High performance GaN-based monolithic bidirectional switch using diode bridges

GaN-based power electronics have attracted great attention for next generation high-efficiency power conversion applications, such as AC–AC matrix converters and multilevel converters, due to their superior high breakdown voltage, high switching frequency, and high temperature operating capability compared with conventional Si-based power devices. ^1–3) AC–AC matrix converters can realize the lower cost, smaller size, and greater reliability due to without DC energy storage. ^4,5) The key component in an AC–AC matrix converter is a bidirectional switch that can switch with bidirectional conducting and blocking capability. However, the conventional bidirectional switch is composed of insulated gate bipolar transistors and power diodes, ⁶⁾ which leads to large a switch size and high parasitic resistance and inductance. In order to significantly improve the bidirectional switch performance, monolithic integration concepts will be an effective method. Besides, the bidirectional switch with high bidirectional breakdown voltage, low conduction loss, and high reliability are critical for high power applications.

Benefiting from the lateral structure of AlGaN/GaN high electron mobility transistors (HEMTs), the bidirectional switch based on HEMTs can realize the real monolithic integration and thus effectively reducing the number of discreet elements and conduction losses. Recently, a monolithic AlGaN/GaN bidirectional switch using a double-gate structure has been successfully demonstrated, ^7,8) and TCAD simulations of a non-ohmic GaN monolithic bidirectional switch has been carried out. ⁹⁾ However, both devices require two gate drivers, which will increase the overall gate control circuit size. Therefore, a diode bridge embedded AlGaN/GaN bidirectional switch has been developed to simplify the gate drivers. ¹⁰⁾ Unfortunately, this switch exhibits a large on-state voltage due to adopt the planar Schottky barrier diode (SBD) structure. Moreover, the normally-off operation is realized by a metal oxide semiconductor (MOS) structure, which specifically impacts both the channel mobility and the threshold voltage stability. ^11–16) In fact, recessed anode SBDs with low turn-on voltage have been widely investigated at presently. ^17–19) p-GaN gate techniques, as one of some approaches realizing the normally-off operation, have successfully commercialized. ^20–22) These approaches provide an important and valuable reference to further improve the performance of the bidirectional switch with diode bridges, which can be readily transferred to the currently commercialized p-GaN HEMT platform.

In this work, a monolithic bidirectional switch with a diode bridge and one p-GaN HEMT has been experimentally demonstrated. Only one gate driver is needed in the bidirectional switch, which can not only simplify the complexity of commutation process, but also reduce the cost of the gate control circuit. p-GaN gate techniques are employed for normally-off operation, and recessed anode SBDs are utilized to lower the on-state voltage and thus reduce the power loss of bidirectional switch. The fabricated bidirectional switch exhibits a threshold voltage of 1.84 V, an on-state voltage of 1.13 V, a high forward and reverse blocking voltages of ∼1100 V.

The bidirectional switch was fabricated on a GaN-on-Si wafer. The epitaxial structure consisted of a 3.4 μm GaN buffer layer, a 420 nm i-GaN channel layer, a 1 nm AlN insertion layer, a 15 nm Al_0.2Ga_0.8N barrier and a 75 nm p-GaN cap layer with an Mg doping concentration of 3 × 10¹⁹ cm⁻³. The microscope image and equivalent circuit of the fabricated bidirectional switch, as well as the cross-sectional schematics of p-GaN HEMT and recessed anode SBD are shown in Fig. 1.

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Device fabrication began with the removal of p-GaN layer outside the gate region by reaction ion etching. Then, Ti/Al/Ni/Au metal stack was evaporated by the e-beam evaporator and annealed at 830 °C for 30 s in N₂ ambient by rapid thermal annealing, which was used to form the source and cathode ohmic contact. The device isolation was done by mesa etching. After that, the recess etching of anode and drain regions was performed by the low-rate etching process, and the recess depth was approximately 30 nm confirmed by atomic force microscope. Then, the device was annealed at 450 °C for 5 min in N₂ to repair the etching damage. Finally, the Ni/Au metal stack was evaporated as gate contact, anode contact, drain contact, and interconnection metal. The fabricated p-GaN HEMT in bidirectional switch had a gate width of 100 μm, a source-gate distance (L_gs) of 3 μm, a p-GaN gate length (L_g) of 4 μm, a gate-drain distance (L_gd) of 15 μm, and recessed anode SBDs in bidirectional switch had an anode–cathode distance (L_ac) of 15 μm. In addition, the extended length over the AlGaN barrier region of the recessed anode is 2 μm.

The operation principle of the bidirectional switch using diode bridges is illustrated in Fig. 2. The on-state and off-state are fully controlled by the p-GaN HEMT in the bidirectional switch. On the one hand, when the p-GaN HEMT is turned off, the bidirectional switch will operate at the bidirectional blocking mode, which can block voltage at both forward and reverse directions [as shown in Figs. 2(a) and 2(b)]. On the other hand, when the p-GaN HEMT is turned on, the bidirectional switch will operate at bidirectional conducting mode, which can conduct current at both forward and reverse directions [as shown in Figs. 2(c) and 2(d)].

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The transfer characteristics of p-GaN HEMT in the bidirectional switch are shown in Fig. 3(a). The gate threshold voltage (V_th) is ∼1.84 V extracted at the drain current of 1 mA mm⁻¹ in the bidirectional conducting mode. Figure 3(b) plots the I–V characteristics of recessed anode SBDs in the bidirectional switch. The extracted turn-on voltage (V_on) at a forward current of 1 mA mm⁻¹ is 0.56 V, and the reverse leakage current at −100 V is 1.6 × 10⁻⁷ A mm⁻¹. The recessed anode structure effectively reduces the V_on due to the Schottky metal directly contacted with the two-dimensional electron gas. ¹⁸⁾

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The bidirectional conducting and blocking characteristics of the bidirectional switch are shown in Figs. 4 and 5, respectively. Approximately symmetric current conducting and voltage blocking performance in quadrants I and III can be observed. At V_GS = 7 V, the on-state saturation current is ∼140 mA mm⁻¹ in both forward and reverse directions. The on-state voltage (V_b) of ∼1.13 V in forward and reverse conducting characteristics is attributed to twice the V_on of recessed anode SBDs in both directions of the bidirectional switch. In addition, the forward off-state breakdown voltage (BV_F) and reverse off-state breakdown voltage (BV_R) are 1105 V and −1115 V at the substrate floating, respectively. On the off-state blocking state, the leakage current of the normally-off p-GaN HEMT is far smaller than that of recessed anode SBDs in the switch, in which case the current path through the p-GaN HEMT can be visualized as the open circuit. Thus, the equivalent circuit of bidirectional switch can be simplified as the top path (D1 combined with D2) in parallel with the bottom path (D3 combined with D4). The leakage current is dominated by the Schottky contact in recessed anode SBDs. The high blocking voltage will be beneficial for the bidirectional switch in the high-power application. Compared with the reported bidirectional switch with diode bridge structures based on MOSHEMT, ¹⁰⁾ a significant improvement of both the on-state voltage and bidirectional breakdown voltage has been achieved in the proposed bidirectional switch.

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Table I gives the comparison between the proposed bidirectional switch and other reported normally-off bidirectional switch. The proposed device exhibits the greatest V_th, and the highest off-state blocking voltage, with only one gate driver. The bidirectional switch in Ref. 7 has the lowest V_b with two gate drivers, at the expense of the increase in the overall gate control circuit size.

In order to demonstrate the advantage of the bidirectional conducting and blocking characteristics, the bidirectional switch is measured as an AC power chopper, which is one of the fundamental functions of matrix converters. The measurement setup for the bidirectional switch operating as an AC switch is shown in Fig. 6. The voltage amplitude and the frequency of the AC power supply are ±150 V and 60 Hz, respectively. The square wave pulse input with the frequency of 5 kHz and the voltage swing from 0 to 5 V is applied to the gate of the p-GaN HEMT. The oscilloscope with a high voltage differential probe is used to monitor the voltage variation between the S1 and S2. Figure 7 shows the measured voltage waveforms, which demonstrates the successful chopping operation of the bidirectional switch.

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In summary, the low bidirectional on-state voltage and high bidirectional breakdown voltage have been realized in the monolithic bidirectional switch with four recessed anode SBDs and one p-GaN HEMT. The fabricated bidirectional switch shows a V_th of 1.84 V, a V_b of 1.13 V, and a forward and reverse off-state breakdown voltages of ∼1100 V. Besides, the successful chopping operation of the bidirectional switch as an AC power chopper has been confirmed. These results demonstrate a promising application of the proposed monolithic bidirectional switch in high-efficient power conversion.

Acknowledgments