What is Diode? Working and Modes of Diode
A Diode is a two-terminal semiconductor device formed by two doped regions of silicon separated by a PN junction. In this article I will tell you about what is diode and how it works.
As mentioned, a diode is made from a small piece of semiconductor material, usually silicon, in which half is doped as a p region and half is doped as an n region with a pn junction and depletion region in between. The p region is called the anode and is connected to a positive terminal of battery. The n-type is called the cathode and is connected to a second terminal of battery. The basic diode structure and schematic symbol are shown in
If we connect positive terminal of battery to the P-type Material and Negative terminal of battery to the N-type of material then it is called Forward Bias Configuration of Diode.
If we Connect Negative Terminal of Battery to the P-type of Material and Positive terminal of Battery to the N-type Material then it is Called Reverse Bias of Diode.
So lets go to the details:
Forward bias of Diode:
Forward bias is the condition that allows current through the pn junction. The voltage source is connected in such a way that it produces a Forward Bias of Diode. This external bias voltage is designated as VBIAS. The resistor limits the forward current to a value that will not damage the diode.
- Note that the -ve side of VBIAS is connected to the n-region of the diode and the +ve side is connected to the p-region. This is one requirement for forward bias.
- A second requirement is that the bias voltage, VBIAS, must be greater than the barrier potential.
So Now what is Barrier Potential:
A Barrier Potential is the internal potential of particular material incase of Silicon it is 0.7v and incase of Germanium it is 0.3v. It means when we forward bias the diode it should cross the voltage above then 0.7 for silicon and 0.3V for germanium.
As we know N-type material is consist of Electrons and P-type material is consist of Holes. A fundamental picture of what happens when a diode is forward-biased is shown below.Because like charges repel, the negative side of the VBIAS “pushes” the free electrons, which are the majority carriers in the N-region, toward the pn junction. This flow of free electrons is called electron current. Now the VBIAS imparts sufficient energy to the free electrons for them to overcome the barrier potential which is 0.7V (Silicon) of the depletion region and move on through into the p region.
Once they reaches in the P-region, these conduction electrons have lost enough energy to immediately combine with holes in the valence band.
Now, the electrons are in the valence band in the P-region, simply because they have lost too much energy overcoming the barrier potential to remain in the conduction band.
Since unlike charges attract, the positive side of the VBIS attracts the valence electrons toward the left end of the P-region. The holes in the P-region provide the medium for these valence electrons to move through the p region. The valence electrons move from one hole to the next hole toward the left. The holes, which are the majority carriers in the p region, effectively (not actually) move to the right toward the junction, as you can see in Figure. This effective flow of holes is the hole current. You can also view that the hole current as being created by the flow of valence electrons through the p region, with the holes providing the only means for these electrons to flow.
As the electrons flow out of the p region through the positive side of the bias-voltage source, they leave holes behind in the p region; at the same time, these electrons become conduction electrons in the metal conductor.
As we Know that the conduction band in a conductor overlaps the valence band so that it takes much less energy for an electron to be a free electron in a conductor than in a semiconductor and that metallic conductors do not have holes in their structure. There is a continuous availability of holes effectively moving toward the PN junction to combine with the continuous stream of electrons as they come across the junction into the p region.
Now Let Us Discus The Effect of Forward Bias on the Depletion Region:
As more electrons flow into the depletion region, the number of +ve ions is reduced. As more holes effectively flow into the depletion region on the opposite side of the pn junction, the number of -ve ions is reduced. This reduction in +ve and -ve ions during forward bias causes the depletion region to narrow. As shown below in figure.
The Effect of the Barrier Potential During Forward Bias:
Electric field between the +ve and -ve ions in the depletion region on either side of the junction creates an “energy hill” that prevents free electrons from diffusing across the junction at equilibrium. This is known as the barrier potential.
When forward bias is applied, the free electrons are provided with enough energy from the VBIAS to overcome the barrier potential and effectively “climb the energy hill” and cross the depletion region. The energy that the electrons require in order to pass through the depletion region is equal (=) to the barrier potential.
In other words, the electrons give up an amount of energy = the barrier potential when they cross the depletion region. This energy loss results in a voltage drop across the PN junction equal to the barrier potential (0.7 V), additional small voltage drop occurs across the p-region and n-region due to the internal resistance of the material. For doped semi-conductive material, this resistance, called the dynamic resistance, this is very small and can be neglected.
Reverse Bias of Diode:
Reverse bias is the condition that essentially prevents current through the diode. As mentioned above if we connect –ve terminal of battery to P-type material and +ve Terminal of Battery to N-type material this lead to diode in Reverse Bias. note that the depletion region is shown much wider than in forward bias or equilibrium.
A diode connected for reverse bias. A limiting resistor is shown although it is not important in reverse bias because there is essentially no current.
An illustration of what happens when a diode is reverse-biased is shown in below. Because unlike charges attract, the +ve side of the bias-voltage source “pulls” the free electrons, which are the majority carriers in the n-region, away from the pn junction. As the electrons flow toward the +ve side of the voltage source, additional +ve ions are created. This results in a increasing of the depletion region and a depletion of majority carriers.
In the p type region, electrons from the -ve side of the voltage source enter as valence electrons and move from hole to hole toward the depletion region where they create additional -ve ions. This results in a increasing of the depletion region and a depletion of majority carriers. The flow of valence electrons can be viewed as holes being “pulled” toward the +ve side. The initial flow of charge carriers is transitional and remain for only a very short time after the reverse-bias voltage is applied. As the depletion region widens, the availability of majority carriers decreases. As more of the n- region and p-regions become depleted of majority carriers, the electric field between the +ve and -ve ions increases in strength until the potential across the depletion region equals the bias voltage, VBIAS. At this point, the transition current essentially ceases except for a very small reverse current that can usually neglected.
This is the small current that exists in reverse bias after the transition current dies out is caused by the minority carriers in the n-region and p-regions that are produced by thermally generated electron-hole pairs. The small number of free minority electrons in the p-region are “pushed” toward the pn junction by the -ve bias voltage. When these electrons reach the increased depletion region, they “fall down the energy hill” and combine with the minority holes in the n-region as valence electrons and flow toward the +ve bias voltage, creating a small hole current.
conduction band in the p-region is at a higher energy level than the conduction band in the n-region. Therefore, the minority electrons easily pass through the depletion region because they require no additional energy. Reverse current is illustrated in Figure below.
Normally, the reverse current is very small that we can neg late it. However, if we increase the external reverse-bias to a value called the breakdown voltage, the reverse current will also increase. This is what happens. The high reverse-bias voltage transmit energy to the free minority electrons so that they collide with atoms with enough energy to knock valence electrons out of orbit and into the conduction band. The newly created conduction electrons are also high in energy and repeat the process. If one electron knocks only two others out of their valence orbit during its travel through the p region, the numbers quickly multiply. As these high-energy electrons go through the depletion region, they have enough energy to go through the n region as conduction electrons, rather than combining with holes.
Most diodes are not operated in reverse breakdown, but if the current is limited (by adding a series-limiting resistor for example), there is no permanent damage to the diode.
VOLTAGE-CURRENT CHARACTERISTIC OF A DIODE:
As you have learned, forward bias produces current through a diode and reverse bias essentially prevents current, except for a negligible reverse current. Reverse bias prevents current as long as the reverse-bias voltage does not equal or exceed the breakdown voltage of the junction. Now we will examine the relationship between the voltage and the current in a diode on a graphical basis.
When a forward-bias voltage is applied across a diode, there is current. This current is called the forward current and is designated IF. Figure below illustrates what happens as the forward-bias voltage is increased positively from 0 V. The resistor is used to limit the forward current to a value that will not overheat the diode and cause damage. With 0 V across the diode, there is no forward current. As you gradually increase the forward-bias voltage, the forward current and the voltage across the diode gradually increase, as shown in Figure given below. A portion of the forward-bias voltage is dropped across the limiting resistor.
When the forward-bias voltage is increased to a value where the voltage across the diode reaches approximately 0.7 V (barrier potential), the forward current begins to increase rapidly, as illustrated in Figure given below. As you continue to increase the forward-bias voltage, the current continues to increase very rapidly, but the voltage across the diode increases only gradually above 0.7 V. This small increase in the diode voltage above the barrier potential is due to the voltage drop across the internal dynamic resistance of the semiconductive material.
V-I Characteristic for Reverse Bias:
When a reverse-bias voltage is applied across a diode, there is only an extremely small reverse current (IR) through the pn junction. With 0V across the diode, there is no reverse current.As you bit by bit increase the reverse-bias voltage, there’s a awfully little reverse current and therefore the voltage across the diode will increase. once the applied bias voltage is increased to a value where the reverse voltage across the diode (VR) reaches to the breakdown value of diode which is (VBR), the reverse current begins to increase rapidly. As you still increase the bias voltage, the present continues to extend very rapidly, but the voltage across the diode increases very little above VBR. Breakdown, with exceptions, is not a normal mode of operation for most pn junction devices.
The Complete V-I Characteristic Curve of Diode:
Combine the curves for both forward bias and reverse bias, and you have the complete V-I
characteristic curve for a diode, as shown in Figure give below.