In this article, we will look at the internal physics of some transistors (not applications).
How Transistors Work
Recall how a transistor functions. In the famous picture of “transistor man” in Figure 1 below, the little man inside the transistor looks at the gauge measuring base current coming in, do a quick sum in his head to multiply this by the transistors hfe and then turns the pot to set the collector current Ic to match this value. (Ib * hfe).
The model in Figure 1 at the left has been criticized for its many assumptions, but it reminds you that there is a relationship between Ib and Ic.
When you gulp down a can of cola, you don’t know what’s inside, but you do know how it will make you feel. Most transistor designers treat transistors the same way. We have no idea what’s inside or how it works, but we are happy with the result.
Knowing the physics of what’s inside, the transistor may make your eyes glaze over, so bear with me, and you won’t be sorry!
Recalling Basic Concepts
We start with two assumptions that you should know: matter is made up of atoms, and the way they share electrons in their orbiting valency bonds keeps them from flying apart.
Secondly, from an electrical point of view, there are three types of materials: conductors such as copper, silver, etc.; insulators such as ceramic and glass; and semiconductors which are only partly conductive such as silicon and germanium.
The atoms in a material share electrons in their outer valency shells, making them hard to break apart in a diamond. Conductors have lots of electrons in the exterior surfaces making it easy to be pushed out of position, thus, enabling a current flow. Semiconductors have electrons that can be slightly displaced with applied voltage, but minimal current flows as they are still high resistance.
Silicon and germanium are two materials that have four electrons in their outer shell. They make good bonds but are still semiconducting. We can improve this property by adding impurities such as phosphorus or boron to the silicon.
|Figure 2||Figure 3||Figure 4|
Figure 2 shows the pure silicon neatly bonded in a crystalline lattice. Figure 3 shows the doping of phosphorus in purple. As the phosphorus atom has five electrons, it causes one electron to be displaced and free to move about in the material. This makes the molecule more damaging, and so is called N-type material. Applying a voltage across the material encourages more movement.
Figure 4 shows doping with boron, which has one less electron, causing a ‘hole’ that can also be moved. This makes the P-type material.
In both cases, the stronger the doping, the more conductive the material becomes.
These N and P types, when bonded together, result in an ability to control the direction and flow of electrons and, thus, current.
A bipolar NPN transistor shown below uses both electrons and holes as charge carriers. Note that I have indicated electron flow in my drawings since we are more accustomed to thinking of conventional current flow, which is from positive to negative.
The lower P and N junctions are sandwiched together, where the upper N-type section has a built-in heavily doped region joined a lightly doped region. The upper N section is bonded to the collector terminal, the P section to the base, and the lower N section to the emitter.
Transistors at Work: A closer look
Initially (see A below), when the layers are sandwiched, all the free electrons from the N-type material across the junction gather at the bottom of the P-type material filling the holes. Holes form at the top of the N layer, forming what’s known as a depletion region as there are now no charge carriers available. Likewise, a depletion region appears in the upper N-type layer, effectively making it high resistance.
Suppose a slowly increasing positive voltage is applied to the base. In that case, no current flows, but the depletion region becomes thinner and thinner until, at about 0.6V, the barrier is overcome and starts conducting current rapidly. This is now a PN junction diode and behaves exactly like any other diode. Electrons flow from the emitter to the base (red arrow in C). Applying a positive voltage to the collector will attract electrons from the lower N region and start a current flow (blue arrow). The highly dope region forms a lower ‘resistance’ than the lightly dope. This creates a potential divider effect between the light and heavy parts, concentrating the charge nearer the junction. The higher voltage in the narrow light area attracts all the free electrons in the lower N region, which rush towards the collector.
So now, as we have a small current in the base and a large current in the collector, we effectively have current gain. This is how a BJT or bipolar junction transistor works. The PNP version is precisely the same. Just all the layers and applied voltages are reversed.
FETs and MOSFETs
Shown at A is a typical JFET or Junction Field Effect Transistor. Although the construction is very different from the bipolar transistor, what goes on is very similar.
The P and N junction forms a depletion area as before, and no charge carriers are available to conduct. Hence, the drain and source have high resistance in between. Note that the bipolar is a current-driven device, whereas FETs are voltage-driven devices. A field breaks down the depletion layer by applying a positive voltage to the gate, and the current can flow between the drain and source (red arrow).
At B, instead of an electrically bonded connection to the gate, a fragile insulator layer exists, typically metal oxide. This gives the MOSFET its name Metal Oxide Field Effect Transistor. Applying a positive voltage to the gate creates a field in the P-type material, which again clears the depletion region creating a conduction channel. Current can flow between drain and source (with an applied voltage, of course).
The significant advantage here is that no wind is flowing into the gate in both FETs, so the transistor is now a voltage-driven device, in many ways quite similar to a thermionic valve (or tube in America).
Above are the different symbols for N-channel and P-channel FETs (left) and MOSFETs (right). Note the direction of the arrows telling if it is a P or an N-type. Also, note the gate symbolically shown as separate from the drain-source channel in the MOSFET.
We have seen how transistors work internally. When we go back to our roots like this, it reminds us why we have the name electronics at all.