Comparative Study on Reliability of InP/InGaAs Heterojunction Bipolar Transistors with Highly Zn- and C-Doped Base Layer
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1195-B06-02
Comparative Study on Reliability of InP/InGaAs Heterojunction Bipolar Transistors with Highly Zn- and C-Doped Base Layers Atsushi Koizumi, Kazuki Oshitanai, Jaesung Lee, Kazuo Uchida, and Shinji Nozaki Department of Electronic Engineering, The University of Electro-Communications, 1-5-1 Chofugaoka, Chofu, Tokyo 182-8585, Japan ABSTRACT The reliability of InP/InGaAs heterojunction bipolar transistors (HBTs) with highly carbon-doped and zinc-doped InGaAs base layers grown by metal-organic vapor phase epitaxy has been investigated. The Raman spectroscopy reveals that the post-growth annealing for the carbon-doped InGaAs base improves the crystallinity to become as good as that of the zincdoped InGaAs base. However, the photoluminescence intensity remains lower than that of the zinc-doped InGaAs even after the post-growth annealing. The current gains of the carbon- and zinc-doped base InP/InGaAs HBTs are 63 and 75, respectively, and they are affected by the base crystallinity. After the 15-min current stress test, the current gains decreased by 40 and 3% from the initial current gains for zinc- and carbon-doped base HBTs, respectively, are observed. These results indicate that the carbon-doped base HBT is much more reliable than that of zincdoped base HBT, though it has a lower current gain.
INTRODUCTION InP-based heterojunction bipolar transistors (HBTs) have demonstrated excellent highfrequency performance owing to high electron mobilities of the InP-based materials [1, 2] and low power dissipation due to their low turn-on voltages [3]. The base layer of a high speed HBT is heavily doped to reduce the base resistance. Carbon (C) has been most commonly used as a ptype dopant for a base layer because of its low diffusivity compared with those of the other ptype dopants such as zinc (Zn) and beryllium (Be) [4]. However, the amphoteric characteristic of C as a dopant forces us to lower the growth temperature and V/III ratio in order to achieve a high concentration of C as an acceptor in the growth of InGaAs layer by metalorganic vapor phase epitaxy (MOVPE). Since these growth parameters are not usually preferred because of poor crystallinity of the grown InGaAs, the InP/InGaAs HBT with a C-doped base layer may not necessarily demonstrate better transistor characteristic and its stability than that with a Zn-doped base layer. Therefore, it is important to compare the characteristics and stabilities of the HBTs with C- and Zn-doped InGaAs base layers (hereafter referred to as C-doped and Zn-doped HBTs, respectively) and mandatory to understand the device physics of these transistors for fabrication of high-performance InP/InGaAs HBTs. In this study, the authors compare the crystallinities, common-emitter current gains, and the current-stress stabilities of C- and Zn-doped HBTs fabricated with the same layer structure and emitter-base geometry.
EXPERIMENT The epitaxial layers of the C- and Zn-doped HBTs were grown by low-pressure MOVPE on Fe-doped semi-insulating InP (001) substrates. The layer structure used
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