Video Script of The Transistor DePuzzler

Transistor De-Puzzler Video Script

<Start w/ me in front of the de-puzzler>

Since embarrassingly few humans seem to have any idea what goes on inside the most manufactured tool of all time, I hope to help by sharing my copyrighted analogy for transistor operation with you. A key concept is the way the missing square in a dime store puzzle behaves like what is called a "hole" in semiconductors.

Conducting electricity with holes is fundamentally different from the usual method which, in this analogy, is like having extra electrons able to simply slide around on top, like this. Metals typically have lots of these mobile electrons, but semiconductors barely have any. Semiconductors such as crystalline silicon chemically bind up almost all the electrons leaving almost none free to move.

In this model, it's as if there's exactly one square in each position - nothing can move. The squares represent atoms with 4 bonds to their 4 nearest neighbors.

If, however, just one out of every million or so silicon atoms is substituted with a phosphorus atom which has 5 electrons in its outer shell, the crystal will carry a few orders of magnitude more electrical current when voltage is applied. It's a huge effect. The extra electron that each phosphorus atom brings relative to silicon atoms cannot find normal bonds to settle into. The framework of silicon crystal is made for atoms with 4 electrons in their outer shells, not 5. Like the "loser" in musical chairs, electrons will be left quite free to walk about.

If there's a voltage applied to the crystal, electrons will quickly move toward the most positive voltage while new electrons will enter and flow away from the negative voltage contact. Remember: opposites attract and, conversely, like charges repel each other.

Now back to the key concept of "holes":

Boron atoms, have one less electron in their outer shells than silicon and create "holes" in the silicon crystals because one of the neighboring silicon atoms will not be able to share a companion electron from the boron to form a proper bond. However, electrons are interchangeable enough that if a voltage is applied to the entire crystal, the "hole" will move toward the negative side in the same manner as the hole in a dime-store-puzzle does: silicon atom after silicon atom can momentarily go without one electron as the hole moves through the crystal. Notice that this method of conducting electricity requires lots of electrons to move just one position instead of individual electrons moving many positions. The holes behave like positively charged particles without actually being particles at all. Each is just a missing electron which can be passed from one atom to another, denoted here with the “h+” symbol. The effect is very similar to having positively charged particles, but note that the only actual movement is from electrons and that they're always moving toward the most positive voltage.

Now let's consider what happens if a single crystal conducts with excess electrons on one end and with holes at the other. Electrical current will flow if an applied voltage drives the holes and extra electrons toward each other.

Where they meet, electrons fall into the holes and just as sound is given off here, light and heat are released in many semiconductors, This is how LEDs work. If the voltage is reversed, current will quickly halt since the voltage pulls the holes away from the side with excess electrons and vice versa. The transition region will be completely depleted of charge carriers and becomes an insulator.

This simple structure that conducts electricity in just one direction is called a diode and modern electronics utilizes many diodes.

Most of these diodes are used in pairs, one in each direction like this, to construct transistors. Electrical current is blocked by one diode or the other no matter which direction a voltage is applied unless some trick is used.

In this configuration, current will be blocked right here and if the voltage is reversed, flow will be blocked here. The most common "trick" to get current to flow takes advantage of the fact that electrons can't easily move through insulators, yet they can be attracted or repelled through insulators. Consider a conductive plate, called a gate, placed near, but insulated from, the surface of the middle layer of this three layer sandwich made by alternating the two types of conductivity methods. A voltage on this plate can not only repel that middle layer's charge carriers, but if the voltage is large enough, it can attract a thin layer of the opposite type from the outside regions. This is called the “inversion layer”.

After a threshold voltage has been exceeded on this plate, the layer will bridge the outside regions and electric current can begin to flow beneath it. Doubling the voltage beyond this threshold will quadruple the current allowed to flow because both the carrier density of the inversion layer doubles which cuts the resistance in half AND the voltage that can be applied across the whole structure without saturating doubles. Saturation occurs when the voltage at one end of the insulator comes within the threshold voltage of the gate plate. No additional current will flow even if additional voltage is applied across the whole structure. The device is then in saturation.

The best part of how this type of transistor works is that since the plate is insulated, only a tiny amount of current is required to change its voltage -it's insulated! - yet it controls much larger amounts of current and power flowing beneath it.

Now you know the basics of how most of the quintillions of transistors work! There are other types I'll cover in future videos, but they usually exploit the fundamental differences between the two methods of conduction to amplify or switch electrical current on or off.

Thanks for listening!

Video Script of The Transistor DePuzzler