This is the first installment of a two part article that examines CMOS, the nearly perfect technology for implementing the digital logic that powers much of our modern age. The n-type and p-type field effect transistor are introduced along with how these are combined to form basic logic gates. In the second part I’ll examine performance issues and the methods microprocessor circuit designers use to reach today’s GHz clock rates.
The Straight Dope on CMOS
A single technology underlies the modern microprocessor, and indeed, just about every integrated circuit you will find inside computers, telecommunications equipment, and digital consumer electronic devices. That technology is called CMOS, or Complementary Metal Oxide Semiconductor. CMOS is a nearly perfect silicon based technology with which to implement digital logic. It is fast, flexible, dissipates low power, has excellent noise immunity, and scales nicely with shrinking feature size and reduced supply voltage. Constant improvement to photolithographic techniques allows the number of transistors that can be incorporated into a mass-produced silicon chip to grow exponentially in time, a trend commonly known as Moore’s Law. What is less well known is that without CMOS technology the degree to which this trend can be harnessed for practical applications would be severely constrained and many areas of microelectronics would be at least a decade behind where they are today.
Silicon is called a semiconductor because its electrical properties lie in between a good insulator like glass, and a good conductor, like copper. Pure silicon is a poor conductor at room temperature because it has very few free charge carriers. Nearly all electrons in silicon reside in a low energy state called the valence band where they are tied to individual atoms within the crystal lattice, and are thus unable to move and carry electrical current. It takes a relatively large amount of energy to kick an electron from the valence band to the conduction band where it is free to move and thus create electrical current. An electron that moves into the conduction band in pure silicon leaves behind a so-called hole, a missing covalent bond between a silicon atom in the crystal lattice and its neighbors. Remarkably enough, a hole acts as a kind of positive charge carrier. In response to an electric field, an electron from the neighboring atom in the direction of the electric field will shift into the position of the missing covalent bond leaving a gap in its place. Thus a hole effectively moves in the direction of an applied electric field.
In pure silicon every electron in the conduction band has a corresponding hole in the valence band. These are called an electron-hole pair. There is a very strong tendency for a free electron to fall back into the valence band and eliminate the hole, a process called electron hole recombination. Because of this there are relatively few electrons or holes to act as charge carriers in pure silicon, and thus it is a good insulator. By introducing an atom of an element with 3 valence electrons (one less than silicon) into pure silicon a hole will be created in the valence band. Because there is no corresponding electron in the conduction band looking to recombine with this hole it will persist over time. An example of a 3 valence electron (or acceptor) atom is boron. Because a hole acts like a virtual positive charge carrier, silicon that has been deliberately contaminated (or doped) with acceptor atoms is called p-type or p-doped silicon. The greater the doping level (i.e. the concentration of acceptor atoms in the silicon) the more holes there are per unit volume and the greater the conductivity of the doped silicon. In a similar fashion silicon can be doped using an element with 5 valence electrons, one more than silicon. The extra valence electron is free to move in the conduction band because it has no corresponding hole for it to recombine with. A valence 5 element, or donor, is called an n-type dopant because an electron is a negative charge carrier. An example of a donor element is phosphorous.
When a region of n-type silicon is formed next to a region of p-type silicon the interface is known as a pn junction, which is the basis of the diode, an electronic component that only conducts current in one direction. Because current is carried by holes in p-type silicon and electrons in n-type silicon, current normally can only flow in the direction p to n (so called forward bias). This current is composed of a stream of holes in the p-type silicon that flows to the pn junction and recombine with a corresponding stream of electrons in the n-type silicon that also flows to the junction. Current can’t flow in the opposite direction (reverse bias) because that would require electron hole pair generation at the junction rather than electron hole pair recombination. Except in special cases (like a photodiode exposed to light) there is no energy source available to create electron hole pairs at the junction and thus current can’t flow.
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