Common-emitter current–voltage characteristics of a Pnp AlGaN/GaN heterojunction bipolar transistor with a low-resistance base layer

Abstract

We fabricated a Pnp AlGaN/GaN heterojunction bipolar transistor and investigated its common-emitter current–voltage characteristics. The device structures were grown by metalorganic vapor phase epitaxy on a sapphire substrate. The Al mole fraction in an AlGaN emitter layer was 0.18. The thickness of the GaN base layer was 0.12 μm and its Si doping concentration was as high as 1×1019 cm−3, so its base resistance decreased two orders of magnitude compared with the reported Npn nitride heterojunction bipolar transistors. The transistor showed good saturation current–voltage characteristics and the maximum common-emitter current gain of 28 was obtained for collector current of −2×10−5 A at room temperature.

Carrier diffusion length measured by optical method in GaN epilayers grown by MOCVD on sapphire substrates

“…In the literature , a large spread of measured values of L in GaN as well as in InGaN and AlGaN alloys has been reported, ranging from 30 nm or less [9,17] up to 10.5 mm [2]. Usually, L is evaluated from the electron-beam-induced current (EBIC) [1][2][3][4], I-V characteristics of devices [5,6], or luminescence intensity mapping in the vicinity of dislocations [7][8][9][10][11][12][13][14][15], when the intensity nanoscale [9] or microscale mapping of both cathodoluminescence (CL) [7,9,11] and photoluminescence (PL) [8,12,14] is investigated. The first two methods require electric contact formation.…”

Explanation of anomalously high current gain observed in GaN based bipolar transistors

“…From common emitter operation and Gummel plot, current gain varying from 3 up to 100 has been reported [4], [6]- [12]. However, it is commonly seen that high can only be obtained with a collector current smaller than 1 mA (a corresponding average current density of 100 A/cm ) [9], [11]- [13] and that sometimes, exhibits a very high peak at low current level in the microamp range [8], [14], [15]. To understand these issues and characterize devices correctly, we carefully studied the impact of poor ohmic contacts and leakage currents and on the measured device performance, especially on the Gummel plot.…”

High power operation of Pnp AlGaN/GaN heterojunction bipolar transistors

“…The corresponding current density and power density were 1.3 kA/cm 2 and 84.5 kW/cm 2 which are equal to or higher than the reported value of a Npn AlGaN/GaN HBT [2]. The collector current at the maximum current gain is three orders of magnitude larger than that of the previous reported Pnp AlGaN/GaN HBT [3]. This would be ascribed to the use of the AlGaN/(Al)GaN superlattices at the emitter and subcollector layer.…”

“…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.…”