A transistor is a tiny electrical switch. An average CPU contains over 25 million transistors. Transistors are made out of materials know as semiconductors. Semiconductors are the transitional materials between Conductors/Metals and Nonconductors/Nonmetals. These elements don't really prevent or insulate electrical current, and they don't really allow or conduct electrical current. Instead, we can control their behavior and make them selectively conductive and selectively nonconductive. Is how we can use them as a switch.

The element used to make transistors is called Silicon. The major source for silicon is sand. But pure silicon isn't ideal for electronics, so a form of silicon is used called dope silicon. Dope silicon is created by combining silicon with either Boron or Phosphorus, depending on the properties needed.

Boron and silicon form what is know as P-Type dope silicon. P-Type silicon has a positive charge. Phosphorus and silicon form what is know as N-Type dope silicon. N-Type has a negative charge. Using these to materials, a wide variety of transistors and logic gates can be created.

A transistor is a semiconductive device with three leads or connections. At the Gate connection, a very small electrical current can be used to control the amount of current that is allowed to pass from the Drain to the Source. A transistor has purposes, it can be an amplifier or a switch. So there are 2 different types of transistors, Bipolar and Field Effect. In order for a CPU to opperate, the transistors must act as switches and be very small so field effect transistors must be used. Computers only work with on or off signals, 1's or 0's.

The type of transistor used in processor cores are named Metal Oxide Semiconductor Field Effect Transistors. This is because of the way that they are made. MOSFET's can be either N-Type or P-Type.

This is the basic MOSFET layout.

N-Type MOSFET Operation

The Source and the Drain are both made out of N-Type silicon. The is a section of P-Type silicon in between the two connections with the Gate right above it. Without any current at the gate, electrons are not allowed to travel from the drain to the source. When a positive current is applied to the Gate, electrons from the lower sections of the P-Type silicon are attracted to the positive charge and move up to be near the gate. This neutralizes the P-Type's positive charge in the section right under the gate, creating a negative "N-Type" bridge between the drain connector and the source connector. The stronger the voltage that is applied at thegate, the more electrons will be allowed to move from the drain to the source.

The Source and the Drain are both made out of P-Type silicon. The is a section of N-Type silicon in between the two connections with the Gate right above it. Without any current at the gate, electrons are not allowed to travel from the drain to the source. When a negative current is applied to the gate, the extra electrons under the gate spread out into the lower sections of the N-Type silicon being repelled by the negative gate. This neutralizes the N-Type's negative charge in the section right under the gate, creating a positive "P-Type" bridge between the drain connector and the source connector. The stronger the voltage that is applied at the gate, the more electrons will be allowed to move from the source to the drain.

Transistors themselves are of limited use, but they can be arranged into patterns to create Logic Gates that have tremendous functionality. These logic gates have know patterns for know inputs, which can be arranged in what is known as a True Table.

AND OR NOR  NAND   NOT  XOR XNOR ab y 00 0 01 0 10 0 11 1 ab y 00 0 01 1 10 1 11 1 ab y 00 1 01 0 10 0 11 0 ab y 00 1 01 1 10 1 11 0 a y 0 1 1 0 ab y 00 0 01 1 10 1 11 0 ab y 00 1 01 0 10 0 11 1

Truth tables are fairly simple to understand. Logic gates are made out of numerous transistors, and they have been named according to their operation. These gates, except for the NOT gate, all have 2 inputs, labeled a and b, and have one output, labeled y. These circuits can be designed to have more, but this is their simplest form.

The AND Gate : Output for this logic gate is always false, or 0 value whenever both of the inputs are not true, or 1. For output to be 1, both a AND b both have to be true.

The OR Gate : For output to be true, either one or both of the inputs have to be true.

The NOR Gate : This logic gate will always have the opposite value of the OR gate. It is basically can be thought of as an OR gate with a NOT gate right after the output.

The NAND Gate : Output will always be the opposite of what an AND gate would output. Again, this can basically be thought of as an AND gate with a NOT gate right after the output.

The NOT Gate : This gate has one input because it doesn't do "comparisons", the output is just the opposite of the input.

The XOR Gate : This is similar to an OR gate, but differs in that only one input can be 1 for the output to be one. It is exclusive, if both inputs are either 1 or 0, than the output will be zero. Both inputs must be different, although it doesn't matter which one is high, or which one is low.

The XNOT Gate : This is similar to the XOR gate, but the output is reversed. It can be basically thought of as a XOR gate with a NOT gate right at the output. Both inputs must be the same for the output to be 1.

Transistor manufacturing performance have made transistor speeds increase exponentially in the past decades, from the very first processors like the 68000 which operated at a speed of 8MHz, to the recent chips which are breaking the 1GHz barrier.

Transistor Manufacturing
Transistors are made through a process called photolithography. This is where light is used to etch the transistors into a sheet of silicon called a waffer. How this is done is the silicon is coated with a thin sheet of light reactive material. Then light is projected through a negative screen, reacting only portions of the coating. When the coating reacts, it hardens to the silicon. Then the unreacted coating is removed from the silicon waffer, and the waffer is exposed to acid or hot ions. This will disolve the exposed silicon, but will not disolve the silicon under the coating. After this process, the coating is removed by special reactive materials, which do not effect the silicon.

Interconnect Materials
Most of todays processors use connectors made out of aluminum to connect the transistors. This is because aluminum is economical, and can more accurately be placed on the silicon because it has less surface tension. Technology is just beginning to allow for copper based processors, which are far superior than aluminum based ones. Copper produces less heat, and cares higher frequency signals with less resistance.

Transistor Size
Transistor sizes are the main reason that current processors are able to hit such high speeds. One property of electrical currents is that they operate much quicker and with less resistance at lower operating temperatures. The major factor limiting CPU speeds is heat dissipation from the processors. Electricity will always experience resistance when moving through materials. This is why some poorer quality electrical chords get warm when electricity is flowing through them, and the reason for why light bulbs and fuses work. Electrical currents passing through the processor create heat which, if hot enough will cause the processor to stop working, malfunction or burn out.

Smaller transistors in the processor use less voltage to operate, and therefore create less heat. As processor frequencies increase, more voltage is needed to keep up signal strength for each clock signal. The newer top of the line processors use heatsinks that are larger than a medium sized alarm clock just to keep cool.

Transistor size is calculated in microns (u), properly named, micrometers (um), which are 10^-6meters. There are exactly 1000um in one 1mm. When referring to micrometers in regards to transistor size, the unit is represented with a "u". This is referred to as the "micron process" of a processor, because the processor uses transistors which are "X"um's in width. Early processors used a 3.5u process, with today's processors using a 0.18u process.