High-Power Wire Bonded GaN Rectifier for Wireless Power Transmission

A novel wire bonded GaN rectifier for high-power wireless power transfer (WPT) applications is proposed. The low breakdown voltage in silicon Schottky diodes limits the high-power operations of microwave rectifier. The proposed microwave rectifier consists of a high breakdown voltage GaN rectifying element for high-power operation and a novel low loss impedance matching technique for high efficiency performance. Wire bonding method is adopted to provide electrical connection between GaN chip and board which induces undesirable inductance. In order to realize high efficiency performance, an impedance matching network is proposed to exploit the unavoidable inductance along with a single shunt capacitor, resulting in a low loss matching circuit to achieve a compact high-power rectifier. The fabricated GaN rectifier exhibits a good performance in the high-power region and can withstand up to 39 dBm input power before reaching the breakdown limit at the operating frequency of 0.915 GHz and load resistance of $100~\Omega $ . It has a compact size and exhibits high efficiency performance in high-power region (achieved a maximum efficiency of 61.2% at 39 dBm), making it suitable for high-power applications like future unmanned intelligent devices and WPT in space applications.


I. INTRODUCTION
Wireless power transmission has gained much attention in the last few years, due to new charging requirements and the rapid advancement of electronic devices and sensor nodes [1]. Highly efficient non-radiative near field techniques are broadly accepted by industry for wireless power transfer in a limited distance with large coils [2]. However, radiative microwave power transmission is becoming a promising technology for remotely powering devices which are not easily accessible over a long distance [3]. Forthcoming unmanned and intelligent devices demand a wireless power technique to supply actuators or wireless sensors [4]. Input power required for these electronic devices is in the range of watts instead of micro and milliwatts for IoT sensors. Power transmission for low power IoT sensors (−20 to 20 dBm) are extensively studied and demonstrated in literatures using Si and The associate editor coordinating the review of this manuscript and approving it for publication was Tariq Masood .
GaAs commercial Schottky diodes [5]. But very few studies have been reported on the wireless power transmission for high-power applications. Therefore, in order to ensure the reliability of high-power WPT and to handle large quantity of power, a high-power microwave rectifier is an essential circuit element.
A conventional microwave rectifier mainly consists of rectifying elements, impedance matching network, DC pass filter and load resistor [6]. Rectifying elements are usually arranged in single series, shunt, voltage doubler or bridge configurations [7]. Efficiency of the microwave rectifier primarily depends on the rectifying element and the impedance matching network. Conventional rectifier usually utilize Si based Schottky diodes for rectification. Conversion efficiency in high-power region is restricted by the low breakdown voltage in silicon Schottky diodes [8]. Large voltage and current swings arise in Schottky diodes due to the high RF (radio frequency) input power. This triggers a breakdown in the metal-semiconductor junction and can permanently VOLUME 8, 2020 This work is licensed under a Creative Commons Attribution 4.0 License. For more information, see https://creativecommons.org/licenses/by/4.0/ damage a diode. Thus, Si Schottky diode-based rectifiers usually operate below watt-level input power. In order to prevent the breakdown in low-power capability diodes for realizing high-power rectifiers, power divider circuits are utilized to split the high input power into several diode circuits [9]. In [10], a stepped impedance transformer and coupled lines are employed to develop a planar Wilkinson power divider. However, these circuits based on transmission lines are complex in structure and large in size. To solve the low breakdown voltage problem of the Schottky diodes in the zero-bias and full-wave bridge rectifiers, the one-tofour series-dividing transformer and parallel-dividing transformer for impedance matching and power dividing are employed in [11]. Diode arrays were also used to increase the power capacity of microwave rectifiers [12]. However, these approaches introduce additional size and losses, as well as increase circuit complexity.
In this paper, a GaN Schottky diode based high-power microwave rectifier is proposed. High current carrying ability of GaN is utilized to realize the rectifier. A GaN HEMT is turned to a Schottky diode by shorting the source and drain to make the cathode and gate as the anode. Circuit model of GaN diode is estimated for better simulation of the rectifier. A shunt diode topology is adopted in this highpower rectifier to achieve maximum efficiency. Impedance matching is achieved in this rectifier by using a single shunt capacitor along with the bond-wire interconnects. The length of interconnects is properly designed to attain the impedance matching with minimum number of lumped elements, to minimize the losses. Due to the low loss rectifying element and efficient impedance matching technique, the high-power GaN rectifier achieves a peak efficiency of 61.2 % at 39 dBm. Maximum output voltage of 24.5 V is observed at 0.915 GHz with an input power of 40 dBm.
The organization of this paper as follows. The rectifying element selection, wire bonding interconnects and the rectifier design are presented in Section II. The proposed highpower rectifier performance is illustrated and discussed in Section III. Comparison with related high-power rectifiers are also presented in this section. Finally, conclusions are drawn in Section IV.

II. RECTIFIER DESIGN A. RECTIFYING ELEMENT SELECTION
The difficulty in development of high-power rectifier is the limitations of the performance of the rectifying element. A high breakdown voltage and current carrying capabilities of the rectifying element are necessary factors in the design of a high-power rectifier. Parameters of commercially available diodes are summarized in Table 1 for comparison. It can be observed that Si and GaAs based commercially available diodes has low reverse breakdown voltage (≤15 V) and power capabilities, which limits their applicability in high-power applications.
A promising solution to design the high-power rectifier is to use GaN technology. GaN has attracted much attention as a material suitable for operating at microwave frequencies due to the high-power and high conversion efficiency of highpower amplifiers (HPAs) and rectifiers [13]. Properties like wide bandgap, high electron saturation velocity, high critical electric field, and high carrier density in 2-D electron gas (2DEG) channel in the device and high breakdown voltage making it suitable for wireless power transmission applications [14]. GaN Schottky barrier is better suited than Si and GaAs Schottky diodes for increasing the handling power of the high-power wireless transmission.
Based on the above-mentioned properties, a GaN HEMT is proposed for wireless power transfer application. A Schottky barrier diode is implemented by shorting the source and drain of the HEMT to serve as the cathode. Layout of the GaN HEMT is shown in Fig. 1. To design the rectifier circuit, the GaN Schottky diode is measured individually to achieve the diode model for ADS simulation. I-V (current-voltage) and C-V (capacitance-voltage) measurements are carried out and curve fitting is performed for estimating the diode parameters. Circuit model of the GaN diode is shown in Fig. 2. The measured breakdown voltage of the GaN diode Bv is 76 V, which makes it suitable for high-power applications. Due to the large bandgap of GaN material, forward voltage Vf is observed as 1.1 V. Instead of using GaN, a low forward voltage of Si Schottky diodes with similar series resistance and breakdown voltage can only perform at low frequencies.
Due to the larger size of the pads in the GaN chip, the parasitic elements have high values. However, proper impedance matching can cancel out the reactive parasitic elements of the physical diode without affecting the rectifying efficiency.

B. WIRE BONDNG INTERCONNECTS
For realizing a high-power and high efficiency rectifier, it is necessary to eliminate the packaging losses of the semiconductor devices. Therefore, a chip on board method is adopted in this design by directly using the GaN chip in the rectifier circuit. Chip on board method helps to eliminate the packaging of semiconductor devices and thus, the final product can be more compact, lighter, and less costly.
Flip-chip and wire bonding are widely adopted techniques to interconnect chip and PCB board [15]. In flip-chip method, small solder bumps are employed in PCB board. Then the chip is inverted so that the metallized side facing the circuit  board. Electrical connection is realized by reflow soldering process. Flip chip interconnects offer lower inductance however have some drawbacks including poor thermal dissipation to the PCB, and electromagnetic coupling to the PCB that may not have been taken into account during the chip design. In wire bonding, a small wire lead is used to connect the chip and board. This process is quite similar to the way that an integrated circuit is connected to its lead frame, but instead the chip is directly connected to circuit board. For higher frequencies this interconnect leads to higher inductance values which makes the impedance matching difficult [16]. In this GaN rectenna design, wire bond interconnection is employed for connecting GaN chip on the circuit board.
Wire bonding interconnect can be modeled by using an equivalent circuit as shown in Fig. 3. Equivalent circuit mainly consists of a series inductance (L w ) and series resistance (R w ). Input and output capacitances (C w ) depict the electrical representation for the coupling to the substrate. As the operating frequency of the proposed design is 0.915 GHz, the major factor is the series inductance. In order to gain a better understand of bond wire, the performance of the bond wire is analyzed using ADS software. Diameter of the bond wires in this design is considered as 30 um. Fig. 4 illustrates the variation in inductance and resistance value   with respect to the bond wire length. It can be observed that the length of the bond wire is proportional to the inductance and resistance. Inductance is varying from 0.15 to 3.57 nH at 0.915 GHz whereas resistance varies from 0.055 to 0.9 ohms by changing bond wire length from 250 um to 5000 um. The effect of input and output bond wire capacitance is considerably negligible at this frequency.

C. IMPEDANCE MATCHING AND RECTIFIER DESIGN
The topology of the rectifier is a crucial factor in determining the output power and efficiency of a rectifier. For maximum conversion efficiency, a shunt diode topology is selected in this design. A conventional shunt diode configuration is depicted in Fig. 5.
RF power is initially passed through a matching network for delivering the maximum power. The negative half cycle of the wave is rectified by the shunt diode D 1 and the energy is stored in C 1 . C 2 along with load resistor R L acts as a DC pass filter to suppress the ripples from the rectified voltage.  The impedance matching section in a rectifier has an important role in determining the losses of a rectifier. Due to the non-linearity of the Schottky diode, complex impedance matching circuits with lumped elements may act as the main source of loss and could affect the rectifier performance. To implement a high-power rectenna with high efficiency, it is necessary to reduce the losses accompanying the matching network as low as possible. Therefore, designing the impedance matching network with minimum components is essential to realize a high efficiency rectifier.
For providing the electrical connection between chip and circuit board, wire bond is necessary. Hence, the proposed solution is to exploit the undesirable inductance effects of wire bonding in order to ease the impedance matching process. By properly estimating the length required for impedance matching, it is possible to avoid other lossy lumped inductors. Therefore, the unavoidable interconnecting bond wires for GaN chip can be effectively use with a capacitor for impedance matching.
The schematic of the proposed high-power rectifier with a wire-bonding-based impedance matching network is shown in Fig. 6. The circuit includes the rectifying GaN HEMT based diode, matching capacitor C M , energy storage element C 1 , bond wires for attaching the chip to PCB, DC pass filter using capacitor C 2 and output load resistor. Instead of the conventional T-shaped or π -shaped impedance matching networks, a single shunt capacitor C M is used in this proposed high-power rectifier along with the wire bonds for interconnection. L W 1 and L W 2 represents the length of wire bonds used for interconnection of GaN chip in circuit and also serves as inductors for impedance matching. For wireless energy harvesting applications, the expected RF input power from the antenna is in the range of 0 dBm (i.e., 1 mW). But in this design as we are aiming for high-power operation of rectifier, the rectifier is designed to perform well in the high-power region of 20 to 40 dBm. So, the designed high-power rectenna should have a maximum efficiency in this range of input power. Fig. 7 represents the impedance tuning by employing shunt capacitor and bond wire length, L (L = L W 1 = L W 2 ). It can be observed that by varying the capacitor value with accordance to the length of the wire bonds, it is possible to match the impedance properly at different frequencies. Therefore, the length of wire bond is taken as 900 um.  The proposed single shunt high-power rectifier is designed at 0.915 GHz on a 1.6 mm thick FR4 substrate (εr = 4.4, tan δ = 0.02). Rectifier layout is optimized to deliver maximum output DC power in the above-mentioned received antenna power. The final values of C M and R L have been determined through an optimization process performed with the commercial software Agilent ADS; both circuit analysis and full electromagnetic simulations have been carried out for improving results reliability. The load resistor value is selected based on the required DC voltage at the output of the rectifier. While designing the rectifier, the maximum conversion efficiency and maximum output DC power are enhanced based on the value of the load resistor. A 100 resistor is selected as the load after careful design simulations. Final capacitors C M , C 1 and C 2 are 5.1, 10000 and 10000 pF respectively (Murata components have been used).

III. RECTIFIER PERFORMANCE
The proposed microwave rectifier has been made and evaluated using the measurement setup shown in Fig. 9. A Keithley 2920 RF signal generator with an output power up to 13 dBm has been utilized to generate the RF signal at 0.915 GHz. A 40 dB power amplifier has been employed to amplify the signals for the testing of high-power rectifier. To protect the signal generator from any power surge and reflections,  a 3-dB attenuator is connected between the signal generator and power amplifier. For analyzing and estimating the signal power from the power amplifier a Keithley signal analyzer together with a 20-dB attenuator is utilized. A digital multimeter has been used to measure the output voltage across the R L resistor. The measurement setup has been appropriately calibrated considering all device losses to provide the most reliable results. Fig. 10 shows simulated and measured S-parameter of the designed high-power rectifier. Acceptable matching condition was confirmed under large signal S-parameters (LSSP) in the range of 30-40 dBm input power. Measured S parameter has a good agreement with simulated value at 5 dBm input power. Fig. 11 shows the simulated and measured output DC voltage of the GaN rectifier. It can be observed that a maximum voltage of 24.5 V is obtained at 40 dBm input power. Measured output voltage has good agreement with simulated values. The input power to the rectifier has been varied from 0 to 43 dBm, to measure the DC voltage across the load resistor. Input power is restricted to 43 dBm in order to ensure the safe operation of GaN rectifier. The RF-to-DC conversion efficiency (η) of a microwave rectifier is calculated as the ratio of the rectified DC output power to the incident RF  power, and it can be expressed as where P in is the input power and V out is the DC output voltage across the load resistor R L of the microwave rectifier. Fig. 12 shows the simulated and measured conversion efficiencies of the proposed microwave rectifier as a function of input power. Maximum measured efficiency of 61.2 % is observed at 39 dBm. Efficiency is more than 50% from an input power of 28 dBm. A comparison between the proposed GaN rectifier and other relevant high-power rectifiers is given in Table 2.
The proposed GaN rectifier design has achieved high output power with high efficiency. It can be observed that our design can stand high input power of 39 dBm compared to the other published designs. Although the measured results have shown that the proposed rectifier has a high output power, the overall dimension of our design is still the lowest due to the absence of complex and large matching networks. The proposed high-power rectifier is better than the other published designs in terms of the overall performance and compact size due to the novel wire bond matching, and thus is a very good candidate for future unmanned WPT applications. VOLUME 8, 2020

IV. CONCLUSION
In this paper, a novel high efficiency GaN rectifier for highpower WPT applications has been proposed. In order to realize high-power operations, a high breakdown voltage GaN Schottky diode is implemented from GaN HEMT. Chip on board technique is utilized to avoid the packaging losses and to reduce the size. A novel low loss impedance matching is proposed by exploiting the unavoidable inductance effects of bond wires used for providing the electrical connection between GaN chip and board. The microwave rectifier has been optimized for 0.915 GHz frequency and an input power of 39 dBm. The fabricated GaN rectifier achieved a maximum efficiency of 61.2% and a high output voltage of 22.05 V has been achieved at this optimized input power of 39 dBm. The proposed rectenna is better than the other published designs in terms of the high-power operation as well as the peak voltage and power. The fabricated prototype has a compact size and exhibits high efficiency performance in high-power region, making it a good candidate for high-power electronics like future unmanned intelligent devices.