P-N Junction Diode

The P-N Junction Diode

A p-n junction is formed when the p-type semiconductor is joined with an n-type semi conductor. The n-type material has free electron introduced by the donor atoms (group V element dopant) while the p-type material has holes introduced by the acceptor (group III dopant element). These holes and electrons are free to move and they are at high concentrations in their respective materials (regions). 

Due to high concentration of holes in the p-type region and electrons in the n-type region, a very large density gradient exists between both sides of the junction. Some of the free electrons from the donor impurity atoms in the n-type region begin to diffuse across this newly formed junction to fill up the holes in the P-type material.

However, because the electrons have moved across the junction from the N-type region to the P-type region, they leave behind positively charged donor ions on the negative side and now the holes from the acceptor impurity (p-type region) diffuse across the junction in the opposite direction into the n-type region where there are large numbers of free electrons. As holes diffuse into the n-type region, they leave behind negatively charged acceptor ions in the p-type region.

As a result, the P-type region near  the junction becomes negative while the n-type region near the junction becomes positive. This charge transfer of electrons and holes across the junction is known as diffusion.

As this  process continues,   electrons accumulates in the p-type region while positive chage accumulates in the n-type region and at a large enough electrical charge, they repel or prevent any more diffusion of holes and electrons over the junction. Eventually a state of equilibrium (electrically neutral situation) will occur producing a "potential barrier" zone around the area of the junction as the donor atoms repel the holes and the acceptor atoms repel the electrons.

Since no free charge carriers can rest in a position where there is a potential barrier, the regions on either sides of the junction now become completely depleted of any more free carriers in comparison to the N and P type materials further away from the junction. This area around the junction is now called the Depletion Layer.

The PN junction

As the N-type material has lost electrons and the P-type has lost holes, the N-type material becomes positive with respect to the P-type. Then the presence of impurity ions on both sides of the junction causes an electric field to be established across this region with the N-side at a positive voltage relative to the P-side. The problem now is that a free charge requires some extra energy to overcome the barrier (the barrier potential difference) that now exists for it to be able to cross the depletion region junction. That is to say energy is required for electric current to flow through the junction diode. This energy can be provided by connecting the ends of the p-n junction to an external voltage source.

If we make electrical connections at the ends of both the N-type and the P-type materials and then connect them to a battery source, an additional energy source now exists to overcome the barrier resulting in free charges being able to cross the depletion region from one side to the other. 

The behaviour of the PN junction with regards to the potential barrier width produces an asymmetrical conducting two terminal device, better known as the P-N Junction Diode.

PROPERTIES OF A DIODE

A diode is one of the simplest semiconductor devices, which has the characteristic of passing current in one direction only. However, unlike a resistor, a diode is non ohmic; does not behave linearly with respect to the applied voltage as the diode has an exponential I-V relationship and therefore we cannot described its operation by simply using an equation such as Ohm's law.

If a suitable positive voltage (forward bias- positive terminal connected to the p-type side) is applied between the two ends of the PN junction, it can supply free electrons and holes with the extra energy they require to cross the junction as the width of the depletion layer around the PN junction is decreased. 

By applying a negative voltage (reverse bias- positive terminal connected to the n-type side) results in the free charges being pulled away from the junction resulting in the depletion layer width being increased. This has the effect of increasing or decreasing the effective resistance of the junction itself allowing or blocking current flow through the diode.

Then the depletion layer widens with an increase in the application of a reverse voltage and narrows with an increase in the application of a forward voltage. This is due to the differences in the electrical properties on the two sides of the PN junction resulting in physical changes taking place. One of the results produces rectification as seen in the PN junction diodes static I-V (current-voltage) characteristics. Rectification is shown by an asymmetrical current flow when the polarity of bias voltage is altered as shown below.

Junction Diode Symbol and Static I-V Characteristics

But before we can use the PN junction as a practical device or as a rectifying device we need to firstly bias the junction, ie connect a voltage potential across it. On the voltage axis above, "Reverse Bias" refers to an external voltage potential which increases the potential barrier. An external voltage which decreases the potential barrier is said to act in the "Forward Bias" direction.

Biasing a P-N Junction Doide

There are two operating regions and three possible "biasing" conditions for the standard Junction Diode and these are:

1. Zero Bias - No external voltage potential is applied to the PN-junction.
2. Reverse Bias - The voltage potential is connected negative, (-ve) to the P-type material
      and positive, (+ve) to the N-type material across the diode which has the effect of
      Increasing the PN-junction width.
3. Forward Bias - The voltage potential is connected positive, (+ve) to the P-type material and
      negative, (-ve) to the N-type material across the diode which has the effect of Decreasing the
      PN-junction width.

Zero Biased Junction Diode

When a diode is connected in a Zero Bias condition, no external potential energy is applied to the PN junction. However if the diodes terminals are shorted together, a few holes (majority carriers) in the P-type material with enough energy to overcome the potential barrier will move across the junction against this barrier potential. This is known as the "Forward Current" and is referenced as I_F

Likewise, holes generated in the N-type material (minority carriers), find this situation favourable and move across the junction in the opposite direction. This is known as the "Reverse Current" and is referenced as I_R. This transfer of electrons and holes back and forth across the PN junction is known as diffusion, as shown below.

Zero Biased Junction Diode

The potential barrier that now exists discourages the diffusion of any more majority carriers across the junction. However, the potential barrier helps minority carriers (few free electrons in the P-region and few holes in the N-region) to drift across the junction. Then an "Equilibrium" or balance will be established when the majority carriers are equal and both moving in opposite directions, so that the net result is zero current flowing in the circuit. When this occurs the junction is said to be in a state of "Dynamic Equilibrium".

The minority carriers are constantly generated due to thermal energy so this state of equilibrium can be broken by raising the temperature of the PN junction causing an increase in the generation of minority carriers, thereby resulting in an increase in leakage current but an electric current cannot flow since no circuit has been connected to the PN junction.

Reverse Biased Junction Diode

When a diode is connected in a Reverse Bias condition, a positive voltage is applied to the N-type material and a negative voltage is applied to the P-type material. The positive voltage applied to the N-type material attracts electrons towards the positive electrode and away from the junction, while the holes in the P-type end are also attracted away from the junction towards the negative electrode.

The net result is that the depletion layer grows wider due to a lack of electrons and holes and presents a high impedance path, almost an insulator. The result is that a high potential barrier is created thus preventing current from flowing through the semiconductor material.

                  Reverse Biased Junction Diode showing an Increase in the Depletion Layer

                                                        Reverse Characteristics Curve for a Junction Diode

Forward Biased Junction Diode

When a diode is connected in a Forward Bias condition, a negative voltage is applied to the N-type material and a positive voltage is applied to the P-type material. If this external voltage becomes greater than the value of the potential barrier, approx. 0.7 volts for silicon and 0.3 volts for germanium, the potential barriers opposition will be overcome and current will start to flow.

This is because the negative voltage pushes or repels electrons towards the junction giving them the energy to cross over and combine with the holes being pushed in the opposite direction towards the junction by the positive voltage. This results in a characteristics curve of zero current flowing up to this voltage point, called the "knee" on the static curves and then a high current flow through the diode with little increase in the external voltage as shown below.

Forward Characteristics Curve for a Junction Diode

The application of a forward biasing voltage on the junction diode results in the depletion layer becoming very thin and narrow which represents a low impedance path through the junction thereby allowing high currents to flow. The point at which this sudden increase in current takes place is represented on the static I-V characteristics curve above as the "knee" point.

                     Forward Biased Junction Diode showing a Reduction in the Depletion Layer

This condition represents the low resistance path through the PN junction allowing very large currents to flow through the diode with only a small increase in bias voltage. The actual potential difference across the junction or diode is kept constant by the action of the depletion layer at approximately 0.3v for germanium and approximately 0.7v for silicon junction diodes.

Since the diode can conduct "infinite" current above this knee point as it effectively becomes a short circuit, therefore resistors are used in series with the diode to limit its current flow. Exceeding its maximum forward current specification causes the device to dissipate more power in the form of heat than it was designed for resulting in a very quick failure of the device.

Junction Diode Summary

The PN junction region of a Junction Diode has the following important characteristics:

1. Semiconductors contain two types of mobile charge carriers, Holes and Electrons. 
2. The holes are positively charged while the electrons negatively charged. 
3. A semiconductor may be doped with donor impurities such as Antimony (N-type doping), so that it contains mobile charges which are primarily electrons. 
4. A semiconductor may be doped with acceptor impurities such as Boron (P-type doping), so that it contains mobile charges which are mainly holes. 
5. The junction region itself has no charge carriers and is known as the depletion region. 
6. The junction (depletion) region has a physical thickness that varies with the applied voltage. 
7. When a diode is Zero Biased no external energy source is applied and a natural Potential Barrier is developed across a depletion layer which is approximately 0.5 to 0.7v for silicon diodes and approximately 0.3 of a volt for germanium diodes. 
8. When a junction diode is Forward Biased the thickness of the depletion region reduces and the diode acts like a short circuit allowing full current to flow. 
9. When a junction diode is Reverse Biased the thickness of the depletion region increases and the diode acts like an open circuit blocking any current flow, (only a very small leakage current)