2 - VLSI circuit technologies

Abstract

There are two classes of silicon devices—bipolar transistors and unipolar transistors. They differ in that both majority and minority carriers participate in the bipolar transistor action while only minority carriers participate in the unipolar transistor. The main bipolar circuit techniques are emitter-coupled logic/current mode logic (ECL/CML), integrated injection logic (I2L), and transistor-transistor logic (TTL). These circuit techniques are not suitable for large integrated circuits for various reasons. There are several types of unipolar devices, for example, metal oxide semiconductor (MOS) transistor, junction field-effect transistor (JFET), and metal semiconductor field-effect transistor (MESFET). In practice, only MOS transistors are used for very large scale implementation (VLSI) circuits. The main unipolar circuit technique is complementary metal oxide semiconductor (CMOS) circuits, but numerous alternatives exist. CMOS circuits are often augmented with a few bipolar devices, called BiCMOS, to achieve higher speed in critical parts. Gallium arsenide (GaAs)-based VLSI circuits have recently become available. They are important because of their high speed and compatibility with optical components such as lasers. GaAs-based circuits are interesting candidates for many digital signal processing (DSP) applications. The main advantages of CMOS over nMOS circuits are low power dissipation, high noise margin, high speed, robustness, and ease of design. Disadvantages of CMOS compared to nMOS are slightly lower device density and a more complex manufacturing process. However, these drawbacks tend to be small for modern VLSI processes and circuit techniques.