How Semiconductors Work

Silicon Element

Understanding how modern semiconductors work requires a basic knowledge of physics. On a remedial level however, it can be simply explained. That is what I will try to do here.  For all the pomp and circumstance, solid state semiconductors are not all that new. K.F. Braun discovered in 1874 that some crystals posses a unilateral conductance. That is, he discovered that current would flow in only one direction in some crystals. Modern Si (Silicon) based semiconductors are crystal structures.  Other semiconductors crystals exists such as diamond, germanium, silicon carbide, and silicon germanium. However, for our discussion we will focus on silicon.

Silicon has four electrons in its outer shell. These valence electrons form bonds with neighboring silicon atoms. Creating a crystal structure where one electron from each atom is used to form a two electron bond between neighboring silicon atoms. Pure silicon is a very poor conductor because there are no free electrons. To make a semiconductor from silicon impurities must be added to the Si crystal structure. The impurity is chosen to provide free electrons in the material. The process of adding impurities is called doping and the doping element is called a dopant. Boron, arsenic, phosphorus and gallium are often used to dope silicon.

By doping pure silicon with some of these elements extra valence electrons are added. These electrons become unbonded from individual atoms creating an electrically conductive n-type semiconductor. Some doping materials such as boron, are missing the fourth valence electron and are used to create “broken bonds” (holes) in the silicon lattice. These holes give pockets where unbonded electrons are free to move. These types of dopes create p-type semiconductors. In most cases many types of impurities will be present in the final doped semiconductor. If an equal number of donors and acceptors (holes) are present in the semiconductor the extra core electrons provided by the donors will be used to satisfy the broken bonds acceptors. Due to this pairing, doping produces no free carriers of either type. This phenomenon is known as compensation, and occurs at the p-n junction in the majority of semiconductor devices. Partial compensation, where donors outnumber acceptors or vice versa, allows device makers to repeatedly reverse (invert) the type of a given portion of the material by applying successively higher doses of dopants, so-called counterdoping. Most modern semiconductors are made by successive selective counterdoping steps to create the necessary P and N type areas.

Adding phosphorus to silicon as a dopant allows us to turn non-conductive silicon into a semiconductor material. Phosphorus has five electrons, one extra (when compared to silicon) and one extra proton.  By replacing some of the silicon atoms in the crystal lattice with phosphorus atoms we gain a free electron. The phosphorus bonds to the silicon using its four valence electrons and the fifth electron is free to move about. It is these extra unbonded electrons that make the material conductive.

If a power source where connected to a silicon lattice doped with phosphorus, an electric current would flow as electrons from the power source would flow from the negative terminal of the source into the crystal lattice and would exit the crystal lattice to flow into the positive terminal of the power source.  When Boron is used as the dopant, empty spaces are left in the valence bonds between the atoms. This allows some electrons to migrate into the holes while leaving a new hole at its last location. If a power source is applied to a boron doped silicon lattice, electrons will move out of the lattice and into the positive terminal of the source. At the same time, electrons will be leaving the negative side of the source and entering the lattice by plugging some of the holes either provided by the boron atoms or by the holes left when electrons from the silicon atom relocated. In both of these cases, whether there are extra electrons or holes that allow electrons room to move, produce a conductive lattice and electrical current is allowed to flow. This is because both materials now contain charge carriers (holes or unbonded electrons). Materials with unbonded electrons are called N-type semiconductors. Those materials using holes as charge carriers are called P-type semiconductors.

Note that the type is related to the charge carrier and that the lattice doesn’t really take on this charge. This is because the phosphorus atoms not only have an extra electron but also have an extra proton. Thus neutralizing the lattice. The same is similarly true for boron. Boron not only has one fewer electrons but also has one fewer proton, again, neutralizing the lattice charge.

So now let’s consider what happens when we combine these two types of semiconductor materials.  If we take a single silicon crystal and apply a Boron dopant to one half and a Phosphorus dopant to the other half.  With no current applied, the junction of the N and P type regions will form a barrier known as the depletion region. In the depletion region Unbonded electrons move into holes and form a region where the material becomes slightly positively charged near the N-type region and slightly negatively charged near the P-type region. The remaining areas remain neutrally charged. Because the electrons in the depletion region have been used to plug holes, the area looses it’s charge carriers. With no charge carriers this region become non-conductive.

When a forward biased supply is applied to the material electrons on the N side began to collect near the depletion region as electron move from the negative terminal of the power source into the N-type region. At the same time, the positive side of the power source begins to drain off  electrons from the P-type region causing holes to form closer to the depletion region. If the power source applies enough pressure (voltage) some of the electrons will be pushed through the depletion region and into the P-type material to fill holes. For silicon semiconductor material a voltage of 0.6 volts is required to force the electrons through the depletion region.

If we reverse bias the material, then some of the electrons in the N-type region will move out into the positive terminal of the applied source. This will cause the the depletion region on junction with the N-type region to widen. At the same time, electrons leaving the negative terminal of the source will move into the holes in the P-type region and will plug the holes there. This will cause the depletion region junction with the P-type material to widen.  Since the depletion region has no charge carriers it acts as an insulator and stops all current flow through the area. 

You might recognize these actions as that of a rectifier or diode. Ben Eater has a wonderful youtube video that does a very good job explaining all this.

But how do we get from a diode to a transistor?

Transistors are really just two tvs diodes in series. The junction of the two diodes is called the “Base”. One of the outer regions will be a collector and the other the emitter. If we apply a voltage across the emitter and collector, one of the diodes will always be reversed biased and current will not flow. However, if we also apply a voltage strong enough to cause electrons to cross into the the depletion region of the emitter-base diode, then electrons will move from the emitter to the base and out to the base source. However, far more electrons move into the depletion region then can move out into the base source. These extra electrons will cross over into the collector region. Since it only take a slight base current to cause a larger emitter collector current, the transistor acts as a current amplifier.

The description I just gave is for an NPN type bi-junction transistor.  These transistors have N-type material for the emitter and collector and a P-type material for the base. By applying a positive voltage to the base and a high positive voltage to the collector, these devices act as current amplifiers. However, transistors can also be built by placing an N-type base region between to P-type regions. This type of transistor is known as a PNP type bi-junction transistor. PNP type transistors require the polarity of the base-emitter and collector-emitter circuits to be reversed. With PNP type transistors a negative bias is applied to the collector and base to cause current to flow through the collector emitter circuit. In both cases however, a small current applied to the base will cause a greater current to flow in the collector-emitter circuit.

Transistors are often used as switches. When used as a switch, a transistors with large amplification or gain is used. A small current in the base causes a maximum current to flow in the collector-emitter circuit. Since a very small current causes such a large change in the output, it appears as though the transistor is turned on and off.  In another article I will discuss using transistors to build simple circuits and digital gates. 

There is a decent youtube video explaining the basic operation of a transistor here.