Different Types of Diodes

A diode is an electronic component allows current to flow easily in one direction but blocks current in the other direction.

Most diodes are made from a silicon semi-conductor material. To make silicon a more effective semi-conductor the material goes through a process called doping. This involves adding impurities (holes) to the material. By adding holes, it makes two different types of semi-conductor material known as n-type and p-type. N-type materials have a net negative charge due to an excess of electrons and P-type materials have a positive charge to a lack of electrons

When joining both types of material together it forms a p-n junction. The excess electrons in the n-type material flow across the junction to combine with p-type material because of the lack of electrons. The consequence of this is that there are very few charge carriers present at the junction. The junction forms a depletion layer. The depletion layer acts as a barrier preventing the flow of the negatively charged electrons at the cathode end to the opposite side which has the positive potential.

The diode will not conduct until a potential difference is applied between the anode and cathode. This potential difference is required to overcome the electrostatic field formed across the depletion layer. The thinner the depletion layer the more current flows through. For a continuous current to go through the diode, the depletion region must be fully collapsed by the applied voltage to allow the flow of electrons, this is called a forward voltage. This takes a certain minimum voltage to accomplish.

For most silicon diodes, the forward voltage is 0.6 volts. For germanium diodes, the forward voltage is only 0.3 volts. The chemical mixture of the P-N junction in the diode is the reason for the forward voltage figure, which is why silicon and germanium diodes have a forward voltage figure difference.

A diode will also conduct in reverse bias (potential difference) which is when the negative terminal of the battery is connected to the P-type and the positive is connected to the N-type semiconductor. In which the diode blocks the current because of a thick depletion layer. For standard diodes the reverse breakdown is much higher than the forward bias voltage. The point at which the voltage increases but the current stays the same is called the reverse saturation current. This is because it has hit a point where further voltage applied does not increase the electric current. In germanium diodes because of the increase in temperature creating more charge carriers than the silicon diode, the reverse saturation current is greater.

In a forward-biased diode the cathode (n-type semiconductor) connects to a negative potential, and the anode (p-type semiconductor) connects to the positive potential. This doesn’t block the current and has less resistance than a reserve bias, but it does drop the current. The forward-bias voltage drop by the diode is due to the action of the depletion region. If no voltage is applied across the semiconductor diode, a thin depletion region exists around the region of the P-N junction, preventing current flow.

If a reverse-biasing voltage is applied across the P-N junction, this depletion region expands, further resisting any current through it. A very small amount of current can and does go through a reverse-biased diode, called the leakage current. The ability of a diode to withstand reverse-bias voltages is limited. If the applied reverse-bias voltage becomes too great, the diode will experience a condition known as breakdown which the reverse current suddenly increases in value. A diode’s maximum reverse-bias voltage rating is known as the Peak

Inverse Voltage (PIV). Suddenly there is an increased flow in reverse current. No matter how much reverse voltage is applied, the voltage across the diode does not change. Typically, the PIV rating of a common diode is at least 50 volts at room temperature.Ones the diodes have hit the minimum forward voltage you can see from the graphs that the current starts to increase very fast. These diodes are temperature dependent and their characteristics can change in different temperatures. This will affect its limits such as maximum current that can pass through the diode. The diode will eventually heat up and the curve on the graph will drift.

Looking at these results you can see why silicon is used over germanium. Silicon has a higher breakdown voltage and lower reverse current making it a more efficient rectifier. The graphs also show that in forward biased diodes the current that flow through it isn’t proportional to the applied voltage. However, Germanium diodes has a lower forward bias voltage which means smaller power losses and therefore the circuit is more electrically efficient. They would be better when voltage variations must be kept down.