Semiconductors are the foundation for all electronic devices. Without them there would be no home computers, small radios, CDs, or any of the thousands of other devices we take for granted.
Conductors have atoms with lots of loose electrons that can be caused to move in an electrical current when a voltage is applied. Insulators have no free electrons and will not conduct current. Semiconductors have a few free electrons but usually not enough to conduct useful amounts of current.
Before taking a closer look, we need to deal with ions. An ion is an atom that has lost an electron, or gained an electron. Recall that atoms usually have the same number of protons (+ve charge) as electrons -ve charge) and are electrically neutral. An atom that is missing an electron has 1 less -ve charge and is a positive ion. An atom that has an extra electron has one more -ve charge and is a negative ion. Semiconductors take advantage of positive and negative ions. Positive ions can collect electrons, and negative ions can give up electrons.
Most semiconductors are made from silicon (sand) and some are made from germanium. These materials have 4 electrons in the other layer, and they bond with neighbouring atoms to create perfect crystal structures with no free electrons. This all changes when impurities, called donors, are added. Some donors, called n-type (for negative) provide free electrons. Other donors, called p-type (for positive), collect free electrons.
N-type semiconductors have either phosphorus or arsenic added. Both these impurities have 5 electrons in the other layer, and when they become part of the crystalline structure, they have 1 free electron thus creating negative ions in the semiconductor. Negative ions have free electrons to give out and can conduct electricity.
P-type semiconductors have either boron or gallium added. Both of these materials have only 3 electrons in the outer layer. When they become part of the crystalline structure they are missing an electron, and become positive ions. Positive ions can collect electrons and can conduct electricity.
The basic form of semi-conductor is called a diode. Its what you get when you bond a piece of n-type material to a piece of p-type material.
Figure A Diode is a piece of P-type material bonded to a piece of N-type material
When P-type and N-type material are joined, the area on both sides of the junction loses its ability to conduct electrons. This so called 'depletion zone' is the effect that makes the diode what it is. The depletion zone is, in effect, an insulator.
Figure Depletion Zone at N-P Junction
Connecting the diode to a battery has interesting effects. By connecting the negative terminal of the battery to the N-type material and the positive terminal to the P-type material, the depletion zone disappears and current will flow through the diode.
Figure Diode acting as Conductor—Current is Flowing
Connected the other way, the depletion zone gets bigger, the diode acts like an insulator, and current will not flow through the diode.
Figure Diode acting as Insulator—No Current Flow
The diode is the building block for many electronic devices including transistors and integrated circuits.
A transistor is a device that is effectively made from two diodes. There are two types—NPN, and PNP, named for the way the N-type and P-type material is bonded.
Figure NPN and PNP Transistors
Transistors exhibit even more interesting effects. Ordinarily, they will not conduct electricity from one end to the other. When a small current is applied to the middle layer, a large current can be conducted across the whole transistor (from N to N, or from P to P). This effect makes them very useful devices.
Transistors can be used in two basic ways
When used as a switch, turning on and off the current to the center layer switches on and off the two ends. When used as an amplifier, varying the current to the middle layer has an amplified effect on the current across the two ends.
Transistors are used as switches in computers. The CPU (central processing unit), ROM, and RAM are made up of millions of very tiny transistors. Roughly speaking, a transistor in the on position represents a one (1), and a transistor in the off position represents a zero (0). This makes binary (base 2 numbers, or 0s and 1s) the native communications language of computers.
Integrated circuits are electronic circuits designed and fabricated in miniature form on a single chip of silicon. It can have anywhere from a few devices (diodes, transistors, resistors, and capacitors) to millions of devices, all on a chip as big around as a pencil.
Using the switching function of transistors, boolean logic gates are constructed. Logic gates provide a way of dealing with numbers and making logical decisions (logical OR, AND, NOT, NAND). Physical logic gates in microprocessors provide the foundation for interacting with programs and executing instructions.
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