The research of astrophysics and earth science is the primary motivator for the development of high output power and high efficiency terahertz sources.^[1,2] Multiplier based on solid-state sources is the critical technology for generating power at terahertz frequency.^[3,4] GaAs Schottky diodes are the core device of high output power multipliers due to its large electron mobility and uniform epitaxial material quality.^[5,6]

In recent research, the edge field effect on the interface between Schottky metal and semiconductor causes a lower breakdown voltage and a larger leakage current.^[7,8] This can induce lower power handling capability and poor conversion efficiency of multipliers.^[9] A novel Schottky metal–brim structure (SMB) can effectively eliminate the accumulation of charge caused by edge-field effect, which will increase the output power of multipliers.

In this study, GaAs planner SBV with Schottky metal–brim structure (SMB-SBV) which is shown in Fig. 1(a) is fabricated and experimentally studied for the first time. Compared with normal SBV (normal-SBV), the SMB-SBV shows better breakdown characteristics (19.6 % @ Schottky diameter is 3 μm). Combining the electric field and current density distribution extracted from simulations, the physical mechanism is discussed and explained.

Fig. 1

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	Fig. 1 Cross sectional schematic of GaAs SBV (a) Schottky metal–brim structure and (b) normal structure.

The basic structures of the GaAs planner SBV are shown in Fig. 1. The epitaxial wafer is depicted as follows: a 30-μm-thick semi-insulating GaAs substrate serves as a supporting structure for the SBV devices; an epitaxial lattice-matched GaAs material including a 1.5-μm-thick heavily doped n-type GaAs layer (i.e., n⁺ GaAs buffer layer) and a 0.3-μm-thick lightly doped n-type GaAs layer (n-GaAs epi-layer), the doping concentrations of the two layers are ∼ 10¹⁸ cm⁻³ and ∼ 10¹⁷ cm⁻³, respectively.

The fabrications start with mesa isolation using wet etching.^[10] Ti/Au metal-layers are deposited by E-beam evaporation as anode Schottky contacts. Ni/Ge/Au cathode ohmic contacts are then deposited. After metal contacts are finished, SMB structure is formed by wet etching aided a certain power ultrasound. Then, samples are passivated with 300-nm SiO₂ and the dielectric layer existing on top of the metal contacts is removed by SF₆ dry etching. The process is completed with interconnection metal electroplate to form gentle air bridge. Based on this SMB-SBV, the 225-GHz millimeter-wave integrated circuits (MMIC) are also produced. Figure 2(a) shows three anodes series GaAs SBVs and figure 2(b) shows the details of the SMB structure. The diameter of Schottky anodes in this paper is 3 μm. A Schottky anode with a diameter of 40 μm is also designed to analyze the physical mechanism of the SMB structure.

Fig. 2

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	Fig. 2 Scanning electron microscope (SEM) photographs of (a) the fabricated GaAs SBVs, (b) Schottky metal–brim structure details.

The electrical simulations described in this paper are performed using Sentaurus TCAD tools.^[11] The simulations with metal–interfacial layer–semiconductor (MIS) model which is shown in Fig. 3 are based on the literature in Refs. [12,15]. In the simulation, the two-dimensional (2D) structures are depicted in Fig. 1 and the doping levels are described above. Adequate numerical convergence is also reached by an optimized meshing, with nanometer grid spacing for the key electrical layers and their interfaces. The model is based on the basic drift-diffusion and Poisson equations for direct current (DC), breakdown characteristics, and small signal simulation.^[13,14] The drift-diffusion for electrons and Poisson equations are given respectively by

Fig. 3

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	Fig. 3 Energy band diagram of MIS model.

This basic drift-diffusion model includes Fermi statistics, mobility with doping dependence and high velocity saturation model, carrier generation, and recombination, non-local tunneling, and band to band tunneling.

The forward I–V and reverse breakdown characteristics are measured with Agilent 4200 semiconductor parameter analyzer at room temperature. S-parameters is also measured by Agilent E8363B network analyzer to extract depletion layer capacitance. The test frequency range is 0.1 GHz–40 GHz. According to Taking’s studies in Ref. [16] the SBV is embedded in the ground–signal–ground (GSG) coplanar waveguide to measure the S-parameters of the coplanar waveguide. Three auxiliary structures, diode-open, diode-short, and pin-open are used to obtain the junction capacitance at 0-V bias (C_j0) by using the single-port open-short de-embedding method. Figure 4 shows the equivalent circuits of the three structures based on the work in Ref. [17]. Figure 5 shows the photograph and equivalent circuit of the actual SBV. The pad-to-pad capacitance C_pp, the finger-to-pad capacitance C_fp, the finger inductance L_f, and the finger resistance R_f are the parasitic parameters. C_j is the intrinsic voltage-dependent junction capacitance. R_s is the series resistance.

Fig. 4

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	Fig. 4 The equivalent circuit of (a) diode-open, (b) diode-short, and (c) pin-open structure.

Fig. 5

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	Fig. 5 (a) Photograph and (b) equivalent circuit of the actual SBV.

Under small-signal conditions (at an input power of –20 dBm), the magnitude and phase of the S-parameters are measured, which could directly convert to Y-parameters. The steps of extracting C_j0 are as follows: (i)

In diode-open structure, C_pp is determined by

According to the thermal electron emission model,^[15] the current–voltage curve across the Schottky diode can be described by

Figure 6 shows the forward I–V characteristics in a log scale and the turn-on voltage is 0.68 V as measured at a current of 1 μA. The ideal factor is a figure of merit that describes an unnecessary current path except the thermionic electron emission current, which is induced by interfacial disorder, interface state densities, electron tunneling current.^[19–21] The ideal factor is defined as

Fig. 6

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	Fig. 6 Forward I–V characteristics with different structures in log scale; full line: measured; dashed line: simulated. The inset shows the I–V in linear scale.

The breakdown characteristics with different structures of SBV are shown in Fig. 7. In order to investigate the improvements in reverse breakdown characteristics of SMB-SBV, a current of 1 μA for defining the breakdown voltage of SBV is used.^[23] The GaAs SBV with different structures has the breakdown voltage of the order of –8.75 V for SMB structure and –7.31 V for normal one, respectively. The results show that SMB-SBV has the advantage of high breakdown voltage. The leakage current caused by electron tunneling is significantly lower than that of normal-SBV. Simulated results based on the MIS model and measurement results show a good agreement as shown in Figs. 6 and 7. So, multipliers based on SMB-SBV will have larger power handling capability and better power conversion efficiency.

Fig. 7

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	Fig. 7 Variations of breakdown characteristics with different structures.

Furthermore, the breakdown characteristics of SBVs with a Schottky junction diameter of 40 μm are shown in Fig. 8. Due to the large difference of anode area, the normalized current density is used to compare the influence of the edge electric field effect of different areas on the breakdown characteristics. The current value corresponding to the same current density is marked in Fig. 8. It shows that the breakdown voltage is –7.29 V for SMB structure and –6.75 V for normal structure at the same order of leakage current density. It reveals that the improvements in reverse breakdown characteristics of SMB structure obviously decreases with the increasing of anode area. Accordingly, SMB structure might increase the utility of SBV for THz multipliers where the anode area must be decreased in order to achieve greater application frequency.

Fig. 8

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	Fig. 8 Variations of breakdown characteristics with different structures at Schottky diameter of 40 μm. The inset shows the I–V curve in linear scale.

The measurement and fitting data of the S-parameters are shown in Fig. 9, and they are in good agreement. The extracted junction capacitance at 0-V bias (C_j0) is 13.7 fF for SMB structure and 16.5 fF for normal structure, respectively. The C_j0 of SMB-SBV is less than that of normal structure, which shows a larger capacitance modulation ratio (C_max/C_min). The SMB-SBV shows better conversion efficiency because of its uniform capacitance modulation ration.

Fig. 9

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	Fig. 9 Measurement and fitting data of S21 at the frequency range of 0.1 GHz–40 GHz.

Table 1.

Table 1.

Table 1. Parameters of SBVs with different structures. . Parameters SMB-SBV Normal-SBV Diameter/μm 3 3 Ideal factor 1.14 1.15 Breakdown voltage/V –8.75 –7.31 Cj0 by S-parameter/fF 13.7 16.5 Rs/Ω {5.4} 5.3 Reverse saturation current/A 1.14 × 10–13 1.75 × 10–13 Junction potential/V 0.69 0.69

Table 1. Parameters of SBVs with different structures. .

In order to investigate the physical mechanism of SMB with excellent reverse breakdown characteristics, simulations are carried out to precisely extract the distribution of electric field and space charge area at the same reverse bias voltage. The difference between the depletion layer region and the electric field distribution of these two structures is shown in Fig. 10. At the edge of the anode electrode, the maximum electric field of normal structure is approximately 4.5 times of the SMB structure one, which suffers from the electric field crowding near the metal edge. This would induce local breakdown in advance, especially happened between the anode and cathode region.^[24] From Fig. 11, it is obvious that the leakage current mainly comes from the edge of the depletion layer. The normal SBV has a narrow depletion layer near the edge of the anode electrode. It causes an increase in C_j0 calculated from the formula as a function of C_j0

Fig. 10

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	Fig. 10 Depletion layer area (white line) and electric field distribution near the edge of the anode metal of (a) normal-SBV, (b) SMB-SBV, (c) cross-section of the electric field distribution.

Fig. 11

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	Fig. 11 Distribution of leakage current density across the anode depletion layer. The abscissa corresponds to the horizontal depletion layer distribution (x axis in Fig. 10) and the dotted line is the center of the anode.

The SMB structure provides electric field relief and uniform depletion layer distribution, reducing the magnitude of the electric field. The SMB-SBV shows excellent electrical performance because it completely eliminates the accumulation of charge at Schottky electrode edge.

In summary, the GaAs SBV with SMB structure is fabricated and systemically evaluated. With SMB structure, the reverse breakdown voltage increases from –7.31 V to –8.75 V. The C_j0 is reduced from 16.5 fF to 13.7 fF due to SMB structure ignoring the fringe capacitance factor. SMB structure would not cause the degradation of other SBV characteristics, such as ideal factor, reverse saturation current, and series resistor. The distributions of the leakage current and the electric field are extracted by Sentaurus TCAD tools, which reveal that SMB structure can eliminate the accumulation of charge at Schottky electrode edge induced by edge-field effect. The reverse breakdown voltage will be larger if SMB etching process is optimized. Through a series of comparisons, the GaAs SBV with SMB structure shows significantly better electrical characteristics including larger breakdown voltage, lower leakage current, and larger capacitance modulation ratio, which is beneficial to high power and high efficiency multipliers application.