Microprocessors

I-Introduction

Microprocessors have had a monumental impact on our society. Microprocessors are the heart of all computing and most electronic device. They are the basis of all the technological discoveries for the past two decades. A microprocessor contains a semiconductor chip, most often built from silicon, at its heart. In this paper I am going to briefly explain the way semiconductors changed our life then explain their constitution and how they work. But, today we are touching the limits of currents methods and techniques so; I am going to expose what could be the future of Microprocessors.

II-A bit of history

A microprocessor -- also known as a CPU or central processing unit -- is a complete computation engine that is fabricated on a single chip. The first microprocessor was the Intel 4004, introduced in 1971. The 4004 was not very powerful -- all it could do was add and subtract, and it could only do that 4 bits at a time. But it was amazing that everything was on one chip. Prior to the 4004, engineers built computers either from collections of chips or from discrete components (transistors wired one at a time). The 4004 powered one of the first portable electronic calculators.

Intel 8080

The first microprocessor to make it into a home computer was the Intel 8080, a complete 8-bit computer on one chip, introduced in 1974. The first microprocessor to make a real splash in the market was the Intel 8088, introduced in 1979 and incorporated into the IBM PC (which first appeared around 1982). If you are familiar with the PC market and its history, you know that the PC market moved from the 8088 to the 80286 to the 80386 to the 80486 to the Pentium to the Pentium II to the Pentium III to the Pentium 4. All of these microprocessors are made by Intel and all of them are improvements on the basic design of the 8088. The Pentium 4 can execute any piece of code that ran on the original 8088, but it does it about 5,000 times faster!

All these improvements were only possible with many technological and industrial discoveries involving new production techniques, more precise engraving, faster clock speeds and more. Below is a table that shows the evolution of Intel microprocessors since the 70s. It displays various data; Date: The year that the processor was first introduced. Many processors are re-introduced at higher clock speeds for many years after the original release date. Transistors: the number of transistors on the chip. You can see that the number of transistors on a single chip has risen steadily over the years. Microns: the width, in microns, of the smallest wire on the chip. For comparison, a human hair is 100 microns thick. As the feature size on the chip goes down, the number of transistors rises. Clock speed: the maximum rate that the chip can be clocked at. Data Width: the width of the ALU. An 8-bit ALU can add/subtract/multiply/etc. two 8-bit numbers, while a 32-bit ALU can manipulate 32-bit numbers. An 8-bit ALU would have to execute four instructions to add two 32-bit numbers, while a 32-bit ALU can do it in one instruction. MIPS: stands for "millions of instructions per second" and is a rough measure of the performance of a CPU. Modern CPUs can do so many different things that MIPS ratings lose a lot of their meaning, but you can get a general sense of the relative power of the CPUs from this column.

Microprocessor Progression: Intel

Name Date Transistors Microns Clock speed Data width MIPS

8080 1974 6,000 6 2 MHz 8 bits 0.64

8088 1979 29,000 3 5 MHz 16 bits

8-bit bus 0.33

80286 1982 134,000 1.5 6 MHz 16 bits 1

80386 1985 275,000 1.5 16 MHz 32 bits 5

80486 1989 1,200,000 1 25 MHz 32 bits 20

Pentium 1993 3,100,000 0.8 60 MHz 32 bits

64-bit bus 100

Pentium II 1997 7,500,000 0.35 233 MHz 32 bits

64-bit bus ~300

Pentium III 1999 9,500,000 0.25 450 MHz 32 bits

64-bit bus ~510

Pentium 4 2000 42,000,000 0.18 1.5 GHz 32 bits

64-bit bus ~1,700

Compiled from The Intel Microprocessor Quick Reference Guide

III - Semiconductors:

Microchips are constituted of semiconductors. So, in order to understand how they work, let's take a look into semiconductors. Today, most semiconductors are constituted by silicon. Silicon is a very common element it sits next to aluminum, below carbon and above germanium in the periodic table. Carbon, silicon and germanium (germanium, like silicon, is also a semiconductor) have a unique property in their electron structure -- each has four electrons in its outer orbital. This allows them to form nice crystals. The four electrons form perfect covalent bonds with four neighboring atoms, creating a lattice. In carbon, we know the crystalline form as diamond. In silicon, the crystalline form is a silvery, metallic-looking substance.

In a silicon lattice, all silicon atoms bond perfectly to four neighbors, leaving no free electrons to conduct electric current. This makes a silicon crystal an insulator rather than a conductor.

Metals tend to be good conductors of electricity because they usually have "free electrons" that can move easily between atoms, and electricity involves the flow of electrons. While silicon crystals look metallic, they are not, in fact, metals. All of the outer electrons in a silicon crystal are involved in perfect covalent bonds, so they can't move around. A pure silicon crystal is nearly an insulator -- very little electricity will flow through it.

You can change the behavior of silicon and turn it into a conductor by doping it. In doping, you mix a small amount of an impurity into the silicon crystal. There are two types of impurities:

* N-type - In N-type doping, phosphorus or arsenic is added to the silicon in small quantities. Phosphorus and arsenic each have five outer electrons, so they're out of place when they get into the silicon lattice. The fifth electron has nothing to bond to, so it's free to move around. It takes only a very small quantity of the impurity to create enough free electrons to allow an electric current to