Why did CMOS become the dominant technology?

Ken Deevy

Why did CMOS become the dominant technology? Before I get to that question, let’s rewind a bit and consider the origins of CMOS.

CMOS is an acronym which stands for Complementary Metal Oxide Semiconductor. It is so named because circuits fabricated in a CMOS technology utilise both NMOS and PMOS transistors, i.e., using complementary p-type and n-type transistors. Originally CMOS technology was used mainly for constructing digital logic circuits, microprocessors, and memory chips. Nowadays CMOS is used for a wide spectrum of functions which also includes analog circuits, image sensors, data converters, RF circuits, and a wide range of mixed signal circuits (digital and analog circuits).

The ‘metal oxide semiconductor’ part of the CMOS acronym refers to the physical structure of MOS field-effect transistors, having a Metal gate control electrode, Oxide insulator and a Semiconductor channel material. Aluminium was originally used as the gate material, hence the use of M in the name, but this was later replaced by a polysilicon gate because of its compatibility with the self-aligned process (SAP), an important innovation in the development of CMOS fabrication technology. However, in more recent years metal gates have made a comeback with the advent of the 45 nm process node and smaller sizes.

The CMOS process was originally conceived by Frank Wanlass at Fairchild Semiconductor around 1963. Prior to the development of CMOS, PMOS and later NMOS technology was used in the manufacture of digital logic and microprocessors. However, CMOS overtook NMOS as the dominant MOSFET technology for logic and VLSI ICs in the 1980s, and significantly also replaced the earlier TTL and LS-TTL logic technologies, which were based on the bipolar transistor. Currently, 99% of all integrated circuits, including digital VLSI, microprocessor, analog, and mixed-signal ICs, are fabricated using CMOS technology. Other technologies such as Bipolar, BiCMOS, and Gallium Arsenide are still available because of their use in high speed and low noise specialist applications, but these technologies contribute only a small proportion of the overall semiconductor market.

Dominance
There are several reasons why CMOS is so important and dominant. High among these reasons are, higher circuit integration, lower static power (digital), lower cost of manufacturing (due in large part to the higher circuit densities achievable) and good noise margin.

Other Features
But CMOS also has some other interesting features. Because it is an electric field operated device a MOS transistor has an extremely high input resistance. This is due to the gate oxide which isolates the gate electrode from the conducting channel. This contrasts with the bipolar transistor where there is a base current to contend with. High input impedance also greatly improves the fan-out capability of logic devices.

MOS transistors scale better with reducing power supplies than do bipolar transistors due to the manufacturer being able to determine the threshold voltage during the fabrication process. A MOSFET has four terminals compared to a Bipolar transistor which has only three. What’s more important here though is that the analog circuit designer has an extra degree of freedom in configuring the design because both the device width (W) and length (L) are available as selectable design parameters. The same flexibility is not available with bipolar transistors. Moreover, a MOS transistor is symmetrical and bidirectional, i.e., the source and drain are physically the same which gives flexibility to the physical layout of a chip as well as the fabrication.

Disadvantages
It’s not all plain sailing when it comes to CMOS technology. CMOS devices generally have much poorer transconductance when compared to their Bipolar cousins which presents a challenge when trying to design high gain amplifiers. MOS transistors are slower (lower fT) than bipolars and depending on the current that they are operating at they are generally much noisier than their bipolar counterparts due to high levels of flicker noise (1/f noise).

Summary
In summary CMOS logic circuits draw extremely low current in standby operation, mainly because of the complementary nature of their circuit arrangement. The static power consumption of a conventional CMOS logic chip is extremely low. However, power consumption during switching is increased and is directly proportional to the clock frequency, referred to as dynamic power consumption. Overall, this tendency for low power operation combined with their inherent simplicity of fabrication and high integration levels has made CMOS the natural choice for manufacturing very high-density VLSI digital and mixed-signal microchips.

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