P-N Junction

Let's look at the foundation for transistors, diodes and many other semiconductor components: the p-n junction.

Semiconductors, Doping and the P-N Junction

In the previous tutorial on semiconductors, we learned about their special conductivity behavior. The conductivity of semiconductors can be heavily increased by introducing foreign atoms into the silicon, which is also referred to as doping. We learned that there are two different types of doping: n-type and p-type doping.

The image above shows n-type doped silicon with phosphorous used as dopant and p-type doped silicon with boron used as dopant. In n-type doped silicon the phosphorous provides additional electrons. These can be relatively easily excited into the conduction band. This allows current to flow through semiconductor material. Electrons are the majority charge carriers in n-type doped semiconductors. Boron used for p-type doping has one electron less than silicon. This creates spots where an electron is missing to form the fourth covalent bond. These spots are also called electron holes. An electron from the valence band can jump into such a hole and fill it. This creates a vacancy in the valence band, which allows the electrons to move step-by-step just like one can move the parts in a 15 puzzle. A charge transport becomes possible within the valence band. It looks like the empty spot is moving in the opposite direction. It is possible to interpret the hole in a different way: as a positively charged defect electron that moves in the opposite direction of the electron flow. Defect electrons are the majority charge carriers in p-type doped semiconductors.

Doped semiconductors are not charged in any way. It is, however, favorable for the additional electrons to recombine with electron holes and thereby enter a lower energetic state. This fact becomes important when n- and p-type doped regions are joined together. The spot where two regions of different doping touch each other is referred to as p-n junction. In this tutorial we will look at the special properties of this junction which allows components like diodes, LEDs or transistors to work.

The P-N Junction

Let's assume that a piece of n-type and p-type doped silicon has just been joined together, even though this is not how it works in practice. In reality this junction would not be perfect enough. This is why in reality a single piece of silicon is used in which different regions are doped differently. Anyway, let's imagine freshly created p-n junction. At first sight there seems to be nothing special here. However, in the n-type doped region there is a higher concentration of electrons due to the additional electrons of the phosphorous. This causes a diffusion process to start. Electrons will diffuse from the n-type doped region into the p-type doped region. There they recombine with the holes in the p-type doped region.

Due to the diffusion process phosphorous atoms in the n-type doped region now miss their additional electron. They become positively charged. The boron atoms in the p-type doped region now have an additional electron. They become negatively charged. A potential difference exists between the p-type and the n-type doped region in proximity to the junction. The electric field caused by it forces electrons to drift in the opposite direction. This drift process counteracts the diffusion process. At some point an equilibrium is reached. In sum, no current flows across the junction anymore. The figure below illustrates both diffusion and drift process, as well as the small charged area that has formed around the junction.

The further one goes into the depletion zone the less free majority charge carriers are available. This region is thus called depletion zone. The depletion zone acts like a barrier. No significant amount of current can flow trough it, due to the force the electric field opposes on the majority charge carriers. The size of the depletion zone depends on the doping level it is smaller for highly doped semiconductors. The depletion zone doesn't have to be symmetrical. It is not, if the doping level is different in the p-type and n-type doped region. In this case the depletion zone extends further into the more lightly doped region than into the highly doped.

The million-dollar question is how can a current flow across the p-n junction? We need to enforce the diffusion process by providing enough energy to the electrons that they can diffuse across the junction besides the potential difference. This can be achieved by applying a voltage.

P-N Junction with Applied Voltage

To strengthen the diffusion process the direction of the external electric field has to be the opposite of the direction of the internal electric field. This can be achieved by connecting - to the n-type doped region and + to the p-type doped region. This is also called connecting in forward bias configuration.

The applied voltage pushes electrons in the n-type doped material and defect electrons in the p-type doped material towards the junction. The size of the depletion zone decreases. If the applied voltage is high enough, this neutralizes the internal electric field created by the diffusion process. Electrons from the n-type and defect electrons from the p-type region can now move across the junction. This is also called minority charge carrier injection as the electrons from the n-type doped region enter into the p-type doped region where they are the minority charge carriers. The same is true for the defect electrons which enter the n-type doped region as minority charge carriers. Due to this semiconductor devices that make use of a p-n junction are also called minority carrier devices.

The minority charge carriers only exist for a short time, until electrons and the electron holes recombine. After that the charge is transported by the free majority charge carriers in the semiconductor material. No electric field building up as the applied voltage continuously pushes more electrons and electron holes towards the junction.

The resistance of the p-n junction drastically decreases above the forward voltage, as shown in the graph below. A resistor is needed to limit the current flow or the semiconductor component might be damaged due to overheating. This should sound familiar to you. It is the reason why it is recommended to always use a series resistance together with LEDs. The voltage required to make the p-n junction conductive is called forward voltage \(V_f\). Its value is dependent on the band gap of the semiconductor, the doping levels and temperature. For silicon and typical doping levels it is roughly around 0.7 V. For germanium it is roughly around 0.3 V.

As we know energy is released during the recombination process, typically as heat. Where does this energy come from? It originates from the work done by the externally applied electric field. The power loss at the p-n junction can be calculated by multiplying the forward voltage \(V_f\) with the current \(I\):
\(P_{loss} = V_f \cdot I\)

What happens if we apply a voltage to the p-n junction in the opposite direction? This is called operating the p-n junction in reverse bias configuration. The external electric field drags the majority charge carriers away from the junction. The size of the depletion zone increases.

No significant amount of current can flow trough the depletion zone because there are no majority charge carriers available there. A p-n junction acts like a one way road. This property is used in diodes, which will be the topic in the next tutorial.

In practice, there is a small leakage current. This leakage current is caused by minority charge carriers. If an electron is excited in the depletion region and a new electron-hole-pair is generated, the electric field accelerates the and drags them away from the junction. The electron is dragged into the p-type doped region and the defect electron into the n-type doped region. This process allows a small current to flow through the junction. Under normal conditions not enough electrons are excited for a significant amount of current. A few micro amperes are a normal value. There are some semiconductor devices where the leakage current caused by electron excitement has a use. Examples for this are photo diodes and solar cells, in which the incoming light will excite electrons. They will be covered in a later tutorial as well.

When taking a look at the I-V characteristics, one can see that the above only holds true until a certain reverse voltage is exceeded: the breakdown voltage. There are different physical process can cause the barrier between the p-type doped and n-type region to collapse. The most important ones are the avalanche and the zener breakdown. I will not explain them here. What is important to know is that in theory a breakdown is reversible and does not necessarily damage the p-n junction. The resistance decreases drastically above the breakdown voltage, however. The resulting current likely leads to the thermal damage. Local damage inside the component might occur even for smaller currents, if the component is not designed to be operated in reverse bias configuration above the breakdown voltage. There are special zener and avalanche diodes that are specially designed for this, but this is a topic for its own tutorial.

Let's sum up:

• the p-n junction acts like a one way road
  • forward bias: current can flow if the voltage exceeds the forward voltage
  • reverse bias: no current flows, if the breakdown voltage not exceeded
• resistance decreases rapidly above the forward or breakdown voltage
  • thermal damage can occur for too high currents
  • the power loss at the p-n junction is \(P_{loss} = V_f \cdot I\)
  • for safe operation a series resistance is needed