Design and Realization of GaN Trench Junction-Barrier-Schottky-Diodes CNF Project Number : 2307-14

We present the design principle and experimental demonstrations of GaN trench junctionbarrier-Schottky-diodes (trench JBSDs), where the Schottky contact within the patterned trenches is at the same plane as the adjacent p-n junctions. Assisted by the TCAD simulations, the leakage current reduction mechanism is identified as the reduced surface field (RESURF) effect due to the barrier-height difference between the p-n junction and Schottky junction. Design space for the width of stripe-shaped trenches is found to be < 0.5 μm for a drift layer doping level of 1e15~1e16 cm-3, while for circular trenches the size requirement is relaxed. In the fabricated devices with 1-4 μm diameter circular trenches, approximately a 20X reduction in the reverse leakage is observed with a characteristic shift in the turn-on voltage, which are signatures of the trench JBSD with desired RESURF. The experimental observations are in excellent agreement with the simulation results. This JBSD design shows promising potential in further improving the performance of Schottky-based GaN power devices without the need for ion-implantation or material regrowth. Summary of Research: As shown in Figure 1(a), the trench JBSD epi structure is similar to our previous high­BV PNDs grown by metal­ organic chemical vapour deposition on freestanding GaN substrates with a threading dislocation density of ~ 2 × 106 cm­2 [1]. A net doping concentration of ~ 1 × 1016 cm­3 in the drift layer is extracted by the capacitance­voltage (C­V) measurement [Figure 1(b)]. Trench JBSDs are designed to have circular trench patterns with a diameter of 1, 2, 3, and 4 μm. The trench JBSDs are fabricated by dry­etch first to form trenches and reveal the n­GaN surface, followed by deposition of circular Pd­based anodes. Pd forms an ohmic contact to p­GaN and a Schottky contact to n­GaN in the trench. Figure 1(c) shows the representative transmission line method I­V characteristics of the ohmic contact on p­GaN. An excellent specific contact resistivity of 3.9 × 10­5 Ω∙cm2 is extracted. Conventional SBDs are made on the etched n­GaN surface. No additional field plate (FP) structures are used for edge termination, since the additional leakage often associated with the FP process [2] might mask the trend in the leakage current of trench JBSDs designed with varied trench sizes. Figure 2 shows the measured I­V characteristics of the fabricated trench JBSDs. An ideal Schottky turn­ on behavior is observed for both trench JBSDs and the SBD at ~ 1 V with an ideality factor of ~ 1.00­1.05. A clear shift in the turn­on voltage is observed in the log plot in Figure 2(a) and highlighted in the inset, which agrees well with the simulation results. The linear­ scale I­V curves in Figure 2(b) show a two­step turn­ on for the trench JBSDs, similar to [3]. The reverse I­V characteristics in Figure 2(c) show a clear reduction of the leakage current density as the trench size decreases from 4 μm to 1 μm. Up to 20 times reduction in leakage current is achieved in the 1 μm trench JBSDs compared with conventional SBDs, reaching the PND leakage level. As the total trench area is designed to have the same total area, the reduction in leakage current is due to the RESURF effect arising from the trench JBSD design. Finally, the benchmark plot of RON­BV is shown in Figure 4. The figure­of­merit of the SBD and trench JBSD in this paper is comparable to that of the best SBD reported without field plate.