There are certain inventions, such as the discovery of fire and the invention of the wheel, which have changed the course of human history.
One such invention is the use of semiconductors.
The use of semiconductors is by comparison a relatively new invention,
as it is only in 1947 that semiconductors were used to create the first working transistor.
New devices made from semiconductors are constantly being made
and we still do not know the full extent of where this will lead.
We divide our history based on the technology used at the time,
such as the Stone Age, the Bronze Age, and the Iron Age.
Several thousand years from now, the discovery of semiconductors may be viewed as one of the great demarcation lines of history.
In order to understand semiconductors, we need to first understand what makes some materials conductors, and other materials insulators.
The height of this graph represents the energy of each electron.
Inside a material, some energy levels are allowed, whereas other energy levels are forbidden.
In this graph, electrons are allowed in the energy levels at the top and at the bottom, but there is a large gap of forbidden energy levels in between.
In this example, the top band of allowable energy levels is partially full.
This gives the electrons room to move around.
As a result, this material is an example of an electrical conductor.
In this other example, the bottom energy band is completely full and the top energy band is completely empty.
Here, the electrons have no room to move around.
As a result, this material is an example of an electrical insulator.
Now suppose we have a material where the gap between allowable energy levels is much smaller.
We call this material a semiconductor.
Let’s just show one row for the top energy band, and one row for the bottom energy band.
In a semiconductor, the gap is small enough for the thermal vibrations to allow an electron to sometimes jump from the lower energy band to the higher energy band.
The electron in the higher energy band is now free to move around.
This electron leaves behind a hole in the bottom energy band,
and the location of the hole is also free to move around.
When the electron at the top and the hole at the bottom find each other,
the electron falls back down to the lower energy level.
The semiconductor in this example is not a very good conductor of electric current.
We can change this by adding impurities to the semiconductor.
For example, we can add impurities which create extra available energy levels near the bottom energy band.
This allows the electrons in the bottom energy band to easily jump to these new levels, thereby leaving behind holes that can move around.
This material can now easily conduct electric current.
Each blue sphere represents a negatively charged electron and each red sphere represents the associated positively charged proton.
The total number of protons and electrons in the material are exactly equal,
and the material therefore contains an equal amount of positive and negative electric charge.
The locations showing a red sphere without a blue sphere can be thought of as a "hole" with a positive charge.
Since it is the holes with a "Positive" charge that can be thought of as moving around, we call this a "P-Type" material.
We call this other example an "N-Type" material.
The height of the graph only represents the energy of the electrons, not the energy of the protons.
Each proton was shown next to its associated electron just to make it clear that the total number of protons and electrons in the material are exactly equal,
and that the material therefore contains an equal amount of positive and negative electric charge.
The electrons which are free to move around in the top energy band have a "Negative" charge, which is why we call this an "N-type" material.
Just like a P-type material, an N-type material can also easily conduct electric current.
Now suppose that we place an N-type material next to a P-type material.
The electrons from the N-type material will migrate into the P-type material,
and holes from the P-type material will migrate into the N-type material.
The electrons in the top energy band will then fall down into the holes of the bottom energy band.
This results in a region with a net positive charge in the N-type material, and a region with a net negative charge in the P-type material.
This creates an electric field preventing additional electrons and holes from crossing over.
This is represented by an energy difference between the two materials,
indicating that if the electrons try to go "uphill", they will feel an opposing force.
Suppose we apply an external voltage as shown.
This voltage increases the difference in the energy between the N-Type material and the P-type material,
making it even harder for electrons and holes to cross over.
In this situation, the device blocks the flow of current.
Now, suppose that we apply a voltage in the opposite direction.
This decreases the energy difference between the two sides, allowing some electrons and holes to cross over.
Since the density of electrons is higher in the N-type material
and the density of holes is higher in the P-type material,
the net flow of electrons is from the N-type into the P-type,
and the net flow of holes is from the P-type into the N-type.
A hole moving to the left requires an electron moving to the right,
which means that inside both the N-type and the P-type materials,
there is a net flow of electrons moving from the left to the right.
We have created a device called a "diode", which allows current to flow in one direction, but blocks current from flowing in the opposite direction.
Let’s now consider what happens when we place a P-type material in between two different N-type materials.
If we apply a voltage as shown, the device will block the flow of current.
Let’s now apply just a small voltage in this other location.
The device is now allowing current to flow.
Here, a very small current through the middle terminal controls a much larger current through the other two terminals.
We call this device an N-P-N transistor, and this is just one of many different ways to construct a transistor out of semiconductors.
Transistors are the foundation of modern electronics and transistors are only one example of the many different types of devices that can be made from semiconductors.
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