Compositionally graded III-nitride alloys: building blocks for efficient ultraviolet optoelectronics and power electronics

As members of the III-nitride family, AlGaN alloys crystallized in the wurtzite (WZ) structure are most thermodynamically stable at room temperature at standard atmospheric pressure, and have therefore been investigated the most for device applications [52]. In a WZ AlGaN structure, Ga/Al and N atoms are tetrahedrally bonded with each other, and based on the different stacking sequences of the atomic layers, metal-polar (Ga/Al-polar) and N-polar AlGaN alloys are separately defined, as shown in figures 2(a) and (b). In a single tetrahedron, there is a large electronegative difference between neighboring Ga/Al and N atoms. Therefore, each bond can be treated as a microscopic electric dipole P ₀. Since WZ AlGaN alloys deviate from an ideal centrosymmetric tetrahedron along the c-axis, the vector sum of four electric dipoles in an AlGaN tetrahedron leads to non-neutralized components Δ P along the [000-1] orientation, as annotated in figures 2(a) and (b). Besides, the WZ structures lack inversion symmetry centers, so the residual Δ P can accumulate along the c-axis and generate macroscopic polarization fields, known as spontaneous polarization (denoted by P _sp ). Furthermore, when AlGaN thin films are under external stress, two types of homogenous in-plane strain, i.e., tensile strain and compressive strain, are produced. Accordingly, additional net polarization fields Δ P _add are generated in the c-axis direction, as annotated in figures 2(c) and (d). This net polarization, derived from external stress, is known as piezoelectric polarization (denoted by P _pz ).

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Two types of polarization effect, including spontaneous and piezoelectric polarization, are significant properties of AlGaN alloys. One of the most successful uses of such effects is the production of a polarization-induced two-dimensional electron gas (2DEG) in GaN-based heterojunction field-effect transistors (HFETs), also known as high electron mobility transistors (HEMTs) [17–19]. Figure 3(a1) shows a schematic of an Al-polar AlGaN/GaN HFET. Because of polarization effects, equivalent polarization charge distributions are exhibited in the HFET in figure 3(a2), where σ_sp and σ_pz stand for the sheet charge densities generated by spontaneous polarization and piezoelectric polarization, respectively. Due to the discontinuity of polarization fields across the heterostructure, net-positive sheet polarization charges (σ_pol) are formed at the AlGaN/GaN heterointerface; the charge density can be calculated by σ_pol = (${\sigma }_{\text{sp}}^{\text{AlGaN}}+{\sigma }_{\text{pz}}^{\text{AlGaN}}$) − ${\sigma }_{\text{sp}}^{\text{GaN}}$ [53]. Those polarization charges can further induce electrons ionized from the surroundings such as remote dopants and surface states, and therefore produce 2DEGs as dense as ∼10¹³ cm⁻² at the heterointerface without incorporating any intentional impurity dopants [54–56]. Besides, strong polarization fields can largely bend the band structure of the AlGaN/GaN HFET, forming a narrow and deep conductive channel at the interface to confine the polarization-induced 2DEG, as illustrated in figure 3(a3). Similarly, HFETs based on a GaN/AlGaN heterostructure, as shown in figure 3(b1), can generate a two-dimensional hole gas (2DHG) at the heterointerface. Figures 3(b2) and (b3) are the separate corresponding equivalent polarization charge distribution and band diagrams of a GaN/AlGaN HFET. Recently, Chaudhuri et al successfully demonstrated a polarization-induced 2DHG in an undoped GaN/AlN heterostructure with a high density of 5 × 10¹³ cm⁻² [57], providing an alternative p-type doping strategy for AlGaN-based optoelectronic and power electronic devices.

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Furthermore, instead of forming abrupt AlGaN/GaN heterojunctions, we can spread out the fixed polarization charges at the heterointerfaces in a three-dimensional way by steadily increasing or decreasing the Al composition in AlGaN alloys along the growth direction. The charge profiles are determined by the divergence of polarization fields, as given by $\rho \left(z\right)=\nabla P\left(z\right)=\partial P/\partial z$, where $\rho \left(z\right)$ is the charge density along the growth direction z, and P is the total polarization field of the graded AlGaN alloy [58, 59]. Consequently, mobile three-dimensional electron and hole carriers are generated by those unbalanced polarization charges; importantly, this type of carrier concentration and profile can easily be modulated by specially designing the grading scheme of AlGaN alloys [38, 58, 59].

A polarization-induced three-dimensional electron slab, also referred to as a three-dimensional electron gas (3DEG), in graded AlGaN alloys was first verified by Jena and Mishra et al in the early 2000s [58, 59]. They achieved a 10¹⁸ cm⁻³ 3DEG concentration in an AlGaN alloy that was linearly graded from GaN to Al_x Ga_1−x N in the [0001] growth direction, as shown in figure 4(a1) [58]. Unbalanced positive polarization charges were generated in the whole graded structure (figure 4(a2)), which were able to attract a mass of electrons from the surroundings to form a 3DEG in the graded AlGaN alloy, as illustrated in figure 4(a3). This type of linear grading profile leads to an approximately uniform doping profile $\rho \left(z\right)=\left[{P}_{A{l}_{x}G{a}_{1-x}N}-{P}_{\text{GaN}}\right]/t$, where ${P}_{A{l}_{x}G{a}_{1-x}N}$ is the total polarization of the Al_x Ga_1−x N surface and t is the thickness of the graded AlGaN layer. Correspondingly, the band diagram and 3DEG carrier profile in graded AlGaN alloys with an increased Al composition along the growth direction are plotted in figure 4(a4). So far, the carrier concentration of a polarization-induced 3DEG has been able to reach an order of 10²⁰ cm⁻³ following structural optimizations [60]. Similarly, Zhang et al confirmed that a 3DHG could also be induced by polarization by linearly decreasing the Al composition in the [0001] direction, as shown in figure 4(b1) [61]. The corresponding charge distribution, polarization-induced 3DHG, and band diagram with the carrier profile of such graded AlGaN structures are plotted in figures 4(b2)–(b4), respectively. The observed hole concentration in the graded AlGaN alloys (from Al_0.3Ga_0.7N to GaN) reached a level of ∼10¹⁸ cm⁻³ [61]. Subsequently, Li et al demonstrated 3DHGs in Al-rich graded AlGaN alloys (from AlN to Al_0.7Ga_0.3N) with a hole concentration of 10¹⁸ cm⁻³, showing great potential for realizing highly conductive Al-rich AlGaN films [62].

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Intriguingly, it is noted that the generative mechanism of 3DEG and 3DHG strongly depends on the polarity of the AlGaN films. By simply flipping the polarity of AlGaN structures, Simon et al successfully demonstrated polarization-induced 3DHG in N-polar graded AlGaN alloys, as illustrated in figures 5(a1)–(a4) [30]. They further incorporated such graded alloys into UV light-emitting diodes (UV LEDs) as a highly conductive p-type current injection layer. The device exhibited a six-fold higher hole concentration and a roughly thirty-fold enhancement in electroluminescent (EL) intensity, compared with non-graded AlGaN-based UV LEDs. A 3DEG can also be generated in an N-polar downward-graded AlGaN alloy (from Al_x Ga_1−x N to a GaN surface), as illustrated in figures 5(b1)–(b4). In particular, N-polar GaN/AlGaN heterostructures have attracted widespread attention for device applications. In comparison to Al-polar AlGaN/GaN heterojunctions, the N-polar GaN/AlGaN structure possesses lower Al composition and barrier height at the semiconductor's surface, which is beneficial in reducing the contact resistance of AlGaN UV light emitters and power electronic devices [45, 63–65]. More importantly, researchers have further confirmed that the carrier densities of 3DEG and 3DHG are nearly temperature-independent, since those carriers are mostly polarization-induced. On the contrary, the carrier concentration of conventional Si-doped n-AlGaN and Mg-doped p-AlGaN alloys decreases exponentially at low temperatures, due to the freeze-out effect of thermally activated electrons and holes [30, 63].

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The previous section elaborated how carriers can be generated via graded AlGaN alloys. In this section, we focus on how such alloys can control and affect carrier transport behavior. In general, the aims of carrier transport manipulation in AlGaN optoelectronic and power electronic devices can be categorized as: (1) to achieve efficient carrier migration and injection, and (2) to realize sufficient carrier confinement to avoid carrier leakage and thus suppress the leakage current. Such carrier manipulation largely depends on the energy-band structures of devices, since band offsets between different layers directly determine the carrier transport behavior. Conversely, traditional non-graded AlGaN heterostructures have inherently strong band bending and offsets in their energy diagrams which can deteriorate the carrier transport/mobile efficiency and carrier confinement capability.

Figures 6(a) and (b) schematically show the typical structure of an AlGaN UV LED along the [0001] orientation. Due to the discontinuity of strong polarization fields in the MQWs, net polarization charges accumulate at the QW/QB interface and lead to strong internal electric fields in the opposite direction, which are annotated E _QW and E _QB . As a result, the band structures of the MQWs are undesirably bent, as shown at the top of figure 6(a). The effective barrier height for electrons in the sub-bands (green and purple lines) is lowered and becomes less effective at confining electrons inside the QWs, resulting in severe leakage overflow from the active regions [21]. To prevent electrons overshooting and recombining with holes in the p-type region, a p-EBL is normally inserted between the last quantum barrier (LQB) and the p-AlGaN layer, as illustrated in figure 6(b). However, such structures can form a large hole barrier height, ϕ_h, and block efficient hole injection into the active regions.

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To simultaneously achieve efficient current injection and sufficient carrier confinement in AlGaN UV light emitters, compositionally graded AlGaN quantum structures (QWs [66–70], QBs [71–74], a LQB [75, 76], and p-EBLs [77–79]) have been tentatively investigated and employed. Such structures can tailor the polarization fields, alleviate band bending, and consequently either increase the electron barrier height or decrease the hole barrier height, to effectively control the carrier transport, confinement, and recombination process. Figures 6(c) and (d) illustrate the grading profiles of QWs/QBs/p-EBLs commonly used for band engineering in the development of efficient UV LEDs [21]. These step-like and linear grading profiles of AlGaN alloys can easily be achieved using relatively mature growth techniques, such as metal-organic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE).

In regard to power electronic devices, studies have proven that the incorporation of graded AlGaN structures can achieve effective band engineering to overcome the intrinsic disadvantages of conventional AlGaN-based HEMTs [27, 46]. On the one hand, an Al-rich AlGaN barrier layer forms a large electron barrier height at the metal–semiconductor interface, which leads to a large contact resistance when the device is externally connected via ohmic contacts. Consequently, the current density and power density are suppressed. Alternatively, figure 7(a) shows an AlGaN-based HEMT with the incorporation of a graded AlGaN ohmic contact layer on top of the AlGaN barrier layer [46]. Obviously, the surface barrier height is significantly lowered at the surface, leading to efficient carrier transport which benefits the formation of ohmic contacts with lower contact resistance. On the other hand, a traditional bulk GaN layer cannot form an effective back-barrier height that sufficiently confines a 2DEG to the conductive channel. Therefore, electrons can easily drift into the GaN bulk and cause a large leakage current under high-voltage conditions, which is one of the main causes of early device punchthrough and failure. With an inserted graded AlGaN back-barrier layer, as illustrated in figure 7(b), the back-barrier height can be elevated, leading to better carrier confinement, so that the leakage current can be significantly suppressed [25].

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The commonly-used acceptor impurity for p-type AlGaN alloys, Mg, requires extremely high thermal activation energies from 200 meV in GaN to 630 meV in AlN at room temperature [80]. This primarily leads to a low level of thermally activated hole concentration and highly resistive p-AlGaN current injection layers with inefficient hole injection into the active regions. An exciting breakthrough occurred when Simon et al first demonstrated a UV LED with a graded AlGaN hole injection layer [30]. The graded AlGaN layer was in-situ doped with Mg but grown on N-polar n-GaN native substrates to further enhance the doping efficiency of the polarization-induced 3DHG, since N-polar AlGaN alloys tend to possess a higher background electron density, hindering the formation of p-type conductive layers. Compared with non-graded Mg-doped p-type AlGaN layers, the graded AlGaN layers exhibited a six-fold higher hole concentration and doubled p-type conductivity at room temperature. Hence, their fabricated graded AlGaN UV LED showed a thirty times higher EL intensity at 40 mA and a greatly improved LOP.

Recently, graded AlGaN structures have been implemented in tunnel junction-based UVA LEDs emitting at ∼320 nm [32]. As shown in figure 8(a), the tunnel junction consisted of both n- and p-type graded AlGaN layers with polarization-induced 3DEG and 3DHG. When an additional impurity dopant (Mg) was doped into the graded p-AlGaN layers, hole concentrations as high as ∼10¹⁹ cm⁻³ were produced. Figures 8(b) and (c) separately plot the I–V curves and the WPEs of four LEDs with different Mg dopant concentrations (samples A–D). Sample C, with the highest Mg concentration (3 × 10¹⁹ cm⁻³), exhibited the highest current density and the maximum WPE of 1.62%, which was 10³ times higher than that of sample A (without Mg doping). The external quantum efficiencies (EQEs) of samples C and D were 3% and 3.37% without an obvious droop effect. This type of graded AlGaN tunnel junction has also been implemented in UVB (290 nm) [81] and UVC (257 nm) [82] AlGaN LEDs, and their optical and electrical performances were among the highest levels for UV LEDs with similar emission wavelengths, showing the great potential of graded AlGaN-based tunneling injection for efficient UV LEDs.

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Apart from conventional planar AlGaN UV LEDs, graded AlGaN alloys have also been utilized as building blocks in nanowire-based AlGaN UV LEDs [35–37]. Importantly, AlGaN nanowires can be grown on any substrate, regardless of lattice mismatch, and normally possess high crystal quality thanks to their large surface-to-volume ratios [83–85]. In addition, p-type AlGaN nanowires were found to be more conductive than thin-film AlGaN layers thanks to the reduced lattice strain imposed by surface dopants [85–87]. Therefore, such nanowire structures have emerged as an alternative device architecture for efficient UV light-emitting devices [88, 89]. Figure 8(d) shows a single AlGaN nanowire representing the device architecture of a fabricated UVC LED. The upward and downward linear grading profiles can be visually observed using the transmission electron microscopy image. The devices exhibited a low turn-on voltage of 6 V and an EL peak intensity centered at 250 nm, as displayed in figures 8(e) and (f). Additionally, a peak power of 0.22 μW at 400 mA was measured in pulsed operation mode.

3.2.1. Graded AlGaN structures in the active region

To facilitate the radiative recombination of electrons and holes in the active region, MQW structures are widely utilized as the active regions in AlGaN-based UV LEDs. However, researchers have recognized that the existence of strong polarization effects in AlGaN MQWs has adverse effects on the carrier confinement and recombination process, namely the notorious quantum-confined Stark effect [21]. To address this issue, active regions with incorporated linear or step-like graded AlGaN quantum structures have been proposed, which were found to be effective in the tailoring of internal polarization fields, the modulation of band structures, and therefore, the manipulation of carrier transport behavior [66–73].

Figures 9(a) and (c) illustrate the step-like graded QBs [71] and linearly graded QWs [66] used to re-engineer the energy band. Figure 9(b) plots the calculated band diagram of sample C shown in figure 9(a), namely Al_{0.54→0.52→0.50}GaN step-like graded QBs. The raised effective barrier heights of the QBs for electrons (ϕ_CB) and holes (ϕ_VB) provided sufficient carrier confinement in the QWs for radiative recombination. Furthermore, the modulated p-EBL possessed the highest electron barrier height (ϕ_e) of 338.8 meV and the lowest hole barrier height (ϕ_h) of 358.0 meV among the three samples, indicating that such band engineering can effectively prevent electron overshooting without hindering hole injection. Following the same principles, sample B in figure 9(c) with increased-Al-composition (IAC) graded Al_0.5→0.6GaN QWs was also able to effectively manipulate carrier transport and enhance carrier confinement capability. The corresponding UV LEDs exhibited the highest IQE without an obvious droop effect and a 67.04% higher LOP than reference sample A, as compared in figure 9(d).

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In particular, the band engineering of LQBs stands out as an important realm, since such structures act as a bridge between the active regions and p-type regions of AlGaN UV LEDs. Such LQBs can deeply influence the carrier injection efficiency and confinement capability. Figure 10(a) shows three separate samples that incorporate non-graded Al_0.15Ga_0.85N, upward-graded Al_x Ga_1−x N (x = 0.15→0.3), and downward-graded Al_x Ga_1-x N (x = 0.3→0.15) LQBs [75]. The inset of figure 10(b) shows the UV light emission of the fabricated devices with a downward-graded LQB. The main device characteristics, including the EL intensity (figure 10(b)), IQE (figure 10(c)), and LOP (figure 10(d)) of such devices are significantly boosted, demonstrating the feasibility of this type of band engineering for efficient carrier behavior manipulation.

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3.2.2. A p-EBL in the form of a graded AlGaN structure

A p-type EBL is widely adopted in AlGaN UV LEDs to prevent electrons from overflowing the active regions and diffusing into the p-type regions. However, large valence band offsets that form between the Al-rich p-EBLs and the p-AlGaN layers can also severely block hole injection. Similarly to the graded QWs and QBs, a graded AlGaN structure has recently been adopted as a p-EBL to tailor the band structure, thereby facilitating hole injection. Figures 11(a) and (b) separately show two devices fabricated without (LED A) and with (LED B) graded p-EBLs [78]. According to numerical calculations, the modulated band structure of graded p-EBLs can lead to reduced hole energy loss, and consequently, accelerated hole injection. Simultaneously, the graded p-EBL exhibited a higher electron barrier height of 682 meV, compared with non-graded EBLs (386 meV). As a result, overflown electrons were able to be sufficiently blocked, and holes were be efficiently injected into the active regions. The measured EL spectra show that LED B had a smaller parasitic emission intensity, as compared in figure 11(c). Figures 11(d) and (e) show the improved I–V characteristics and output power of LEB B. In particular, a high EQE of 7.6% was measured, thanks to the effectiveness of the carrier transport manipulation performed by this type of band engineering. Additionally, other types of novel EBL in the form of graded AlGaN structures were demonstrated in [73, 79]. All the simulated and experimental results reveal that this type of graded AlGaN EBL can facilitate much higher hole injection efficiencies with reduced energy losses by lowering the effective hole barrier height and screening the strongly polarizing electric fields.

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In recent decades, with the enormous effort devoted to the development of AlGaN UV LEDs, remarkable progress has been achieved, including the demonstration of the shortest wavelength of 210 nm [77], the highest IQE of 92% [90], an EQE higher than 20% [91], and a large WPE of 21.6% [92]. However, the development of UV LDs has fallen behind that of LEDs. For example, electrically injected UV LDs emitting at 336 nm [93] and 342 nm [94] were reported more than a decade ago, with high operating voltages of more than 25 V in the lasing mode due to poor hole injection efficiency. To overcome these challenges, the exploration of graded AlGaN alloys by leveraging their polarization-induced carrier doping could be a promising and effective approach for enhancing current injection in UV LDs, in addition to the conventional impurity (Mg) doping technique. Moreover, such graded structures naturally possess a graded refractive index along their grading profiles, according to Vegard's law [95]. Therefore, graded AlGaN layers can be employed to enhance UV optical-mode confinement in the active regions for larger optical gains and to achieve better control of carrier behavior.

As an example, the configuration of a UV LD with a graded AlGaN layer is schematically shown in figure 12(a). A graded cladding layer is employed to enhance current injection efficiency and optical mode confinement. An additional graded AlGaN contact layer is inserted to reduce the contact resistance. Utilizing this type of device architecture, Sato et al tentatively demonstrated an electrically pumped 298 nm UV LD with a current density of 41.2 kA cm⁻² [33, 96, 97]. The optical mode confinement factor was calculated to be 3.5%, as shown in figure 12(b). More excitingly, a 271.8 nm electrically pumped UV LD was recently realized based on the same structure, which is the shortest wavelength recorded for thin-film based AlGaN UV LDs [34]. The pulsed I–V characteristics shown in figure 12(c) exhibit a super-linear increase of output power above the threshold current of 0.4 A. A sharp EL spectrum peak emerges at 271.8 nm, indicating successful UV-B lasing with a pulsed current at room temperature.

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Another recently proposed configuration with graded AlGaN alloys intended for UV LD implementation is the well-known graded-index separate confinement heterostructure (GRINSCH), which has been widely adopted as the building block of cubic III–V compounds, for example, in GaAs and InP-based LDs [98–100]. GRINSCH consists of two graded AlGaN layers on both sides of MQWs. The grading profiles are specially designed so that the refractive index is highest in the active region, where the optical mode should be well confined for UV lasing [101]. Figures 13(a) and (b) show an Al-polar GRINSCH structure and the calculated optical mode, corresponding to a high optical confinement factor of ∼13% [102]. Simultaneously, polarization-induced 3DEGs and 3DHGs can be generated in the graded AlGaN layers, leading to the automatic formation of a p–n junction in the GRINSCH structure. Figure 13(c) shows the calculated band diagram and carrier concentration of a GRINSCH structure under thermal equilibrium. Both electrons and holes have a high concentration of ∼3 × 10¹⁸ cm⁻³ in the p–n junction with external doping. Moreover, the AlGaN GRINSCH diode shows a turn-on voltage reduced by more than 20 V, compared with conventional UV light emitters, for the thin-film-based structure [31].

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Additionally, this type of GRINSCH AlGaN UV LD has been explored in a nanowire-based device architecture, as shown in figure 13(d) [37]. The nanowires were epitaxially grown on conductive Si substrates coated with metals and then fabricated into vertically conductive devices. A large optical confinement factor of 17.9% was obtained by numerical simulation. Compared with a p-i-n diode without graded AlGaN layers, the GRINSCH diode exhibited significantly improved electron and hole concentrations, by 29.5% and 81.1%, respectively. Combining excellent carrier generation and optical mode manipulation, the GRINSCH diode had a smaller turn-on voltage (6.5 V) and a four-fold reduced series resistance compared with the p-i-n diode, as plotted in figure 13(e). Figure 13(f) demonstrates that the GRINSCH diode possessed significantly enhanced output power and EQE without an obvious droop effect.

4.1.1. Graded AlGaN channels for linearity improvement in RF applications

AlGaN/GaN-based HEMTs have achieved great success in microwave power applications [103–107] due to their high electron mobility and large power capacity. However, conventional HEMTs with 2DEG channels inherently suffer from severe transconductance (g_m ) drop-off under large gate biases (V_gs ) [24–26]. This property is detrimental to the linear transfer characteristics between input, modulated, and output signals and can introduce unwanted signal compression, offset, and intermodulation distortion into communication systems [18]. One effective approach for improving transistor linearity is to intentionally design the doping profiles in the conductive channels to tailor the g_m –V_gs curves and thus produce a constant g_m within a wide range of bias voltages [38–42, 59]. Such principles can be realized by adopting graded AlGaN alloys, in which the 3DEG carrier profiles can be modulated according to $\rho \left(z\right)=\nabla P\left(z\right)=\partial P/\partial z$, as discussed in section 2. Ancona et al carried out a comprehensive numerical study of graded AlGaN channels with different grading profiles (linear, quadratic, quartic, tanh, and linear-tanh functions) for RF applications [108]. All the transistors with graded AlGaN channels, which are normally referred to as polarization-induced FETs (PolFETs), exhibited flat and bias-insensitive g_m values, compared with conventional HEMTs.

So far, PolFETs with linearly graded AlGaN channels have already been experimentally demonstrated, as schematically shown in figure 14(a) [38]. Figure 14(b) presents the calculated doping profiles in the linearly graded channel with different thicknesses and Al composition gradients. Uniform doping profiles are observed, which are consistent with $\rho \left(z\right)=\left[{P}_{A{l}_{x}G{a}_{1-x}N}-{P}_{\text{GaN}}\right]/t$. The experimentally measured g_m − V_gs curves (figure 14(c)) exhibit a constant g_m value over a wide range of bias voltages, confirming the effectiveness of doping profile manipulation for linearity improvement. Besides, the contour plots of cut-off frequency (f_T ) and maximum oscillation frequency (f_max) in figures 14(d) and (e) also exhibit bias-insensitive behaviors for high-voltage operation, further proving the suitability of PolFETs with graded AlGaN channels for RF applications.

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In particular, such graded AlGaN channels are very suitable for highly scaled devices with short gate lengths and source-to-drain spacings. In such devices, electrons move at their saturated velocity (v_sat) in the conductive channels and thus support a higher switching speed. However, the v_sat of a 2DEG largely decreases with increased carrier density at large biases [24–26]. Since g_m is proportional to v_sat in short-channel devices, the RF performance of conventional HEMTs inevitably rolls off. In this regard, linearly graded AlGaN channels which can produce bias-insensitive g_m are highly desirable for improving the linearity of RF applications. Figure 15(a) shows a PolFET with a gate length of 250 nm [39]. A 20 nm reversely graded AlGaN layer was employed as the contact layer for better ohmic contact. A tailored g_m –V_gs curve with a flat g_m as high as 240 mS mm⁻¹ was obtained, along with flat frequency features over a wide range of drain currents, as shown in figures 15(b) and (c), respectively. Apart from laterally scaled PolFETs, a vertical device scaling scheme was proposed by Park et al [40]. They suggested incorporating a hybrid 2DEG/3DEG channel, so that the device was able to take advantage of the higher mobility of 2DEGs as well as a manipulated 3DEG doping profile. As shown in figure 15(d), the vertical PolFET had a narrow channel as thin as 8 nm [3 nm Al_0.9Ga_0.1N + 5 nm linearly graded Al_xGa_1-xN (x: 0→15%)]. The measured flat g_m values indicate that the linearity was significantly improved, in comparison with a conventional HEMT with a non-graded Al_0.9Ga_0.1N channel, as indicated in figure 15(e). Quite recently, a scaled PolFET with a gate length of 50 nm was demonstrated, exhibiting improved f_T and fmax of 170 GHz and 363 GHz, respectively, indicating the great potential of graded AlGaN-channel based PolFETs for RF applications [109].

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4.1.2. Graded AlGaN channels for high-voltage operation

Apart from the higher-frequency operation, another trend of AlGaN power electronic devices is a movement towards higher-voltage applications. Therefore, ultrawide bandgap (UWBG) AlGaN devices incorporating Al-rich AlGaN channels are emerging, aiming to simultaneously enhance the BV and improve the power capacity [110–113]. There are three main device architectures for UWBG AlGaN transistors, including impurity-doped metal-semiconductor FETs (MESFETs), 2DEG-channel HFETs, and graded AlGaN channel PolFETs. In MESFETs, the high activation energy of n-type impurity dopants in Al-rich AlGaN alloys cannot meet the requirements for highly conductive channels [80]. In addition, strong impurity scattering introduced by ionized impurities is also undesirable for achieving high electron mobility. In regard to the HFETs, the 2DEG density can be enhanced by increasing the Al composition contrast between the barrier and the channel. However, the 2DEG mobility of Al-rich HEMTs was reduced in a lower Al-composition AlGaN channel [111, 114], not to mention that 2DEGs are not suited to linear transfer behaviors in RF applications. To overcome these drawbacks in MESFETs and HEFTs, scientists have proposed and investigated PolFETs by adopting graded AlGaN channels, which can not only generate a high-density 3DEG via polarization-induced doping, but also enhance the 3DEG mobility due to the absence of impurity scattering. In particular, the 3DEG carrier profile can be manipulated by controlling the graded AlGaN alloys to improve transistor linearity. Therefore, the PolFET has emerged as a promising device architecture and has shown great potential in the UWBG AlGaN device community.

Figures 16(a)–(c) present the schematics of three UWBG AlGaN PolFETs and their corresponding output I–V curves. Figure 16(a) shows a PolFET with an average 3DEG mobility, μ, as high as 210 cm² V⁻¹ s⁻¹, which was among the highest value for UWBG AlGaN transistors with Al compositions >70% [43]. However, the electron density was measured as 7 × 10¹⁷ cm⁻³, which was relatively low, compared with other doping strategies. Besides, the high-Al-composition AlGaN surface resulted in poor Ohmic contact characteristics. As a result, the maximum drain current (I_dmax) was suppressed to only 24 mA mm⁻¹. Figure 16(b) shows a PolFET with further improved channel mobility of 340 cm² V⁻¹ s⁻¹ [44]. Besides, the higher Al composition contrast (Al_0.6Ga_0.4N to AlN) in the graded AlGaN alloys led to a higher 3DEG density of 1.3 × 10¹⁸ cm⁻³ as well as a significantly enhanced I_dmax of 200 mA mm⁻¹. Recently, Lemettinen et al demonstrated an N-polar PolFET with a graded Al_0.8Ga_0.2N→AlN channel for the first time, as shown in figure 16(c) [45]. The electron density was measured at 1.3 × 10¹⁸ cm⁻³, indicating an integrated sheet charge density as high as 1.2 × 10¹³ cm⁻². This value is comparable to the typical 2DEG density of abrupt AlGaN heterojunctions, confirming the effectiveness of the polarization-induced doping strategy via graded structures in N-polar Al-rich AlGaN devices. The measured I_dmax was 60 mA mm⁻¹, which can be further improved by optimizing the growth and design of N-polar graded AlGaN alloys and the ohmic contacts.

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4.2.1. Graded AlGaN ohmic contact layers for efficient electron transport

Another critical challenge faced by AlGaN HFETs is the relatively high electron barrier height formed at the surfaces of AlGaN barrier layers, especially for UWBG AlGaN transistors, due to the much smaller electron affinity of AlN (∼0.4 eV) than that of GaN (∼4.1 eV). Therefore, it is difficult to form an ohmic contact with low contact resistance. The most straightforward way to lower the electron barrier height is by decreasing the Al composition at the semiconductor's surface. Thus, downward-graded AlGaN alloys (from Al-rich to Al-less AlGaN) along the growth direction can possibly be used as additional ohmic contact layers. Such graded structures should be doped with donor impurities to compensate for the 3D negative polarization charges and to flatten the conduction band for efficient electron transport through the ohmic contacts.

Figure 17(a) represents a schematic of an AlN transistor with a graded AlGaN ohmic contact layer. This device used Ti/Al/Ni/Au metal stacks as the source and drain contacts [46]. The linear I–V curves (as shown in figure 17(b)) and enhanced current density shown in the inset confirm the formation of good ohmic contacts by the 150, 220, and 340 nm thick graded AlGaN layers. However, the 60 nm thick graded structure did not perform effectively as an ohmic contact layer, exhibiting a lower current density than the non-graded structure. As illustrated in figure 17(c), the polarization-induced charge density in a 60 nm thick graded AlGaN layer was so high that Si compensation doping was not sufficient. Consequently, a high electron barrier was formed, which severely hindered electron transport. For comparison, figure 17(d) shows the charge density and the band diagram of the 340 nm thick graded layer. The compensation doping was able to effectively neutralize the negative charges and

form a smooth conduction band. The specific contact resistance measured was as low as ∼10⁻² Ω·cm². It should be noted that the gate region should be recessed down to the AlN channels during device fabrication, as shown on the right of figure 17(a). The large electron barrier height formed at the AlN channel's surface can suppress undesired gate leakage current and benefit the formation of a good Schottky contact at the gate region.

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Owing to the successful electron transport manipulation via band engineering, this type of graded AlGaN ohmic contact layer has attracted extensive attention as a building block for UWBG AlGaN transistors to improve the contact characteristics [47–51]. Figures 18(b) and (c) separately show the band diagrams of a PolFET (figure 18(a)) without and with the incorporation of a graded AlGaN ohmic contact layer [47]. It can clearly be seen that the electron barrier height at the semiconductor's surface was significantly reduced and that the conduction band was flat for efficient electron transport, thanks to the incorporation of the graded contact layer. The measured contact resistance of this PolFET was 73 mΩ·mm. Figure 18(d) shows a fabricated Al_0.7Ga_0.3N/Al_0.5Ga_0.5N HFET with a recessed gate region [51]. Figures 18(e) and (f) are separate the band diagrams of the gate Schottky contact with a raised barrier height and their source/drain ohmic contact with a specific contact resistance of 6.2 × 10⁻⁵ Ω·cm². Besides, an MESFET with a downward-graded AlGaN contact layer also exhibited a low specific contact resistance, which was measured at 3.3 × 10⁻⁵ Ω·cm². All these values represent the lowest reported contact resistance for Al-rich AlGaN channel transistors, suggesting the excellent carrier transport manipulation of graded AlGaN alloys via band engineering for ohmic contact applications.

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4.2.2. Graded AlGaN back-barrier layers for sufficient electron confinement

Back-barrier layers act as an important device architecture in AlGaN HEMTs to confine electrons inside conductive channels and thus suppress leakage current at large bias voltages, preventing early device punchthrough and mitigating short-channel effects [27]. Graded AlGaN alloys have band structures that can be tuned by designing their grading profiles and which can be utilized as back-barrier layers to lift the electron barrier height beneath the 2DEG channels and thus improve the electron confinement capacity [115–117]. Figure 19(a) shows AlGaN/GaN HEMTs fabricated on a graded AlGaN back-barrier (called Str_A) and a non-graded AlGaN back-barrier (called Str_B), and their calculated electron distributions and BVs in the buffer layer. Apparently, less leakage current was observed in Str_A, confirming the effectiveness of an increased back-barrier height for carrier confinement. Therefore, the BVs of Str_A were significantly enhanced, as shown in figure 19(b). Furthermore, with increased buffer layer thickness, the BV can be further enhanced to 350 V. Additionally, it has been confirmed that graded AlGaN buffer layers can simultaneously improve the crystal quality by modulating the internal strain [118–121]. Therefore, this type of graded AlGaN back-barrier layer has been widely implemented in AlGaN electronic devices on Si [122–124], SiC [125], and sapphire substrates [126–128], contributing to the improved crystal quality of AlGaN epilayers and enhanced device performance.

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Moreover, the graded p-AlGaN back-barrier layers have lately been employed in vertically conductive power devices, including trench MOSFETs [129] (figure 20(a)) and vertical double-diffused MOS (VDMOS) devices [130] (figure 20(b)). Such vertical devices consist of thick drift layers for vertical carrier transport and high voltage endurance. However, the trade-off between BV and specific on-resistance (R_on,sp) of the drift layers persists as a limitation for those devices [131–133]. Fortunately, with an inserted polarization-doped graded p-AlGaN layer (PI-doped p-AlGaN), the back-barrier height beneath the source/n-GaN region can be effectively elevated, suppressing leakage current through the p-type region into the n-GaN drift layer. Plus, 3DHGs in the linearly graded layer are uniformly distributed, so that the charge crowding effect at the p–n junctions can be avoided [134]. Combining the modulated band diagram and optimized carrier distribution, the introduction of graded p-AlGaN back-barrier layers can enhance the critical electric fields of vertical power devices for higher-voltage applications.

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