Effect Transistor (MESFET) and a regular PIN-PD device that can convert the incident light with photon energy greater than the semiconductor’s bandgap to the regulated photocurrent by changing the gates bias which mounted over the absorption region. The significant parameters such: responsivity, output current in both the ON and OFF states, dark current, and ION/IOFF ratio that play key roles in the optical applications have been extracted. To extract and illustrate the electrical and optical results of both the regular PIN-PD and the proposed FEPD in this work, we have used TCAD tools as a semiconductor simulator.


Introduction
Photodiodes are optoelectronic devices that can absorb light and penetrate the absorbing region. The photon energy greater than the semiconductor's bandgap energy excites electrons from the valance band to the conduction band, generating electron-hole pairs, and results in the output photocurrent (Chuang 1995;Shaban et al. 2018). For description this mechanism, we should seek some optical and physical phenomena; internal quantum

FEPD structure and modeling
The proposed device structure in this work is generally based on self-aligned n-GaAs as an absorption region material with indium tin oxide (ITO) as Schottky metal gate electrodes. When the surface of the device is illuminated by optical radiation between intervals from 300 nm to 800 nm from an external source, it couples into the device through the transparent ITO metal contact as a Schottky-gate electrode to modulate the electrical and frequency characteristics of the device. Some limitations like increasing the recombination at the surface device lead to decreasing the absorption distance at shorter wavelengths and increasing dark current. Therefore, we tend to use the absorbing materials with bandgaps that are only modestly smaller than the energy of the incident photons to obtain the best efficiency and lower noise. For reaching the best results of the designed device, it should be noted that the detector designs, specifically engineered to ensure from the drift-dominated transport mechanism away from the illuminated surface area, have been demonstrated to improve the short-wavelength performance (Sun et al. 2003). For the combination of the mentioned factors; interested in the use of hetero-junction structure such GaAs/AlGaAs for high-speed detectors systems since mole fractions of Al x Ga 1−x As provide a good match to the required bandgap and GaAs/AlGaAs hetero-junction provides acceptable lattice-matched materials either has a larger bandgap as a material in the absorption region as well as good electron transport properties and increasing the rate of the electron-hole pair generation.
To explain the reasons of using Gallium Arsenide as an absorption region material can be referred to the figure of merits this material such as the high electron mobility that enables superior high-frequency operation achieved and favor for using in the fast optical devices such GaAs lasers, Light Emitting Diode, Photodetectors, and Optical switches. Below the n-GaAs layer, we have set a layer of AlGaAs to ensure the separation of the hetero-junction interface from the doped GaAs region (Dagli 2003). This is critical if high electron mobility is to be achieved. To provide the lower parasitic capacitance, we have set SiO 2 as a substrate. The structures of the regular PIN photodiode and the proposed devices have illustrated in Fig. 1a and b.
To design the proposed, we have used two semiconductors, GaAs and AlGaAs, because these semiconductors have a direct energy bandgap and do not require phonon interactions (crystal lattice vibrations) for the necessary momentum to absorb photons. The reason for using the hetero-junction GaAs/AlGaAs was to have the highest lattice matching between the two materials. The substrate layer is made of SiO 2 . The doping densities of the drain and source areas are n + and p + with a density of 10 19 × cm −3 , respectively, and

Absorption Region GaAs
LGS LGD LSpacer SiO2 tGaAs (b) Fig. 1 a The regular PIN photodiode, b the novel field effect photodiode structures the absorption region, which is the same as the depletion region, has n-type doping with a lightly doping concentration or intrinsic with a density of 10 14 × cm −3 .
Due to some of the figure of merits of the indium tin oxide (ITO), such: transparent to visible light, relatively high electrical conductivity due to a low electrical resistivity of ~10 −4 Ω cm, a thin film of ITO can have an optical transmittance of greater than 80% (Chen et al. 2013). It can be used as a thin film for glass substrate for glass windows to conserve energy (Kim et al. 1999). We have chosen two transparent ITO Schottky gate electrodes mounted over the absorption region. To ensure the proper Schottky contacts, we have set a 4.67 eV for each contact as a work-function.
In choosing these contacts, it has been taken into account that the Schottky contact has the lowest contact with its substrate and thus increases the response speed of the device in optical applications and fast switching.
The adjacent gate electrode to the drain contact is a Gate-Drain (GD), and the adjacent gate electrode to the source contact is a Gate-Source (GS), respectively. Two gates are in the same length L GD = L GS and are used to control the density of the carriers in the absorption region by changing their biases and polarities. For having the proper performance, both the gates must be located near together for the FEPD. In this paper, we have chosen the FEPD structure like asymmetrical structure, meaning that the source and drain lengths can have an equal length L D = L S .

FEPD simulations and operation modes
To simulate and extract the FEPD parameters, we have used TCAD tools as a semiconductor drift-diffusion solver. Models include Shockley-Read-Hall (SRH), Auger (AUGER), the dependence of the carrier mobility on the doping concentration, Lombardi mobility model (CVT), Bandgap narrowing, Fermi statistic dependence (FERMIDIRAC), and Lateral electric field-dependent mobility. To validate the accuracy of the simulation results, the TCAD simulator (Device Simulator ATLAS 2015) has been calibrated by experimental data (Jiang et al. 2016). We observed that there is a good agreement between the experimental results and the simulation data.

ON-state
By illuminating the absorption region with visible light with photon energy greater than n-GaAs layer bandgap on top of the FEPD, the absorbed photons transfer energy to the atoms of the radiated n-GaAs layer, moving these holes and electrons carriers to their conduction bands. Once there, the individual carrier may or may not contribute to the current flow (Graeme 1996). Carriers release within the absorption region and produce electronhole pairs (EHP). By applying V D > 0 while the source is grounded, the FEPD is in the reverse bias. In this case, the width of the depletion region increases, and the electric field causes more electrons to be transported to the drain contact and more holes transported to the source contact.
In the first mode, by biasing the V GS < 0 while V GD > 0, more generated holes accumulated under the GS contact, and more generated electrons accumulated under the GD contact. Here we have a p + -p-n-n + structure and the FEPD works in its ON-state. Hence the magnitude of the output current versus optical wavelength of the FEPD in its ON-state is several orders of magnitude higher than in comparison with the regular PIN-PD, as shown in Fig. 2.
The collected charges at these contacts produce an electric signal in the external circuit. Fig. 3a, b illustrate the energy band diagrams of the regular PIN-PD and FEPD in the ON-state.

OFF-state
In the second mode, by biasing the V GS > 0, V GD < 0, V D > 0 while the source remains grounded, the width of the depletion region increases due to reverse bias, and the electric field cause more electrons transported to the drain contact and more holes transported to the source contact. But regarding the gates bias; more generated holes accumulated under the GD contact, and more generated electrons accumulated under the GS contact. Here, we have a p + -n-p-n + structure and the FEPD works in the OFF-state, which considerably decreases the output current magnitude in the defined wavelength interval. Fig. 4 shows the energy band diagrams of the FEPD in the OFF-state. Figures 5 and 6 illustrate the carrier densities versus device length of the FEPD in both the ON and OFF states, respectively.
According to the simulation results adopted from Fig. 5, the heavily doped regions in the source/drain of the novel FEPD change the potential distribution. Regarding the gates bias in the first mode (V GS < 0 and V GD > 0), we have a p + -p-n-n + structure and the FEPD works in the ON-state. The gates induct a much higher potential barrier in the absorption region by changing the gate bias polarities in the second mode (V GS > 0 and V GD < 0). Hence, a large temporary reversed-junction and perfect p + -n-p-n + structure have formed inside the novel FEPD as illustrated in Fig. 6.
The output current versus optical wavelength in the visible range results of simulating the FEPD in the OFF-state has been demonstrated in Fig. 7.

ION/IOFF ratio
After calculating the ION and IOFF in both ON and OFF modes, we have obtained the main parameter of the proposed device, which has a high ION/IOFF ratio and plays a crucial role in the fast optical switching applications, as shown in Fig. 8. Table 1 shows the various structures according to gates polarity and work modes for FEPD versus optical wavelength.

Dark current
Dark current is a sum of the various phenomena such as non-radiative currents generated by SRH recombination and band-to-band tunneling current. In some references, the dark current is considered as a leakage current in the absence of light. Fig. 9 shows the value of the dark current relative to the drain voltage for the gate biases, V GS = + 3 V and V GD = − 3 V, in the off state and without the presence of radiation.

Spectral response in on and off modes
To better understand the concept of sensitivity of a photodiode, it is necessary to introduce a parameter called the spectral response, which is the change of the output signal as a function of the changes in the wavelength of the input signal. We have demonstrated this parameter according to the wavelength range of the absorbed photon and considering the different applied biases to the gates installed on the absorption region.
The spectral response of the device in the OFF state of the device has shown in Fig. 10 at the wavelength range from λ= 300 nm to λ = 800 nm. Fig. 11 shows the spectral response of the device in the ON state at gates biases V GS = − 3 V and V GD = + 3 V at the wavelengths from λ = 300 nm to λ = 800 nm.
According to the magnitude of the spectral response in the ON-state and the wavelengths from λ = 300 nm to λ = 800 nm, it is observed that the device in the wavelengths below λ =520 nm has shown good responses.

Bandwidth
For indicating the bandwidth of the device, we have simulated the proposed structure in the different values of the gate's biases from the frequency range of 100 KHz to 100 GHz then the cutoff frequency occurred at about 10 GHz, where it accounts as a bandwidth of the device as mentioned in Fig. 12.

State-of-the-art comparison
In the final section of this work, we have compared the typical characteristics of the recent published results (Song et al. 2021;Chen et al. 2020;Megherbi et al. 2021) with the proposed device, as shown in Table 3.

Conclusions
A competing interests declaration is mandatory for publication in this journal. Please confirm that this declaration is accurate, or provide an alternative.To control output current and fast optical switching of a regular PIN photodiode (PIN-PD), a novel device named Field Effect Photodiode (FEPD) was introduced in this paper. The proposed device can be fabricated by the standard technology of the regular PIN photodiode and conventional MESFET structures despite two Indium tin oxide (ITO) Schottky contact gates over the absorption region. For controlling an output current either properly turning the device off in the visible wavelength interval. The top layer is an n-GaAs that has an Eg = 1.44 eV. Below the n-GaAs layer, we have set a layer of AlGaAs. The GaAs/AlGaAs hetero-junction can provide acceptable lattice-matched materials regarding the larger bandgap for controlling the absorption in the structure as well as good electron transport properties. The simulation with TCAD tools software has been performed to obtain the output parameters in different bias and optical wavelength conditions. The I-V characteristics such as output current versus wavelength, carrier density, Internal Quantum Efficiency, energy band diagrams, I ON and I OFF , I ON /I OFF ratio, dark current, spectral response for the proposed FEPD and the regular PIN-PD and bandwidth of the device have extracted. The characteristics and the results were compared and shown graphically, then the novel FEPD exhibited good results to control and turn off the device output current. According to the results of simulating, to improve the output parameters; suggestions are provided below: (1) In order to increase the efficiency of the device, which is directly related to the ability to receive incident photons on the surface of the absorption region, it is suggested to use an anti-reflection layer on the surface of the absorption region to receive maximum incident light power and prevent reflection.
(2) One of the problems of all electronic and optoelectronic semiconductor devices is the noise caused in the process of mobility and transfer of electrons inside the material or passing from one material to another, which has its effect as heat or decreasing in the linear response of the device. For this device, it is suggested to use an area next to the absorption region as an augmentation or multiplication region, such as avalanche photodiode structures, which have an internal gain based on impact ionization. (3) Proposing to change the molar fraction of binary and ternary semiconductors used in the structure of the device and to obtain the best combination for designing the absorption region that is directly related to the molar mass of the semiconductor and can be an effective parameter. The amount of absorption and reduction of undesirable quantum effects is the light-absorbing layer.