# On the origin and suppression of parasitic channel in AlN/GaN/AlGaN back barrier RF devices

L. Ben Hammou, F. Grandpierron, E. Carneiro, Katir Ziouche, Etienne Okada, F. Medjdoub

► To cite this version:

L. Ben Hammou, F. Grandpierron, E. Carneiro, Katir Ziouche, Etienne Okada, et al. On the origin and suppression of parasitic channel in AlN/GaN/AlGaN back barrier RF devices. APL Electronic Devices, 2025, 1 (4), pp.046101. 10.1063/5.0288132. hal-05293333

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On the origin and suppression of parasitic channel in AlN/GaN/AlGaN back barrier
RF devices.

L.Ben Hammou,ª F.Grandpierron, E.Carneiro, K.Ziouche, E.Okada, and F.Medjdoubª

CNRS-IEMN, Institute of Electronics, Microelectronics and Nanotechnology,
Av. Henri Poincaré, Villeneuve d'Ascq, France

(Dated: 9 September 2025)

This work investigates the origin and physical location of a parasitic conduction path in
GaN devices incorporating an AlGaN back barrier (BB), through a combined approach of
calibrated TCAD simulations and experimental C-V and I-V measurements on Schottky
diodes and HEMTs. The phenomenon was observed to directly depend on the carbon dop-
ing in the AlGaN BB, which significantly affects its resistivity. Insufficient doping leads
to an uncompensated n-type BB, enabling electron accumulation at the GaN channel/BB
interface and the formation of a parasitic charge sheet. Conversely, high carbon concen-
trations result in a semi-insulating behavior, preventing vertical leakage current and thus
effectively suppressing the conduction path. This mechanism, which is not intrinsic to the
heterostructure but emerges from inadequate doping compensation, contrasts with previ-
ous hypotheses invoking defect-mediated hopping. The study highlights the critical role
of BB resistivity, not just polarization design, in determining vertical confinement. These
findings offer valuable insight into the optimization of BB doping strategies for leakage
control and enhanced electron confinement in next-generation GaN HEMTs

<footnote>ª Authors to whom correspondence should be addressed : lyes.benhammou@univ-lille.fr and farid.medjdoub@univ-lille.fr</footnote>
The integration of an AlGaN back barrier (BB) into GaN-based RF high electron mobility transistors (HEMTs) has emerged as an effective strategy to enhance vertical electron confinement and suppress buffer leakage, particularly in aggressively scaled devices for high-frequency and high-power applications. Several prior studies have demonstrated that carefully engineered BB structures can significantly improve device performance and robustness such as drain-induced barrier lowering (DIBL), dynamic on-resistance, and output power at microwave frequencies. For instance, a 100 nm-thick Al<sub>0.25</sub>Ga<sub>0.75</sub>N BB led to a reduction in drain-induced barrier lowering (DIBL) to below 30 mV/V for 100 nm gate lengths with state-of-the-art power performances at 40 GHz.<sup>1</sup> Likewise, it was demonstrated that a 25% Al-content BB effectively blocks electric field penetration into the buffer, thereby mitigating current collapse and supporting robust operation at drain voltages exceeding 20 V.<sup>2</sup> Similarly, back-barrier design was linked to reduced resistance dispersion and improved dynamic Ron behavior.<sup>3,4</sup> Furthermore, improved cut-off-frequencies ft/fmax performance using AlGaN BBs has been reported.<sup>5,6</sup> Despite these advances, the presence of unintended parasitic conduction paths in AlGaN BB-based HEMTs remains a critical challenge. Improper BB doping or structural design can compromise vertical electron confinement, leading to increased leakage and degraded gate control. Recent studies, have observed such a parasitic conduction channel, attributing them to localized states and variable-range hopping mechanisms near or within the back barrier region.<sup>7,8</sup> However, no physical evidence of the proposed localized defect states or their spatial distribution has been experimentally demonstrated. Thus, the physical origin and spatial localization of the parasitic conduction path remain to be conclusively established.

In this work, we address this gap by presenting a comprehensive, experimentally calibrated TCAD simulation and measurement-based investigation of the parasitic conduction behavior in AlGaN BB HEMTs. Two heterostructures, identical except for a two-order-of-magnitude difference in carbon doping concentration in the back barrier, were systematically studied using capacitance-voltage (C-V) and current-voltage (I-V) measurements, as well as 2D device simulations. We demonstrate, for the first time, that the parasitic channel originates from a potential well formed at the GaN channel/AlGaN BB interface in low-doped samples. This electron accumulation, driven by insufficient resistivity in unintentionally n-type AlGaN layers, leads to the formation of a secondary charge sheet and a distinctive second capacitance plateau. The phenomenon is fully suppressed in heavily doped samples due to the semi-insulating nature of the carbon-rich BB.
<figure><img src="image_2.png" /><figcaption>FIG. 1. Schematic cross section of the AlN/GaN HEMT (a) wafer A with a high carbon doping. (b) wafer B with a low carbon doping. SIMS profiles showing C, Si, O concentrations and Al, Ga contents in (c) wafer A (d) wafer B.</figcaption></figure>

Two AlN/GaN/AlGaN heterostructures were studied, sharing the same epitaxial stack as presented in Fig. 1(a) and 1(b), they were grown on 4-inch silicon carbide (SiC) substrates by Metal-Organic Chemical Vapor Deposition (MOCVD). The layer sequence consists of an AlN nucleation layer, a carbon doped GaN buffer, a 100 nm Al<sub>0.28</sub>Ga<sub>0.72</sub>N back barrier, a 100 nm unintentionally doped (UID) GaN channel layer, a 3 nm AlN barrier layer, and a 6 nm in situ silicon nitride (Si<sub>3</sub>N<sub>4</sub>) passivation. As verified by secondary ion mass spectrometry (SIMS) in Fig. 1(c) and 1(d) the primary difference between the two structures lies in the carbon doping concentration in the back barrier layer, wafer A with high doping at 5×10<sup>19</sup> cm<sup>-3</sup> and wafer B with low doping around 1×10<sup>17</sup> cm<sup>-3</sup>, representing a two-order-of-magnitude difference.

Ohmic contacts were fabricated using a Ti/Al/Ni/Au metal stack deposited directly on top of the AlN barrier after etching the in-situ SiN cap layer, followed by rapid thermal annealing at 850° C. Ni/Au Schottky contacts were used and defined via electron beam lithography.

DC transfer characteristics(Id-Vg) measured on 2×50 µm HEMTs with a 100 nm gate length and a gate-to-drain distance of 1.5 µm revealed a significantly different electrical behavior between
<figure><img src="image_3.png" /><figcaption>FIG. 2. (a) Measured transfer characteristics comparing wafer A and B. (b) Transfer characteristics from V<sub>DS</sub>=2V to 20V of a 100 nm HEMT from wafer A indicating excellent electron confinement. (c) Capacitance-voltage (C-V) characteristics of Schottky diodes measured on both wafers, showing the presence of second capacitance in wafer B, reflecting a parasitic conduction path.</figcaption></figure>

the two wafers. As shown in Fig. 2(a) and 2(b), devices fabricated on wafer A, with high carbon doping in the back barrier, exhibited excellent electron confinement with low off state leakage currents in the 10<sup>-6</sup> A/mm range. In contrast, devices fabricated on wafer B (Fig. 2(a)), with a lower back-barrier carbon doping concentration, showed significantly higher leakage currents and failed to pinch-off, indicating poor electrostatic gate control and possible parasitic conduction. Capacitance-voltage (C-V) measurements on circular Schottky diodes with a diameter D=54 µm, illustrated in Fig. 2(c), show the presence of an additional capacitance in wafer B, not present in wafer A. This extra capacitance is indicative of a parasitic 2DEG, suggesting the formation of a secondary parasitic conduction path, responsible for the high leakage observed in the transfer characteristics. Its presence in wafer B only suggests a strong dependence on the concentration of carbon doping level in the AlGaN BB. A sufficiently high carbon doping effectively suppresses this undesired parasitic conduction.

To understand the physical origin of this behavior, Silvaco ATLAS was used to simulate and calibrate the C-V and I-V characteristics on the Schottky diodes and the HEMTs, aiming to investigate the source of the leakage path observed experimentally. The device structure and layer stack were reproduced based on the actual epitaxial parameters. The simulation framework solved self-consistently the Poisson equation and drift-diffusion transport equations. To account for polarization effects at the III-nitride heterointerfaces, the built-in polarization model was used, instead of setting fixed charges at the surface and interfaces. The physical models activated are Fermi-Dirac carrier statistics for carrier distribution and field dependent mobility (FLDmob) for high-field mo-
bility and the velocity saturation. Shockley–Read–Hall was used for carrier recombination. At the metal-semiconductor interface, thermionic emission was used to model carrier injection across the Schottky barrier, both electron tunneling and surface recombination were enabled to account for the non-ideal behavior. Carbon doping concentration in the AlGaN BB was identified as the main structural difference between the two device variants. This significantly affects the resistivity of this layer. To reflect this in the simulations, doping traps were introduced to control the Fermi level and adjust the back-barrier from n-type to semi-insulating behavior. A simplified model was used, consisting of one donor and acceptor like traps, each defined by a fixed energy level and capture cross-section. Trap parameters were assigned differently in GaN and AlGaN to account for the difference in band gap. We did not implement a more advanced coordination trap model, capable of accounting for multiple donor and acceptor states, such as the complex behavior of carbon incorporation at multiple energy levels within the bandgap.

The incorporation behavior of carbon in nitride materials depends strongly on its concentration. At low doping levels as in wafer B (1×10¹⁷ cm⁻³), carbon predominantly substitutes at nitrogen sites (C<sub>N</sub>), forming deep acceptor levels above the valence band maximum.<sup>9–13</sup> As the doping level increases, donor-type defects such as C<sub>Ga</sub> (carbon substituting for Gallium) form, introducing shallow donor levels. This result in a self-compensation regime, where both acceptors and donors coexist. However, at sufficiently high concentrations (≥1×10¹⁹ cm⁻³), the formation energy of C<sub>N</sub> is again more favorable, leading to a strong acceptor-dominant regime where the donor-to-acceptor ratio (DAR) drops well below unity and the Fermi level pins near the valence band.<sup>10,11,14</sup> While it is well established that the high carbon concentrations in wafer A leads to a resistive back barrier layer, the effectiveness of the low concentration in wafer B in achieving similar compensation is limited. Although carbon is incorporated as an acceptor at low doping levels, its concentration may be insufficient to fully neutralize background donors originating from residual impurities such as oxygen or silicon and native defects like nitrogen vacancies. Our previous study on structures grown with the same epitaxial stack but without the AlGaN back barrier<sup>15</sup> provides important insight. We found that carbon doping up to 5×10¹⁸ cm⁻³ in the GaN buffer was insufficient to suppress short-channel effects and ensure proper electron confinement, indicating that the material remains weakly n-type. A significant increase in resistivity was only observed when the carbon doping was raised to 2×10¹⁹ cm⁻³. This suggests either a high background concentration of residual donors (e.g., oxygen, nitrogen vacancies) on the order of 1×10¹⁷ cm⁻³ or more, or that carbon related donors such as C<sub>Ga</sub> and di- or tri-carbon complexes could be incorporated even at
<figure><img src="image_4.png" /></figure>

FIG. 3. (a) Schematic cross section showing the location of the polarization induced 2DEG and 2DHG in the AlN/GaN/AlGaN/GaN heterostructure. (b) Simulated energy band diagram under equilibrium, highlighting conduction and valence band bending due to polarization.

moderate doping levels, limiting the net acceptors effectiveness.¹⁶

Although these mechanisms are well-established in GaN, similar trends are expected in AlGaN, with deeper trap levels due to its larger bandgap.¹²,¹⁷

Based on this understanding, in our TCAD model, the AlGaN back barrier in wafer B was treated as n-type and a variation in the doping donor traps concentration was implemented to assess the impact of the low resistivity in this layer. For wafer A, the back barrier was simulated using an acceptor-dominant model.

In addition to doping, the GaN/AlGaN/GaN heterostructure configuration introduces polarization discontinuities at the AlGaN interfaces. A sufficiently high net negative polarization at the GaN/AlGaN BB interface can enhance confinement of the main 2DEG and, if large enough, may also promote the formation of a two-dimensional hole gas (2DHG). At the AlGaN/GaN buffer interface, a high net positive polarization can induce an additional 2DEG¹⁸⁻²⁰. Their locations are illustrated in Fig. 3(a), with the corresponding simulated energy band diagram at equilibrium, shown in Fig. 3(b), obtained using the TCAD built-in polarization model. However, for wafer B, the experimentally observed high leakage and absence of pinch-off indicate that the net negative polarization at the GaN/AlGaN BB interface is too low to ensure a minimum 2DEG confinement. Furthermore, the presence of carbon doping at 5×10¹⁸ cm⁻³ in the GaN buffer is expected to have a screening effect on the net positive polarization at the AlGaN/GaN interface, reducing its effect. These observations suggest that polarization contributions in the studied devices are weak. Nevertheless, TCAD simulations were carried out to evaluate the impact of these polarization charges on the formation of the parasitic channel. In the context of this study, the GaN/AlGaN interface
refers to the GaN channel/AlGaN back-barrier (BB) interface, whereas the AlGaN/GaN interface
141
142 corresponds to the lower AlGaN BB/GaN buffer interface.

The first step in the TCAD calibration focused on assessing separately the impact of the low
143
144 resistivity of the back barrier in wafer B. For this purpose, the net polarization charges at both
145 interfaces of the AlGaN BB were neutralized, and the donor trap concentration in the AlGaN BB
146
147 was varied. Shallow donors with an energy level of 0.05 eV below the conduction band (E<sub>C</sub>-0.05)
148 and electron and hole capture cross sections of 10<sup>-15</sup>cm<sup>2</sup> were used.<sup>17,21</sup> The carbon concen-
149 tration of 5×10<sup>18</sup> cm<sup>-3</sup> in the GaN Buffer was incorporated using the same shallow donors and
150 C<sub>N</sub> acceptors with an energy level of 0.9 eV above the valence band and capture cross section of
151 10<sup>-15</sup>cm<sup>2</sup>, a compensation ratio close to the unity was tuned to set the fermi level at the middle
152 of the bandgap, placing the material in an autocompensation model<sup>10</sup>. The extracted conduction
153 band profiles for the different concentrations are shown in Fig. 4(a), illustrating the progressive
154 lowering of the conduction band with increasing donor concentration. The calibrated Schottky
diode produced the capacitance-voltage (C-V) and reverse current (I-V) characteristics shown
155 in Fig. 4(b) and Fig. 4(c), respectively. Concentrations below 1×10<sup>17</sup>cm<sup>-3</sup> (1×10<sup>16</sup>cm<sup>-3</sup> and
156 5×10<sup>16</sup>cm<sup>-3</sup>) showed no signature of a second capacitance step. At 1×10<sup>17</sup>cm<sup>-3</sup>, a second capac-
157 itance step emerged, and with further increases to 3×10<sup>17</sup>cm<sup>-3</sup> and 5×10<sup>17</sup>cm<sup>-3</sup>, the voltage at
158 which this second plateau is fully depleted shifts progressively toward more negative values. The
159 best agreement with the experimental C-V and I-V data was obtained for a donor concentration of
160 3×10<sup>17</sup>cm<sup>-3</sup>.

To further investigate the spatial location of the parasitic conduction path observed in wafer
161
162 B, the calibrated TCAD structures at different donor concentrations were analyzed under reverse
163 bias conditions within the second capacitance plateau. Specifically, the structures were biased at
164 -4V, and the resulting conduction current density distribution was extracted. The cross sectional
165 distribution at 3×10<sup>17</sup>cm<sup>-3</sup> in Fig. 4(d), shows a clear lateral current path at the GaN/AlGaN
166 interface. A vertical cutline was then extracted at the position of this conduction path, and the
167 corresponding energy band diagram was analyzed in Fig. 4(e). The conduction band profile reveals
168 a potential well at the GaN/AlGaN interface, that coincides with the peak of the conduction current
169 density of the parasitic channel. The well originates from the conduction band offset between GaN
170 and AlGaN. This well acts as an electron accumulation site, forming a secondary charge sheet
171 consistent with the experimentally observed second capacitance. Fig. 5(f) presents the extracted
172 conduction current density distribution for all simulated donor concentrations. The results show
<figure><img src="image_5.png" /><figcaption>FIG. 4. (a) Simulated conduction band energy diagram for different donor concentrations (b) Simulated vs. Measured Capacitance-voltage (C-V) characteristics for wafer B for different donor concentrations (c) Simulated vs. measured reverse leakage current (I-V) characteristics for the same device at different donor concentration (d) cross-sectional conduction current density distribution at VGS = -4V for donor concentration of 3×10¹⁷ cm⁻³, highlighting a parasitic path at the GaN/AlGaN BB interface. (e) The corresponding simulated energy band diagram and conduction current density profile at -4V, showing a localized potential well at the GaN/AlGaN BB interface (inset) responsible for electron accumulation and parasitic channel formation. (f) Conduction current density profile at -4V for different donor concentrations.</figcaption></figure>
that electron accumulation at the potential well, appear only in cases where a second capacitance plateau is present in the C-V characteristics. Moreover, the peak electron density in the well increases systematically with donor concentration, consequently, a larger reverse bias is required to fully deplete this charge, explaining the observed shift of the depletion voltage for the second plateau toward more negative values as the donor concentration increases.

These findings demonstrate that the parasitic conduction path in wafer B is not an intrinsic property of the GaN/AlGaN/GaN heterostructure. Instead, it arises from the electrostatic characteristics of the uncompensated n-type AlGaN back barrier. The insufficient resistivity in this region allows for electron accumulation at the GaN/AlGaN interface, ultimately enabling the formation of a parasitic leakage channel and degrading vertical confinement. Furthermore, at low donor concentrations, no electron accumulation is observed at the interface, even in the absence of polarization charges, indicating that the GaN buffer alone is sufficient to prevent at least charge accumulation in this region, this means that the absence of polarization charges alone is not sufficient

<figure><img src="image_6.png" /><figcaption>FIG. 5. (a) Simulated Energy band diagrams at different polarization (b) Simulated vs. measured Capacitance-voltage (C-V) characteristics for wafer B at different polarizations (c) Conduction current density profile at -4V for all polarization cases.</figcaption></figure>
to create the parasitic channel, a sufficiently low-resistivity back barrier is required.

The effect of polarization charges at the AlGaN BB interfaces is then investigated. The donor trap concentration in the AlGaN BB was fixed at $3 \times 10^{17} \text{cm}^{-3}$, the value that provided the best fit in the resistivity variation. The polarization charges are then gradually increased by adjusting the polarization scale parameter in TCAD. Fig. 5(a) compares the simulated energy band diagrams for the baseline case with polarization excluded and for three scaled cases (Pol-1, Pol-2, Pol-3), showing a progressive elevation of the energy bands at the GaN/AlGaN interface with increasing polarization charge. The corresponding simulated C-V characteristics are shown in Fig. 5(b). As the polarization charge increases, the parasitic capacitance level decreases. More importantly, the plateau shape becomes less distinct, transitioning toward a sloped profile. This behavior can be explained by analyzing the conduction current density distribution for all structures, biased within the second capacitance. As shown in Fig. 5(c), higher polarization charge leads to a broader spread of electrons along the GaN/AlGaN interface and a reduction in both the peak and total density. Since the second capacitance plateau originates from the filling of this interfacial well via leakage, a broader and less confined distribution means that depletion removes charge more gradually with reverse bias. This leads to a lower measured capacitance and a non-plateau shaped, consistent with the effect of negative polarization charge at the GaN/AlGaN interface. When the polarization scaling is further increased (Pol-3), the second capacitance is not observed, and no mobile charges accumulate at the well. These results show that reproducing the experimentally observed plateau requires very low net polarization charge, as higher values alter the C-V shape or strengthen confinement. Based on this, polarization at this interface is neglected in further simulations.

Because the primary role of the back barrier is to enhance confinement through a back polariza-
tion, we emphasize that the measured C-V/I-V do not show any confinement benefit attributable
to a net negative sheet at the GaN/AlGaN interface. This suggest that this net negative charge is
strongly screened. Possible mechanisms include compensation by ionized donors in the AlGaN
BB and GaN channel, a partial relaxation reducing the piezoelectric component of the polariza-
tion, or interface states. Such mechanisms are not included explicitly in our model; we represent
their screening effect by neutralizing the net polarization sheet at GaN/AlGaN interface as an
electrostatic-equivalent simplification. This choice should not be interpreted as removing the un-
derlying polarization physics at this interface.

To validate the robustness of the calibrated TCAD model, the simulation framework initially
<figure><img src="image_7.png" /><figcaption>FIG. 6. (a) Simulated and measured transfer characteristics of HEMTs from wafer B with $L_{GD}=1.5$ µm and various $L_G$. (b) Simulated cross-sectional conduction current density distribution at $V_{GS}=-5$V, showing parasitic path at the GaN/AlGaN BB interface under the gate for HEMT with $L_G=500$nm.</figcaption></figure>

developed for the Schottky diode was directly extended to HEMT structures, using identical layer stacks, material parameters, physical models and doping concentrations. This approach ensured that no essential additional adjustments were introduced beyond those established during the diode analysis. The first validation step involved HEMT with a gate length $L_G=500$ nm using the same trap-based doping and polarization settings extracted from the diode analysis. The experimentally measured $I_D V_{GS}$ transfer characteristics exhibited a two-threshold behavior, with a shallow subthreshold bump appearing at $V_{GS}=-6.5$ V followed by the main threshold voltage at $V_{GS}=-3.1$V. This distinct dual transition behavior was accurately reproduced in simulation, as shown in Fig. 6(a). To identify the physical origin of the subthreshold bump, the conduction current density distribution was analyzed at $V_{GS}=-5$V within the region of the first transition. The simulated cross section in Fig. 6(b) reveals a clear parasitic conduction path localized at the GaN/AlGaN interface, directly beneath the gate region. This supports the mechanism previously identified in diode simulations, and confirms that the first threshold corresponds to partial channel depletion, where inadequate led to accumulation in the interface well. Critically, without any further modification of the model, the same calibrated structure was used to simulate HEMTs with a gate length of 100nm. The simulated $I_D V_{GS}$ characteristics closely matched experimental observations, including a pronounced gate leakage and incomplete pinch-off behavior (Fig. 6(a)). This scaling-dependent behavior highlights the effectiveness of the model in capturing the electrostatic limitations introduced by low-resistivity back barriers, especially in aggressively scaled device
<figure><img src="image_8.png" /><figcaption>FIG. 7. (a) Capacitance–voltage (C–V) characteristics measured from Schottky diodes of wafer A, showing a single capacitance. (b) Simulated and measured reverse leakage current (I–V) profiles from the same device (c) Simulated energy band diagram at –5V, confirming the absence of electron accumulation at the GaN/AlGaN BB interface.</figcaption></figure>

geometries.

To examine the effect of the heavy carbon doping, additional TCAD simulations were con-
ducted on the Schottky diode structure corresponding to wafer A, in which the AlGaN BB was
doped with carbon at 5×10¹⁹ cm⁻³. Both shallow donors with an energy level of 0.05 eV below
the conduction band (E_c-0.05) and a capture cross section of 10⁻¹⁵cm².¹⁷,²¹ And C_N acceptors
with an energy level of 1.1 eV above the valence band (E_v+1.1)¹²,¹⁷ and a capture cross sections of
10⁻¹⁵cm², were included in the AlGaN BB, using a donor-to-acceptor ratio (DAR) of 0.4, placing
the material in an acceptor-dominant regime, consistent with semi-insulating behavior. The simu-
lated C-V characteristics in Fig. 7(a) showed a single capacitance transition, with no evidence of
a second threshold, in perfect agreement with experimental measurements. The reverse leakage
current profile also matched closely with measured data, as seen in Fig. 7(b). Further insight is
provided by the energy band profile extracted under reverse bias at V GS = -5V (Fig. 7(c)), clearly
demonstrating the impact of the high-resistivity AlGaN BB. The conduction band edge at the
GaN/AlGaN interface remains well above the Fermi level, indicating strong electron confinement
and the absence of electron accumulation at that interface. This behavior is in stark contrast to
the low-doped wafer B and directly illustrates how heavy carbon doping effectively suppresses the
parasitic conduction through the back barrier.

In summary this work provides a comprehensive experimental and simulation-based investiga-
tion into the origin of the parasitic conduction path observed in GaN HEMTs featuring an AlGaN
back barrier. By calibrating TCAD models against diode and HEMTs measurements, we demon-
strate that this parasitic channel is not intrinsic to the GaN/AlGaN heterostructure but instead emerges from inadequate compensation in unintentionally n-type AlGaN layers. In low carbon-doped structures, insufficient resistivity in the back barrier permits electron accumulation at the GaN/AlGaN interface, forming a secondary charge sheet responsible for the observed second capacitance and degraded confinement. In contrast, heavily carbon-doped AlGaN layers exhibit acceptor-dominant behavior and semi-insulating properties, effectively eliminating the parasitic path and preserving the 2DEG confinement. These results confirm that the electrostatic properties of the AlGaN back barrier governed by its carbon doping profile play a critical role in device leakage and channel control. The findings offer actionable guidance for optimizing BB design in advanced GaN HEMT technologies.

The authors would like to acknowledge the support of the French RENATECH network, the DGA/CNRS funded GREAT project as well as SOITEC-Belgium for material delivery.

## DATA AVAILABILITY

The data that support the finding of this study are available from the corresponding author upon reasonable request.

## REFERENCES

¹K. Harrouche, S. Venkatachalam, L. Ben-Hammou, F. Grandpierron, E. Okada, and F. Medjdoub, "Low Trapping Effects and High Electron Confinement in Short AlN/GaN-On-SiC HEMTs by Means of a Thin AlGaN Back Barrier," *Micromachines* **14**, 291 (2023).

²A. Shanbhag, F. Grandpierron, K. Harrouche, and F. Medjdoub, "Physical insight of thin AlGaN back barrier for millimeter-wave high voltage AlN/GaN on SiC HEMTs," *Applied Physics Letters* **123**, 142102 (2023).

³H. Yu, B. Parvais, U. Peralagu, R. Y. ElKashlan, R. Rodriguez, A. Khaled, S. Yadav, A. Alian, M. Zhao, N. De Almeida Braga, J. Cobb, J. Fang, P. Cardinael, A. Sibaja-Hernandez, and N. Collaert, "Back Barrier Trapping Induced Resistance Dispersion in GaN HEMT: Mechanism, Modeling, and Solutions," in *2022 International Electron Devices Meeting (IEDM) (2022) pp. 30.6.1–30.6.4*.

⁴W. Wang, L. Li, L. He, F. Yang, Z. Chen, Y. Zheng, L. He, Z. Wu, B. Zhang, and Y. Liu, "Influence of AlGaN back barrier layer thickness on the dynamic ron characteristics of AlGaN/GaN
HEMTs," in *2016 13th China International Forum on Solid State Lighting: International Forum on Wide Bandgap Semiconductors* (2016) pp. 77–80.

⁵B. Liu, Z. Feng, S. Dun, X. Zhang, G. Gu, Y. Wang, P. Xu, Z. He, and S. Cai, "An extrinsic $f_{max} > 100$ GHz InAlN/GaN HEMT with AlGaN back barrier," *Journal of Semiconductors* **34**, 044006–4 (2013).

⁶L. Yang, B. Hou, F. Jia, M. Zhang, M. Wu, X. Niu, H. Lu, C. Shi, M. Mi, Q. Zhu, Y. Lu, X. Ma, and Y. Hao, "The DC Performance and RF Characteristics of GaN-Based HEMTs Improvement Using Graded AlGaN Back Barrier and Fe/C Co-Doped Buffer," *IEEE Transactions on Electron Devices* **69**, 4170–4174 (2022).

⁷S. Liu, Y. Zhuang, H. Li, P. Cui, Q. Xie, Y. Wang, H. Xie, K. Ranjan, and G. I. Ng, "Analysis of Leakage Channel with Different Al Composition back Barriers in AlN/GaN High-Electron-Mobility Transistors on Silicon," *physica status solidi* (a), 2400983 (2025).

⁸S. Liu, Y. Zhuang, H. Li, Q. Xie, Y. Wang, H. Xie, K. Ranjan, and G. I. Ng, "Variable range hopping-assisted parasitic channel leakage in AlN/GaN/AlGaN HEMTs on Si," *Applied Physics Letters* **125**, 023504 (2024).

⁹P. Deák, M. Lorke, B. Aradi, and T. Frauenheim, "Carbon in GaN: Calculations with an optimized hybrid functional," *Physical Review B* **99**, 085206 (2019).

¹⁰C. Koller, L. Lymperakis, D. Pogany, G. Pobegen, and C. Ostermaier, "Mechanism leading to semi-insulating property of carbon-doped GaN: Analysis of donor acceptor ratio and method for its determination," *Journal of Applied Physics* **130**, 185702 (2021).

¹¹Z. Shen, X. Yang, S. Wu, H. Huang, X. Yan, N. Tang, F. Xu, X. Wang, W. Ge, B. Huang, and B. Shen, "Mechanism for self-compensation in heavily carbon doped GaN," *AIP Advances* **13**, 035026 (2023).

¹²J. L. Lyons, A. Janotti, and C. G. Van de Walle, "Effects of carbon on the electrical and optical properties of InN, GaN, and AlN," *Physical Review B* **89**, 035204 (2014).

¹³S. Wu, X. Yang, H. Zhang, L. Shi, Q. Zhang, Q. Shang, Z. Qi, Y. Xu, J. Zhang, N. Tang, X. Wang, W. Ge, K. Xu, and B. Shen, "Unambiguous Identification of Carbon Location on the N Site in Semi-insulating GaN," *Physical Review Letters* **121**, 145505 (2018).

¹⁴S. Wu, X. Yang, H. Huang, Z. Shen, Y. Xue, H. Yang, L. Wang, F. Xu, X. Wang, W. Ge, and B. Shen, "The movement of the Fermi level in heavily C doped GaN," *Japanese Journal of Applied Physics* **61**, 090901 (2022).

¹⁵K. Harrouche, S. Venkatachalam, F. Grandpierron, E. Okada, and F. Medjdoub, "Impact of un-
321 doped channel thickness and carbon concentration on AlN/GaN-on-SiC HEMT performances,”
322 Applied Physics Express **15**, 116504 (2022).

323 <sup>16</sup>R. Kaneko, J. Tajima, A. Yoshioka, T. Hikosaka, and S. Nunoue, “Carbon-Related Trap Dynamics in Semi-Insulating Buffer Layer of GaN-on-Si Substrate,” physica status solidi (a) , 2401019
324
325 (2025).

326 <sup>17</sup>B. Rackauskas, M. J. Uren, S. Stoffels, M. Zhao, S. Decoutere, and M. Kuball, “Determination of
327 the Self-Compensation Ratio of Carbon in AlGaN for HEMTs,” IEEE Transactions on Electron
328 Devices **65**, 1838–1842 (2018).

329 <sup>18</sup>L. Méchin, F. Médard, J. Leymarie, S. Bouchoule, J.-Y. Duboz, B. Alloing, J. Zuñiga-Pérez, and
330 P. Disseix, “Experimental demonstration of a two-dimensional hole gas in a GaN/AlGaN/GaN
331 based heterostructure by optical spectroscopy,” Physical Review B **109**, 125401 (2024).

332 <sup>19</sup>C. Beckmann, J. Wieben, T. Zweipfennig, A. Kirchbrücher, J. Ehrler, R. Stamm, Z. Yang,
333 H. Kalisch, and A. Vescan, “MOVPE-grown GaN/AlGaN heterostructures on sapphire with
334 polarization-induced two-dimensional hole gases,” Journal of Physics D: Applied Physics **55**,
335 435102 (2022).

336 <sup>20</sup>Y. Gu, D. Chang, H. Sun, J. Zhao, G. Yang, Z. Dai, and Y. Ding, “Theoretical Study of In-
337 AlN/GaN High Electron Mobility Transistor (HEMT) with a Polarization-Graded AlGaN Back-
338 Barrier Layer,” Electronics **8**, 885 (2019).

339 <sup>21</sup>G. Pozina, S. Khromov, C. Hemmingsson, L. Hultman, and B. Monemar, “Effect of silicon and
340 oxygen doping on donor bound excitons in bulk GaN,” Physical Review B **84**, 165213 (2011).