A.5.2 Bipolar and BiCMOS

Considering, e.g., a $\rm 1\mu m$ technology the bipolar junction transistor (BJT) has many advantages over the MOSFET in terms of speed, current drive, transconductance, matching of and noise figure [1,37,42,28]. Emitter-coupled logic (ECL) is a well-established circuit technique to exploit these advantages in digital circuits and has been in use for high-performance computers since the early days of IC technology. On the negative side is the high static power consumption and the small packing density of ECL in comparison to CMOS.

The idea to combine the advantages of BJT and MOSFET to have digital circuits with (almost) zero static power consumption and high speed and low output resistance is fairly old (in terms of IC history) but it was not until the early 90s that Bipolar CMOS (BiCMOS) found broad interest and application. Fig. A.31 shows a typical BiCMOS inverter. Although the unloaded delay of this circuit is larger than that of the plain CMOS inverter the BiCMOS circuit is much faster for large fan-out due to its small sensitivity to capacitive loading. In a digital BiCMOS VLSI circuit typically only a few 0.1-1 percent of the devices are BJTs, which are used in time critical paths and IO circuits.

Despite the advantages of BiCMOS technology in general there are two fundamental limitations to digital BiCMOS in particular: poor voltage scalability and poor speed improvement. The fact that a base-emitter voltage of $V_\mathit{BE} \approx \rm0.8V$ is needed for a fast turn-on imposes a lower limit of about 2.0-2.5V on the supply voltage and even complementary BiCMOS maintains its advantage over CMOS only down to 1.5V [33,17]. Furthermore, despite the fact that for larger feature sizes the BJT is faster than the MOSFET the speed of bipolar devices does not scale as fast so that the speed of CMOS devices will reach that of bipolar devices at a feature size of about $0.2\rm\mu m$ [37].

Both limitations preclude BiCMOS as a candidate for ULP as well as for future high-performance digital VLSI technologies. In analog, high-frequency, and mixed applications, however, BiCMOS offers still great advantages even at smaller feature sizes because of the good noise properties of the BJT.

Figure A.31: BiCMOS inverter