Metal oxide semiconductor-based Schottky diodes: a review of recent advances

Metal-oxide-semiconductor (MOS) structures are essential for a wide range of semiconductor devices. This study reviews the development of MOS Schottky diode, which offers enhanced performance when compared with conventional metal-semiconductor Schottky diode structures because of the presence of the oxide layer. This layer increases Schottky barrier heights and reduced leakage currents. It also compared the MOS and metal-semiconductor structures. Recent advances in the development of MOS Schottky diodes are then discussed, with a focus on aspects such as insulating materials development, doping effects, and manufacturing technologies, along with potential device applications ranging from hydrogen gas sensors to photodetectors. Device structures, including oxide semiconductor thin film-based devices, p-type and n-type oxide semiconductor materials, and the optical and electrical properties of these materials are then discussed with a view toward optoelectronic applications. Finally, potential future development directions are outlined, including the use of thin-film nanostructures and high-k dielectric materials, and the application of graphene as a Schottky barrier material.


Introduction
Despite the emergence of the modern semiconductor devices in the last 70 years, these have substantially revolutionized the human society [1]. The core of these devices are the physical characteristics of the semiconductor material; including its integration of electronic and optical properties which allows interaction and regulation of the photon, electrons, and holes using different electronic infrastructure and operating environments. Most studies confirm the metal-semiconductor (MS) as a prime component of the modern electronic components [2][3][4]. Initially, the metal-semiconductor solid-state device was discovered in 1874 and referred as whisker contact rectifier, which included a wire tip pressed onto a lead sulfide crystal. Ever since, these devices continued to capture great attention in monitoring and testing due to their low cost, high sensitivity, and simplicity [2].
The realization of the metal-semiconductor junction is needed for appropriate use of electronic devices such as diodes, varactor diodes, metal-semiconductor field-effect transistors (MESFETs), high electron-mobility transistors (HEMTs) and heterojunction bipolar transistors [5][6][7][8]. The wide range of MS junction and its application highlights is significant. Researchers have investigated different MS junctions over a long period, and hence their physics are well understood. However, there lacks a comprehensive analysis of MS junction and its use, application, and challenges in the research. To reinforce the significance of MS, this review is conducted which helps enlighten the progress that has MS junction has undergone since its initial discovery in the 1847, along with its application, use, challenges, and potential. Seol et al [9] have recognized that the contact of semiconductor with metal provides two types of the junction, namely: the ohmic junction or the rectifying (Schottky) junction. In an ohmic junction, the current (I) changes linearly with the applied voltage (V) and follows Ohm's law [10].
defined film-fed growth (E FG ). It also presents better growth as a high quality single-crystal which makes it a potential and promising material for the semiconductor devices like Schottky barrier diode as well as metaloxide-semiconductor field-effect transistor (MOSFET) [7,8].
The structure of MOS (also referred to as MOS capacitor) is simple and like plate capacitors, where an oxide material is sandwiched between a metal and a semiconductor. Depending on the various applications, the basic MOS structure can act as a MOS Schottky diode, capacitor, or as a gate in MOSFET transistors [46]. The schematic diagram of a MOS structure is shown in figure 2. Reprinted from [28], Copyright (2017), with permission from Elsevier.  Chen et al [39] coproduced Pd/GaAs and Pd/InP MOS Schottky diodes as hydrogen sensors, with the crosssection of the structures. It was shown that interfacial oxide significantly improves barrier height resulting from the Fermi level pinning weakening for both GaAs and InP Schottky structures [47]. Thus, the Pd/InP MOS Schottky diode performs better and shows a large modulation in barrier height and sensitivity.
The study of radiation effects on MOS devices is important, as radiation exposure of these devices may alter the electrical and dielectric properties of the oxide layer. The radiation may cause the creation of new charge states at the SiO 2 /Si interface [48]. In addition, the high-energy particles can introduce lattice defects that act as recombination centers for the majority/minority charges, resulting in degradation of device performance.
Recently, Juang et al [49] fabricated an Au/SnO 2 /n-LTPS (n-type low-temperature polysilicon) MOS Schottky diode to prepare a glass substrate for carbon monoxide sensing applications (figure 3). SnO 2 with a large bandgap of 3.0 eV is a low cost and highly sensitive material to CO gas. The experimental results show that the Schottky diode with a SnO 2 layer exhibits a high relative response ratio of ∼546% to 100 ppm CO ambient under conditions of 200°C and −3 V bias. The Schottky barrier height is given by equation (1) and the barrier height lowering (ΔΦ B ) under various CO gas concentrations is expressed as follows: [49], Different MOS materials, such as ZnO, SnO 2 , WO 3 , and TiO 2 , are utilized for sensing films as they offer low costs, long lifetimes and better sensitivity and selectivity over conventional solid-state gas sensors [50]. One research study [51] involved the fabrication of a Pd/TiO 2 /Si MOS-based detector utilized for the detection of hydrogen and hydrocarbons, such as ethanol, acetone, and TCE in different ambient atmospheres (O 2 , N 2 , and Ar). The results show that the MOS sensor displayed a maximal response toward acetone in oxygen ambient among the other vapors detected.

Comparison between MS and MOS devices
Since the performance and stability of MS devices depend on the surface conditions, it is integral to understand its underlying mechanism. The addition of the interfacial layer and interface surface states can considerably change the device characteristics. For the ideal behavior of an MS Schottky diode approaching an ideality factor to unity, low series resistance and low reverse leakage current are required. However, the electrical properties of the MS Schottky diodes are determined by various non-idealities, such as interface states, the interfacial oxide layer, and series resistance. Direct deposition of metal on a semiconductor generates many interface states at the semiconductor surface. The higher interface state density is a cause for non-ideal current-voltage characteristic behavior. The crucial effect of adding an oxide layer on the semiconductor surface is to passivate the dangling bonds. This passivation can reduce the anomalies in the diode current-voltage characteristic behavior by minimizing the surface states [52]. Another important aspect of the oxide interfacial layer between the MS Schottky diode is to achieve a low leakage current. For high power, high frequency and high-temperature application devices, MS junctions may suffer from high leakage-current and low breakdown voltage, which limit device performance, reliability, and stability. This is where the deposition of a thin oxide/insulator layer can restrain the leakage current. Rajagopal and Venkata Prasad [53] studied the effects of a high-k ZrO 2 thin insulating layer on the electrical and carrier transport properties of an Au/ZrO 2 /n-GaN MIS junction. The measured I-V characteristics of Au/n-GaN MS and Au/ZrO 2 /n-GaN MIS junctions are shown in figure 4.
I-V characteristics show that the reverse leakage currents are 2.301×10 -8 and 5.566×10 -11 A at −1 V for the MS and MIS junctions, respectively. About a three orders of magnitude reduction in the reverse leakage current is observed for the MIS junction compared with the conventional MS junction. The results also show the increase in barrier height (0.94 eV) for MIS junction compared with the MS junction (0.73 eV) and a decrease in interface state density (Nss) figure 5 (right) for the MS junction inserted with ZrO 2 is observed. Figure 5, 6 and 7 show a comparison between the energy level diagrams of Au/n-GaN and Au/ZrO 2 /n-GaN junctions with interface states and an interfacial layer.

Equations to MOS Schottky diode: carrier transport mechanisms
For an ideal Schottky diode, the voltage drop bias across the depletion is presented by V, whereas, in the presence of the interfacial layer, the bias voltage drop depletion is presented as V/n at semiconductor or metal interface or in the case of non-ideal Schottky diyot [55]. According to the thermionic emission theory, in forward bias current voltage (I-V) characteristics when the applied bias V3kT/q for a MOS Schottky diode is given by the following relation [56,57].
where n is the ideality, k is the Boltzmann constant, T is the absolute temperature, q is the electron charge, R S is the series resistance, and I 0 is the saturation current determined by: Where A is the diode area, A * is the effective Richardson constant (32 A.cm −2 K −2 for p-type Si), and qΦ B0 is the barrier height at zero bias. The ideality factor and saturation current I 0 can be determined using the slope and intercept of the semi-log forward bias ln I-V characteristics. The following relation gives the ideality factor (n), For an ideal MOS Schottky diode, the value of n is unity. Another model for extracting the Schottky diode characteristic parameter, I-V characteristics can be used in the following function [58]; Equations (4) and (5) resemble a straight-line equation. The device parameters, such as ideality factor (n), barrier height (Φ B ) and series resistance (R S ), can be determined from the plots dV/d (ln I) versus I and H (I) versus I. The Norde function also provides the diode parameters and barrier height, as defined by the following relations [59]: The reverse leakage current conduction mechanism can be investigated based on Fowler-Nordheim (F N ), Poole-Frenkel emission (P FE ), and Schottky emission (S E ) models across the Schottky junction. Each model gives a current-voltage relationship describing the conduction mechanism. More than one conduction mechanisms may exist at the same time. According to F N , the reverse current was defined by [60,61]: ) / The reverse current, according to the Poole-Frenkel emission theory, is defined by the relation [62]: and the Schottky emission theory gives the following reverse current relation [60]: where d is the thickness of the film and S PFE and S SE are the Poole-Frenkel and Schottky field lowering coefficients, respectively. The theoretical value of the SPFE (β=1) is twice the value of SSE (β=2) and can be defined as [63]: The non-idealities in the Schottky diode behavior occurs due to the defects at the oxide-semiconductor interface, resulting in large density of interface states that are continuously distributed within forbidden energy gap. It is a non-ideality condition, which causes leakage currents to flow. The barrier height of a Schottky diode due to the presence these interface states is strongly dependent on the electric field in the depletion region and thus it depends on the applied bias. The effective barrier Φ e is defined as [64]: Where β is the voltage coefficient of the effective barrier height Φ e . Hence, the effective barrier is a parameter that take cares of the effects of interface states in equilibrium with the semiconductor, as described by the theory in [65]. The ideality factor relation for a MOS Schottky diode having interface states Nss in equilibrium with semiconductor is given by: where ε i , ε s , δ, W D and N ss are the interfacial layer permittivity, the semiconductor permittivity, the thickness of the interfacial layer, the depletion layer width and the density of the interface states, respectively. The density of interface states (N SS ) considering series resistance and voltage dependent ideality factor n(V) is given as follows [65]: Furthermore, the energy of the interface states E ss (for p or n-type semiconductors) with respect to the bottom of the conduction or top of the valance band, at the surface of the semiconductor is given by: From the C-V characteristics of the MOS Schottky diode depletion layer, the capacitance per unit area can be expressed as [66]: where A is the area of the diode, V is the applied voltage, ε o is the permittivity of free space, ε s is permittivity of semiconductor, q is the electron charge, V bi is the diffusion potential at zero bias and N A is the acceptor concentration. The value of V o can be determined from the intercept of the C -2 versus V plot.
The barrier height Φ B from the C-V characteristics can be obtained by the following relation: where E F is the energy difference between the bulk Fermi level and is given by the following relation [65]: Where N V is the effective density of the states in the valence band and is given by: where mh * is the effective mass of wholes and mo is the rest mass of electron. ΔΦ B is the image force barrier lowering and is expressed by [66]: where E m is the maximum electric field and is given as: The voltage and frequency dependence of the C-V characteristics is due to the non-ideal behavior of a Schottky device and a series resistance effect. The series resistance for a MOS Schottky device is obtained by Nicollian and Goetzberger method [67]: where G m and C m are the measured conductance and capacitance, respectively. This series resistance effect must be adjusted to obtain the correct conductance value and capacitance. Thus, the adjusted values of capacitance and conductance are given by the following relations [66]: Hill-Coleman [68] gave another method to find the density of states, which is defined as: where C ox is the oxide layer capacitance, ω=2πf is the angular frequency, A is the diode area, q is the electrical charge, G adj,max. is the maximum from corrected G adj -V curve and C m is the diode capacitance corresponding to G adj,max.

Recent progress of MOS devices and their applications
Many research efforts have been dedicated to finding a suitable insulating material for MOS/MIS Schottky diodes. In recent years, oxide semiconductors with high-k dielectrics, such as TiO 2 , WO 3 , MoO 3 and ZrO 2 [69,70] have received significant attention as interface layers in SBDs. Besides the unique electrical and optical properties of oxide semiconductors, the tunability of these properties by an appropriate doping level, the ability to grow a variety of nanostructures and the number of growth methods make them superior compared to conventional silicon dioxide used in SBDs. For instance, WO 3 is an insulating layer and was employed as an intermediate layer for a metal and semiconductor in an SBD for hydrogen sensing applications [71]. Marnadu et al [40] studied the effect of strontium doping on a Cu/Sr-WO 3 /p-Si Schottky diode. A thin film of strontiumdoped tungsten oxide with various concentrations (0, 4, 8 and 12 wt%) was spray coated onto a p-Si substrate. It revealed that optical, morphological, and structural properties of the film change with Sr doping concentration.
The XRD patterns showed a higher average crystallite size for 12 wt% Sr-WO 3 . The I-V characteristics for SBD show the minimum ideality factor (n=2.39) and maximum barrier height (Φ B =0.57 eV) values for a higher concentration (12 wt%) of Sr film.
GaN-based HEMTs are required for high frequency, high power, and low noise applications. Despite these wonderful applications, these HEMTs show a limit in their performance due to gate leakage current that limits the gate voltage swing and the maximum channel current that may be reached, which can be resolved with the addition of a dielectric layer between the gate metal and semiconductor. Ye et al [72] deposited a 10 nm thin film of ZrO 2 with the ALD method as a gate dielectric for metal oxide semiconductor high electron mobility transistors (MISHEMTs). The study found that the proposed ZrO 2 AlGaN/GaN MISHEMTs have max drain current density with high transconductance and four orders of magnitude reduced gate leakage current compared to Schottky barrier HEMTs. Kim et al [73] reported a near-infrared photodetector based on a Ni/SiO 2 /Si Schottky diode. The reports show that the highest performing detector showed a high rectification ratio of 19560 with an improved barrier height of 0.75 eV. The ideality factor of the photodetector was reported to be 1.14.
Hydrogen is an ecofriendly alternative fuel, which has potential to replace fossil fuels but is volatile in nature. Despite it, increased demand for hydrogen sensors is found for efficiently detecting hydrogen gas leakage. Chen et al [39] fabricated a Pd/HfO 2 /GaN MOS Schottky diode for hydrogen sensing and showed that the response time decreases from 39 to 5.3 s and 42 to 2.5 s when temperature increases from 300 to 383 K, respectively. However, it reported a lower detection limit of 5 ppm H 2 /air and showed a higher sensing response of 4.9×10 5 under 1% H 2 /air gas at 300 K at a 0.5 V forward voltage. In contrast, devices in comparison Pd/AlGaO x /AlGaN [74] and Pt/SiO 2 /GaN [75] showed sensing responses of 3.9×10 5 and 4.5×10 4 , respectively, at 300 K.
Thapaswini et al [76] used a high-k Ba 0.6 Sr 0.4 TiO 3 (BST) interlayer to study the electrical properties of an MS Schottky diode. The atomic force microscopy images showed a smooth BST insulating thin film on an n-InP substrate. The values for n and Ф B were calculated from I-V characteristics for the Au/n-InP MS and Au/BST/ n-InP MIS Schottky diodes as 1.94 and 0.74 eV and 2.05 and 0.83 eV, respectively. The reported reverse leakage current of the Au/BST/n-InP MIS diode (5.01×10 -10 A at −1 V) is lower than the Au/n-InP (2.76×10 -9 A at −1 V) MS diode, which is a very low reverse leakage current compared to other studies. It was also found that the Poole-Frenkel emission dominates the reverse current in both diodes, indicating the presence of structural defects and trap levels in the dielectric film. The density of states values for Au/BST/n-InP with Rs was found to be 2.97×10 11 eV -1 cm -2 , which is of the same order as in [44].
Racko et al [77] have extended the Shockley-Read-Hall recombination-generation theory of trap-assisted tunneling and highlighted the current leakage that occurs in Schottky structures with a high Schottky barrier (above 1 eV) and a high traps density. It showed that in some conditions that trap assisted tunneling (TAT) is crucial than the direct tunneling such as the charge transport is dominated by the TAT mechanism in a reversebiased Schottky structure, whereas, direct tunneling maintains its dominance in the forward-biased structure. It emphasizes that the simulation of real Schottky diodes I-V curves need to be simultaneously considerate to thermionic emission-diffusion transport theory (TED), direct tunneling, and trap-assisted tunneling.

Oxide semiconductor-based thin-film electronic devices
Oxide semiconductors can provide a replacement to conventional semiconductors, such as amorphous silicon materials, transparent conducting oxides, and organic semiconductors [78,79]. Their useful electrical and optical properties in a single material make them useful for several application fields. Oxide semiconductor thin films have huge applications in the field of electronics devices, such as flat screens, photovoltaic devices, display devices (liquid crystal displays), touch screens, gas sensors, electro-chromic devices, dilute magnetic semiconductors, ozone sensing devices, dye-sensitized solar cells and light-emitting diodes (LEDs) due to their high conductivity and good transparency [79][80][81][82][83][84][85].
The n-channel MOSFET exhibits good electrical characteristics with a maximum transconductance of 135 mS mm −1 and electron channel mobility of 275 cm 2 /V s. In another study [86], Jin et al [86] used sulphur passivation the InP by rapid annealing the substrate under a H2S atmosphere and a MOS capacitor was fabricated with a HfO2 film using ALD. The report shows the electrical properties of the device were improved. A serious issue with the III-V MOSFETs is the high defect density between the III-V material and the high-k dielectric layers. Therefore, the solution to this problem is to create a shallow InGaAs buried channel with an InP barrier layer, which moves the channel away from the oxide material and mitigates the problem of high density of states at the high-k/InP interface. Another study by Ahmadi et al [87] highlighted the passivation of the surfaces as well as the Fermi level depinning with the use of hydrogen and H-sensors. The measurement in the study revealed that the Schottky barrier height can possess similar values for the varied β-(Al x Ga 1−x ) 2 O 3 composition. It stated that this might be due to changes in alloy's composition.
Although, these buried channel techniques work, a significant defect density at the barrier layer and channel interface still exist, which affect the subthreshold swing and threshold voltage [88]. Zhuo et al [89] studied the electrical properties of the Al 2 O 3 /InP interface of a MOS capacitor and showed that an interface with reduced positively charge defects was achieved.
Zhang et al [90] reported Co-doped SnO 2 thin films and explored their hydrogen gas sensing properties. The results indicate that the best performance was obtained for a 1 mol% Co-doped SnO 2 thin film at 225°C with a response time of 7 s at 2000 ppm H 2 gas pressure. The researchers in [89] fabricated Al-doped ZnO (AZO) thin films by DC sputtering to study the electrical characteristics of an AZO/p-Si heterojunction. The rectify ratio at 5 V was found to be as high as 19.7. The results show that the diode can be used as a photodetector. Park and Kim. [91] demonstrated a transparent photodetector based on a TiO 2 film. Li and N co-doped ZnO films were produced by a molecular epitaxy technique, and their electrical properties were studied in [92]. The film was used in fabricating a photodetector. The photodetector showed an excellent response even for very weak signals with a power density as low as 20 nW. The report states that the high performance of the photodetector could be attributed to the relatively low carrier concentration of the ZnO:(Li, N) films caused by the compensation of the incorporated acceptors to the residual donors. Van Meirhaeghe et al [93] study showed that the for GaAs and InP Schottky's barrier height can be increased through deposition technique.

N-type metal oxide semiconductors
Both n-and p-type thin-film oxide semiconductors are required for various optoelectronic applications. Doping is a tool for controlling the properties of these oxide thin films. Ga 2 O 3 , SnO 2 , In 2 O 3 , CdO and ZnO are widely known n-type semiconductor oxides [94]. Zinc oxide-based metal oxide semiconductors have been investigated extensively due to their various applications such as a diode, sensors, and solar cells in recent years. Zinc oxide is a promising material for electronic devices due to its optical bandgap and electrical conductivity. This material is a direct bandgap with n-type electrical conductivity. There are several studies on ZnO thin film properties with IIIA (In, Al and Ga) metals and transition (Cu, Co, Ni, Mn, Ti, and Cd) metals doped, to enhance the electrical and optical properties of ZnO thin films [95,96].

P-type metal oxide semiconductors
In contrast, transparent p-type conductors have been less explored but are becoming more popular recently. The conductivity values for p-type materials are less compared to n-type materials for the same transparency because of the lower mobility values for p-type carriers [97]. A NiO thin film with a bandgap of 3.6-4.0 eV was employed to fabricate an Au/NiO/MgZnO/In structured MIS photodetector [98]. The results show that inserting the NiO not only lowered the dark current but also enhanced the photo response of the photodetector. Fortunato et al [99] also reported p-type conductivity in a CuO 2 thin film. The workers also reported a thin-film transistor based on a CuO 2 thin film, showing improved electrical performance in this research. Singh et al [100] obtained p-type ZnO thin films by bismuth (Bi) doping using a sol-gel method. The p-type nature of the Bidoped ZnO thin film was confirmed by Hall measurements and a hot point probe method. Finally, a Pd/Bidoped p-type ZnO Schottky diode was fabricated successfully. Several methods have been reported to prepare high-quality oxide semiconductor thin films in the literature, such as pulsed-laser deposition, MOCVD, molecular beam epitaxy, magnetron sputtering, electron beam evaporation, spray pyrolysis, vacuum evaporation, chemical deposition, ALD, successive ionic layer adsorption and reaction, electrochemical techniques and sol-gel spin coating methods.

Electrical and optical properties of oxide semiconductors
Davoodi et al [101] studied the effect of Ti and Al co-doping on the electrical and optical properties of ZnO thin films. They found that with the increase in Ti content doping level into the ZnO thin films, the crystal size decreases from 23 to 15 nm, and the surface roughness decreases. Yilmaz [102] investigated the properties of ZnO: Ga thin films because Ga 3+ is considered as an efficient dopant due to its advantages in being less reactive and more resistant to oxidation compared to Al 3+ . In addition, with its lower diffusivity, it is less affected by diffusion problems. Jung et al [103] also reported the electrical and optical properties of Ga-doped ZnO thin films. The films show an average transmittance above 90% in the 550 nm wavelength region. Highly c axisoriented films with the lowest resistivity of 1.46×10 −3 Ω cm were achieved.
Mimouni et al studied the electrical and dielectric behaviors of Mn-doped ZnO films with a spray method [104]. They found that a redshift in the bandgap of the ZnO films occurs due to the substitution of Mn 2+ in the acceptor level, which causes bandgap narrowing. Giri and Chakrabarti [105] studied the structural and optical properties of Mg-doped (0-5 at%) ZnO thin films using an RF sputtering method. The optical bandgap was initially found to increase from 3.02 to 3.74 eV by increasing the Mg content from 0 to 3 at% but further increasing the Mg content decreases the bandgap to 3.43 eV for a concentration of 5 at.%. The XRD results show that the maximum crystallite size of 21.73 nm was calculated for the 3% Mg-ZnO thin film and structural parameters degrade after 3% Mg doping in ZnO. It was shown that the Mg (3 at%) doped ZnO thin film could be employed as a buffer layer in optoelectronic device applications.
Karabulut et al [17] study paired the Au/Ti/HfO 2 /n-GaAs Schottky structures with the use of magnetron dc sputter technuique. It showed the dependence of the temperature of the electircuial and dielectirci proerpties, which was obtained using the G/ω-V dates and C-V. It also showed that the dielectric values (ε′, ε″, tanδ and σ ac ) decreases at high temperature.
Çetinkaya, Sevgili, and Altındal [106] assessed the effects of the Al/p-Si (MS) type photodiodes interlayer fabrication using (%2 ZnO-doped CuO) for its electrical properties. It used an I-V (reversed and bias) measurements for dark and other illimination intensities (10-100 mW cm −2 ). The results showed that values increase with the illumination intensity, particularly for the reverse bias because of high electric field. Altındal et al [107] study fabricated the Au/Ag-doped ZnO/polyvinyl pyrrolidone (PVP)/n-Si SBDs. X-ray diffraction and SEM results showed that small nanocrystals formation. While the formation of ZnO nanostructures was sheet like.
Among III-V compound semiconductors, InP is an attractive material for optoelectronic and highfrequency applications but has a major issue of leakage current. This issue can be resolved by adding an insulating layer between the MS junction and reduce the effective barrier height. For comparison purposes, various parameters extracted from the I-V and C-V characteristics of the Au/n-InP MS and Au/ZrO 2 /n-InP MIS diode are given in table 3. The results show that intermediate oxide layer plays an important role and effectively reduced the leakage current in the Au/n-InP Schottky diode.
The primary challenges are observed in electronic system such as material challenges, material diffusion, intermetallic formation, along with temperature-gradient-induced mechanical stress. Consequently, some challenges emerge at device level such as contacts with high performance, dielectrics, interconnects, controlled doping, encapsulation materials, complementary field-effect-transistors as well as robust imperfections. The problem or opportunity is to use semiconductors for producing extreme-environment sensors that can colocated power as well as process data. Accordingly, material development along with device optimization are required for Schottky diodes to operate at an extremely high voltage. This can be done with integration of alloys in the unipolar devices that have high Al composition.
Extensive research activities have focused on the development of various high sensitivity capable gas sensors for industrial and environmental applications. There is a huge demand for highly sensitive gas sensors for various industrial and environmental applications. Hydrogen sensors are one such example, which is used in industry, medical treatment, and hydrogen-fueled vehicles, where the detection of hydrogen gas leakage is required.

Future directions
With the continuous growth of electronic consumer products, there is an increasing demand for high performance and reliable electronic devices, including MOSFETs, solar cells, photovoltaic devices, gas sensors, photodetectors, display devices, and LEDs. Therefore, there is still a need to develop an efficient and near-ideal diode for these essential applications. Continuous research on this topic will lead to new paths and discoveries. Nanostructures, owing to their interesting properties and higher surface to volume ratio, have attracted the attention of many researchers. As reported in thin-film nanostructures [41,108], they can be used effectively as interfacial layers in MOS Schottky diodes. The grains and grain boundaries at the interface can significantly affect the Schottky diode properties. One-dimensional nanorods or nanowires can be employed efficiently for gas detection. Further doping of these nanostructures can increase the surface to volume ratio and hence better sensing properties can be obtained by fabricating nanostructured diode-based detectors. High-k dielectrics may have the potential to replace conventional SiO 2 . High-k dielectrics offer enhanced capacitance, high-energy storage, high breakdown voltage, and low leakage current. Some reports have been reviewed in this article for their use in MOS Schottky diode applications. Metal oxides, hybrid and organic polymer-based materials or graphene may also be used as future materials for Schottky diode applications. In recent years, conjugated conducting polymers have become popular for optoelectronic applications. Several research reports have focused on modifying the optical and electrical properties of polymers by doping with transition metals, such as Zn, Co, Ni, Cu, and Fe [108][109][110]. Investigations and research are being carried on metal-doped polymer-based Schottky barrier diodes to develop low leakage and electrically superior diodes. Polyvinyl alcohol (PVA) with high dielectric strength (>1000 kV mm −1 ) is potentially an attractive candidate to study as an organic intermediate layer in Schottky barrier diodes. For example, [108] reports the fabrication of PVA doped with Ni, Zn nanofiber film on a silicon substrate using an electrospinning technique as an intermediate layer for an MS diode.
Mousavi et al [111] explored ZnO with graphene quantum dot core-shell structures induced as filler into a PVA matrix for optoelectronic applications. Yuksel et al [112] studied perylene-monoimide as an interlayer in an Au/p-Si Schottky diode. There is a wide variety of polymers available, which are investigated as interfacial layers in Schottky diodes [113][114][115], and this area of composite polymers still demands greater research focus. Graphene oxide can be doped with different elements (e.g., B, N, S, and Si) to control its electrical properties [116]. This makes it a favorable material for several applications, such as transistors, diodes, supercapacitors, and batteries. Studies have shown the use of graphene and a graphene oxide-doped NiO nanocomposite as an interfacial layer in Schottky diodes [117,118]. Little attention has been given to transition metal oxides (TMOs), and there are few reports of TMO-based MOS Schottky diodes [119], with no significant reports found on TMO/polymer hybrid composite based Schottky diodes. To fully exploit the potential of these materials, there is a need for further research and investigations.
Nanostructured graphene material is one of the suitable candidates for an appropriate metal barrier that might help improve the limit of junction temperature as it has metallic behavior with outstanding electronic and thermal properties [119]. A graphene nanowall as a Schottky barrier material was used on a trench MOS barrier Schottky diode. It was grown by plasma enhanced chemical vapor deposition, using n-type epitaxial silicon wafer as a substrate. This material gives an excellent result in the performance of high-temperature leakage current [119]. This gives a high breakdown voltage and is capable of handle much higher temperatures without thermal runaway for the examined device.

Conclusion
The knowledge obtained through research on semiconductor junctions has opened doors for several new inventions and devices. It helps to develop a better understanding of MOS and MS devices, which has led to new and better technology products. MS Schottky diodes, which may be considered as core devices and are one of the earliest solid-state devices invented, has several applications. MS Schottky diodes suffer from poor electrical performance due to strong Fermi-level pinning and metal induced gap states present at the interface. Adding an interfacial layer plays a significant role in determining the electrical properties of the Schottky diode. The interfacial oxide layer in high frequency and high-power devices can effectively reduce the leakage current, but on the other hand, it may have drawbacks such as the lack of stability and drift effects. This detailed study demonstrated that metal oxide/insulator semiconductor Schottky devices are characteristically dominated by the intermediate layer and interface properties. Since the ultimate device reliability and performance is intimately related to the surface conditions, understanding and investigating the device surface physics is very important in a MOS Schottky diode.