Bipolar Technology

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Introduction

The development of a bipolar technology for integrated circuits (ICs) went hand in hand with the steady improvement in semiconductor materials and discrete components during the 1950s and 1960s. Consequently, silicon bipolar technology formed the basis for the IC market during the 1970s. As circuit dimensions shrink, the MOSFET (or MOS) has gradually taken over as the major technological platform for silicon ICs. The main reasons are the ease of miniaturization and high yield for MOS compared with bipolar technology. For VLSI circuits the low standby power of complementary MOS (CMOS) gates is a significant advantage compared with integrated bipolar circuits.

The evolution of MOS technology has followed the famous Moore’s law that predicts a steady decrease in gate length. Bipolar technology has also benefited from the progress in lithography and is currently fabricated using deep UV tools with feature sizes close to 100 nm. The scaling has led to a

significant performance improvement and is further illustrated in Figure 1.1, where the reported gate delay time for emitter coupled logic (ECL) and current mode logic (CML) circuits is plotted for a 10-year period. In addition to the reduced dimensions, the introduction of SiGe epitaxy for the base region has further pushed the performance limits. SiGe bipolars are now considered a mature technology and is mainly offered as a high-speed complement to the low-power MOS in the so-called BiCMOS technology. By adding a small amount of carbon to the SiGe epitaxial base, better profile control and compatibility with MOS process flows have been obtained [1].

A mature Si bipolar technology with implanted base at the 0.25 µm MOS technology node offers 12 ps gate delay and can be used to realize 10 Gb/s ICs [2]. The continuous performance increase owing to reduced dimension is illustrated in Table 1.1, where several generations of a commercial BiCMOS technology are compared [3]. As the dimensions are reduced, the traditional local oxidation of silicon (LOCOS) isolation technology is replaced by shallow and deep trenches to increase the packing density, and also to optimize the process flow by getting a more planar structure. As seen in the table, the epitaxial SiGe base markedly improves the device performance at the same technology node.

Apart from high-speed performance, the bipolar transistor is recognized by its excellent analog prop- erties which feature high linearity, superior low- and high-frequency noise behavior as well as very high transconductance [4]. Such properties are highly desirable for many RF applications, both for narrow- band and broad-band circuits [5]. The high current drive capability per unit silicon area makes the bipolar transistor suitable for input/output stages in many IC designs (e.g., in fast SRAMs). The dis- advantage of bipolar technology is the low transistor density, combined with large power dissipation.

Bipolar Technology-0000

High-performance bipolar circuits are therefore normally fabricated at a modest integration level (MSI/LSI). By using BiCMOS design, the benefits of both MOS and bipolar technology are utilized [6]. One example is mixed analog/digital systems, where a high-performance bipolar process is integrated with high-density CMOS [7]. This technology forms a vital part in several system-on-a-chip designs for telecommunication and wireless circuits.

In this chapter, a brief overview of bipolar technology is given with an emphasis on the integrated silicon bipolar transistor. The information presented here is based on the assumption that the reader is familiar with bipolar device fundamentals and basic VLSI process technology. Bipolar transistors are treated in detail in well-known textbooks by Ashburn [8] and Roulston [9]. Section 1.2 will outline the general concepts in bipolar process design and optimization. Three generations of integrated devices representing state-of-the-art bipolar technologies for the 1970s, 1980s, and 1990s will be presented in Sections 1.3, 1.4, and 1.5, respectively. Finally, some future trends in bipolar technology are outlined.

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