What is a Diode?
A diode is defined as an electronic two-terminal component that conducts current in one direction only (as long as it is operated at a specified voltage level). An ideal diode in one direction will have zero resistance and infinite resistance in the opposite direction.
Although it’s not possible to achieve zero or infinite resistance in the real world. Instead, a diode will have negligible resistance in one direction (to allow current flow) and very high resistance in the opposite direction (to prevent current flow). A diode is like an electrical circuit valve.
The most common type of diode is the semiconductor diodes. Such diodes only continue to conduct electricity when there is a certain threshold voltage in the forward direction (i.e. the direction of “low resistance”). When conducting current in this direction, the diode is said to be “forward biased.” The diode is said to be “reverse biased” when aligned in the reverse direction within a circuit (i.e. the “high resistance” direction).
A diode only blocks the current in the reverse direction (i.e. if it is biased in the reverse direction) while the reverse voltage is within a given range. The reverse barrier breaks over this range. The voltage at which this breakdown happens is called “reverse breakdown voltage.” The diode will conduct electricity in the reverse direction (i.e. the “high resistance” direction) when the circuit voltage is higher than the reverse breakdown voltage. That’s why we say diode’s have a high resistance in the opposite direction –not an infinite resistance.
A PN junction is the semiconductor diode’s simplest form. Ideally, this PN junction acts as a short circuit when forward biased, and as an open circuit when reverse biased. The name diode is derived from “di-ode,” meaning a device that has two electrodes.
The diode symbol is shown below. The arrowhead points in the forward biased condition in the direction of the conventional current flow. That means connecting the anode to the p side and connecting the cathode to the n side.
By doping pentavalent or donor impurity in one portion and trivalent or accepting impurity in another portion of the silicon or germanium crystal block, we can create a simple PN junction diode. Such dopings make the middle part of the block a PN junction. By joining a p-type and n-type semiconductor with a special manufacturing technique, we can also form a PN junction. The anode is the terminal attached to the p-type. The cathode is the terminal attached to the side of the n-type.
Working Principle of Diode
The operating theory of a diode relies on the interaction between semiconductors of n-type and p-type. There are plenty of free electrons and very few holes in a n-type semiconductor. In other words, we can say that in a n-type semiconductor the concentration of free electrons is high and the concentration of holes is very low. Free electrons are referred to as majority charging carriers in the n-type semiconductor, and holes are referred to as minority charging carriers in the n-type semiconductor.
A semiconductor p-type has a high holes concentration and low free electrons concentration. Holes in the p-type semiconductor are mostly charging carriers, and free electrons are minority charging carriers in the p-type semiconductor.
Forward Biased Diode
Now let’s see what happens if a source’s positive terminal is connected to the p-type side and the source’s negative terminal is connected to the diode’s n-type side and this source’s voltage is slowly increased from zero.
There’s no current flowing through the diode at the beginning. This is because although there is an external electrical field applied throughout the diode, most charging carriers still do not have enough external field control to cross the depletion area. As we said the region of depletion acts as a possible barrier against the carriers of the majority charge. This potential barrier is said to be forward potential barrier. Some charging carriers only start to cross the forward potential barrier when the cost of the externally applied voltage across the junction reaches the forward barrier potential. The forward barrier potential for silicon diodes is 0.7 volt and 0.3 volt for germanium diodes. When the forward voltage applied externally across the diode becomes more than the forward barrier potential, the free majority carriers begin to cross the barrier and contribute the forward diode current. In this case, the diode will act as a short-circuited path, and only externally attached resistors to the diode limit the forward current.
Reverse Biased Diode
Now let’s see what happens when we connect the voltage source’s negative terminal to the p-type side and the voltage source’s positive terminal to the diode’s n-type side. Because of the electrostatic attraction of the source’s negative potential, the p-type region’s holes would be shifted more away from the junction, leaving more uncovered negative ions at the junction. Similarly, the free electrons in the n-type region would be moved further from the junction to the voltage source’s positive terminal, leaving more positive ions in the junction. The depletion region becomes wider as a result of this phenomenon. This diode condition is called the reverse biased condition. No majority carriers cross the junction at that point as they leave the junction. In this way, a diode blocks current flow when it is biased in reverse.
As we said at the beginning of this article in the p-type semiconductor there are always some free electrons and some holes in the n-type semiconductor. Such opposite charging carriers are considered minority charging carriers in a semiconductor. In the reverse biased condition the holes will easily cross the reverse biased depletion region on the n-type side as the field across the depletion region is not present, rather it allows minority carriers to cross the depletion region. As a result, a tiny current flows from the positive side to the negative side through the diode. This current’s amplitude is very small due to the very low number of minority charge carriers in the diode. This is called the current of reverse saturation.
If, due to higher electrostatic force and a higher kinetic energy of minority charging carriers colliding with atoms, the reverse voltage across a diode is increased beyond a safe value, a number of covalent bonds are broken to contribute a huge number of free electron-hole pairs in the diode and the process is cumulative. The unusually large number of such charging carriers would contribute to the diode’s large reverse current. If an external resistance connected to the diode circuit does not limit this current, the diode may be permanently destroyed.
Now let’s see what happens when you come into contact with a n-type region and a p-type region. Because of the disparity in density, most carriers are diffusing from one side to the other. Since the density of holes in the p-type region is high and low in the n-type area, the holes begin to spread from the p-type region to the n-type region. Again, the concentration of free electrons in the n-type region is high and low in the p-type region, which is why free electrons tend to migrate from the n-type region to the p-type region.
The free electrons from the n-type region diffusing into the p-type region would recombine with the available holes and create uncovered negative ions in the p-type region. Likewise, the holes spreading from the p-type region into the n-type region will recombine with the available free electrons and generate exposed positive ions in the n-type region.
Therefore, a layer of negative ions would appear on the p-type side and a layer of positive ions would appear in the n-type area along the junction line of these two semiconductor types. The layers of exposed positive ions and uncovered negative ions form an area in the center of the diode where there is no charge carrier as all the charge carriers here in this region are recombined. This area is called depletion region due to the shortage of charge carriers.
After the depletion region has been formed, charge carriers are no longer diffused in the diode from one side to the other. This is because it will prevent further migration of charge carriers from one side to another due to the electric field appearing across the depletion area. The potential of the layer of uncovered positive ions on the side of the n-type would repeal the holes on the side of the p-type and the potential of the layer of uncovered negative ions on the side of the p-type would repeal the free electrons on the side. This means that a potential barrier is created across the junction to prevent further charging carriers from spreading.
Types of Diode
P-N junction diode
Light emitting diode
Also Read – Transistor as a switch
Also Read – Transistor as an amplifier