PN- JUNCTION:
By themselves, P-type and N-type materials taken separately are of very limited use. It we Join a piece of P-type material to a piece of N-type material such that the crystal structure remains continuous at the boundary, a PN-Junction is formed. As such a PN-Junction created a very useful devices and systems. It is called a semiconductor (or crystal) diode.
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A PN-junction cannot be made by simply pushing the two pieces together; this would not lead to a single crystal structure. some special manufacturing techniques are adopted to form a PN-Junction. during the previous paragraph we already describe how a PN-Junction is formed.
A PN-junction itself is an important device. Furthermore, practically all semiconductor devices contain at least one PN Junction. For this reason, it is very necessary to understand how a PN-Junction behaves when connected in an electrical circuit. In this chapter we shall discuss the properties of a PN-Junction. It will help us to understand even those devices Which have more than one PN-Junction.
JUNCTION THEORY
The most important characteristic of a PN-junction is its ability to conduct current in one direction only. In the reverse direction it offers very high resistance to flow of current. How this happens is explained in the sections that follow.
PN- JUNCTION WITH NO EXTERNAL VOLTAGE:
As show in above figure a PN-junction just immediately after it is formed. Note that it is a single crystal. It’s one side is P-type and other side is N-type. The P region having holes and negatively charged impurity ions. The N region having free electrons and positively charged impurity ions. For simply, minority charge carriers are not shown in the figure.
Holes and electrons are the moving or mobile charges, but the ions are unmovable or immobile. The simply as the whole is electrically neutral and so are the P-type region and N-type region considered independently, therefore, in the P region, the charge of moving holes equal the total charges on its free electrons and unmovable or immobile ions.
Holes and electrons are the mobile charges, but the ions are immobile. The sample as a whole is electrically neutral and so are the P region and N region considered separately. Therefore, in the P region, the charge of moving holes equal the total charges on its free electrons and immobile ions. Similarly, in the region, the negative charge of its majority carriers is compensated by the charge of its minority carriers and immobile ions.
Note that no external voltage has been connected to the PN-junction of as show in figure as soon as the PN-junction is formed, the following processes are initiated:
1. Holes from the P region diffuse into the M region. They then combine with the free electrons in the region.
2. Free electrons from the N-type region diffuse in the P-type region.
3. The diffusion of holes (from P-type region to N-type region) and electrons (from V region to P region takes the place because they move unexpectedly due to thermal energy, and also because there is a difference in their concentrations in the two P-type region and N-type region. The P-type region having more holes and the N-type region having more free electrons.
4. One would normally expect the holes in the P-type region and free electrons in the N-type region to flow towards each other and combine. it is therefore, all the holes and free electrons would have been eliminated. But in practice this does not occur. The diffusion of the holes and free electrons across the junction point is occurs for a very short time. After a few recombination of holes and electrons in the immediate neighborhood of the junction, a restraining force is set up automatically. This force 1s called a barrier. Furthermore, this diffusion of holes and electrons from one side to the other is stopped by this barrier. However, this barrier force is developed is explained in the paragraphs that follow.
5. Some of the holes in the P region and some of the free electrons in N region diffuse towards each other and recombine. Each recombination in eliminates the hole and the free electron. n this process, the negative acceptor ions in the P region and positive donor ions in the N region in the immediate neighborhood of the junction are left uncompensated. This situation is shown in the down figure. Additional holes trying to diffuse into the N-type region are repelled by the uncompensated positive charge of the donor ions. The electrons trying to diffuse into the P-type region are repelled by the uncompensated negative charges on the acceptor ions. As a result, total recombination of holes and electrons cannot occur.
7. The barrier discourages the diffusion of majority carriers across the Junction point. But what happens to the minority carriers? There are a few free electrons in the P-type region and a few hole in the N-type region. The barrier helps to these minority carriers for drift across the PN- junction. So these minority carriers are constantly produced due to thermal energy. it means there would be a current due to the movement of these minority carriers? definitely not. Electric current can’t flow because there is no supply or circuit connected with PN-junction.
The drift of minority carriers across the junction is counterbalanced by the diffusion of the same number of majority carriers across the PN-junction. These few majority carriers have sufficiently high kinetic energy to overcome the barrier and across the junction. Actually, the barrier height itself adjusted so the flow of minority carriers is exactly balanced by the flow of majority carriers across the PN- junction.
Therefore, we conclude that a barrier voltage is produce when across the PN-junction in the absence of external battery.
PN-JUNCTION WITH FORWARD BIAS:
let suppose a battery is connected to the PN-junction diode such that the positive terminal is connected with P-type region and negative is connected with N-type region, as shown in the following figure. So in this type of biasing or condition the PN junction is called forward-biased.
When the PN-junction is forward-biased, the holes are repelled from the positive terminal of the battery and are compelled to move towards the at the electrons are repelled negative terminal of the battery and drift towards the junction point. Because of their acquired energy, some holes and free electrons penetrate the depletion region.
This reduces the potential barrier. then the width of the depletion region reduced so does the barrier height. As a result of this, more majority carries diffuse across the junction. These carriers recombine and make a reason due movement to charge carriers in the space-charge region.
To each recombination of the free electron and the hole occurs, an electron from the battery negative terminal enters the N-type material. Then drifts towards the junction. Similarly, in the P-type material near the positive terminal of the battery, an electron breaks a bond in the crystal and enters the positive terminal of the battery. For each and every electron that breaks the its own bond, resultant a hole is created. This hole drifts towards the junction. Note that there is a continuous electron current in the external circuit. The current in the P-type region of the material is due to the mobility or movement of the holes. The current in the N-type material is due to the movement of electrons. The current continuously as long as the battery is connected with circuit. If the battery volts increased, the barrier potential is more reduced. More majority carriers diffuse across the junction point. This results continually increased the current throughout the PN-junction.
PN-JUNCTION WITH REVERSE BIAS:
As show in figure what happens when a battery with the indicated polarity Is connected to a PN-junction. Note that the positive terminal of the battery is connected to the N-type material and negative terminal is connected to the P-type material. The holes in the P-type region are attracted towards the negative terminal of the battery. The holes in the P-type region are attracted towards the negative terminal of the battery. The holes in the P-type region are attracted towards the negative terminal of the battery. The electrons in the N-type region are attracted to the positive terminal of the battery. therefore, the majority carriers are drawn away from the PN-junction point. This action widens the depletion region and increases the barrier potential (compare this with the unbiased PN-junction as show in the following figure.
The increased barrier voltage or potential makes it furthermore difficult for the majority carriers to diffuse across the PN-junction point. Therefore, this barrier potential is helpful to the minority carriers to crossing the junction. In fact, as soon as minority carrier is generated, it is swept (or drifted) across the junction because of the barrier potential. The rate of produced minority carriers depends on the temperature. If the temperature is stabled, the rate of producing of minority carriers remains constant. thus, the current due to flow of minority carriers remains the same temperature of the battery so voltage is low or high. For this reason, the same current is called reverse saturation current.
This miner current is very small as the numbers of minority carrier is small. It is in to the order of microamperes in germanium diodes and Nano-amperes in silicon diodes.
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