GaN-based Schottky barrier diodes (SBDs) exhibit outstanding power handling capabilities in power electronic applications [1, 2] and multipliers [3] due to the physical properties of wide bandgap and high breakdown electric field [4–6]. In particular, AlGaN/GaN SBDs exhibit excellent characteristics of high electron mobility and high electron density due to the 2D electron gas (2DEG) [7, 8], offering wide applications for high-frequency and high-power devices [9, 10]. SBD is one of the most essential components in microwave power transmission systems [3, 10–12]. However, the high-frequency performance of SBD requires low turn-on voltage (V _on ) to reduce conduction loss, and high cut-off frequency to improve operating frequency. In addition, the cut-off frequency of SBD is limited by the product of capacitance and the on-resistance (R _on ), hence reducing the anode size cannot effectively improve the cut-off frequency. Conventional planar AlGaN/GaN SBDs have undesirable high V _on and large capacitance. The metals (TiN, Mo, and W) of the low Schottky barrier can reduce V _on , which are accompanied by a higher reverse leakage current (J _r ) [13–15]. The recessed anode structure is an effective solution to avoid these two shortages because the 2DEG directly contacts the anode metal [16, 17]. Even so, this design still has a large capacitance due to the parallel plate capacitor caused by the anode field plate (FP) and 2DEG [18, 19]. Compared with the SBD of the recessed anode structure, the capacitance of the lateral SBD can be further reduced. Moreover, the lateral GaN-based SBDs also have low V _on because the carriers directly contact the anode metal. Many researchers have reported studies about the transport mechanism [16, 20, 21], Schottky barrier heights (SBHs) [22, 23], metal-induced gap states (MIGS) [24–26], and barrier inhomogeneity [27–30] on planar n-GaN SBDs or AlGaN/GaN recessed-SBDs. Nevertheless, the electrical properties in lateral GaN-based SBDs are rarely reported.

In this paper, the strategy of lateral GaN-based SBDs was used to eliminate the FP capacitance and reduce V _on without sacrificing the performance of R _on and reverse leakage. The simulated results show the characteristics of the energy band and carrier distribution. The analysis of electrical properties in lateral GaN-based SBDs was described by current-voltage (I-V) and capacitance-voltage (C-V) characteristics. According to the results of temperature-dependent I-V measurements, the barrier inhomogeneity of the lateral SBDs is more intensive than that of planar SBDs, which is attributed to etching damage. In addition, anode resistance (R _A ) was present, which is possibly due to the worsening anode contacts by MIGS.

The lateral GaN-based SBDs in this study were fabricated on n-GaN and AlGaN/GaN heterostructure and were shown in Figure 1A. The wafers were grown by metal-organic chemical vapor deposition (MOCVD) on 2-inch c-sapphire substrates. The epitaxial layers consist of a ∼40 nm-thick AlN, a 4 μm-thick high resistive (HR) GaN buffer layer, and a 300 nm-thick i-GaN. Based on this template, the n-GaN and AlGaN/GaN heterostructure continued to grow 500 nm-thick n-GaN and 24/0.7 nm-thick Al_0.25Ga_0.75N/AlN, respectively. The electron mobility, electron density, and channel thickness of n-GaN and AlGaN/GaN are summarized in Table 1. The ring cathode metals (Ti/Al/Ni/Au = 20/150/50/80 nm) were deposited on the wafer surface by E-beam evaporation and annealed at 850°C for 30 s in N₂ ambient. Using the transfer length method (TLM), the ohmic contact resistances (R _C ) of n-GaN and AlGaN/GaN are 0.65 Ω mm and 1.13 Ω mm, respectively. The anode metals (Ni/Au = 50/80 nm) were deposited on the wafer surface. Figure 1B shows the cross-sectional schematic of the fabricated planar SBD. For lateral SBD, the anode region was etched to the HR GaN buffer layer by inductively coupled plasma (ICP), followed by direct deposition of anode metal to the sidewall, as shown in Figure 1C. The diameter of the circular anode metal was 200 µm. The distance between the anode metal and the ring cathode metal was 40 µm. The I-V and C-V characteristics were measured by the Agilent B1500A at room temperature.

According to the n-GaN and AlGaN/GaN SBDs structures, the characteristics of the energy band and carrier distribution are simulated by Synopsys’ Sentaurus technology computer-aided design (TCAD), and the results are shown in Figure 2. The depletion directions of the planar and lateral SBDs are perpendicular and parallel to the (0,001) plane, respectively. The energy band and carrier distribution are very similar between planar and lateral n-GaN SBDs because both are Ni/n-GaN contacts. The simulation results show that the sample with SBH of about 1.18 eV has a depletion width of about 44.5 nm for n-GaN with doping concentration of 3.4×10¹⁷ cm⁻³, as shown in Figures 2A,B. However, the energy band structures of the planar and lateral AlGaN/GaN SBDs are very different, as shown in Figures 2C,D. The energy band structure of the planar AlGaN/GaN SBD displays that anode metal cannot deplete the 2DEG channel at 0 V, and its SBH is 1.60 eV. The 2DEG is distributed in the i-GaN layer of the AlGaN/GaN heterostructure, which is only a ∼2.8 nm-thick sheet channel. The simulation results of the energy band and carrier distribution of the lateral AlGaN/GaN SBD are shown in Figure 2D. For lateral AlGaN/GaN SBD, the lateral contact of Ni-2DEG can be treated as SBD of a heavily doped n⁺-GaN sheet channel. The Ni-2DEG forms a SBH of 1.08 eV, which only depletes 4.5 nm 2DEG.

As shown in Figures 3A–H, we measured the I-V and C-V characteristics of each SBD at different temperatures from 298 to 504 K. The I-V and C-V curves conform to traditional SBD depletion characteristics for planar n-GaN SBD, lateral n-GaN SBD, and lateral AlGaN/GaN SBD. However, the I-V and C-V curves of the planar AlGaN/GaN SBD are different from the characteristics of traditional SBD depletion. Because anode metal and 2DEG channel are separated by AlGaN barrier layer. The V _on of the planar AlGaN/GaN SBD is as high as 1.64 V since the device conduction requires electrons to pass through the AlGaN barrier layer, while the other three devices show very low V _on because electrons only need to overcome the barriers to flow to the anode. The J _r of the planar AlGaN/GaN SBD reaches saturation at pinch-off voltage (-4.2 V) due to 2DEG depletion, as shown in Figure 3C. The capacitance characteristic of planar AlGaN/GaN SBD is fitted to the parallel plate capacitor model of the anode metal and 2DEG [18]. At lower reverse bias voltage, the planar AlGaN/GaN SBD maintains a stable capacitance value due to the characteristics of parallel plate capacitors. With increasing reverse bias voltage, the 2DEG is gradually depleted, which causes the capacitance to drop rapidly due to the disappearance of the plate capacitor. At higher reverse bias voltage, the capacitance tends to zero, the 2DEG is completely depleted, as shown in Figure 3G. The capacitance approaching zero also shows the HR characteristics of the buffer layer. The n-GaN and AlGaN/GaN SBDs of different FP widths are measured, the values of zero bias capacitance are linear with the area of the FP, so the lateral SBD without FP can effectively eliminate the FP capacitance. Compared with the planar SBDs, the capacitances of the lateral SBDs are reduced by two orders of magnitude without sacrificing the performance of the R _on and J _r . This is attributed to two characteristics of lateral SBDs: the small anode area and the elimination of spreading resistance under the anode metal. The capacitances of lateral n-GaN SBD and lateral AlGaN/GaN SBD are 1.35 pF/mm and 0.70 pF/mm, respectively.

In order to extract the SBH Φ _B , the ideality factor n, and the R _on from the I-V curve, the thermionic emission (TE) can be described as the major forward current contribution [28, 31], I = A A * T 2 exp ( − q Φ B k T ) { exp [ q ( V − I R o n ) n k T ] − 1 } (1) where A is the area of the anode metal, A ^* is the effective Richardson constant (226.4 Acm⁻²K⁻²), k is the Boltzmann constant, and T is the temperature. From the I-V and C-V curves at room temperature, the device parameters of each SBD are summarized in Table 2. The SBHs extracted by the I-V characteristics are lower than the simulation results, which may be due to electron tunneling and inhomogeneity SBHs [27–29]. The ideality factors of each SBD are larger than 1, which indicates that in addition to TE, other transport mechanisms include thermionic field emission (TFE) and field emission (FE) [32, 33]. For planar AlGaN/GaN SBD, the ideality factor of ∼2 illustrates the existence of various transport types other than TE, such as trap-assisted tunneling (TAT) and recombination [34].

The ideality factors of each SBD are extracted from the I-V curve at different temperatures and shown in Figure 4A, which are larger than 1. Hence, the transport mechanism includes other mechanisms other than TE. What’s more, TE is more sensitive to temperature, and the ideality factors are positively related to 1,000/T due to the domination of the TE mechanism at high temperatures.

Figure 4B shows SBHs for each SBD at temperatures ranging from 298 to 504 K. The SBHs of each SBD were extracted from the forward I-V curves by Eq. 1. The SBHs decrease with increasing 1,000/T due to the inhomogeneous nature of the barrier. The electrons only overcome lower barriers at lower temperatures. With increasing temperature, the energy of the electrons increases to overcome the higher barriers at the Ni/GaN interface, where the higher SBHs were exhibited. The inhomogeneous SBHs can be expressed as a Gaussian distribution with a mean barrier height Φ B 0 ¯ and a standard deviation σ, as shown in Eq. 2 [30]. Φ B ( T ) = Φ B 0 ¯ − σ 2 2 k T (2)

In GaN-based SBDs, inhomogeneous SBHs are attributed to multiple factors such as material defects, interface dipole layers, morphological features, and surface Fermi level pinning [27–30, 35]. According to the linear fits of the SBHs vs 1,000/T curve, the values of σ were estimated to be 145, 199, 200 and 239 meV for planar n-GaN SBD, lateral n-GaN SBD, planar AlGaN/GaN SBD and lateral AlGaN/GaN SBD, respectively. Compared to the planar SBDs, the lateral SBDs exhibit larger σ may be due to the etching damage caused by ICP. The lateral AlGaN/GaN SBD has the largest σ value, which is attributed to the higher trap concentration at the contact of anode metal and 2DEG. Thermal annealing is an effective step to reduce the damages induced by ICP etching, and wet treatment methods are used to further treat etching damage, such as hydroxide (KOH) and (NH₄)₂S [36, 37].

At the voltage of -15 V, the J _r vs 1,000/T (J _r -T) characteristics are shown in Figure 4C, which appears linear on the Arrhenius plot. This suggests that the thermal activation mechanism has a dependence on the exp ⁡ ⁡ ( − E A / k T ) function, where E _A is the activation energy. Therefore, a possible reverse leakage mechanism is defined as TAT, which is associated with thermally activated current [38]. The values of E _A were estimated to be 215, 214, 387 and 156 meV for planar n-GaN SBD, lateral n-GaN SBD, planar AlGaN/GaN SBD, and lateral AlGaN/GaN SBD, respectively, according to linear fitting J _r -T data. In addition, the J _r is related to the trap concentration on the interface of anode metal and semiconductor.

The withstand voltage of SBD is a very important parameter for power electronic applications. The reverse breakdown voltages of these SBDs are shown in Figure 5. The planar n-GaN SBD has no breakdown phenomenon at -28.5 V. The lateral n-GaN SBD shows a relatively low leakage current, which is breakdown at -73 V. Compared with n-GaN SBD, the AlGaN/GaN SBDs exhibit a breakdown voltage of more than 2300 V, which is mainly attributed to the undoped AlGaN/GaN materials.

For planar and lateral SBDs, the R _on consists of the R _C , channel resistance (R _channel ) and anode resistance (R _A ). Using the TLM, we calculated the R _C and R _channel for each SBD. Unitized R _on distributions of each SBD are summarized in Figure 6A. The experimental results show that the R _A of lateral n-GaN SBD and lateral AlGaN/GaN SBD are 10.5 Ω mm and 9.2 Ω mm, respectively. The R _A are much larger than the R _C in each SBD, which is probably due to the worsening anode contacts by MIGS. The energy diagram of n-GaN SBD with MIGS is displayed in Figure 6B. The planar AlGaN/GaN SBD exhibits larger R _A due to the electrons passing through the AlGaN barrier layer under the anode metal [39].

In summary, the lateral SBDs based on n-GaN and AlGaN/GaN heterostructure were fabricated, and the temperature-dependent I-V and C-V characteristics were used to evaluate the electrical properties, such as transport mechanisms of the forward and reverse current, SBHs, R _on distribution, MIGS, inhomogeneous SBHs, etc. For the planar and lateral n-GaN SBDs, the values of the V _on are similar. However, compared with conventional planar AlGaN/GaN SBD, the V _on of lateral AlGaN/GaN SBD is reduced from 1.64 V to 0.87 V. The capacitances of lateral n-GaN SBD and lateral AlGaN/GaN SBD are 1.35 pF/mm and 0.70 pF/mm, respectively, which are much smaller than the capacitance of planar SBDs. According to the analysis of inhomogeneous SBHs, etching damage degrades the uniformity of SBHs. Compared with the breakdown voltage of 73 V in the lateral n-GaN SBD, the lateral AlGaN/GaN SBDs exhibit a breakdown voltage of 2322 V. In addition, the R _A is much greater than the R _C due to worsening anode contact by MIGS from the analysis of the R _on distributions of each SBD.