Journal of Semiconductors, 2022, 43 (6): 062803, Published Online: Jun. 10, 2022  

Optimization of recess-free AlGaN/GaN Schottky barrier diode by TiN anode and current transport mechanism analysis

Author Affiliations
1 The Institute of Future Lighting, Academy for Engineering and Technology, Fudan University (FAET), Shanghai 200433, China
2 Institute of Microelectronics of the Chinese Academy of Sciences, Beijing 100029, China
3 Beijing Const-Intellectual Core Technology Co. Ltd, Beijing 100029, China
Abstract
In this work, the optimization of reverse leakage current (IR) and turn-on voltage (VT) in recess-free AlGaN/GaN Schottky barrier diodes (SBDs) was achieved by substituting the Ni/Au anode with TiN anode. To explain this phenomenon, the current transport mechanism was investigated by temperature-dependent current–voltage (I–V) characteristics. For forward bias, the current is dominated by the thermionic emission (TE) mechanisms for both devices. Besides, the presence of inhomogeneity of the Schottky barrier height (b) is proved by the linear relationship between b and ideality factor. For reverse bias, the current is dominated by two different mechanisms at high temperature and low temperature, respectively. At high temperatures, the Poole–Frenkel emission (PFE) induced by nitrogen-vacancy (VN) is responsible for the high IR in Ni/Au anode. For TiN anode, the IR is dominated by the PFE from threading dislocation (TD), which can be attributed to the decrease of VN due to the suppression of N diffusion at the interface of Schottky contact. At low temperatures, the IR of both diodes is dominated by Fowler–Nordheim (FN) tunneling. However, the VN donor enhances the electric field in the barrier layer, thus causing a higher IR in Ni/Au anode than TiN anode, as confirmed by the modified FN model.

1. Introduction

AlGaN/GaN Schottky barrier diode (SBD) features excellent performance for high-power, high-frequency, and high-temperature applications, attributed to the outstanding properties of GaN material[1-5]. Recently, some researchers have demonstrated that AlGaN/GaN SBD has great potential for future wireless power transfer (WPT) applications[6-7]. The SBD converts the received RF signal into DC power, which determines the efficiency of the WPT system. To increase the conversion efficiency, AlGaN/GaN SBD requires a low turn-on voltage (VT) while maintaining a low reverse leakage current (IR).

The Schottky metal is well recognized to have a considerable impact on the VT and IR of Schottky junction (SJ). For AlGaN/GaN SBD, the most common electrode used as the anode is Ni/Au and TiN metal[2-5, 8-10]. Some researchers have compared the performance of AlGaN/GaN SJ based on Ni/Au and TiN Schottky metal. The related results show that the electrical characteristics of TiN-based SJ are better than Ni/Au-based SJ[11-13], but the mechanism is not well understood. A common explanation for this result is that the interaction between Ni and AlGaN(GaN) causes many defects or interface states, thus increasing the IR[12-14]. To confirm this hypothesis, a clear understanding of the current transport mechanism is necessary to determine whether and how the related defects act on the electrical characteristics of SJ. However, the comparative analysis on the current transport mechanism of AlGaN/GaN SBDs with Ni/Au and TiN anodes is still lacking, and it would help obtain deeper physical insights into the impact of Schottky metal on the electrical properties of SBDs.

The recess-free technique can improve device uniformity and reliability by eliminating the plasma damage to the AlGaN barrier layer from the etching process[15-17]. In previous work, we have demonstrated high-performance recess-free AlGaN/GaN SBDs based on Ni/Au anode[16, 17]. However, recess-free AlGaN/GaN SBDs based on TiN anode have never been reported. The TiN anode can be compatible with CMOS processes[18], thus significantly reducing the fabrication cost of recess-free AlGaN/GaN SBDs.

In this paper, we have achieved the simultaneous improvement of VT and IR for recess-free AlGaN/GaN SBDs by substituting the Ni/Au anode with the TiN anode. The impact of Schottky metal on the electrical properties of SBDs is revealed by analyzing the current transport mechanism.

2. Device structure and fabrication

The epitaxial wafer is a commercial product from the Enkris Semiconductor, grown by MOCVD on a 3-inch sapphire <0001> substrate, consisting of a 1.5 μm C-doped GaN buffer layer, a 400 nm UID GaN channel layer, and a 7 nm Al0.25Ga0.75N barrier layer. With a 24 nm LPCVD-SiNx passivation (780 °C, with RCA pretreatment), the 2DEG channel in the access region was effectively preserved[19]. After passivation, the sheet resistance is reduced to 303 Ω/sq.

The schematic cross-section of SBD is shown in Fig. 1, and the device fabrication flow is the same except for the anode metal. First, the LPCVD SiNx cap in anode and cathode region was opened with a fluorine-based ICP etch. Then, the metal layer of Ti/Al/Ni/Au (20/150/45/55 nm) was deposited, followed by rapid thermal annealing (RTA) at 870 °C for 50 s in N2 ambient to form the ohmic cathode. Finally, Ni/Au (50 nm/150 nm) was deposited by evaporation to form the anode electrodes for device A, and TiN metal was deposited by reactive sputtering in Ar:N2 (7 : 3 sccm) mixed gas atmosphere to form the anode electrodes for device B. The devices in this work have the same dimensions of LSC/LAC/LFP/W = 2/6/0.75/100 μm. The electrical characteristics of the device were measured with Keysight B1500A.

Fig. 1. (Color online) (a) Schematic cross-section of the fabricated recess-free AlGaN/GaN SBD. IV characteristics of device A and B at RT on the (b) log scale and (c) linear scale. (d) IV characteristics of 8 devices for A and B.

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3. Overview of electrical characteristics

The I–V characteristics at room temperature (RT) on the log scale and linear scales are shown in Figs. 1(b) and 1(c), respectively. The SBD with TiN anode exhibits a low VT of 0.94 V (defined as the voltage corresponding to current = 1 mA/mm), a low differential specific on-resistance (RON,diff) of 0.52 mΩ·cm2 and a low IR of ~ 0.5 μA/mm. Compared with SBD with TiN diode, the Ni/Au SBD exhibits similar RON,diff, while presenting a higher VT of 1.16 V and a higher IR of ~1 μA/mm. I–V curves of 8 devices are also plotted in Fig. 1(d), suggesting a good device uniformity attributed to the recess-free technique. The lower VT in TiN SBD can be attributed to the difference between the work function of Ni (~5.15 eV) and of TiN (~4.7 eV)[13]. However, the lower IR in TiN SBD is difficult to understand. To further explain the phenomenon, the temperature-related I–V measurement was carried out.

4. Current transport mechanisms at forward bias

The I–V characteristics of devices A and B measured from 223 to 473 K are shown in Figs. 2(a) and 2(b) and used to analyze the current transport mechanisms. The I–V characteristics of both devices can be divided into three regions, namely, region-I (low forward bias), region-II (high forward bias), and region-III (reverse bias). In region-I, the current show weak temperature and voltage dependence on the logarithmic axis, indicating that the tunneling-assisted transport mechanism dominates. Obviously, the tunneling effect in this region is more significant for Ni/Au SBD, although the metal work function of Ni is higher than that of TiN. The strong tunneling effect in Ni/Au SBD may be attributed to the interface states existing at the Ni/AlGaN interface caused by the interaction[12, 20]. In region-II, the current show strong temperature and voltage dependence, indicating that the thermionic emission (TE) is a possible transport mechanism. The equation of TE model can be simplified as[21]

Fig. 2. (Color online) Temperature-dependent IV characteristics of (a) device A and (b) device B. (c) Dependence of n and b on the temperature for both devices. (d) The dependence of b on n for two diodes; the extrapolation at n = 1 of the linear fit of the data gives a value of the mean barrier height.

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$ I = S{A^*}{T^2}\exp \left( { - \frac{{q{\varphi _{\rm b}}}}{{kT}}} \right)\exp \left( {\frac{{qV}}{{nkT}}} \right), $  (1)

$ \ln I = \frac{{qV}}{{nkT}} + {\rm{ln}} \left( {S{A^*}{T^2}} \right) - \frac{{q{\varphi _{\rm b}}}}{{kT}}, $  (2)

where S is the SJ area, A* is the effective Richardson constant of 30 A/(cm·K)2 for Al0.25Ga0.75N[22], T is the absolute temperature, q is the fundamental electronic charge, V is the forward-bias voltage, k is the Boltzmann constant, b is the apparent Schottky barrier height, and n is the ideality factor. Then, the experimental values of b and the n can be extracted from intercepts and slopes of the forward lnI versus V plots in Figs. 2(a) and 2(b).

Fig. 2(c) shows the b and n as a function of temperature. It can be observed that both b and n show a strong temperature dependence for our devices. This observation is inconsistent with the ideal TE mechanism, in which the b and n should be almost constant with temperature. Besides, the calculated values of b and n are also far from the ideal value (n = 1, b = MχAlGaN), especially at low temperatures. Several studies have suggested that such behavior is attributed to the spatial non-homogeneous distribution of the Schottky barrier height[23, 24]. At low temperature, the current is dominated by the low barrier regions because the electrons do not have enough energy to cross over the high barrier regions and thus lead to the low extracted b at low temperatures. According to Schmitsdorf et al.[25] and Tung[26], the presence of inhomogeneity of the Schottky barrier height can be proved by the linear relationship between b and n. Fig. 2(d) shows a linear reduction of b with increasing n, which suggests the presence of inhomogeneity of the Schottky barrier height. By the extrapolation of b to n = 1 in Fig. 2(d), the mean barrier height of 1.14 eV for Ni/Au anode and of 1.0 eV for TiN anode was determined. The extrapolated values agree with the experimental value of 1.10–1.30 eV for Ni/AlGaN SBD[27] and of 0.88–1.13 eV for TiN/AlGaN[13], thus validating our analysis. Moreover, TiN SBD exhibited lower n and higher b compared with Ni/Au at low temperature (Fig. 2(c)), indicating the homogeneity of the Schottky barrier height of TiN/AlGaN contact is better than that of Ni/AlGaN contact. The poor homogeneity of Ni/AlGaN may be attributed to the defects induced by the interaction between Ni and AlGaN.

5. Current transport mechanisms at reverse bias

In region-III, it can be roughly seen that the temperature dependence of IR varied with temperature for both devices, which indicated IR could be dominated by different transport mechanisms at different ranges of temperature. To identify how many transport mechanisms work, the Arrhenius plots of IR at VA = –0.5 V are drawn in Fig. 3(a). Then, the activation energy (EA) was extracted from the Arrhenius plots, as shown in Fig. 3(b).

Fig. 3. (Color online) (a) Arrhenius plot of IR for both devices. (b) EA extracted from the Arrhenius plot.

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The physical meaning of EA is the energy barrier for electrons at the metal fermi-level to pass through the AlGaN barrier. Therefore, different EA corresponds to different reverse leakage mechanisms. According to the extracted EA in Fig. 3(b), the reverse leakage behavior can be roughly divided into two regions. At high temperatures, as temperature increases, the leakage current shows the apparent temperature dependence, indicating that the thermal emission process could dominate the leakage current in this region. The extracted EA (< 0.6 eV) in this region (Fig. 3(b)) is much smaller than the Schottky barrier height reported in the literature (~ 1 eV)[11], suggesting that the emission of electrons is associated with the trap. At low temperatures, the temperature dependence of IR is weaker, and EA is almost zero (Fig. 3(b)), which indicates that the tunneling process could dominate the leakage current. A detailed analysis of reverse leakage mechanisms is presented in the next section.

Electric field (E) estimation is necessary to analyze IR quantitatively. E across the AlGaN barrier layer can be calculated using the equation E(V) = q[σpns(V)]/εsε0[28]. The fixed polarization charge density (σp) at the hetero-interface is estimated to be 1.35 × 1013 cm–2[29]. εs = 8.9[30] is the static dielectric constant of Al0.25Ga0.75N, and ε0 is the vacuum dielectric constant. The 2DEG concentration (ns) at the hetero-interface can be extracted from 1 MHz capacitance–voltage (CV) curves in Fig. 4(a). The calculated E as a function of reverse bias is shown in Fig. 4(b), which will be used to analyze the leakage mechanism. Moreover, it can be seen in Fig. 4(a) that the pinch-off voltage (Vpin-off) in Ni SBD is higher than that in TiN SBD, which proves that the actual barrier height of Ni is higher than TiN.

Fig. 4. (Color online) (a) 1 MHz CV characteristics under the reverse bias voltage. (b) Calculated EV characteristics under the reverse bias voltage.

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5.1. Reverse leakage mechanisms at high temperature

Considering the strong temperature dependence at high temperatures, Poole–Frenkel emission (PFE) is the most probable carrier transport mechanism. The PFE refers to electric-field enhanced thermal emission from a trap state into a continuum of electronic, and the current density of PFE can be described as[31]:

$ {J_{{\text{PF}}}} = CE\exp \left[ { - \frac{{q({\varphi _{\text{t}}} - \sqrt {{{qE} \mathord{\left/ {\vphantom {{qE} {\pi {\varepsilon _{{\text{s(h)}}}}{\varepsilon _0}}}} \right. } {\pi {\varepsilon _{{\text{s(h)}}}}{\varepsilon _0}}}} )}}{{kT}}} \right], $  (3)

where C is a constant associated with trap concentration, E is the electric field across the barrier, t is the barrier height for the electron emission from the trap state, εs(h) = 5.1[32] is the relative dielectric constant at high frequency for Al0.25Ga0.75N. Eq. (3) can be rearranged as given below

$ \begin{gathered} {\rm{ln}} \left( {{J_{{\text{PF}}}}/E} \right) = \frac{{q\sqrt {{{qE} \mathord{\left/ {\vphantom {{qE} {\pi {\varepsilon _{{\text{s(h)}}}}{\varepsilon _0}}}} \right. } {\pi {\varepsilon _{{\text{s(h)}}}}{\varepsilon _0}}}} }}{{kT}}\sqrt E - \frac{{q{\varphi _{\text{t}}}}}{{kT}} + {\rm{ln}} C \\ = m(T)\sqrt E - \frac{{q{\varphi _{\text{t}}}}}{{kT}} + {\rm{ln}} C, \\ \end{gathered} $  (4)

or

$ \begin{gathered} \ln \left( {{J_{{\text{PF}}}}/E} \right) = - \frac{{q({\varphi _{\text{t}}} - \sqrt {{{qE} \mathord{\left/ {\vphantom {{qE} {\pi {\varepsilon _{{\text{s(h)}}}}{\varepsilon _0}}}} \right. } {\pi {\varepsilon _{{\text{s(h)}}}}{\varepsilon _0}}}} )}}{{kT}} + {\rm{ln}} C \\ = - \frac{{q{\varphi _{{\text{eff}}}}(E)}}{{kT}} + {\rm{ln}} C. \\ \end{gathered} $  (5)

From Eq. (4), ln(JPF/E) should be a linear function of E0.5. Figs. 5(a) and 5(b) show that the experimental plots of ln(J/E) versus E0.5 fit well with the linear dependence at different temperatures for both devices, which proves the possibility of PFE. To further confirm the PFE, the εs(h) in Eq. (4) were extracted from the m(T) at different temperatures, as shown in Fig. 5(c). It is found that the εs(h) extracted at high temperature (T > 400 K) is consistent with the theoretical value of Al 0.25Ga0.75N, which strongly indicates that PFE is the dominant mechanism of IR at high temperature. However, at medium and low temperatures (T < 400 K), the extracted εs(h) is much higher than the theoretical value, indicating that different transport mechanisms dominate in this case.

Fig. 5. (Color online) ln(J/E) versus E0.5 at different temperatures for (a) device A and (b) device B. (c) Extracted εs(h) at different temperatures for both devices.

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To identify the defect origin of the trap-involved PFE, the extraction of t is necessary. According to Eq. (5), ln(JPF/E) should be a linear function of 1/T, and eff(E) can be extracted from the slope. The ln(JPF/E) versus 1000/T plots were shown in Figs. 6(a) and 6(b), it can be seen that the data fit well with straight lines at high temperatures (T > 400 K) with various E. At medium and low temperatures (T < 400 K), the dependence of ln( JPF/E) on temperature is weak, suggesting that the tunneling mechanism dominates in this case, although the data also follows a linear relationship. Then, the extracted E-related eff(E) are shown in Figs. 6(c) and 6(d), and the t obtained by extrapolating eff(E) to zero electric field is 0.65 and 1.02 eV for Ni SBD and TiN SBD, respectively. The value of t in Ni SBD and TiN SBD is respectively consistent with the energy level of threading dislocation (TD)[33, 34] and of N vacancy (VN)[35, 36] in AlGaN. Thus, the PFE from the VN to conduction band and the TD to conduction band may dominate the IR of Ni SBD and TiN SBD at high temperatures, respectively.

Fig. 6. (Color online) ln (J/E) versus 1000/T at various temperatures for (a) device A and (b) device B. Extracted eff(E) at various temperatures for (c) device A and (d) device B.

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The difference in electrically active PFE defects between the two devices can be explained by the N diffusion mechanism based on Fick’s law[12]. A significant difference in concentration or chemical potential between two materials facilitates the diffusion process from the region of high concentration to low concentration. The TiN anode with a high N concentration effectively suppresses the N diffusion process from the AlGaN layer to the Schottky metal, which reduces the VN density in AlGaN, so PFE was dominated by natural defects such as threading dislocations. In contrast, the Ni anode with low N concentration facilitates the N diffusion process from the AlGaN layer to the Schottky metal, increasing the VN density in AlGaN, so the VN dominates the PFE. The N diffusion at Ni/AlGaN interface has been reported in as-fabricated HEMT[37], supporting our finding. Besides, the formation of Ni-nitrides has also been reported by annealing at the temperature of 200 °C[38]. In short, it can be considered that the PFE induced by the high-density VN is responsible for the higher IR in Ni SBD at high temperatures.

5.2. Reverse leakage mechanisms at low temperature

At low temperatures, the weak temperature dependence of IR indicates that the tunneling process dominates. A possibility is Fowler–Nordheim (FN) tunneling, which is weakly related to temperature but strongly related to barrier height and electric field. The FN current density is given as[32]:

$ {J_{{\text{FN}}}} = A{E^2}\exp \left( { - \frac{{8\pi \sqrt {2m_{}^*q} }}{{3hE}}\varphi _{\rm b}^{3/2}} \right), $  (6)

where A = ((q2(m0/m*))/(8πhφb)) is constant, h is Planck’s constant, m0 is the free-electron mass, m* ~ 0.27 is the conduction band effective mass in the barrier layer, estimated by linear interpolation of the values in AlN and GaN[39]. Rearranging Eq. (7), we get

$ {\rm{ln}} ({J_{{\text{FN}}}}/{E^2}) \propto - \frac{{8\pi \sqrt {2m_{}^*q} \varphi _{\rm b}^{3/2}/3h}}{E}, $  (7)

which indicates that the ln(J/E2) versus 1/E plot should follow a linear dependency if FN tunneling process is the dominating mechanism, and the slope should be a weak function of T. Figs 7(a) and 7(b) show that the calculated ln(J/E2) versus 1/E plot is very consistent with FN tunneling characteristics, indicating that FN tunneling dominates the leakage current at low temperatures for both devices. However, it is found that the b of Ni SBD extracted from the slope is much lower than the b of TiN SBD (Fig. 7(c)), which is inconsistent with the difference between the work function of Ni (5.15 eV) and TiN (4.7 eV). Moreover, the CV characteristics (Fig. 4(a)) and the forward I–V characteristics (Figs. 2(c) and 2(d)) show that the b of Ni SBD is higher than that of TiN SBD, which is also contradictory to Fig. 7(c).

Fig. 7. (Color online) ln(J/E2) versus 1/E at low temperature for (a) device A and (b) device B. (c) Extracted b from the slope at various temperatures for both devices. (d) Impact of β on b extracted by FN model for Ni SBD.

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It has been found that the ionization of VN donor can cause the thin surface barrier (TSB) effect[40], thus leading to an increase of E in the barrier layer. Therefore, the low value of b for Ni SBD extracted by the FN model may be attributed to the underestimation of the E. Considering the TSB effect, Eq. (7) should be corrected to:

$ {\rm{ln}} ({J_{{\text{FN}}}}/{(\beta E)^2}) \propto - \frac{{8\pi \sqrt {2m_{}^*q} \varphi _{\rm b}^{3/2}/3h}}{{\beta E}} ,$  (8)

where β is the electric field enhancement coefficient defined as the ratio of the actual electric field to the theoretical electric field. According to Eq. (8), the slope in Figs. 7(a) and 7(b) can be expressed as:

$ {\text{slope}} = - \frac{{8\pi \sqrt {2m_{}^*q} \varphi _{\rm b}^{3/2}/3h}}{\beta }. $  (9)

Using Eq. (9), the relationship between β and b for Ni SBD can be obtained, as shown in Fig. 7(d).

Fig. 7(d) shows that the extracted b increases as β increases. When the β is between 1.5 and 2, the b0 for Ni SBD is close to the value reported in the literature[11, 27]. The large β indicates that the strong TSB effect exists in Ni SBD, which leads to the high IR for Ni SBD compared to the TiN SBD at low temperatures. The TSB effect in Ni SBD is mostly caused by the donor-like VN defect, which is introduced by the N diffusion process. For TiN SBD, the N diffusion process is effectively suppressed, and thus the extracted value of b by the FN model is relatively normal (Fig. 7(c)).

Curiously, for reverse characteristics, the b of TiN is higher than Ni (Fig. 7(c)), while for the forward characteristics, the b of TiN is lower than Ni (Fig. 2(d)). This phenomenon can be explained as follows: at high forward bias, the quasi-Fermi level in semiconductor shifts above the level of VN donor, and the ionized VN will be neutralized, so the TSB effect in Ni/AlGaN will be weakened. Besides, the E in AlGaN barrier is decreased with forward bias, and the thermal emission related mechanism will gradually dominate the current transport, so the TSB effect has less influence on the forward characteristics.

In short, the carrier transport mechanisms of both devices are shown by the schematic energy band diagram in Fig. 8. According to the leakage mechanism analysis, it can be concluded that the main reason for the high IR of Ni SBD is that the high-density VN induced by the N diffusion enhances the PFE and FN tunneling. TiN SBD with high N concentrations is efficient to mitigate the N diffusion, thus obtaining a better trade-off between good VT and IR. Therefore, to reduce VN density and improve SBD performance, conductive metal nitride, such as TiN, TaN, and WNx, is more suitable as Schottky metal for AlGaN/GaN SBD.

Fig. 8. (Color online) Schematic band diagram of carrier transport mechanisms at reverse bias for TiN SBD and Ni/Au SBD.

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6. Conclusion

In summary, we have comparatively investigated the reverse leakage mechanisms of recess-free AlGaN/GaN SBD with Ni/Au and TiN anodes. It is found that TiN SBD exhibits lower VT and IR than Ni SBD. For forward characteristics, the current of both devices is dominated by the TE mechanism. It is found that both b and n show a strong temperature dependence. Such temperature dependence can be explained by the presence of inhomogeneity of the Schottky barrier height, which is proved by the linear relationship between b and n. For reverse characteristics, two types of mechanisms work at different temperature ranges. At high temperatures, the IR for Ni/Au anode is dominated by the PFE from VN to the conduction band, while the IR for TiN anode is dominated by the PFE from TD to the conduction band. The decrease of VN can explain the observed low IR in TiN SBD due to the suppression of N diffusion. At low temperatures, the IR of both devices is dominated by FN tunneling. By modifying the FN model, we concluded that the electric field of the barrier layer in the Ni SBD is higher than in the TiN SBD, thus leading to the relatively higher FN leakage in Ni SBD. The TSB effect caused by VN may be responsible for the increased electric field of the barrier layer in Ni SBD.

7 Acknowledgements

This work was supported in part by Natural Science Foundation of China (Grant No. 61804172), in part by GuangDong Province Key Technologies Research and Development Program (No. 2019B010128001) and in part by the Youth Innovation Promotion Association of CAS.

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Hao Wu, Xuanwu Kang, Yingkui Zheng, Ke Wei, Lin Zhang, Xinyu Liu, Guoqi Zhang. Optimization of recess-free AlGaN/GaN Schottky barrier diode by TiN anode and current transport mechanism analysis[J]. Journal of Semiconductors, 2022, 43(6): 062803.

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