Reliability of InP-based HBTs at High Current Density
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1195-B04-01
Reliability of InP-based HBTs at High Current Density Yoshino K. Fukai NTT Photonics Laboratories, NTT Corporation, 3-1 Morinosato Wakamitya, Atsugi-shi, Kanagawa Pref. 243-0198, Japan, [email protected] ABSTRACT InP-based HBTs for ultrahigh speed optical communications systems operation at over 40 GHz require a long-term stability under high current injection conditions, such as current densities of 2 or 5 mA/µm2. We achieved high reliability by suppressing surface recombination and emitter-metal-related crystalline degradation. Changes in the electric properties of devices due to temperature and bias stress were evaluated. The reduction in DC current gain due to surface recombination had the activation energy of 1.7 eV without current density dependence, and the lifetime of HBTs for this degradation mode is predicted to be over 1x108 hours at 125°C. The emitter metal diffusion and disruption of uniformity of the atomic composition were observed by transmission electron microscopy and energy dispersive X-ray spectroscopy in HBTs with the conventional Ti/Pt/Au emitter, whereas suppression of those degradations was observed in HBTs with refractory metal of Mo and W. The emitter resistance was estimated to evaluate the contact layer degradation. The critical time was one order larger for HBTs with refractory metal than for HBTs with conventional metal. The activation energies for resistance increases were 2.0 and 1.65 eV for the current density of 2 and 5 mA/µm2, respectively, for all types of emitter electrodes. The effectiveness of the refractory metal electrode for improving device reliability was confirmed, especially in high-current-density operation, which is essential for applying InP HBTs in high-speed ICs. INTRODUCTION High-speed electrical devices are being developed with the aim of achieving terahertz-level performance. Progress in research and development in InP-based heterostructure bipolar transistors (HBT) technologies have led to the development ultrahigh-speed integrated circuits (ICs) [1]. Since InP/InGaAs HBTs have excellent advantages of low turn-on voltage and low power consumption, in addition to compatibility with optical devices, such as lasers and photo detectors [2], their introduction into optical communications systems has been given a boost. A remaining issue for InP-based HBTs is their reliability, which must be improved before they can be used in practical applications. In the early stage of development of InP- based HBTs, p-type dopants Be and Zn were commonly used, whose abnormal diffusion made transistor performance worse. Performance improved by replacing these p-type dopants with carbon [3] or by modifying epitaxial growth procedure [4]. Device performance often suffers due to surface degradation. Since defects or impurities introduced during fabrication processes such as dry etching and the deposition of metal or dielectric materials, cause surface defects, surface treatments or passivation techniques have been widely studied [5,6]. Recently the reliability of InP-b
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