High power operation of Pnp AlGaN/GaN heterojunction bipolar transistors

Abstract

PACS 85.30. De, 85.30.Pq We fabricated a Pnp AlGaN/GaN heterojunction bipolar transistor and investigated its common -emitter current -voltage characteristics at room temperature. The device structures were grown by metalorganic vapor phase epitaxy on the sapphire substrates. The buffer layer was a newly developed Al 2 O 3 /AlN/AlON/ Al 2 O 3 , resulting in the dislocation density of 6 × 10 8 cm -2 in MOVPE-grown GaN layer. This relatively low dislocation density led to the high voltage operation in the devices, corresponding to the breakdown field of 2.4 MV/cm. AlGaN/(Al)GaN superlattices were applied to the emitter and subcollector to increase the hole concentrations in these layers. An n-type GaN base width was 80 nm. The sheet resistivity and the specific contact resistance were 900 Ω/square and 2.6 × 10 -5 Ω-cm 2 for a 80 nm base, respectively. The base sheet resistivity of Pnp AlGaN/GaN HBT was two orders of magnitude smaller than that of Npn AlGaN/GaN HBTs. The maximum current gain was 8 at the collector current of 11.5 mA for the 30 µm × 50 µm device. It operated at the collector current of 20 mA at the collector -emitter voltage of 65 V with a current gain of 5. The corresponding current density and power density were 1.3 kA/cm 2 and 84.5 kW/cm 2 . High power operation was achieved by using the relatively low dislocation density GaN and low resistance superlattices. 1 Introduction Nitride-based heterojunction bipolar transistors (HBTs) have the potential to operate in high-temperature and/or high-power conditions with uniform threshold voltages and high current densities. The first Npn AlGaN/GaN HBT was reported in 1998 with the current gain of 3 [1], and recent performance was that the AlGaN/GaN HBTs operated at the current density of 1 kA/cm 2 with the current gain of ∼ 18 [2]. Pnp nitride-based HBTs have some advantages over Npn nitride-based HBTs. First, a large emitter -base conduction band offset, which is larger than the valence band offset, prevents a backward injection current from the base to the emitter. Second, we can easily achieve a high base doping concentration above 10 18 cm -3 , which results in low base and contact resistances. Third, we can form an ultra-thin base with heavy Si doping, which improves device characteristics. Moreover, hightemperature operation is promising because it improves the conductivity of the p-type layer at high temperature. The common-emitter current -voltage (I-V) characteristics of Pnp AlGaN/GaN HBT were reported in 2002 for the first time [3]. At that time, the collector current at the maximum current gain and the breakdown voltage at room temperature were 0.02 mA and 10 V, respectively, for a 90 µm × 150 µm device. There were two main problems for this device. One is relatively small collector current and the other is low breakdown voltage. In this paper, we report the high-power operation of Pnp AlGaN/GaN HBTs with p-type superlattice emitter and subcollector toward high power operation.

GaN/InGaN heterojunction bipolar transistors with ultra‐high d.c. power density (>3 MW/cm2)

“…The power switching figure of merit (BV CEO × J C ) is 16.5 MW/cm 2 . This value also represents greater than 10 times of those achieved on pnp AlGaN/GaN HBTs grown on FS GaN substrates 19 and more than 1.5 times higher than any state‐of‐the‐art HBTs 14–16.…”

“…Figure 5 shows a comparison of the current gain versus J C from various reports 4, 9, 17–26. pnp HBTs are also included in this chart for comparison.…”

Electronic Devices

“…GaN multilayer BJT devices with n-p-n structures have been studied [49][50][51][52]. GaN multilayer BJT devices with n-p-n structures have been studied [49][50][51][52].…”

High‐performance GaN/InGaN heterojunction bipolar transistors using a direct‐growth approach

“…In view of simplicity and low-cost manufacturing, however, a direct-growth approach is still preferred. The pnp HBTs using the direct-growth approach were studied and demonstrated with lower base resistance and with higher current gain when compared to the III-N npn HBTs using base or emitter regrowth approaches [9,10]. On the other hand, III-N npn HBTs using direct-growth approach to achieve high current drive and high current gain are not frequently reported because of tremendous technical challenges in device growth and fabrication development such as resistive base layer and etching-damaged surface.…”