Nanoelectronic Devices and Applications

Advancements in GaN Technologies: Power, RF, Digital and Quantum Applications

A. Mohanbabu¹, *, S. Maheswari², N. Vinodhkumar³, P. Murugapandiyan⁴, R. Saravana Kumar⁵

¹ SRM Institute of Science and Technology, Ramapuram, Chennai, India

² Panimalar Engineering College, Chennai, India

³ Vel Tech Rangarajan Dr. Sagunthala R&D Institute of Science and Technology, Chennai, India

⁴ Anil Neerukonda Institute of Technology & Sciences, Visakhapatnam, Andhra Pradesh, India

⁵ School of Electronics Engineering, VIT University, Chennai, India

Abstract

Quantum well devices based on III-V heterostructures outperform Field Effect Transistors (FETs) by harnessing the exceptional properties of the two-dimensional electron gas (2DEG) in various material interface systems. In high-power electronics, III-V-based Gallium Nitride (GaN) HEMTs can have a great influence on the transport industry, consumer, RADAR, sensing systems, RF/ power electronics, and military systems. On the other hand, the devices made of HEMTs and MIS-HEMTs work in enhancement mode, having very low leakage current, which can conserve energy for more efficient power conversion, microwave/ power transistors and high-speed performance for wireless communication. The existing physics of the well-established AlGaN heterostructure system imposes constraints on the further progress of GaN-based HEMTs. Some of the scopes include: Initially, the semiconductor materials made of SiC, GaN, and AlGaN allow a device that is resistant to severe conditions, such as high-power /voltage-high temperature, to operate due to its effective dielectric constant and has a very good thermal conductivity, which makes this device well-suited for military applications. Secondly, with the urgent need for high-speed internet multimedia communication across the world, high transmission network capacity is required. GaN-based HEMT devices are suitable candidates for achieving high-speed limits, high gain and low noise performance. In conclusion, GaN and related interface materials exhibit chemical stability and act as robust semiconductors, exhibiting remarkable piezoelectric polarization effects that lead to a high-quality 2DEG. Integrating free-standing resonators with functionalized GaN-based 2DEG formation reveals the potential for designing advanced sensors.

Keywords: AlGaN, GaN, SiC, Power devices and switching, III-V materials.

* Corresponding author A. Mohanbabu: SRM Institute of Science and Technology, Ramapuram, Chennai, India; E-mail: [email protected]

Introduction

Gallium Nitride High Electron Mobility Transistors (GaN HEMTs) exhibit a set of unique characteristics that make them increasingly attractive for high-frequency, high-power applications in the field of electronics. One of the key features of GaN HEMTs is their exceptional electron mobility, which allows for high-speed operation and efficient power handling. This characteristic, coupled with a wide band gap, contributes to their ability to operate at elevated temperatures without significant performance degradation. GaN HEMTs also have high breakdown voltage, enhancing their resilience to voltage spikes and enabling robust performance in power electronics applications. Furthermore, GaN HEMTs exhibit low on-resistance and parasitic capacitance, contributing to their efficiency and suitability for applications such as radio frequency (RF) amplifiers and power converters. The combination of these characteristics renders GaN HEMTs a promising technology for advancing the capabilities of electronic devices and systems in various industries.

In addition to their impressive electrical characteristics, GaN HEMTs offer notable advantages in terms of miniaturization and size reduction. Their inherent ability to operate at high frequencies enables the design of compact and lightweight devices, making them particularly well-suited for applications where space constraints are critical. GaN HEMTs also exhibit excellent power density, allowing for the development of more energy-efficient and power-dense electronic systems. The material's high thermal conductivity further enhances its performance, facilitating effective heat dissipation even in compact configurations. This combination of high-frequency operation, power density, and thermal management makes GaN HEMTs highly desirable for emerging technologies such as 5G wireless communication, radar systems, and satellite communications, where the demand for compact and efficient electronic components is paramount. As researchers and engineers continue to explore and optimize the potential of GaN HEMTs, their impact on advancing electronic capabilities across various applications is poised to grow significantly.

However, III-V GaN devices face hurdles in achieving E-mode operation with a positively biased threshold voltage (VT) and finding a low-defect material interface with low OFF-state leakage. Advancements in power density, breakdown degradation modes, and operational capabilities have not been fully recognized yet, and these factors currently hinder the widespread commercialization of these devices in the market [1, 2, 3]. Hence, the primary objective is to achieve exceptionally high-performance GaN power devices capable of efficient power conversion and operation at high frequencies. This involves implementing enhancement-mode HEMT operation for fast switching capabilities while simultaneously reducing switching losses to a minimum. Different performances need to be worked upon and are shown below:

• OFF-state breakdown voltage (VBR,OFF): It needs to be over 1000 V, taking the surge capacity into consideration.

• ON-state resistance (RON): It should be as low as possible to minimize power loss, size and cost. Since a decrease in the ON-state resistance defines a reduction in the generation of heat inside devices, this makes cooling systems simpler, such as heat sinks, thereby making a significant contribution to the enhancement of efficiency and the development of simpler power supply circuits.

• Normally-OFF: Enhancement mode (E-mode) operation of the power switch allows for a simpler and more efficient design of the driving and control circuit.

Characteristics of GaN:

GaN is a semiconductor material that exhibits distinct characteristics, making it a compelling choice for a variety of electronic applications. One key feature of GaN is its wide band gap, which allows for efficient electron mobility and high-speed operation. This property makes GaN particularly suitable for applications requiring high-frequency switching, such as RF amplifiers and power converters. GaN devices also demonstrate a remarkable balance between high breakdown voltage and low on-resistance, contributing to their efficiency and reliability in power electronics. Additionally, GaN's excellent thermal conductivity allows for effective heat dissipation, enabling the design of compact yet high-performance electronic systems. The combination of these characteristics positions GaN as a crucial material for advancing technologies in areas like telecommunications, power electronics, and emerging fields such as electric vehicles and renewable energy systems.

Motivation for the Present Research Work

Investigations described in this work are motivated by the search for developing highly safe E-mode operation devices with positive VT for low leakage and enhancing the OFF-state breakdown voltage and high-frequency operation in advanced III-V HEMT devices for the applications of efficient power-switching converter’s/inverter’s and low-noise amplifier’s (LNAs) in microwave/RF systems. Although the technology is being studied since the early 1979s, there is still a vast room and expectations in its yet unachieved findings. The devices' performance remained restricted due to the effects of high-injection velocities and high-field nonlinearities, as discovered during the evaluation by Enrico Zanoni in 2013 [4]. Hence, to improve the devices’ performance further, it is necessary to minimize the potential parameters responsible for these effects under high-current and high-voltage operation conditions.

• One of the major challenges faced by GaN-based HEMTs is achieving E-mode operation in the devices. These devices typically exhibit depletion-mode characteristics, which are less desirable for applications in integrated circuits.

• The effects of space charge in the access gap of the gate-source region during injection of high current cause a non-linear source resistance, which creates degradation in the linearity, the analog/ RF performance, and the OFF-state breakdown voltage of the device. Furthermore, the source carrier injection increases the leakage current in the device [5].

• The device's breakdown voltage is influenced by the presence of high field breakdown phenomena and impacts ionization in the conducting channel situated at the drain side [6, 7].

• The problem of gate leakage and device reliability in these components arises due to electron tunneling from the gate to the substrate, leading to a progressive decline in both DC current and RF output power as time goes on. This electron tunneling is caused by the high electric field at the drain side of the gate edge, which occurs due to the application of high drain bias (Vds).

• The effects of positive polarization (σ) at the interface of GaN cap/passivation and the defects in the surface decrease the 2DEG density n2DEG, drain current Ids, as well as the power capability of the device [8, 9].

These effects are responsible for the device's OFF-state breakdown degradation and performance in advanced III-V material-based HEMT devices. Researchers are working to enhance the characteristics and power performance of the device by reducing the potential parameters responsible for efficiency degradation.

Review of GaN-Based Devices

This systematic review presents a thorough examination of the most recent advancements in GaN-based compounds, encompassing the developments in materials, electronic devices, and circuits. It offers a comprehensive insight into the state-of-the-art progress in this field. The final part discusses nitride semiconductor devices in terms of application-specific requirements from a more practical point of view.

The observation of remarkable transport characteristics and high breakdown voltage has led to significant interest in heterojunction field-effect transistors

based on nitride materials at high fields, as reported by Khan et al. in 1993, making them appropriate for high-power and high-frequency applications [10].

In their study, Ambacher et al. (2000) examined the substantial impact of the built-in electric field arising from polarization-induced charges and the crystal growth direction of this material system on the electrical and optical characteristics of nitride heterostructures. The wurtzite crystal structure of (Ga, Al, In) N exhibits a lack of inversion symmetry. The attainment of polarization direction in nitride-polarity (N-face terminated) materials is opposite to that found in cation (Gallium)-polarity (Ga-face terminated) materials along the (0001) direction. The determination of Ga-face and N-face materials is based on the growth directions and terminations of atomic layers, resulting in the establishment of positive polarization directions as depicted in Fig. (1). In Ga-face terminated layers, the positive polarization charges are oriented toward the surface, which consists of Ga-cations. On the other hand, in N-face terminated layers, the positive direction is determined by the top surface containing N-anions. According to Ambacher et al. (2000), there is a complete reversal in the polarization directions between Ga-face devices and N-face devices. In Ga-face devices, the polarization direction is flipped compared to N-face devices. N-polarity (N-face) heterojunctions offer a unique advantage, as the 2DEG accumulates above the wide band gap layer, which in turn acts as a back-barrier. The polarization effect of N-polarity materials also enables device structures that do not require the electron flow from the metal to the 2DEG to go through a WBG AlGaN layer, resulting in lower ohmic contact resistances (Wong et al. 2007).

Fig. (1))

Effects of piezoelectric (PPE) and spontaneous (PSP) polarization in Ga-face and N-face AlGaN/GaN heterostructures [6].

Wu et al. (2004) stated that the power performance of GaN-based devices was 10 times higher than GaAs-based heterostructures [11]. The AlGaN/GaN Single-heterojunction (SH)-HEMT with 2DEG channel was initially discussed by Asif Khan et al. (1993), along with DC and RF performances [10]. Nowadays, the maximum output achieved is more than 40 W/mm. High frequency and power density operations are performed with great success in these devices, and they boast impressive breakdown strength and electron velocity in saturation. This ensures excellent performance and reliable operation.

Micovic et al. (2004) discussed the Double heterostructure (DH) devices and their advantages [12]. For applications in the microwave and millimetre-wave power range, a double-heterojunction (DH) AlGaN HEMT grown on Si/SiC by molecular beam epitaxy (MBE) at HRL is more effective than a single heterojunction (SH) device. DH-HEMTs have superior transconductance (gm) and reduced output conductance compared to SH-HEMTs, leading to improved power performance and higher RF-frequencies. For instance, an AlGaN/InGaN/GaN DH-HEMT yielded an output power of 6.3 W mm-1 at 2 GHz and Vds = 30 V. At 10 GHz, a 10% improvement in power-added efficiency was observed for a gate length (Lg) of 150 nm with a Vds of 30 V, while power density remained unchanged.

To enhance GaN's performance from diodes to nitride HEMT, Khan et al. (2003) investigated the Metal-Insulated Semiconductor (MIS)-HEMT using metal/semiconductor Schottky gates [13]. At a temperature of 300⁰C, the low-leakage current of the MIS-HEMTs found at RT is similarly decreased. Additionally, the DH AlGaN/InGaN/AlGaN structures are subjected to the MIS-HEMT, which significantly decreases the impact of the current collapse.

Hahn et al. (2014) and Colon et al. (2014) presented a method to reduce gate leakage by inserting an insulating layer between the semiconductor and the metal gate, leading to the formation of a MIS-HEMTs structure [5, 14]. By employing this method, a broad gate voltage range and a positively shifted threshold voltage can be achieved, thereby facilitating the development of E-mode HEMTs and MIS-HEMTs with favorable attributes like high VT (threshold voltage), high drain current (Idsat) and minimal device leakage current. Overcoming these challenges is crucial in device design. A notable challenge arises from the scarcity of insulators that possess both excellent quality and thermodynamic stability, which are necessary for III-N semiconductors, especially those with a low value of interface states.

According to the studies conducted by Sonia Sadeghi et al. (2013) and Nsele et al. (2013), MIS-HEMTs exhibit enhanced subthreshold behavior, reduced current leakage, improved microwave performance, and decreased noise levels compared to HEMTs [15, 16]. Consequently, investigating the low-frequency noise (LFN) properties of these devices has become highly important for ensuring reliable RF/microwave operations.

Felice Crupi et al. (2016) investigated the role of trap distribution in GaN HEMTs and highlighted the importance of low-frequency noise (LFN) analysis as a powerful tool for studying the reliability of these devices, particularly in relation to gate oxide integrity [17]. Gaining a deep understanding of the LFN mechanism in AlGaN HEMTs and MIS-HEMTs is essential for comprehending the reliability characteristics of these devices.

Terashima et al. (2006) and Dimakis et al. (2007) examined the utilization and popularity of N-polar InN-based HEMTs compared to other III-N transistors [18, 19]. Theoretical analysis suggests that by replacing the GaN channel with an InN channel in transistors with Lg = 0.1 µm, there is an expected improvement in fT from 480 to 680 GHz.

The specific advantages of N-polar GaN devices over Ga-polar devices have been listed by Rajan et al. (2007) and M.H. Wong et al. (2013) [20, 21]. These advantages encompass the ability to operate in enhancement mode, enhanced channel confinement, and reduced resistance in both the access and contact regions.

Singisetti et al. (2011) presented their findings on a GaN HEMT with N-polar E-mode characteristics [22]. The device showcased self-aligned source/drain contact regions and demonstrated a gm ̴ 225 mS/mm along with an RON of 2 Ω∙mm.

In their study, Nidhi et al. (2009) achieved significant results by developing an N-polar heterostructure [23]. They observed a peak gm of 350 mS/mm at Vds = 0.5 V and 530 mS/mm at Vds = 5 V, with a gate length (Lg) of 120 nm. Notably, they employed a highly scaled self-aligned procedure, which enabled them to achieve an impressively low intrinsic delay time (τ) of 0.6 ps.

Shinohara et al. (2013) examined how the performance of scaled N-polar HEMTs is affected by varying the lengths of the access regions between the gate, source, and drain [24]. Their study focused on assessing the relationship between these access region lengths and the overall performance of the HEMTs. The specific focus of the study was on N-polar GaN devices that were synthesized using metal-organic chemical vapor deposition. Notably, they reported a high RON of 1.02 Ω∙mm and impressive frequency performance with fT/fmax values of 103/196 GHz at Vds = 5.5 V for an N-polar MIS-HEMT. These findings highlight the potential of their device in advanced electronic applications.

The development of N-polar HEMTs with E-mode operation, possessing both positive VT and high gm, has been limited according to the findings of previous studies by [22, 24]. The presence of a high positive VT provides a reliable and secure operation for the device, while a high gm contributes to superior high-frequency performance.

Recently, Normally-OFF GaN devices have been the focus of interest mainly because of their potential use in next-generation power electronic switches due to safe and simple circuit design operation. Moreover, they provide ultra-low loss during switching. So far, different approaches have been taken by different groups to realize E-mode or Normally-OFF GaN HEMTs. Until now, considerable effort has been made on AlGaN/GaN-based HEMTs. However, these devices exhibit a negative threshold voltage, resulting in Normally-ON operation. Consequently, an additional drive circuit is necessary to regulate the gate bias, resulting in increased costs and added complexity to the circuit [25]. Substantially, in view of power device applications, the normally-OFF type transistor is highly required from the application requiring low-loss power switching and converter/ inverter applications.

Several methods have been suggested to manufacture E-mode devices. These include recess techniques [26], p-type gate structures [27], and thin AlGaN barrier layers [28], which have been reported. Nonetheless, the VT of these devices is not sufficiently high to effectively prevent noise interference in power applications. In high-voltage operations, even slight noise can lead to uncontrollable device behavior. In line with the demands of circuit design for power electronics, typically, a VT of 3-5 V is considered essential [25]. In HEMTs, E-mode is achieved when VT is lower than the built-in potential of the material. In this condition, the intrinsic depletion region of the gate acts as a barrier, blocking the 2DEG layer and causing the device to be in the OFF state. However, by applying a positive bias, the depletion region reduces, leading to the creation of a conducting channel and switching the device to the ON state.

Zhikai Tang et al. (2013) reported SiNx/AlGaN/GaN MISHEMTs with VT ~ 3.6 V using Fluorine (F-) ion treatment [29]. In their study, Cen Tang et al. (2015) introduced a gate recess technique using a one-step pure wet etch method, utilizing a combination of hydrogen peroxide and potassium hydroxide [30]. This approach led to the fabrication of Normally-OFF MOS-HEMTs with VT of around 3 V and a maximum drain current (Idsat) of less than 200 mA/mm.

Very recently, Qi Zhou et al. (2015) reported an AlGaN/GaN MIS-HEMT with Al2O3 gate dielectric operating with VT ~ 5 V and field effect mobility of 70 cm²V-1s-1 [31]. The dielectric/GaN interface in this method suffers from a large number of interface states and interface roughness due to the physical etch process, therefore resulting in large hysteresis and low mobility.

In a recent study by Ting-Hsiang Hung et al. (2015), they presented an approach aimed at reducing the fixed charges at the dielectric/AlGaN positive interface [32]. The method involved utilizing oxygen plasma pre-treatment and post-metallization annealing in forming gas. However, the reported work highlighted certain fabrication issues and drawbacks associated with their approach. These included drawbacks such as low threshold voltage (VT), reduced mobility, decreased Ids,sat, compromised VBR,OFF, and increased drain/gate leakage current. Addressing these limitations in existing HEMTs is essential for attaining a higher VT, which is crucial for achieving goals such as minimizing power consumption, simplifying circuit design, and ensuring fail-safe operation.

According to Mishra et al. (2002), the significant advantages of GaN in the realm of analog/RF applications are primarily seen in the development of high-performance HEMTs and Microwave Millimeter Integrated Circuits (MMICs) operating within the microwave frequency range of 300 MHz to 300 GHz [33]. These devices are utilized for microwave mixing, enhancing power density, amplifying signals with low noise, and transmitting high power. GaN analog/RF devices find applications in a broad array of end uses, including broadband communication systems, radar technology, telecom base stations, military communications, and satellite communications.

In addition, AlGaN/GaN-based HEMTs are not suitable for power-switching applications because of current collapse or drain current dispersion effects. This effect leads to increased dynamic ON resistance and decreased saturation drain current. Addressing this challenge is of paramount importance for high-power switching applications. Meanwhile, there is a better way to improve drain current and decrease current collapse using remarkable grown methods [34, 35].

In their research, Wu et al. (2004) identified that the occurrence of current collapse in devices is primarily linked to trapping phenomena near the gate-to- drain region [11]. The study highlights the importance of selecting an optimal passivation film and utilizing a longer field-plate length (LGFP) spanning from the gate to drain in AlGaN devices to mitigate current collapse. By implementing these measures, the study suggests that the adverse effects of the current collapse can be effectively reduced. The current collapse (CC) phenomenon (also known as dynamic resistance) is the degradation of the ON-state current because of applied voltage stress. Consequences of an applied voltage stress are the reduction in the Ids,sat and increase in the RON, as shown in Fig. (2).

Fig. (2))

Effect of trapped electrons and current collapse in GaN HEMT with various drain voltage [36].

The occurrence of current collapse is linked to the existence of traps, either at the device's surface or within its bulk regions. Trapping at the surface from the gate contact to the surface states (acts as a virtual gate effect) creates a negatively charged region, as shown in Fig. (2). These trapped electrons act as a virtual gate, depleting the channel beneath it when the voltage stress is removed [36]. Primarily, passivation of AlGaN/GaN HEMTs shows better transient response compared to un-passivated devices [34]. The electrons in the trapped states are depleted by AlGaN Field-plate (FP) length. The results using FP show that applying higher positive gate bias shows reduced dynamic ON resistance and trapped charges [37, 38]. So, we can conclude that FP devices are responsible for drain current dispersion reduction and field effect recovery process. Very little experimental evidence proves the suppression of the density of states and trap charges near the gate region.

Traps responsible for the current degradation mechanism can have two different origins. They can be related to the quality of the layers already present on the surface when the stress is applied, or they are generated by the inverse piezoelectric effect created as a consequence of the applied stress [39]. The formation and flow of the inverse piezoelectric effect are given below in Fig. (3).

Fig. (3))

Flow of formation of the inverse piezoelectric effect.

The simulation model for field-plated device characterization is essential to overcome these problems. The utilization of a field plate (FP) design presents a solution for reducing the inverse piezoelectric effect and enhancing the OFF-state breakdown voltage in AlGaN/GaN HEMTs. By incorporating a field plate, these effects can be mitigated effectively. Among all, the peak electric field near the gate-to-drain region is assumed as a dominant factor for trap occupation near the gate region. The FP in HEMTs increases breakdown voltage, and at the same time, it reduces the effect of the peak electric field near the gate edge.

David Reusch et al. (2014) demonstrated that GaN switches exhibit exceptional performance in high-frequency power converter/inverter switching applications [40]. They showcase advantages such as low switching loss, higher efficiency in DC-DC/AC-AC boost converters, and superior performance compared to Si-IGBT and MOSFET switches. In Fig. (4), it is evident that HEMT switches, despite their relative novelty, exhibit outstanding performance when compared to the existing Si and SiC-based FETs. This finding highlights the remarkable capabilities of HEMT switches, which surpass the performance of their established counterparts in various applications. A 25% decrease in power loss is seen when the Si-MOSFETs are replaced with GaN HEMTs, which results in a one percent increase in efficiency in a 48-12 V resonant intermediate bus converter operating at 1.2 MHz. A 30% reduction in the size of the power stage is reported as a result of lower ON-resistance and improved packaging of GaN HEMT switches.

Masahiro Ishida et al. (2013) compared the performance of GaN switch with Si-MOSFET and IGBT in a 500 W, 120-210 V boost converter and a 1-KW, 100 V, 50 Hz half-bridge inverter, respectively [41]. Because of low switching loss in the GaN switch, a monotonic decrease in efficiency is seen with an increase in switching frequency compared to a sharp decrease in Si and SiC-IGBT diode.

Transistors are essential components found in a wide range of electronic devices, serving purposes such as switches, amplifiers, and oscillators. HEMT transistors offer distinct advantages over ordinary transistors, as they can operate at millimeter wave frequencies. HEMTs find extensive application in a diverse array of products, including but not limited to satellite television receivers, cell phones, radar and voltage converters. Their versatility and performance enable high-speed and high-frequency digital circuits while also being essential components in microwave circuits. Notably, HEMTs excel in applications where minimizing noise is of paramount importance. Their utilization plays a significant role in ensuring the optimal functioning of various electronic systems, supporting efficient communication, signal processing, and power conversion. The seminal work by Lee and Webb (2004) highlights the pivotal role played by HEMTs in advancing these technologies and meeting the requirements of modern electronic devices [42].

Fig. (4))

Efficiency comparison of GaN HEMT switches with Si/SiC IGBT switches [43].

Reports on measurements first started to appear in 1998. Because LFN measurements can be used as a way of monitoring crystal quality, some reports came from materials scientists [44]. Physicists have measured GaN LFN and are using some of the following arguments to explain it: tail states near the band gap edge, mobility fluctuations, and tunneling of electrons from the channel to traps in surrounding layers.

Lee and Webb (2004) presented a numerical device solver that enabled the simulation of intrinsic noise sources in HEMTs [42]. Specifically, they employed capacitive coupling and the impedance field concept (previously introduced by Shockley et al., 1996) to estimate the spectral densities of gate and drain noise current sources, as well as their correlation. This approach provides a valuable tool for analyzing and understanding the noise characteristics of HEMTs. The first published NF report for a GaN HEMT was performed by Ping et al. (2000). For a 0.25 × 100 µm gate device, a minimum noise figure (NFmin) of 1.06 dB and a gain of 12 dB at a frequency of 10 GHz was demonstrated. No theory is yet accepted as the best explanation.

Literature reveals that GaN-based converters exhibit lower losses compared to Si-based converters when operated at the same switching frequency. This makes GaN-based converters advantageous in applications where device efficiency is a crucial factor. Furthermore, GaN-based converters offer improved power density, lower NFmin, and significantly higher switching frequencies compared to their Si-based counterparts while maintaining similar levels of switching loss. These attributes make GaN-based converters a favorable choice in applications that prioritize power density, noise performance, and higher switching frequencies.

N-Polarity GaN/InN/GaN/In0.9Al0.1N Heterostructure E-Mode HfO2 Insulated MIS-HEMTs

The III-V (nitride) material with a wurtzite crystal structure gives rise to significant polarization effects. Among these materials, InN, a binary compound, stands out as a promising channel layer due to its exceptional characteristics. InN possesses an exceptional electron velocity and exhibits low value of effective electron mass compared to other semiconductor alternatives, surpassing Silicon by a factor of roughly four. This makes InN-based channel HEMTs strong candidates for reaching frequencies in the terahertz (THz) range [45]. InN, belonging to the III-N semiconductor group, generates a high electron density (n2DEG) within the quantum well (QW) region of HEMTs due to robust polarization effects. Remarkably, this high electron density is achieved without the need for additional doping, highlighting the unique electronic properties of InN and its potential advantages in terms of carrier density and device performance [46, 47]. However, it is worth noting that the development of InN is still in its nascent stage and is considered to be in an early phase of maturity.

To gain a comprehensive understanding of the impact of InN on the DC and RF performance, extensive research and analysis was undertaken. Our focus was to analyze the sensitivity of the DC-RF performance in InN heterostructures and enhance our knowledge in this area. By employing polarization engineering techniques, these devices exhibited remarkable enhancements in crucial parameters such as Ids,Sat, VT, gm, ft, fmax, τ, and RON under low voltage bias conditions. Our study scrutinized the influence of induced polarization charges (σint) at all material interfaces. However, it should be noted that InN heterostructures are still in an early stage of development, and as of now, they exist primarily as theoretical constructs. As a result, no optimization steps or performance evaluations have been carried out to assess the device's behavior, including its electrical properties and performance at high frequencies of MISHEMTs with an E-mode InN channel [48].

Fig. (5) illustrates the schematic representation of the N-polar GaN heterostructure MIS-HEMT used in our investigation. The structure comprised various layers to achieve the desired device performance. The bottom layer consisted of a relaxed 1 µm In0.9Al0.1N back-barrier layer, which is placed on top of a 0.6 µm N-polar GaN layer. A strained 0.6 nm GaN spacer was then added above this layer. The main function of this spacer was twofold: firstly, to mitigate potential scattering effects caused by alloy disorder originating from the In0.9Al0.1N buffer layer, and secondly, to confine the channel electrons effectively. The In0.9Al0.1N layer was chosen as the back-barrier/buffer material due to its lattice-matched composition, making it an excellent choice for enhancing conductivity in channels for N-polar devices. The InN active channel, which served as the threshold control layer (TCH), was strained, and its dimensions varied from 0.5 to 3 nm. Lastly, the InN channel was capped by a 0.4 nm strained GaN spacer.