PN JUNCTION DIODE
INTRODUCTION
Materials can be classified in a myriad of ways among them their ability to conduct electricity. Materials fall under three categories in this respect;
- Conductors
- Insulators
- Semiconductors
Conductors are materials with properties that enable them pass various types of energy, among then electric current. Metal for example is a good conductor of electricity. Metals contain free electrons which help in the movement of current.
Insulators on the other hand do not contain free electrons and are therefore non-conductors of electricity. Examples include wood and plastic.
Semiconductors are materials whose conductivity lies between that of an insulator and a conductor. Semiconductors are metalloids. Their chemical and physical properties lie between those of metals and non-metals (the converse is however not the case; all metalloids are not semiconductors). The conductivity of semiconductors is temperature dependent. When the temperature of the semiconductor increases, the ability to conduct increases and when the temperature falls, so does the conductivity.
A semiconductor in its pure form is called and intrinsic semiconductor. Examples of intrinsic semiconductors are silicon, germanium, gallium and arsenide. The conducting ability of a pure semiconductor can be increased by adding a impurity, called a dopant, to it through a process called doping. The resulting semiconductor is referred to as an extrinsic semiconductor or doped semiconductor. There are two different types of extrinsic semiconductors categorised on the type of doping carried out. These are:
- P-type semiconductor
- N-type semiconductor
A P-type semiconductor has an excess of positively charged carriers. It is formed when a trivalent element (impurity) is added to an intrinsic (pure) semiconductor. An N-type material on the other hand has an excess of negatively charged carriers. It is formed when an intrinsic semiconductor is doped with a pentavalent element
A semiconductor fabricated in such a way that it allows current to flow in one direction while restricting flow of current in the reverse direction is called a diode, also called a rectifier. A diode essentially acts as a one-way switch. There are different types of diodes among them:
- Light Emitting Diode (LED)
- Laser diode.
- Avalanche diode.
- Zener diode.
- Schottky diode.
- Photodiode
- PN junction diode
The PN junction diode is discussed is this section.
PN JUNCTION DIODE
A PN junction diode is a 2-terminal semiconductor device that conducts electricity in one direction only. This means that it has minimum resistance when connected one-way and maximum resistance when connected the other way. A PN junction diode therefore controls the flow of current in a circuit. The diode has a positive side (P-side, also called P-type) and a negative side (N-side, also called N-type) The interface or boundary between the P-type and N-type is called a junction.
A P-side (P-type material) has an excess of positively charged carriers (holes). It’s an extrinsic semiconductor formed when a trivalent element (impurity) is added to an intrinsic (pure) semiconductor. The N-side (N-type material) on the other hand has an excess of negatively charged carriers. It is an extrinsic semiconductor formed when an intrinsic semiconductor is doped with a pentavalent element.
The symbol for a PN junction diode is a triangle pointing to a line across one of the apexes. The apex with the bar points in the direction of current and is taken as the N-side (negative terminal)). The triangle is taken as the P-side (positive terminal.
A diode conducts when connected one way and not the other way. When conducting, the diode is said to be forward biased and when not conducting it is said to be reverse biased. A diode is said to be forward biased when the P-type region is connected to the positive terminal and N-type region to the negative terminal of a power source.
It is reverse biased when the N-type region is connected to the positive terminal and P-type region to the negative terminal of a power source.
Depletion region
A short while after fabrication of a PN junction diode, a thin depletion region is formed between the P-type and N-type materials (without the aid of external power source).
The depletion region is formed due to diffusion of negative charge carries in to the P-region and holes into the n-region on account of concentration gradient. The holes and electrons diffusing towards each other merge, forming a region devoid of free charge carriers. The depletion region, acts as an insulator.
The thickness of the depletion region is reduced when the diode is forward biased as more electrons are pushed across the junction. An increase in forward voltage leads to a further decrease in the thickness of the depletion region leading to an increase in the diode conductivity.
If the voltage is increased beyond a certain threshold value, the thickness of depletion layer is reduced so much so that the diode becomes a super conductor and the current increases sharply.
Thickness of the depletion region is increased by reverse biasing the diode as holes (positive charge carriers) move to one end of the diode due to attraction by the negative terminal (well, holes appear to move but in reality, it is the few electrons in the P-region that are repelled by the negative voltage leaving holes behind) while the negative charge carriers move to the other extreme end under the action of the positive terminal.
If the reverse voltage is increased beyond a certain threshold value, the diode breaks up and starts conducting. This threshold voltage is called breakdown voltage.
Uses of diodes:
There are many diode applications among them
- Rectification
- Isolating signals from a supply
- Mixing signals
Rectification
This refers to the process of turning alternating current into a direct current. There are two forms of rectification:
- half-wave rectification
- full-wave rectification.
Half-wave rectification:
The positive cycle is passed though the load (L) while the negative cycle is blocked. Half wave rectification can be achieved using one diode as shown.
The diode conducts only during the positive (clockwise) cycle but does not conduct during the negative (anticlockwise) cycle.
Full-wave rectification:
Happens when current through the load flow in the same direction during both cycles. This can be achieved through the use of;
(i) Two diodes
During the positive cycle, the current passes through diode A then through the load in XY direction and during the negative cycle, the current passes through diode B then through the load in the direction XY, similar to the positive cycle.
(ii) Four diodes:
Current through the load in the positive cycle is in the direction NM (XANMBY), same as in the negative cycle (YCNMDX).
EXAMPLES
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