Introduction

Abstract

Of the many technologies available for designing and building integrated circuits (IC), the complementary metal oxide semiconductor (CMOS) transistor technology has the dominant share of the IC market [3]-[5]. The word complementary refers to the existence of two different types of MOS transistors that complement each other. These two transistors are typically connected so as to eliminate any static power consumption. The two types of transistors are the N-channel (or NMOS) transistor and the P-channel (or PMOS) transistor. These two types of transistors are shown in Figure 1.1. The transistors are termed metal oxide semiconductor (MOS) due to the physical structure of the transistors. The gate is composed of a conducting material, typically polycrystalline silicon (polysilicon), although historically metal was originally used [3]-[5]. The gate is separated from the semiconductor silicon channel by a silicon dioxide (SiO₂) insulating layer. The channel connects the drain to the source of the transistor. For an NMOS transistor, a high voltage (usually V _DD in a digital circuit) applied between the gate and the source turns the transistor on and creates a path (or channel), permitting electrons to flow between the drain and the source. A low voltage (usually ground in a digital circuit) eliminates the path (or channel), turning the transistor off. These conditions of operation are reversed in polarity for a PMOS transistor. The gate electrode acts as the input or control point of the transistor. An insulating layer of silicon dioxide (SiO₂) between the gate and the channel eliminates any current from passing into the channel or substrate from the input gate. This feature significantly reduces the power consumption of a MOS transistor-based circuit as compared to the power consumption of circuits based on bipolar or Metal Semiconductor Field Effect Transistors (MESFET) in Gallium Arsenide (GaAs) technologies as well as provide isolation among the transistors.