Semiconductors and Transistors 101
Image Credit: Wikipedia
As a professor I would occasionally have a student ask, “but how can something be a SEMI conductor, surely it either conducts or insulates”, a not unreasonable question. Well, you are about to find out what a semiconductor is and discover how it is used to control virtually all of your electronics.
This series of posts provides a lower level introduction for the newcomer wanting to know a little more.
Due to the amount of material on these topics I have split this post into 3 bite sized reads, hopefully making them a little more digestible.
- Semiconductors and the diode
- Bipolar junction transistors – BJT
- Field effect transistors – JFET, MOSFET
As always if you need any clarifications please e-mail me.
Semiconductors
The word semiconductor is often used in everyday conversations to describe the active electronic parts of our everyday AV devices. So lets see what a semiconductor is.
We saw from this post what a conductor is and by inference what an insulator is. Conductors have lots of available free electrons, insulators have virtually no free electrons. A semiconductor is not a conventional conductor material like copper, silver, gold, aluminum etc. or insulator like plastic, rubber etc. that have in some way been atomically modified, it is a whole new class of material.
Most semiconductor materials are based upon two raw types of base materials known as intrinsic materials:
- Germanium (original material but will be not considered here as it is rarely used today due primarily to its intolerance to high temperatures)
- Silicon – now the most common semiconductor material
(A third semiconductor material is also growing in popularity particularly for very high speed and high power switching for both RF and audio. Gallium Nitride (GaN) is extensively used to make blu-ray lasers and field effect transistors that are finding their way into Class D power amplifiers due to their very high switching speeds and low power dissipation.)
It is the atomic structures of these materials that make them suitable to being turned into semiconductors. We will not delve too deep into those structures other than to say that they can readily be modified at the atomic level to have excess electrons or holes (missing electrons) without disrupting the materials atomic structure and atomic bonding.
(Advanced note: These materials each have 4 valence electrons. Valence electrons are those in the outer most orbits of an atom that are available to share with, or be transferred to another atom.)
These materials in their natural state are poor conductors of electricity. The question is what do we do to them to turn them into semiconductors? The simple answer is that we add, at the atomic level, impurities to them that cause them to either have an excess of electrons or a shortage of electrons. In semiconductor jargon a missing electron is called a hole, is defined as having a positive charge and to the onlooker looks like a +ve electron.
The addition of these impurities is known as doping and may include the following materials; boron, phosphorous, aluminum, indium, arsenic and antimony. Once doped the materials are referred to as extrinsic semiconductors.
When a semiconductor material has an excess of electrons it is referred to as N type. When a semiconductor material has an excess of “holes” (a shortage of electrons) it is referred to as P type.
N-type material may be made by doping with phosphorous or arsenic, materials with 5 valence electrons. This creates 5-4=1 extra electron per additional atom of doping material. P-type materials may be made by doping with gallium or indium, materials with 3 valence electrons. This creates 3-4=-1 electron, a shortage or one hole per additional atom of doping material.
N-Type Silicon P-Type Silicon
The importance of these two extrinsic semiconductor materials is only realized when they are in intimate atomic contact with each other.
The Diode
When an intrinsic material is doped P type on one side and N type on the other side, something very special happens where the P and N types of extrinsic material meet. The excess electrons from the N-type migrate to the P-type and leave holes in the N-type material. Eventually the movement of the electrons and holes sets up an electrical field across the PN junction stopping any further migration of electrons and holes. This area is now depleted of mobile charge carriers (electrons and holes) and is known as a DEPLETION layer.
The electrical charge imbalance creates a depletion layer voltage of 0.30-0.35 volts for germanium and 0.60-0.70 volts for silicon.
So what is so special about the P-N junction? Lets see what happens when we apply an external DC voltage across it.
With the battery connected as above (forward biased) as the voltage is continually increased it causes the depletion layer to become narrower and narrower, eventually removing it all together. This occurs at the depletion layer voltage. If the voltage continues to increase electrons will start to flow through the junction and the device becomes conductive. If the battery polarity is reversed (reverse bias) and the voltage increased, the depletion layer gets wider and wider and no current flows until the device breaks down. The devices resulting characteristic is as shown below:
Typical Si Diode Characteristic
Once we exceed the depletion layer voltage, the device only passes current in one direction, similar to a diode valve. A PN junction is therefore referred to as a diode or rectifier having an anode (+ve) and cathode (-ve).
Diode Symbol
There are about fifteen basic classes of semiconductor diodes optimized for performance as rectifiers in DC/switched mode power supplies, RF rectification/detection, light emitting diodes (LED), photo electric diodes, variable capacitance diodes, logic diodes etc. and each class has hundreds of types. Also more than one can be fabricated into a package, e.g. for AC rectification in DC power supplies they can be fabricated as a pair or as a bridge of four. One of the more common silicon power rectifiers is the 1N4001 – 1N4007 series.
A Selection Of Typical Symbols
Typical Individual Devices
So now you have an understanding of what a semiconductor is, the types and what a p-n junction depletion layer is. In the next two posts we shall look at how these extrinsic materials, this p-n junction and depletion layer are used to create the different types of transistors.
Here are the other two parts to this post:
- Semiconductors and Transistors 101 – The BJT (Bipolar Junction Transistor)
- Semiconductors and Transistors 101 – The FET (Field Effect Transistor)
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