ABSTRACT
CONTENTS 19.1 Introduction ...................................................................................................................... 552 19.2 Device Structures ............................................................................................................. 553
19.2.1 Vertical npn BJTs ................................................................................................ 553 19.2.2 Vertical, Lateral, and Substrate pnp BJTs........................................................ 553
19.3 Radiation-Induced Defects in BJTs................................................................................ 554 19.3.1 Overview.............................................................................................................. 554 19.3.2 Oxide-Trapped Charge ...................................................................................... 554 19.3.3 Interface Traps..................................................................................................... 554 19.3.4 Displacement Damage ....................................................................................... 555 19.3.5 Relationship between Defects and Device Degradation ............................... 556
19.4 SRH Recombination ........................................................................................................ 556 19.5 Bipolar Devices................................................................................................................. 558
19.5.1 Current Components.......................................................................................... 558 19.5.2 Recombination in the Emitter-Base Depletion Region ................................. 559 19.5.3 Recombination in the Neutral Base.................................................................. 560 19.5.4 Current Gain........................................................................................................ 561
19.6 Enhanced Low-Dose-Rate Sensitivity (ELDRS) .......................................................... 562 19.6.1 Overview.............................................................................................................. 562 19.6.2 ELDRS Mechanisms ........................................................................................... 563
19.6.2.1 Physical Models ................................................................................... 563 19.6.2.2 Temperature Effects on ELDRS......................................................... 565 19.6.2.3 Electric Field Effects on ELDRS......................................................... 566
19.7 Impact of Hydrogen on Defects in BJTs....................................................................... 567 19.8 Electron Capture, Hydrogen Release, and ELDRS..................................................... 568 19.9 Summary........................................................................................................................... 571 Acknowledgments ..................................................................................................................... 571 References.................................................................................................................................... 571
Bipolar junction transistors (BJTs) play critical roles in modern integrated circuits (ICs), ranging from low-cost analog ICs to high-performance silicon-germanium (SiGe) bipolar complementary metal-oxide-semiconductor (BiCMOS) technologies. In applications such as space systems, defense systems, medical facilities, and high-energy particle accelerators these ICs may be exposed to radiation. The principal effect of total ionizing dose (TID) and displacement damage on bipolar transistors is an increase in the number of defects that participate in Shockley-Read-Hall (SRH) recombination (either at the Si=SiO2 interface or in the Si bulk) [1,2]. These defects are manifested at the device level as increases in the base current, which lead directly to reductions of the current gain. Bipolar and BiCMOS technologies are particularly important for analog, radio fre-
quency (RF), and mixed-signal ICs. High-speed bipolar transistors (fT> 100 GHz) based on SiGe are particularly important for the wireless communications market [3]. However, much of the market for BJTs remains in the traditional silicon-based analog IC area. Many of these analog ICs are fabricated in relatively old technologies with relaxed feature sizes compared to state-of-the-art CMOS ICs. Irradiated bipolar transistors may exhibit long-term degradation of their electrical prop-
erties, possibly resulting in functional failure, or transient effects that occur due to collection of radiation-generated charge at device junctions. Transient radiation effects caused by individual ionizing particles (single-event effects) are a serious problem for electronics operated in space, and are becoming an increasingly important issue for advanced technologies at sea level. The charge deposited by a single ionizing particle can produce a wide range of effects, including single-event upset, single-event transients, single-event functional interrupt, single-event latchup, single-event dielectric rupture, and others. The physical structure of SiGe heterojunction bipolar transistors (HBTs), with a large reversebiased collector junction surrounded by deep trench isolation, makes them particularly vulnerable to single-event effects [4]. While these transient effects are important for many applications, they are not considered further in this work. The focus of this work is on defects produced by the cumulative amount of energy
that goes into electron-hole pair creation. The effects of this radiation, typically called TID radiation, are manifested as semipermanent changes in the BJT current-voltage characteristics. The TID response of BJTs is dominated by defects created at the Si=SiO2 interface over the base region or by charge trapping in the oxide that covers the base. In addition to TID effects, we also consider the effects of displacement damage in the Si substrate. This damage is usually related to the amount of nonionizing energy loss (NIEL), and it is much more significant for particle irradiation than for photon irradiation. The oxides that affect the TID radiation response of bipolar transistors are generally
field or isolation oxides that have properties significantly different from those of highquality gate oxides. They are much more likely to see high temperatures during the fabrication sequence and to serve as masking layers during ion implantation. The density of defects in these oxides is frequently quite high and there is usually a great deal of hydrogen present. The radiation response of these oxides may be much more complicated than that observed in MOS-quality oxides; in particular, the amount of degradation that occurs at a given total dose may depend on the dose rate at which the energy is deposited.
