How Atoms are Arranged to Make Transistors

Electricity

So, let's see if this works: imagine that electricity flows by moving electrons among the atoms. Most atoms hold strongly to their electrons, so they aren't free to move, so it takes a very strong force to break an electron free -- as much as millions of volts of force. Atoms, and combinations of atoms, that behave like this have been grouped together and called "insulators". For normal voltages, up to several hundred volts, atoms in such materials keep nearly all their electrons tight and effectively no current flows. (Just to fill in, only 17 elements act as insulators, but the majority of materials around us are composed of those elements or combinations of elements that together act as insulators. So, most things insulate.)

Other elements have a structure to their atoms that makes one or two of them quite loose, and easy to take off. When a bunch of these atoms are together, they effectively end up sharing these loose electrons. Each electron sort of equally belongs to the atoms closest to it. That means that a very small force can cause electrons to move. Elements, and combinations, that behave like this have been grouped together and called "conductors".

Now comes the important part. Elements have been observed that fit neither the insulator behavior nor the conductor behavior. These, and combinations, have been grouped into the "semiconductor" category. These are the elements used to make transistors.

Just to be complete, it has been observed that combinations of elements that separately are in one category, when combined create a material that fits with a different category. For example, Iron is a conductor, but when combined with oxygen the pair behaves like an insulator (that's what rust is). Likewise, Gallium conducts and Arsenic does not, but the combination acts like a semi-conductor. Also, plastics have complex polymer molecules and normally act as insulators, but special combinations have been found that conduct electricity well.

Basic Idea of a Field Effect Transistor

Now, we want to make a transistor, which changes from acting like an insulator to acting like a conductor. That means we need some way to make the atoms change from holding tight to their electrons over to being loose. How to do this?

For one kind of transistor, it's accomplished with a field of force, which attracts loose electrons over to a portion of material that normally doesn't have any. When the force is applied, electrons are available to use, and the transistor acts like an "on" light switch. When the force is absent, no electrons are available, and the transistor acts like an "off" light switch.

Notice that a light switch works by physically separating two conductors (pieces of metal) by placing air between them, which has no free electrons. In this configuration, no electricity flows between the conductors and the switch is "off". When the air is replaced by a conductor that touches both ends, then electrons can flow from one, into the middle piece, and out the other end, and the switch is "on". So, it's the same principle at work in both places: there are two conductors with a region between them that changes from having free electrons to not having any. In one case, the change is done by physically moving a conductor into that region, instead of air. In the other case, the change is done by applying a field that attracts free electrons to that region.

What is a Field of Force?

Movement happens when a force puts energy into movement. The three things -- Force, Energy, Movement -- are elements of a model that people have made up, and correlate to observations people have made. They lie at the bottom of the model, as the names of directly observed behaviors. For water flowing, force is taken per area, and called Pressure, movement is number of volume units per second, and energy is total volume moved times the difference in pressure at start point versus end point. Electricity also uses pressure and flow. For electricity, pressure is the force per electron (rather than force per area), and a difference in pressure is called Volts; flow is number of electrons per second (instead of number of volume units per second), called Amps; and energy is number of electrons moved times the difference in pressure at start versus end, which is number of electrons moved times the volts between start and end points.

So, voltage is a difference in the force that acts on electrons. A voltage across a material represents a change in force from one side to the other, and inside, the force changes bit by bit throughout the material. At each point inside the material, the force has a strength and a direction. The strength of the force at a given point in the material can be measured. Do this by taking pairs of points from a small ring around the given point. Measure the force generated between each pair. The result will be a direction in which the force is strongest, and the size of the strongest force. The size depends on how far apart the measurement points are.. the further apart, the more force between them. So, divide the force by the distance, to get a value that doesn't depend on the way it was measured, only the point around which it was measured. That gives just force-at-a-point. Each point in the material has such a force-at-a-point and a direction. The collection of all those force and direction values is called a field. Each point of the field has a direction and a strength.

In other words, applying something that creates a voltage across a material, also creates a field of forces within the material. If an electron were placed at any point within that field, then released, it would travel. At each point, it would move in the direction of the field, and would feel force proportional to the strength of the field (direction and strength at that point). The field gives energy to the electron. The energy is calculated in tiny little pieces -- at each point, the energy is the field strength, to get the force, times a small distance, to get the force-through-distance, which is energy. Adding those up along the path the electron takes gives the total energy per electron.

So that's what people have made up, and called a field of force. People get lazy and usually just say "field", as in "the electric field". When given an electric field, to get volts between two points in the field, draw a line between the points, then at each end, take the amount of field that points along the line. The difference between those is the difference in force, along the line, between the two ends. That difference in force is the volts between the points.

Moving electrons makes a flow of electrons, which is called electric current. A current flows within a material that has free electrons, and is measured between two end-points in the material. Causing an electric current to flow puts energy into that current. The energy is the voltage between the end-points, times the total electrons that flow between the end-points. Power is voltage times electrons-per-time moving between the end-points, which is Volts times Amps.

How to Arrange Materials so they Act Like a Switch

Now we have the idea that some materials have only tightly bound electrons, while others have loosely bound electrons, and this determines whether the electrons can move. So, we need to arrange materials such that a voltage is applied across a piece of material that normally has no free electrons, and then arrange so that a different voltage can be applied that changes the number of free electrons in the material. This figure shows the arrangement:

The area labelled "channel" has few free electrons. The area labelled "gate" is covered in metal, and a voltage is applied between the gate and the channel. The insulator between the gate and channel prevents electrons from flowing between gate and channel. However, one model that is fairly consistent with behavior is that electrons are taken off the metal of the gate and placed into the channel. These electrons are free, and exist in the channel, and so are available for current to flow across the channel. Hence, when a voltage exists on the gate, current can flow through the channel, which is like an "on" light switch. When no voltage exists on the gate, then no current can flow, which is like an "off" light switch.

Putting it all together

According to this simple model, the number of free electrons determines how many electrons per second can flow. So, the higher the gate voltage, the more current can flow in the channel. In this way, the gate voltage can be used to change the amount of current flowing through the channel, even when the voltage across the channel stays constant.

This is how an amplifier works. In an amplifier, a small voltage whose value goes up and down is turned into a larger voltage whose value goes up and down in the same pattern. For example, radio waves going through the air cause a voltage to appear between the ends of an antenna. The shape of change of the voltage is the same shape as the sound waves collected by a microphone. An amplifier then turns the tiny voltage from the antenna into a large voltage that changes in the same shape.

A Field Effect Transistor can be used as an amplifier because it only takes a small voltage to move electrons into the channel and then to take them away again. But a much larger voltage can be placed across the sequence of the channel plus something connected to the channel, such as a resistor. As the gate voltage changes, the current flowing through the channel changes, and so the variation in current has the same pattern as variation in voltage on the gate. The resistor connected to the channel turns the current back into a voltage. By choosing the value of the resistor properly, the current can create much larger voltage across that resistor than the voltage that is on the gate. That voltage has the same shape as the current, which has the same shape as the gate voltage, and so the small gate voltage is turned into a replica, but much larger.

When a transistor is placed into a circuit, each of the circuit elements affects the current and voltage flowing through itself plus the elements connected to it. In this way, each element can affect current and voltage for elements far away, so all the elements in the circuit have to be considered together, in order to understand what function the circuit accomplishes. Understanding these interactions becomes more of an art than a science. Many techniques have been invented to help people understand the interactions, and so to predict the behavior that will be observed from a circuit.

Models

Something to keep in mind: this model of "electron" and "atom" is just that -- a model. They exist only in people's minds. The only thing outside people's heads would be observations we've made, which is the world outside our heads talking to us. We've found the electrons and atoms model to be simple to use, and at the same time has pretty good consistency with the observations people have made. But that doesn't make electrons and atoms "real" outside our heads. In fact, this model breaks for many experiments, and people have to shift to a different model in order to get a model that stays consistent with the observations from the experiment.

Other models that have consistency with people's observations can be created in our heads. If we find a new model that has consistency with a wider set of observations, then we wouldn't have to change models, we'd have one model that was consistent with everything ever observed. That's the goal of the "Grand Unified Theory" of physics.

So, just always keep in mind that the equations are just models that exist in our heads, that we've randomly stumbled across, that happen to be consistent with a subset of all observations. The equations aren't real. Observations are real. When the two differ, observations win, and the equations and models have to change.

The model used here is consistent with the majority of behaviors of electric current and Field Effect Transistors. However, the model described here fails to explain behaviors such as how Bipolar Junction Transistors work, and how Diodes work, and the reason for a "threshold" voltage for Field Effect Transistors. A much more complicated model based on Quantum Mechanics has to be used in order to be consistent with those finer-level observations.