A Novel Bidirectional AlGaN/GaN ESD Protection Diode

Despite the superior working properties, GaN-based HEMTs and systems are still confronted with the threat of a transient ESD event, especially for the vulnerable gate structure of the p-GaN or MOS HEMTs. Therefore, there is still an urgent need for a bidirectional ESD protection diode to improve the ESD robustness of a GaN power system. In this study, an AlGaN/GaN ESD protection diode with bidirectional clamp capability was proposed and investigated. Through the combination of two floating gate electrodes and two pF-grade capacitors connected in parallel between anode or cathode electrodes and the adjacent floating gate electrodes (CGA (CGC)), the proposed diode could be triggered by a required voltage and possesses a high secondary breakdown current (IS) in both forward and reverse transient ESD events. Based on the experimental verification, it was found that the bidirectional triggering voltages (Vtrig) and IS of the proposed diode were strongly related to CGA (CGC). With CGA (CGC) increasing from 5 pF to 25 pF, Vtrig and IS decreased from ~18 V to ~7 V and from ~7 A to ~3 A, respectively. The diode’s high performance demonstrated a good reference for the ESD design of a GaN power system.


Introduction
Currently, GaN-based high-electron-mobility transistors (HEMTs) have attracted a great deal of research attention in high-power applications, owing to their low specific on-resistance, high breakdown voltage, high switching frequency and, especially, the more convenient integration (just as the GaN-based monolithic integrated circuits (MICs), which are characterized with low parasitic parameters and high performance) [1][2][3][4][5][6]. Despite the superior operation properties, the GaN-based HEMTs and MICs are still confronted with the threat of failure caused by a transient electrostatic discharge (ESD) event, especially for the vulnerable gate structure of the p-GaN HEMTs, metal-oxide-semiconductor (MOS) HEMTs and Schottky-gated GaN-based HEMTs. In some reports [7][8][9][10][11], it was comprehensively demonstrated that the Schottky-gated GaN-based HEMTs can withstand extremely high transient ESD voltages in the drain-to-source, drain-to-gate and gate-to-source conditions. However, things go differently for the p-GaN (or MOS) HEMTs. We comprehensively investigated the ESD robustness of the p-GaN HEMTs in different conditions [12]. In drain-to-source and drain-to-gate conditions, the equivalent human body model (HBM) failure voltage (V HBM ) of the p-GaN HEMTs can meet the industrial standard (2 kV) [13,14]. However, owing to the lack of discharge path in the gate electrode of the p-GaN HEMTs, the devices exhibit poor ESD robustness in the gate-to-source condition, with an equivalent V HBM of only 0.2~0.33 kV. E. Canato [15] and Yiqiang Chen [16,17] reported the gate-tosource ESD failure and degradation mechanisms of p-GaN HEMTs, which mainly rely on Figure 1a,b shows the schematic structure and equivalent circuit of the proposed AlGaN/GaN B-ESD-PD. The device features two floating gate electrodes (FG1 and FG2), a floating ohmic contact (FO) between the floating gate electrodes, two ohmic contacts as the anode/cathode electrodes (A/C) and two pF-grade capacitors parallelly connected between the anode or cathode electrodes and the adjacent floating gate electrodes (called as C GA and C GC ). As it can be seen, the proposed AlGaN/GaN B-ESD-PD is similar to two E-mode HEMTs connected in series with the sources tied together. Furthermore, the fabrication process of the proposed AlGaN/GaN B-ESD-PD can be fully compatible with the traditional E-mode p-GaN HEMTs (as shown in Figure 2). Therefore, the proposed AlGaN/GaN B-ESD-PD can be easily implemented in state-of-art GaN technology, demonstrating a good reference for the ESD design of the GaN power system. Moreover, the required C GA can be easily integrated into the state-of-art GaN technology by changing the area of the capacitor's metal plate. For example, when the second SiN passivation layer is 100 nm, to obtain a 10 pF capacitor, the required area of the capacitor's metal plate is 0.0144 mm 2 (120 µm × 120 µm), which accounts for less than 0.1% of the total area of the traditional p-GaN HEMT in [19]. The working mechanism of the proposed AlGaN/GaN B-ESD-PD is given in Figure 3a  The equivalent structure configured using the chip capacitor and the commercially p-GaN HEMT (EPC2036) from the EPC Corporation [20].  During a forward transient ESD event, a high dv/dt can induce a capacitive coupling current from the anode electrode to the cathode electrode ( Figure 3a). The capacitive coupling current will carry a certain amount of positive transition charges (Qtran1 and Qtran2) to FG1 and FG2, and the positive transition charges will be stored at FG1 and FG2 [12], which can pull down the energy band in the floating gate regions and force the electrons to gather under FG1 and FG2. When the gates' potentials induced by the positive transition charges exceed the threshold voltage (Vth) of the 2DEG channel, the 2DEG channel under FG1 and FG2 will be turned on. Then, the large current can be passed through the 2DEG channel under FG1 and FG2. Consequently, in a transient ESD event, the proposed structure is similar to two series-connected lateral field-effect rectifiers (L-FER). Therefore, the ESD-event-induced accumulated electrostatic charges can be effectively released through the proposed AlGaN/GaN B-ESD-PD, which can effectively avoid damage to the GaN power system, thereby enhancing the system's ESD robustness. Similarly, the proposed   To reduce the cost of the validation experiment, an equivalent structure configured using the chip capacitors and the commercially p-GaN HEMTs (EPC2036) [20] was used to verify the operating principle of the proposed AlGaN/GaN B-ESD-PD, as shown in Figure 1c. The area of EPC2036 and the required capacitor (as stated above) were only about 0.81 mm 2 and 0.0144 mm 2 , respectively. Therefore, the total area of the proposed Al-GaN/GaN B-ESD-PD was about 1.85 mm 2 , which accounted for less than 5% of the total During a forward transient ESD event, a high dv/dt can induce a capacitive coupling current from the anode electrode to the cathode electrode ( Figure 3a). The capacitive coupling current will carry a certain amount of positive transition charges (Q tran1 and Q tran2 ) to FG1 and FG2, and the positive transition charges will be stored at FG1 and FG2 [12], which can pull down the energy band in the floating gate regions and force the electrons to gather under FG1 and FG2. When the gates' potentials induced by the positive transition charges exceed the threshold voltage (V th ) of the 2DEG channel, the 2DEG channel under FG1 and FG2 will be turned on. Then, the large current can be passed through the 2DEG channel under FG1 and FG2. Consequently, in a transient ESD event, the proposed structure is similar to two seriesconnected lateral field-effect rectifiers (L-FER). Therefore, the ESD-event-induced accumulated electrostatic charges can be effectively released through the proposed AlGaN/GaN B-ESD-PD, which can effectively avoid damage to the GaN power system, thereby enhancing the system's ESD robustness. Similarly, the proposed AlGaN/GaN B-ESD-PD can also effectively avoid damage to the GaN power system during a reverse transient ESD event. Typically, in a forward transient ESD event, the voltage needed to simultaneously turn on the 2DEG channels under FG1 and FG2 (V trig_F ) is positively correlated with V th × C 1 /(C ga + C GA ) and V th × (C gc + C GC )/C 2 [12], where C 1 (C 2 ) is the parasitic capacitance between FG1 (FG2) and FO, and C ga (C gc ) is the parasitic capacitance between FG1 and the anode electrode (the cathode electrode). A required V trig_F can be obtained by changing C 1 (C 2 ), C ga (C gc ) and C GA (C GC ). Similarly, in the reverse transient ESD event, the voltage needed to simultaneously turn on the 2DEG channel under FG1 and FG2 (V trig_R ) is positively correlated with V th × C 2 /(C gc + C GC ) and V th × (C ga + C GA )/C 1 . A required V trig_R can also be obtained by changing C 1 (C 2 ), C ga (C gc ) and C GA (C GC ).

Structure and Mechanism
To reduce the cost of the validation experiment, an equivalent structure configured using the chip capacitors and the commercially p-GaN HEMTs (EPC2036) [20] was used to verify the operating principle of the proposed AlGaN/GaN B-ESD-PD, as shown in Figure 1c. The area of EPC2036 and the required capacitor (as stated above) were only about 0.81 mm 2 and 0.0144 mm 2 , respectively. Therefore, the total area of the proposed AlGaN/GaN B-ESD-PD was about 1.85 mm 2 , which accounted for less than 5% of the total area of the traditional p-GaN HEMT in [19]. As analyzed above, the ESD protection capability of the proposed AlGaN/GaN B-ESD-PD are related to the continuous working current, threshold voltage and parasitic capacitance between the gate electrode and the drain/source electrode of the commercially p-GaN HEMTs, which were 1.7 A, 2 V and~10 pF/75 pF, respectively. Moreover, the parasitic capacitances caused in BOEL was less than 1 pF, which did not significantly influence the ESD behavior of the proposed AlGaN/GaN B-ESD-PD. More detailed device characteristics of the commercially p-GaN HEMTs can be found in the datasheet of EPC2036 [20]. In this work, the transient ESD events were produced by our self-developed transmission line pulsing (TLP) measurement system ( Figure 3c). The pulse width and rising time in the TLP tests were set to be 100 ns and 2 ns, respectively. To capture the effective transient TLP voltage and current waveforms, the averaged values over the time span from 70% to 90% of the TLP pulse width were extracted. Furthermore, the bidirectional TLP current-voltage (I-V) characteristics of the proposed AlGaN/GaN B-ESD-PD were extracted from two of the same devices. The reason for this is explained in Section 3. Moreover, during the TLP test, the sudden obvious decrease in voltage between the anode electrode and cathode electrode was used as a failure criterion. Figure 4 shows the bidirectional leakage current characteristics of the proposed Al-GaN/GaN B-ESD-PD with different C GA (C GA ), accompanied by that of the gate-floating bidirectional GaN diode and GS-shorting bidirectional GaN diode. The gate-floating bidirectional GaN diode is similar to two anti-series connected E-mode p-GaN HEMTs with two gate electrodes floated, and the GS-shorting bidirectional GaN diode is similar to two anti-series connected E-mode p-GaN HEMTs with two gate electrodes shortly connected to the source electrodes. The proposed diodes exhibited a relatively low DC leakage current in different directions; therefore, after the proposed diodes were integrated into the GaN power systems, the forward or reverse DC leakage current of the GaN power systems was not markedly increased. Especially, under the conventional gate working voltage of the traditional p-GaN HEMT (less than 5 V), the DC leakage current of the proposed AlGaN/GaN B-ESD-PD was less than 1 µA. For now, the DC gate leakage current of the traditional p-GaN HEMT was in the range from 20 µA to 320 µA [20]. Among them, the DC gate leakage currents were 160 µA and 320 µA for the devices with static working currents of 30 A and 60 A, respectively. Predictably, the device with higher static working current possessed a higher gate leakage current. Therefore, integrating the proposed AlGaN/GaN B-ESD-PD into the traditional p-GaN HEMT did not obviously increase the DC gate leakage current of the traditional high-current p-GaN HEMT. In addition, as stated above, the fabrication process of the proposed AlGaN/GaN B-ESD-PD can be fully compatible with the traditional E-mode p-GaN HEMTs, making the ESD design more convenient. Although the gate-floating and GS-shorting bidirectional GaN diodes also exhibited a relatively low leakage current in different directions, the diodes were not suitable as the ESD protection diode due to their high triggering voltage (V trig ) and low secondary breakdown current, which is described later. Figure 4 shows the bidirectional leakage current characteristics of the proposed Al-GaN/GaN B-ESD-PD with different CGA (CGA), accompanied by that of the gate-floating bidirectional GaN diode and GS-shorting bidirectional GaN diode. The gate-floating bidirectional GaN diode is similar to two anti-series connected E-mode p-GaN HEMTs with two gate electrodes floated, and the GS-shorting bidirectional GaN diode is similar to two anti-series connected E-mode p-GaN HEMTs with two gate electrodes shortly connected to the source electrodes. The proposed diodes exhibited a relatively low DC leakage current in different directions; therefore, after the proposed diodes were integrated into the GaN power systems, the forward or reverse DC leakage current of the GaN power systems was not markedly increased. Especially, under the conventional gate working voltage of the traditional p-GaN HEMT (less than 5 V), the DC leakage current of the proposed AlGaN/GaN B-ESD-PD was less than 1 μA. For now, the DC gate leakage current of the traditional p-GaN HEMT was in the range from 20 μA to 320 μA [20]. Among them, the DC gate leakage currents were 160 μA and 320 μA for the devices with static working currents of 30 A and 60 A, respectively. Predictably, the device with higher static working current possessed a higher gate leakage current. Therefore, integrating the proposed Al-GaN/GaN B-ESD-PD into the traditional p-GaN HEMT did not obviously increase the DC gate leakage current of the traditional high-current p-GaN HEMT. In addition, as stated above, the fabrication process of the proposed AlGaN/GaN B-ESD-PD can be fully compatible with the traditional E-mode p-GaN HEMTs, making the ESD design more convenient. Although the gate-floating and GS-shorting bidirectional GaN diodes also exhibited a relatively low leakage current in different directions, the diodes were not suitable as the ESD protection diode due to their high triggering voltage (Vtrig) and low secondary breakdown current, which is described later.    fore, in the transient ESD event, the gate-floating and GS-shorting bidirectional Ga odes could not effectively clamp the potential to be a required value for the key po of the GaN power system, and the low positive secondary breakdown current coul effectively release the accumulated electrostatic charges in the transient ESD eve other words, the gate-floating and GS-shorting bidirectional GaN diodes may be no able as ESD protection diodes to enhance a system's ESD robustness and to prote GaN power system from being damaged in a transient ESD event.  As stated in Section 2, the bidirectional TLP I-V characteristics of the proposed Al-GaN/GaN B-ESD-PD were extracted from two of the same devices. The reason for that can be explained in Figure 6 in which the bidirectional leakage current characteristics and TLP I-V characteristics before and after the occurrence of the forward ESD breakdown (FB) are exhibited. It can be seen that, after the occurrence of the forward ESD breakdown, there was an obvious change in the bidirectional leakage current and TLP I-V characteristics for the proposed AlGaN/GaN B-ESD-PD. However, to capture the bidirectional secondary breakdown current of the proposed AlGaN/GaN B-ESD-PD, the device was always tested until the occurrence of an ESD breakdown. Therefore, to obtain the bidirectional TLP I-V characteristics of the proposed AlGaN/GaN B-ESD-PD, two of the same devices were needed.  It can also be seen from Figure 5 that the change in C GA (C GC ) had an obvious impact on the bidirectional TLP I-V characteristics of the proposed AlGaN/GaN B-ESD-PD. The slope of the snapback region showed a gradual decrease with the increase in capacitor values; this was because increasing the capacitor values decreased the charging rate of the capacitor, which reduced the opening speed of the floating gate structure, subsequently increasing the transient load resistance of the TLP load-line. Furthermore, the influences of C GA (C GC ) on the triggering voltage and the secondary breakdown current are summarized in Figure 7. As analyzed above, in the TLP test, the triggering voltages (V trig_F and V trig_R ) of the proposed AlGaN/GaN B-ESD-PD were decreased with the increase in C GA (C GC ). With C GA (C GC ) increasing from 5 pF to 25 pF, the triggering voltages (V trig_F and V trig_R ) decreased from 18 V to 7 V. Therefore, through changing C GA (C GC ), the desirable triggering As stated above, through changing CGA (CGC), the desirable triggering voltages (Vtrig_F and Vtrig_R) could be obtained for the proposed AlGaN/GaN B-ESD-PD. To make the dependence of the triggering voltage on CGA and CGC clear, the bidirectional TLP I-V characteristics of the unidirectional AlGaN/GaN ESD protection diode with different CGA values were studied, as shown in Figure 8. The change in CGA only had an obvious impact on the positive TLP I-V characteristics of the unidirectional AlGaN/GaN ESD protection diode and had no effect on its reverse TLP I-V characteristics. In other words, only the forward triggering voltage of the unidirectional AlGaN/GaN ESD protection diode depended on CGA, and the reverse triggering voltage was not related to CGA. Because the proposed bidirectional AlGaN/GaN ESD protection diode was similar to two anti-series connected unidirectional AlGaN/GaN ESD protection diodes, it can be inferred that the forward triggering voltage of the proposed bidirectional AlGaN/GaN ESD protection diode is related to CGA and not related to CGC, and its reverse triggering voltage is related to CGC and not related to CGA.  As stated above, through changing C GA (C GC ), the desirable triggering voltages (V trig_F and V trig_R ) could be obtained for the proposed AlGaN/GaN B-ESD-PD. To make the dependence of the triggering voltage on C GA and C GC clear, the bidirectional TLP I-V characteristics of the unidirectional AlGaN/GaN ESD protection diode with different C GA values were studied, as shown in Figure 8. The change in C GA only had an obvious impact on the positive TLP I-V characteristics of the unidirectional AlGaN/GaN ESD protection diode and had no effect on its reverse TLP I-V characteristics. In other words, only the forward triggering voltage of the unidirectional AlGaN/GaN ESD protection diode depended on C GA , and the reverse triggering voltage was not related to C GA . Because the proposed bidirectional AlGaN/GaN ESD protection diode was similar to two anti-series connected unidirectional AlGaN/GaN ESD protection diodes, it can be inferred that the forward triggering voltage of the proposed bidirectional AlGaN/GaN ESD protection diode is related to C GA and not related to C GC , and its reverse triggering voltage is related to C GC and not related to C GA . forward triggering voltage of the unidirectional AlGaN/GaN ESD protection diode d pended on CGA, and the reverse triggering voltage was not related to CGA. Because th proposed bidirectional AlGaN/GaN ESD protection diode was similar to two anti-serie connected unidirectional AlGaN/GaN ESD protection diodes, it can be inferred that th forward triggering voltage of the proposed bidirectional AlGaN/GaN ESD protection d ode is related to CGA and not related to CGC, and its reverse triggering voltage is related CGC and not related to CGA. In order to make a comprehensive comparison, the TLP I-V characteristics of the pr posed AlGaN/GaN B-ESD-PD with different CG1 (CG2) values were also investigated. Firs the leakage current and TLP I-V characteristics of the two reverse-series connected E mode p-GaN HEMTs with two gate electrodes connected to the floating ohmic electrod through CG1 and CG2 are presented in Figure 9. The device is called diode 1 in the followin work, and its equivalent circuit is shown in the inset of Figure 9a. As indicated in Figu In order to make a comprehensive comparison, the TLP I-V characteristics of the proposed AlGaN/GaN B-ESD-PD with different C G1 (C G2 ) values were also investigated. First, the leakage current and TLP I-V characteristics of the two reverse-series connected E-mode p-GaN HEMTs with two gate electrodes connected to the floating ohmic electrode through C G1 and C G2 are presented in Figure 9. The device is called diode 1 in the following work, and its equivalent circuit is shown in the inset of Figure 9a. As indicated in Figure 8, although diode 1 exhibited a relatively low static leakage current in different directions, the device possessed a high triggering voltage over 200 V and an extremely low positive secondary breakdown current of 0.01 A. In addition, the triggering voltages of diode 1 were increased with the increase in C G1 . Therefore, diode 1 could not effectively clamp the potential to be a required value for the key position of the GaN power system. Moreover, that high triggering voltage is much higher than the safe gate working voltage of the traditional p-GaN HEMT, which will damage the p-GaN gate of the traditional p-GaN HEMT. Furthermore, that low positive secondary breakdown current cannot effectively release the accumulated electrostatic charges in the transient ESD event. On the whole, diode 1 may be not suitable as the ESD protection diode to enhance a system's ESD robustness and to protect the GaN power system from being damaged in a transient ESD event.

Results and Discussion
Micromachines 2022, 13, x FOR PEER REVIEW 9 of 8, although diode 1 exhibited a relatively low static leakage current in different direction the device possessed a high triggering voltage over 200 V and an extremely low positi secondary breakdown current of 0.01 A. In addition, the triggering voltages of diode were increased with the increase in CG1. Therefore, diode 1 could not effectively clamp t potential to be a required value for the key position of the GaN power system. Moreove that high triggering voltage is much higher than the safe gate working voltage of the tr ditional p-GaN HEMT, which will damage the p-GaN gate of the traditional p-Ga HEMT. Furthermore, that low positive secondary breakdown current cannot effective release the accumulated electrostatic charges in the transient ESD event. On the who diode 1 may be not suitable as the ESD protection diode to enhance a system's ESD r bustness and to protect the GaN power system from being damaged in a transient ES event.  Figure 11a. It is shown in Figure 10 that the triggerin voltages and secondary breakdown currents of diode 2 were related to CG1 (CG2). With C (CG2) increasing from 10 pF to 100 pF, the triggering voltages of diode 2 decreased from  Figure 10a, and the diode with a C GA (C GC ) of 10 pF was called diode 3, shown in the inset of Figure 11a. It is shown in Figure 10 that the triggering voltages and secondary breakdown currents of diode 2 were related to C G1 (C G2 ). With C G1 (C G2 ) increasing from 10 pF to 100 pF, the triggering voltages of diode 2 decreased from 18 V to 36 V, and the secondary breakdown currents slightly decreased from 7 A to 6.5 A. Correspondingly, the equivalent HBM failure voltages of diode 2 decreased from 10.5 kV to 9.75 kV, as summarized in Figure 12a. Therefore, through changing C G1 (C G2 ), the desirable triggering voltages (V trig_F and V trig_R ) could also be obtained for diode 2. Meanwhile, for diode 3, the change in C G1 (C G2 ) had nearly no effect on the secondary breakdown currents, only leading to a slight increase in the triggering voltages. Therefore, to obtain desirable triggering voltages (V trig_F and V trig_R ) for diode 3, the way of changing C G1 (C G2 ) was not particularly effective. Moreover, compared with diode 2 and diode 3, the proposed AlGaN/GaN B-ESD-PD may be more desired. This is because when using a traditional GaN HEMT, increasing the capacitance should be avoided as much as possible.

Conclusions
In conclusion, a novel AlGaN/GaN B-ESD-PD featuring two floating gate electrodes and two pF-grade capacitors was proposed for enhancing the ESD robustness of a GaN power system. Through the TLP tests, it was demonstrated that the proposed AlGaN/GaN B-ESD-PD could be triggered by a required voltage (~10 V) and possessed a high secondary breakdown current in both forward and reverse transient ESD events. Furthermore, it was also found that the required triggering voltages and secondary breakdown currents of the proposed AlGaN/GaN B-ESD-PD were strongly related to CGA (CGC). With CGA (CGC) increasing from 5 pF to 25 pF, the positive triggering voltages decreased from 18 V to 7 V, and the positive secondary breakdown currents decreased from ~7 A to ~3 A. In addition,

Conclusions
In conclusion, a novel AlGaN/GaN B-ESD-PD featuring two floating gate electrodes and two pF-grade capacitors was proposed for enhancing the ESD robustness of a GaN power system. Through the TLP tests, it was demonstrated that the proposed AlGaN/GaN B-ESD-PD could be triggered by a required voltage (~10 V) and possessed a high secondary breakdown current in both forward and reverse transient ESD events. Furthermore, it was also found that the required triggering voltages and secondary breakdown currents of the proposed AlGaN/GaN B-ESD-PD were strongly related to C GA (C GC ). With C GA (C GC ) increasing from 5 pF to 25 pF, the positive triggering voltages decreased from 18 V to 7 V, and the positive secondary breakdown currents decreased from~7 A to~3 A. In addition, the fabrication process of the proposed AlGaN/GaN B-ESD-PD can be fully compatible with the traditional GaN HEMT, demonstrating a good reference for the ESD design of GaN monolithic integrated circuits.