p-InGaN/n-GaN Heterojunction Diodes and their Application to Heterojunction Bipolar Transistors
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p-InGaN/n-GaN Heterojunction Diodes and Their Application to Heterojunction Bipolar Transistors Toshiki Makimoto, Kazuhide Kumakura, Toshio Nishida and Naoki Kobayashi NTT Basic Research Laboratories 3-1 Morinosato Wakamiya, Atsugi-shi, Kanagawa 243-0198, Japan ABSTRACT p-InGaN/n-GaN heterojunction diodes were grown by metalorganic vapor phase epitaxy and characterized using current-voltage (I-V) and capacitance-voltage (C-V) measurements. We changed the In mole fraction in p-InGaN from 0 to 25% to investigate diode characteristics. All the diodes showed rectified I-V characteristics at room temperature. The ideality factors obtained from forward I-V characteristics were around 2, meaning that the recombination current is dominant instead of the tunneling current through the defects in depletion layers of the diodes. The breakdown voltage in reverse I-V characteristics depends on the net donor concentration (ND - NA) in n-GaN instead of the In mole fraction in p-InGaN. This result also means that the defects in p-InGaN do not influence the breakdown voltage. The built-in potential from C-V measurements decreases with the In mole fraction in p-InGaN, meaning that the valence band discontinuity increases with the In mole fraction. This valence band discontinuity realizes the hole confinement in an HBT with an p-InGaN base. Using these InGaN/GaN heterojunction diodes, an InGaN/GaN double heterojunction bipolar transistor was fabricated for the first time. The maximum current gain of 1.2 was obtained at room temperature.
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Hole Concentration (cm )
INTRODUCTION Wide-bandgap group III-nitrides are drawing a lot of attention due to their application to electronic devices operating at high temperatures and those with high output power. Furthermore, these group III-nitrides are friendly to environment, compared with other III-V compound semiconductors, since they have nontoxic nitrogen atoms as a column-V element. For bipolar transistors, high hole concentrations in p-type layer are desirable and process damage, especially to p-type layers for group III-nitrides, should be as small as possible. Several GaN/AlGaN
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Figure 1. Hole concentration at room temperature as a function of In mole fraction in p-InGaN.
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M g Concentration = 3x10
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In M ole Fraction in p-InGaN (% ) G13.10.1
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Figure 2. Mg acceptor activation energies of group III-nitrides such as GaN, AlGaN, InGaN.
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heterojunction bipolar transistors (HBTs) were fabricated and characterized [1-3]. Their characteristics, however, severely suffered from relatively high base resistances and process damage. To eliminate the process damage, the base layer was re-grown to obtain a current gain of 3 at room temperature [1]. Recently, we have reported that Mg-doped InGaN layers show high hole concentrations above 1×1018 cm-3 at ro
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