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Review

Surface Dispersion Suppression in High-Frequency GaN Devices

by
Pengfei Zhu
,
Xianfeng Ni
,
Qian Fan
and
Xing Gu
*
Institute of Next Generation Semiconductor Materials, Southeast University, Suzhou 215123, China
*
Author to whom correspondence should be addressed.
Crystals 2022, 12(10), 1461; https://doi.org/10.3390/cryst12101461
Submission received: 27 September 2022 / Revised: 11 October 2022 / Accepted: 14 October 2022 / Published: 16 October 2022
(This article belongs to the Special Issue Recent Advances in III-Nitride Semiconductors)
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Abstract

:
GaN-based high electron mobility transistors (HEMTs) are shown to have excellent properties, showing themselves to perform well among the throng of solid-state power amplifiers. They are particularly promising candidates for next-generation mobile communication applications due to their high power density, frequency, and efficiency. However, the radio-frequency (RF) dispersion aroused by a high surface-state density inherent in nitrides causes the degradation of GaN devices’ performance and reliability. Although various dispersion suppression strategies have been proposed successively—including surface treatment, passivation, field plate, cap layer, and Si surface doping—outcomes were not satisfactory for devices with higher frequencies until the emergence of a novel N-polar deep recess (NPDR) structure broke this deadlock. This paper summarizes the generation of dispersion, several widespread dispersion containment approaches, and their bottlenecks under high frequencies. Subsequently, we highlight the NPDR structure as a potential substitute, evaluate its technical benefits, and review the continuous exertions in recent years.

1. Introduction

Millimeter-wave technology is undoubtedly an ongoing research focus for fifth-generation mobile communication technology (5G) and beyond. GaN-based high electron mobility transistors (HEMTs) are taking the lead in the radio-frequency (RF) industry, especially for those applications that require abundant power density, thanks to the unparallel combined material properties of a wide bandgap (Eg ~ 3.4 eV), large critical breakdown electric field, high electron saturation velocity, high electron mobility and sheet density achieved by 2-Dimensional electron gas (2DEG), and excellent thermal properties when prepared on silicon carbide (SiC) substrates [1,2,3,4]. However, the existence of direct current (DC)-RF dispersion (also known as the “current collapse”) leads to a significant decrease in output power density and causes reliability-related issues in GaN-based HEMTs, especially at higher frequencies, which limits the development and application of such devices. Research has shown that the widespread presence of trapping states (covering the semiconductor surface, barrier layer, interface, channel, and buffer layer) in III/V group compound semiconductors should be mainly responsible for the dispersion of GaN HEMTs [5]. In high-frequency devices, the closer proximity of 2DEG to the surface highlights the role of surface traps (surface states) on dispersion [6,7]. These surface states trapping electrons to form a “virtual gate” (similar to a gate function) further deplete the 2DEG channel, which strongly impacts the device output characteristics and reliability. Engineers have tried to solve this problem by using multiple processes, including surface treatment [8], passivation [9], field plate [10], cap layer [11], and Si surface doping [12] in recent years. With the increasing demand for higher operating frequencies, the reduced gate length and the aspect ratio in need of being maintained drive the 2DEG much closer to the surface compared with the lower frequency HEMT epitaxial structure, thus limiting the effectiveness of those techniques in improving the dispersion. In this vein, researchers working on high-frequency GaN devices have sought novel technologies to better solve this aporia.
In this paper, we explain the origin of surface states and the mechanism causing dispersion in GaN devices. The current dominant methods for suppression dispersion and their bottlenecks in the high-frequency domain are summarized. Finally, we focus on the N-polar deep recess (NPDR) structure, addressing its uniqueness in controlling dispersion and reviewing the research advances in NPDR metal-oxide-semiconductor (MIS)-HEMTs.

2. Surface States and Virtual Gate

In GaN-based HEMTs, surface states are critical sources of 2DEG in the channel [13], allowing the device to achieve a very high electron concentration without the need for intentional doping (unlike GaAs-based 2DEG), which dramatically reduces ionized impurity scattering and enhances carrier mobility. On the other hand, GaN-based HEMTs are also plagued by the dispersion effect generated by surface states. This section mainly discusses the creation of surface states and the principle of dispersion caused by them.

2.1. Sources of Surface States

Surface states, a type of electron-bound state, can be categorized into two sorts based on their formation: intrinsic and extrinsic.
Intrinsic surface states arise due to the termination of the ideal crystal structure at the surface. It is common knowledge that GaN, like any crystal, cannot be extended infinitely in space. Therefore, the periodic potential field will be interrupted at the surface and cause additional energy levels in the forbidden band known as surface energy levels, whose corresponding electronic states are named surface states. Chemical bonding theory explains that the chemical bonds of GaN crystal break at the surface, and the unbonded electron pairs form dangling bonds, the corresponding electronic states being surface states [14]. Besides, those created by threading dislocations accessible at the surface, native oxide, N-vacancies, ions adsorbed from the surrounding environment (such as oxygen, carbon, and hydrogen, each with its unique energy level), and surface damage in the processing are referred to as extrinsic surface states [14,15,16]. These two different forms of generation pathways are shown in Figure 1.

2.2. Virtual Gate Model

Depending on their behaviors, surface states can be classified as either acceptors or donors. Table 1 shows the charge situations of these two categories of surface states in the empty and filled (after trapping the electrons) conditions. Under the strong influence of the polarized electric field, the donor surface state filled with electrons below the Fermi level will be partly ionized, allowing the electrons to escape into the channel to produce a high concentration of 2DEG and leave large positive charges on the surface [17]. As such, the surface of the GaN device should not show a negative surface potential. Vetury et al. [18] observed that a significant number of negative charges reside on the device’s surface, conflicting with the previous theory when they employed the floating gate approach to investigate the surface state impact of AlGaN/GaN HEMTs in 2001. In response, they explained that these positive charge centers (empty surface states) located between the gate and drain act like traps to capture gate leakage electrons or those channel hot electrons escaping via the barrier to the surface [15,19,20]. During the process from off-state to on-state, electron emission from surface states with a slower period differs from RF signals, so the trapped electrons are not released when the device is turned on, which is equivalent to another gate existing between the gate and the drain electrodes, i.e., virtual gate [21,22]. As shown in Figure 2, the trapped electrons move and jump on the surface to form a negative charge region, causing the gate depletion region to expand laterally, depleting the 2DEG in the channel and resulting in the current collapse. This RF dispersion is generally determined by pulsed IDS–VDS measurement, depending on different stressed conditions, and can be defined as:
Dispersion   ( % ) = I DS , DC − I DS , Pulsed I DS , DC × 100 %

3. Methods for Suppression Surface Dispersion and Their Bottlenecks

Based on the principle of extra depletion of the 2DEG channel by the virtual gate model, it can be concluded that one of the most critical tasks in dispersion control is to minimize trapping electron action attributable to surface states. Common solutions in AlGaN/GaN HEMTs are summarized in Figure 3. Five categories of methods have been developed and demonstrated to suppress the RF dispersion with mixed success: (1) surface treatment that intends to diminish surface state density to reduce the odds of surface states trapping electrons; (2) passivation, which intends to bury surface traps by depositing dielectrics on the device surface to prevent gate leakage electrons from being trapped in them; (3) the uses of field plate structure intend to alleviate the gate edge electric field crowding and enhance the emission rate of trapped electrons; (4) adding a GaN cap intends to control the gate-lag caused by the leakage current; (5) si surface doping intends to compensate for the trapping energy level to screen the 2DEG from the traps. This section summarizes the research advances of the above technologies and the bottlenecks they encounter at high frequencies.

3.1. Common Methods for Suppression Surface Dispersion

3.1.1. Surface Treatment

Natural oxides and organic matter adsorbed on the (Al)GaN surface create many interface states between the dielectric used for passivation and the barrier, which reduces the effectiveness of passivation. Consequently, surface treatment prior to dielectric deposition is of extraordinary importance to obtain a high-quality interface.
Organic matter and oxides are mainly composed of carbon and oxygen. UV/ozone surface treatment can proficiently remove carbon [23], while oxygen can be treated by some wet chemical cleaning methods. King et al. [24] found that HF and HCl were able to minimize oxygen coverage on AlN and GaN surfaces, and residual halogen ions (cl− and F−) tie up the dangling bonds preventing the surface from being oxidized again. The same result was found in the experiments of Diale et al. [25], who respectively compared the methods of cleaning GaN surfaces with HCl, KOH, and (NH4)2S. They concluded that KOH was effective for carbon removal but contributed to a rough surface. The sample cleaned in (NH4)2S, a chemical avoiding surface re-oxidation, acquired the optimum cleaning result with the lowest values of both C and O, root mean square (RMS) roughness, and Ga/N ratio. Other wet chemical cleaning efforts mainly focus on HNO3, H2SO4, NH4OH, and NaOH.
Surface treatment using plasma like NH3, N2, and N2O is also valuable for suppressing dispersion to enhance performance [26,27,28,29]. By filling nitrogen vacancies and removing impurity oxygen atoms, nitrogen-related plasma treatment considerably enhances the quality of the SiNx/GaN interface [28]. Romero et al. [27] treated the AlGaN/GaN HEMT (which had been wet cleaned by NH4OH) with in situ N2 plasma (N2PP) at 200 °C for 1 min prior to SiNx passivation of the device. The treated GaN HEMT obtained an IDS, which was 10% higher than the untreated one, and achieved a more advanced knee voltage. A 65% drop in the density of interface states was discovered after applying N2PP, which was responsible for reducing gate leakage current by two to three times (from 7.7 to 2.3 × 10−2 A/cm2 at VGD = −20 V), utilizing capacitance-voltage (C–V) and conductance-frequency (G-F) measurements. Furthermore, NH3 plasma-treated performs better than N2 in GaN HEMTs [26], with RF dispersion respectively decreasing from 63% (without pretreatment) to 1% and 9% (shown in Figure 4). This is probably due to the fact that NH3 plasma eliminates oxide and carbon residues while reducing the density of surface state defects.
In addition, plasma can also be combined with heat treatments such as high-temperature annealing in vacuum [30] or N2 [31] and NH3 [32] atmospheres to enhance the desorption of contaminants further and assist in the elimination of remaining surface defects, thereby improving the surface quality of the device. It has to be emphasized, however, that the surface treatment also increases the risk of unreliability and is, therefore, mostly involved in laboratory research efforts.

3.1.2. Passivation

Since 2000, when Green et al. [15] deposited a layer of Si3N4 for passivation on the surface of AlGaN/GaN HEMT to obtain the highest power density (4 W/mm at 4 GHz) during that period, passivation has become one of the most commonly used and effective techniques to solve RF dispersion after more than 20 years of development.
Previous studies have reported that SiNx, AlN, SiO2, SiNO, Al2O3, and HfO2 (high-κ dielectric) can all be applied to GaN-based HEMTs [28,31,33,34,35]. However, some researchers believe that nitrides (compared to oxides) probably are more suitable for GaN devices because oxygen impurities can easily diffuse into the GaN crystal and generate a high density of deep (or slow) interface states, producing additional current collapse [36,37]. This conclusion is consistent with the report of Geng et al. [31], which evaluated AlGaN/GaN MIS-HEMTs utilizing PECVD deposition of 20 nm SiNx, SiO2, and SiNO as passivation layers, respectively. It is found that the interface-trapped charge density (or the deep energy level defects density with long emission time of electrons) for ∆E > 0.657 eV in SiO2 is 4.13 × 1012 cm−2 (three times larger than that of SiNx, ~1.38 × 1012 cm−2 in SiNx). Compared to only 11.06% dispersion in SiNx, the current collapse in SiO2 reaches 84.14% at Vgs = −18 V, proving the success of SiNx passivation and the fact that SiO2 would not be appropriate for GaN devices. Based on taking SiNx as a dielectric, Huang et al. [38] proposed an improved structure using bilayer LPCVD SiNx; a Si-rich SiNx layer (Si/N = 59/41) with a thickness of ~10 nm was first deposited, followed by a ~67 nm Si-poor SiNx layer (Si/N = 50/50, high-resistive). Figure 5 exhibits a TEM cross-sectional image of this bilayer SiNx passivation, and the black dotted line refers to the boundary between the first and second layers. The Si-rich SiNx layer upper to surface is argued to remove the deep-level traps at the AlGaN/SiNx interface, which makes microwave GaN HEMTs show a much smaller degradation of about 9.7% (~30% dispersion in 70 nm Si-poor SiNx sample) while maintaining a low gate leakage current owing to the second high-resistive layer.

3.1.3. Field Plate

The field plate is applied in GaN-based HEMTs to reduce the electric field crowding at the gate corner of the drain side, preventing electron injection from the gate into surface states, and forming a virtual gate. Moreover, the horizontal electric field near the end of the field plate will strengthen the process of electrons escaping from the traps and speed up the electrons hopping along the AlGaN surface, further lowering the trapped electron density and, therefore, substantially suppressing current collapse [22]. The effectiveness of the field plate in suppressing current collapse is more robust with increasing field plate length. The trapped charges on the device surface decreased from 1.58 × 1012 cm−2 (no field plate) to 0.8 × 1012 cm−2 after using a 1.5 μm field plate structure [39], the pulsed IDS–VDS characteristics and trapped charge density are displayed in Figure 6. The field plate this method has now been widely adopted and greatly influenced the high-voltage devices. Despite this, it is worth noting that it is thrown into an awkward situation in high-frequency applications. Because this design not only reduces the speed of the channel electrons owing to the weakened electric field but also introduces additional external capacitance that limits the efficiency and gain of the device. Accordingly, in order to ensure anticipant RF performance, the field plate has found very limited usage in high-frequency industries.

3.1.4. GaN Cap

GaN cap, a thin layer of GaN (about a few nanometers) on top of the barrier that forms a sandwich structure (e.g., GaN/AlGaN/GaN), is another well-established technique in industry for managing the current collapse. This cap can be doped GaN (typically n-type doping by SiH4 or p-type doping by Cp2Mg) or i-GaN. No matter what type of GaN cap is used, it degrades gate leakage levels [40], both vertically and horizontally. Vertically, raising the effective Schottky barrier by the GaN cap provides a stronger limitation on the emission and tunneling of hot electrons [11,41]. Horizontally, as presented by Sarkar et al. [11], the GaN cap passivates the access region to suppress lateral electrons hopping via surface traps, significantly reducing the total leakage current (shown in Figure 7a). In other words, the GaN cap can effectively screen 2DEG from dispersion-related traps. As shown in Figure 7b,c, 2 nm of GaN cap layer effectively controls the current collapse in GaN-based HEMTs, while enhancing the device’s frequency performance (62% improving in fT) at the expense of a small amount of gain.

3.1.5. Si Surface Doping

Surface doping of GaN HEMTs with Si also contributes to the suppression of dispersion [42]. Si (as an electron donor) surface doping can prevent the slow process of re-emitting trapped channel electrons from surface states by filling and screening the deep-level traps [12]. However, the method also causes an escalation in leakage levels (electrons can tunnel through the thinned potential barriers more easily), so attention to a concentration should be paid when doping the surface of the device to avoid the impact of poor leakage current and breakdown resistance on device reliability.

3.2. Bottlenecks Encountered at High Frequencies

At lower frequencies (below Ka-band), the previously mentioned techniques can be combined to control RF dispersion in GaN HEMTs. As the gate length is further decreased to increase the operating frequency, the distance from the polarized surface to the 2DEG channel also has to be reduced to maintain the desired aspect ratio and contain the short channel effect, whereas the corresponding cost is the 2DEG channel suffers from the surface RF dispersion more seriously, rendering the effect of all surface engineering techniques less effective. When the operating frequency is pushed over 30 GHz (which means just entering the millimeter-wave range), the capacitance penalty imposed by the field plate causes this structure to be disabled because of the severely degraded high-frequency performance. Therefore, researchers urgently need to find new techniques to alleviate the problem of surface state trapping electrons to ensure the device’s reliability at high-frequency operation.

4. NPDR MIS-HEMTs for High-Frequency Device

Due to the comparatively good material quality and electrical features, Ga-polar GaN has dominated power devices for a long time. In recent years, the material growth technique has constantly been progressing on N-polar GaN; thus, related device research gradually gained significant interest [43,44]. On account of the opposing polarity, N-polar GaN HEMTs offer a series of natural advantages for higher frequency epitaxy and device design over the Ga-polar [45], such as (a) devices with a narrower top layer bandgap achieve lower contact resistance; (b) a natural back-barrier improves 2DEG confinement and minimize short channel effect while giving rise to higher transconductance. Additionally, the presence of the back-barrier allows the device to keep leakage current (from channel to substrate) at a low range even when there is no doping (e.g., Fe, C) in the buffer, avoiding the dispersion introduced by slow deep energy level discharge [46]; (c) flexible device aspect ratios accommodate shorter gate lengths resulting from increased frequency. These features are tailored to the needs of high-frequency devices so that the N-polar GaN transistor stood distinctly above all others in 2012, with a fT = 270 GHz and fmax = 370 GHz [47]. However, the most outstanding value of N-polar GaN materials is the ability to make the deep recess structure dramatically mitigate RF dispersion to guarantee reliability at higher frequencies [48]. Here, the basic structure of NPDR is displayed, the reason why deep recess can only be applied on the N-polar GaN (rather than Ga-polar) is discussed, and the research advances on NPDR are summarized.

4.1. What and Why Is NPDR?

The most basic structure of NPDR is to directly thicken the GaN channel above the back-barrier, as shown in Figure 8a, and the 2DEG at the bottom of the channel is isolated from the surface to control dispersion. The gate is buried deep in the GaN channel to considerably boost vertical scaling and strengthen the gate’s modulation of 2DEG. To avoid gate leakage currents and regulate the dispersion created by the sidewall, a thin layer of dielectric (normally MOCVD SiNx) is commonly placed for initial passivation before depositing gate metal. In the improved design of the NPDR structure (shown in Figure 8b), the insertion of an AlGaN cap layer (as etch stop layer) is considered necessary because the difference in components allows for more precise selective etching, while the cap provides better Schottky contact for the device. Apart from these two causes, the polarization electric field generated by N-polar AlGaN also mitigates the injection of gate leakage electrons during reverse bias [49]. Since the thick GaN cap keeps the surface away from the channel (differs from passivation, thick dielectric does not change the surface position due to the absence of polarization), even if a virtual gate is formed on the surface of the access region, there is almost no depletion of charge in the channel. The scattering associated with the surface is also effectively controlled, thus substantially increasing the 2DEG mobility. As mentioned earlier, the traditional passivation of HEMTs using ex situ SiNx dielectrics still has massive trapping states, whether in bulk or at the interface. However, no significant interfacial states are presented in the NPDR structure because the GaN cap has no difference in lattice constants from the channel. There should also be no more bulk traps within the UID GaN cap than in the channel, which ensures an ultra-low trap density near the 2DEG. To sum up, the deep recess structure is impressive for eliminating the RF dispersion in GaN HEMTs.
In fact, deep recess structures were first proposed for Ga-polar devices [50,51,52], but the results did not meet the expectations. As the polarization of the GaN cap caused the conduction band to be pulled up, the 2DEG would be depleted below the access region (shown in Figure 8c). Shen et al. speculated that suitable Si doping could compensate for the negative polarization charge and prevent the accumulation of holes. They proposed direct Si delta-doping at the GaN/AlGaN interface [52], but unfortunately, the introduction of Si would cause higher gate leakage and, thus, a reduction in breakdown voltage. Another method that insets a Si-doped graded AlGaN between the UID GaN cap and the AlGaN barrier is introduced for similar motivations [50]. Although the lower Al component of AlGaN can partially alleviate the 2DEG depletion by polarization, it is difficult to overcome the process because of the high requirements for precise etching. The additional graded AlGaN is also detrimental to the device aspect ratio. Hence, it can be confirmed that using a deep recess structure on the Ga-polar must be compromised for device performance.
In contrast to the Ga polarity, the electric field created in the access region of the N-polar GaN HEMTs partially counteracts the electric field provided by the gate. It pushes the conduction band to the Fermi energy level (Figure 9), thus augmenting the 2DEG concentration [53]. This provides an intrinsic advantage wherein the Ga-polar devices cannot be duplicated, and demonstrates that only the N-polar GaN HEMTs are appropriate for the deep recess.

4.2. Research Advances

In 2011, Kolluri et al. [54] first designed and fabricated an N-polar GaN HEMT with a thick unintentionally doped (UID) GaN cap, i.e., NPDR MIS-HEMT, and found that a 120 nm UID GaN cap was sufficient to remove any RF dispersion in the device. Since the last decade, extensive efforts have been devoted to NPDR structures to eliminate RF dispersion in solid-state power devices.
Wienecke et al. [55] from UCSB broadened the operating frequency to W-band (80-100 GHz) by fabricating NPDR MIS-HEMT on a sapphire substrate in 2016, with a 75 nm gate length and T-shaped gate. After implementing a T-gate design, weakened parasitic capacitance enhances device RF performance (both fT and fmax) to some extent [56].
The controlling dispersion ability is assessed using DC and 200 ns pulsed I–V tests. Figure 10a supports that the device’s on-resistance (Ron) and Imax are 0.58 Ω and 1.5 A/mm, respectively, under the DC condition. Only a minimal current collapse occurs even in the pulsed measurement, demonstrating the deep recess structure’s efficient dispersion administration in high-frequency devices. Thanks to this, the device achieves a 2.9 W/mm Pout with an associated PAE of 15.5% at a load of 94 GHz. The following year, this team significantly improved the NPDR structure design, i.e., gate-sidewall overlapping [49]; the 3D model is presented in Figure 11. A 30 nm overlap on the sidewall was introduced when depositing the gate metal (the gate was aligned to the bottom of the notch structure in the previous design). This design is believed to assist the drain-side gate in relieving the edge electric field while allowing the device to handle more significant voltage fluctuations (over 12 V) and raise this device’s power density to double the previous density. As can be seen from the pulse diagram (Figure 10b), the wrapping of the metal gate around the sidewall does bring about amazing dispersion management (details of which will be discussed later), resulting in the absence of any current collapse and knee-out phenomenon. Furthermore, in the small-signal RF measurement (shown in Figure 10c), the fT and fmax of this HEMT are extrapolated to be 113 and 323 GHz, respectively, when the VDS and VGS bias are set to 13 V and −1.75 V. N-polar GaN HEMTs passivated by MOCVD and PECVD SiNx during the same period, on the contrary, tend to exhibit an undesirable dispersion behavior [57,58].
The relationship between gate metal coverage of recessed regions and dispersion was further discussed by Wienecke [59]. In the case of a fully exposed sidewall, the dispersion aroused by the gap (refers to the space between the bottom of the gate metal and the groove, schematic diagram as shown in Figure 12) was first determined. Figure 13 exhibits the dispersion vs. the drain-side and source-side gap and the effect of different gate coverage on the output power and gate leakage current, respectively. When the gate metal covers the entire gate depression area (no gap and the sidewall is completely covered), the dispersion induced by those surface states on the sidewall is wholly removed. If the gap is minute (for example, 5 nm), the produced dispersion is negligible and can be acceptable. Once the gap approaches 50 nm or more, the RF dispersion will increase dramatically; hence, wide gaps over 50 nm are forbidden in NPDR structures. The gap discussed above relates to the drain side, whereas the source-side sidewall is completely covered, and a 50 nm field plate is used so that the experimental dispersion is mainly from the drain side. The same test procedure is applied to the source side, and a similar dispersion behavior is obtained. Results of gate coverage contribution to device performance suggest that the full metal coverage sample achieved the highest RF Pout of any transistor under the drain bias. However, it will cause a slight degradation in fT/fmax due to the larger Cgs and Cgd [60]. The Ig drops rapidly with the increase of sidewall coverage, but the device with a 15 nm gap is even lower than that of the one with half of the sidewall covered; thus, the mechanism of gate metal deposition coverage on the gate leakage current should be further investigated. Weighing the device’s frequency characteristics, breakdown and output power performance, Full Metal Coverage is considered the best trade-off solution. Most reports on NPDR structures currently use a self-alignment process to cover the entire gate depression region.
The UID GaN cap thickness is also one of the crucial parameters in the NPDR structure. It is clear that the thicker the cap is, the further the channel is from the device surface, which minimizes the surface charge impact on the 2DEG and achieves superior dispersion control. 47.5 nm GaN cap thickness was found to be the most suited in earlier research until lately, this classic thickness has been replaced by 20 nm, primarily to reduce the fringing capacitance while still maintaining the high access region channel conductivity [61]. This action helps improve the device’s gain and fT/fmax combination but perhaps leads to incomplete dispersion control. While additional PECVD passivation can be used to compensate for the reduced ability to regulate dispersion and obtain the desired results.
UID GaN cap with PECVD SiNx passivation to commonly suppress dispersion has been demonstrated and fabricated many times in N-polar GaN devices [55,61,62,63]. Utilization of the double-layer structure makes it possible to achieve excellent performance without the original thick cap, preventing parasitic channels caused by the strong polarization effect (PECVD SiNx does not produce polarization). At the same time, gate misalignment inside the trench is inevitable during the gate deposition, and PECVD SiNx is also effective in eliminating the remaining dispersion left by gate misalignment [55]. Regardless of the rationale, the additional SiNx passivation does have beneficial effects on the breakdown and power performance of the device. Romanczyk found that depositing 40 nm PECVD SiNx on the surface of a typical 47.5 nm UID GaN cap yielded higher output power at 10 GHz and allowed the HEMT to operate at higher VDS,Q (the SiNx passivation increased VT,access so that the channel can withstand higher voltages without being depleted) [62]. Under pulsed IDS–VDS measurement (650 ns pulse with and 0.065% duty cycle), the ID of the device drops only 8.5%, even with the stress as high as 23 V. Figure 14 compares the performance of this NPDR GaN HEMT with the superior Ga-polar monolithic microwave integrated circuit (MMIC) at W-band. When the load-pull measure is boosted to 94 GHz, a peak output power of 8.84 W/mm with 27% associated PAE (VD = 23 V) made it the best-performing device among the N-polar GaN device family. A slight reduction in voltage (to 20 V) would show a peak PAE of 30.7%, also unprecedented in N-polar GaN devices to date. This result far exceeds the leading level in the W-band reported for Ga-polar devices (an MMIC reported by Niida et al. [64] achieving 3.6 W/mm Pout with a maximum PAE of 12.3%, and an AlN/GaN HEMT reported by Harrouche et al. [65] achieving 4 W/mm Pout with a PAE of 14.3% in 2019), proving the absolute dominance of NPDR structures in the millimeter-wave RF field.
A thin double-cap layer structure, 2.6 nm of AlGaN deposited on top of 10 nm UID GaN as the cap layer, facilitates gain enhancement and suppresses parasitic channels [66]. At a 1 μs gate pulse width and an 800 ns drain pulse width, the test findings demonstrate that only around 10% dispersion (VGS,Q = −6 V, VDS,Q = 0 V) is observed. The gain enhancement due to the thin double-cap design facilitates the modulation effect of the gate on the 2DEG. It reduces the signal distortion of the device, enabling it to be more suitable for receivers that operate at low drain bias voltages.
More recently, Transphorm Inc. [67] has grown NPDR epitaxial stack on a 4-inch off-axis sapphire and a 4-inch off-axis C-face SiC substrates, respectively, and obtained excellent material quality characteristics for both, which demonstrates the feasibility of commercializing GaN NPDR MIS-HEMTS in the future. The STEM and typical AFM topography of the epitaxial stack are indicated in Figure 15. It can be seen in the STEM that the GaN buffer gradually filters out the linear dislocations starting from the nucleation layer to ensure the material quality in the upper HEMT region. In addition, there is a good hierarchical structure between the GaN and Al(Ga)N, with a low center-to-edge non-uniformity of about 0.3%. The low roughness average (Ra) of only 1.1 nm (on 100 mm sapphire) and 1.5 nm (on 100 mm SiC) reveals that the N-polar epitaxial layer has a smooth surface, implying success in the large-size N-polar GaN wafers’ growth. Researchers at Transphorm Inc. also found a low average Rsh of 234.3 Ω/sq for the 2DEG channel (with a 2% non-uniformity) owing to the 2DEG with an electron density of about 1.44 × 1013 cm−2 and high electron mobility (is approximately 1850 cm2/V∙s). It is worth mentioning that these values are the average mobility measured in both the parallel and perpendicular directions to the miscut steps. Thus, higher mobility and lower Rsh may be obtained if the current moves precisely in the parallel direction (along the terrace direction) of the actual deep recess devices.
The parameters and the performance of NPDR MIS-HEMTs operating in a range of frequencies are summarized in Table 2 for reference. The superior results imply that the NPDR structure is an efficient solution for suppressing surface dispersion of high-frequency devices, which may be commercially available and employed in the future RF industry, especially in the growing demand of the 5G market. However, it is necessary first to consider the poor heat dissipation caused by the current recovery. GaN-on-diamond may be a prospective option to address the thermal aggregation of NPDR GaN HEMTs in the future [68,69,70].

5. Conclusions

GaN-based HEMTs have shown great performance in the microwave RF field. Nevertheless, the inherent high-density surface states of GaN and the inevitable defects during fabricating are potential sources of surface traps. In the switching transition of the device, the behaviors of trapping and delayed releasing electrons by the surface states are the leading causes of RF surface dispersion and pose a significant challenge to HEMTs to maintain high output power and stability. Accordingly, how to suppress RF dispersion in GaN devices is an interesting research problem.
Surface treatment, passivation, field plate, GaN cap layer, and Si surface doping are demonstrated to effectively suppress RF dispersion and are widely used in the industry. While these methods provide varying degrees of dispersion suppression, as higher frequencies are explored, the exacerbated surface trapping effect or excessive parasitic capacitance makes these techniques progressively less effective.
N-polar GaN has many special advantages in high-frequency applications due to the opposite polarity, especially allowing the growth of a thick GaN channel (or thick UID GaN cap on the access region), providing an alternative way to minimize the dispersion caused by surface trapping at high frequencies. The utilization of a thick GaN layer keeps the surface far from the 2DEG channel to control the surface states and minimize the impact of surface charging on device operation. The isolation between the channel and the surface enables the NPDR structure to be considered the most promising approach for solving RF dispersion in GaN-based high-frequency power devices. In the past decade, the structural design of NPDR has been continuously improved, such as self-alignment for full coverage of the depression region with gate metal, simultaneous use of deep recess structure and SiNx passivation, and a double-layer structure for the receiver side. From the results, NPDR is expected to significantly contribute to the elimination of RF dispersion in high-frequency GaN HEMTs, restore their stability and inherent high power density, and is likely to be used in commercial 5G millimeter-wave RF devices in the future. However, the high price of SiC substrates, the high-quality requirements for N-polar GaN epitaxy, and heat dissipation are the primary impediments to large-scale industrial fabrication. Therefore, if more substantial progress can be achieved in addressing these obstacles, the application prospects of high-frequency GaN HEMTs will extend further.

Author Contributions

Conceptualization, P.Z. and X.G.; investigation, X.N. and Q.F.; resources, X.N. and Q.F; data curation, P.Z. and X.G.; visualization, P.Z.; writing—original draft preparation, P.Z.; writing—review and editing, X.G.; funding acquisition, X.G. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Nature Science Foundation of China, grant number 62074033.

Data Availability Statement

Not applicable.

Acknowledgments

The authors would like to thank Maxwell QH from Dulwich College Suzhou for his contribution to the English language and style in this work.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Mechanisms for the generation of intrinsic and extrinsic surface states.
Figure 1. Mechanisms for the generation of intrinsic and extrinsic surface states.
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Figure 2. The device model shows the virtual gate’s location, schematic representation, and the electric field distribution in the channel.
Figure 2. The device model shows the virtual gate’s location, schematic representation, and the electric field distribution in the channel.
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Figure 3. Common methods for suppressing RF dispersion in AlGaN/GaN HEMTs.
Figure 3. Common methods for suppressing RF dispersion in AlGaN/GaN HEMTs.
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Figure 4. DC and pulsed IDS–VDS characteristics for the GaN device. (a) Without pretreatment; (b) with NH4OH/N2 pretreatment; (c) with NH4OH/NH3 pretreatment. DC and pulsed IDS–VDS were measured at VDS = 0 to 15 V and VGS = −3 to 1 V. The pulse width and period were 500 ns and 1 ms. Adapted with permission from Ref. [26]. Copyright 2010, The Japan Society of Applied Physics.
Figure 4. DC and pulsed IDS–VDS characteristics for the GaN device. (a) Without pretreatment; (b) with NH4OH/N2 pretreatment; (c) with NH4OH/NH3 pretreatment. DC and pulsed IDS–VDS were measured at VDS = 0 to 15 V and VGS = −3 to 1 V. The pulse width and period were 500 ns and 1 ms. Adapted with permission from Ref. [26]. Copyright 2010, The Japan Society of Applied Physics.
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Figure 5. TEM cross-sectional image shows a distinguishable bilayer structure of the passivation layer. Reprinted with permission from Ref. [38]. Copyright 2018 by IEEE.
Figure 5. TEM cross-sectional image shows a distinguishable bilayer structure of the passivation layer. Reprinted with permission from Ref. [38]. Copyright 2018 by IEEE.
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Figure 6. AlGaN/GaN HEMT characteristics with different field plate lengths. (a) Pulsed IDS-VDS characteristics with Quiescent Voltage (VGS,Q = −8 V and VDS,Q = 50 V); (b) trapped charge (QST). Adapted with permission from Ref. [39]. Copyright 2021 by Elsevier.
Figure 6. AlGaN/GaN HEMT characteristics with different field plate lengths. (a) Pulsed IDS-VDS characteristics with Quiescent Voltage (VGS,Q = −8 V and VDS,Q = 50 V); (b) trapped charge (QST). Adapted with permission from Ref. [39]. Copyright 2021 by Elsevier.
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Figure 7. (a) Characteristics of the double gate. Is is the vertical leakage current through 2DEG, and IG1 is the horizontal leakage current by electron hopping between surface states. IG2 is the total leakage current (IG2 = IS + IG1). Pulsed output characteristics of HEMTs, (b) without GaN cap, and (c) with GaN cap. Adapted with permission from Ref. [11]. Copyright 2022 by Elsevier.
Figure 7. (a) Characteristics of the double gate. Is is the vertical leakage current through 2DEG, and IG1 is the horizontal leakage current by electron hopping between surface states. IG2 is the total leakage current (IG2 = IS + IG1). Pulsed output characteristics of HEMTs, (b) without GaN cap, and (c) with GaN cap. Adapted with permission from Ref. [11]. Copyright 2022 by Elsevier.
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Figure 8. Schematic diagrams of on-state device operation for deep recess GaN HEMT with different structures. (a) N-polar; (b) N-polar with AlGaN cap; (c) Ga-polar.
Figure 8. Schematic diagrams of on-state device operation for deep recess GaN HEMT with different structures. (a) N-polar; (b) N-polar with AlGaN cap; (c) Ga-polar.
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Figure 9. Energy band diagram comparing the N-polar GaN HEMT with and without a GaN cap. Reprinted with permission from Ref. [53]. Copyright 2018 by IEEE.
Figure 9. Energy band diagram comparing the N-polar GaN HEMT with and without a GaN cap. Reprinted with permission from Ref. [53]. Copyright 2018 by IEEE.
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Figure 10. (a) DC and pulsed IDS–VDS measurements of the NPDR MIS-HEMT, fabricated by Wienecke et al. in 2016. Adapted with permission from Ref. [55]. Copyright 2016 by IEEE. (b) DC and pulsed IDS–VDS measurements and (c) small-signal RF performance of the NPDR MIS-HEMT with gate-sidewall overlapping, fabricated by Steven et al. in 2017. Adapted with permission from Ref. [49]. Copyright 2017 by IEEE.
Figure 10. (a) DC and pulsed IDS–VDS measurements of the NPDR MIS-HEMT, fabricated by Wienecke et al. in 2016. Adapted with permission from Ref. [55]. Copyright 2016 by IEEE. (b) DC and pulsed IDS–VDS measurements and (c) small-signal RF performance of the NPDR MIS-HEMT with gate-sidewall overlapping, fabricated by Steven et al. in 2017. Adapted with permission from Ref. [49]. Copyright 2017 by IEEE.
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Figure 11. 3D model of NPDR MIS-HEMTs. (a) Without gate-sidewall overlapping; (b) with gate-sidewall overlapping.
Figure 11. 3D model of NPDR MIS-HEMTs. (a) Without gate-sidewall overlapping; (b) with gate-sidewall overlapping.
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Figure 12. Schematic diagram of the gap between the gate metal and the GaN cap sidewalls.
Figure 12. Schematic diagram of the gap between the gate metal and the GaN cap sidewalls.
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Figure 13. (a) Plots of the dispersion vs. source-side and drain-side gap. (b,c) The performance of output power and gate leakage current at different gate coverage. Reworked from Ref. [59].
Figure 13. (a) Plots of the dispersion vs. source-side and drain-side gap. (b,c) The performance of output power and gate leakage current at different gate coverage. Reworked from Ref. [59].
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Figure 14. Power characteristics of devices in the W-band. (a) NPDR GaN HEMT reported by Romanczyk. Adapted with permission from Ref. [62]. Copyright 2020 by IEEE. (b) Advanced Ga-polar MMIC, the solid and dashed lines are the results in the actual measurement and simulation scenarios, respectively. Adapted with permission from Ref. [64]. Copyright 2016 by IEEE.
Figure 14. Power characteristics of devices in the W-band. (a) NPDR GaN HEMT reported by Romanczyk. Adapted with permission from Ref. [62]. Copyright 2020 by IEEE. (b) Advanced Ga-polar MMIC, the solid and dashed lines are the results in the actual measurement and simulation scenarios, respectively. Adapted with permission from Ref. [64]. Copyright 2016 by IEEE.
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Figure 15. (a) STEM cross-section of the N-polar GaN epitaxy and NPDR MIS-HEMT layers; (b) AFM topography image of N-polar GaN epitaxy on 100 mm sapphire and 100 mm miscut C-face SiC substrates. Adapted with permission from Ref. [67]. Copyright 2021 by IEEE.
Figure 15. (a) STEM cross-section of the N-polar GaN epitaxy and NPDR MIS-HEMT layers; (b) AFM topography image of N-polar GaN epitaxy on 100 mm sapphire and 100 mm miscut C-face SiC substrates. Adapted with permission from Ref. [67]. Copyright 2021 by IEEE.
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Table
Table 1. Electrical properties of donor and acceptor surface states.
Table 1. Electrical properties of donor and acceptor surface states.
TypeEmpty Filled
Donor surface statesPositiveNeutral
Acceptor surface statesNeutralNegative
Table
Table 2. Characterization of NPDR MIS-HEMTs applied to various wave bands, including epitaxial structures, output performances, and dispersion suppression.
Table 2. Characterization of NPDR MIS-HEMTs applied to various wave bands, including epitaxial structures, output performances, and dispersion suppression.
Ref.BandFreq.
(GHz)		Peak PoPeak PAEDisp.
(%)		Cap Thk.
(nm)		SiNx Thk.
(nm)		Substrate
VD(V)Po
(W/mm)		PAE
(%)		VD(V)Po
(W/mm)		PAE
(%)
[55]W94102.915.581.720 11018Sapphire
[60]144.2 81.616.5 22NOSapphire
[61]165.520.6123.725.9<52020Sapphire
[71]113.7 93.027.8<547.5NOSiC
[72]186.233.8165.634.8 47.524SiC
[49]166.714.4154.816.9NO47.5NOSiC
[73]207.125.1144.627.5 47.5NOSiC
[53]207.926.9165.328.8<1047.5NOSiC
[62]238.827207.230.28.547.540SiC
[74]87 82.534.2NO47.5NOSiC
[66]Ka30 1012.620SiC
[53]208.152.5145.655.9<1047.5NOSiC
[63]2610.347.4 47.540SiC
[53]X10207.854.9145.158.1<1047.5NOSiC
[54]C4245.574 NO120NOSapphire
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Zhu, P.; Ni, X.; Fan, Q.; Gu, X. Surface Dispersion Suppression in High-Frequency GaN Devices. Crystals 2022, 12, 1461. https://doi.org/10.3390/cryst12101461

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Zhu P, Ni X, Fan Q, Gu X. Surface Dispersion Suppression in High-Frequency GaN Devices. Crystals. 2022; 12(10):1461. https://doi.org/10.3390/cryst12101461

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Zhu, Pengfei, Xianfeng Ni, Qian Fan, and Xing Gu. 2022. "Surface Dispersion Suppression in High-Frequency GaN Devices" Crystals 12, no. 10: 1461. https://doi.org/10.3390/cryst12101461

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