The diode current is an exponential function of diode voltage in the forwardbias region. Abrupt depletion layer approximation let apply bias and calculate current through the pn. What is difference between drift current and diffusion. When reverse bias voltage applied to the pn junction diode is increased, a large number of holes majority carriers from pside and electrons majority carriers from nside are moved away from the pn junction. In an unbiased junction, due to the diffusion of charge carriers, the diffusion current, which flows from the p to n region, is exactly balanced by the equal and opposite drift. When the diode is forward biased drift current is present, but because diffusion current grows exponentially, it dominates. The drift current, by contrast, is due to the motion of charge carriers due to. A ptype has a lot of holes and an ntype lot of electrons.
Diffusion current is caused by variation in the carrier concentration. Drift current diffusion current drift current is caused by electric fields. Pn junctions consist of two semiconductor regions of opposite type. The graphs shows the ideal iv characteristics of a pn junction diode. The unbiased pn junction is in an equilibrium where the diffusion current is cancelled by the drift current. The currents associated with the diffusion and builtin field drift current cancel for both the electrons and holes. The carrier current at equilibrium cross a pn junction 1. Concept of drift current and diffusion current comes in the semiconductor material. When you form a pn junction, mobile carriers from each side of the junction flow to the other side due to diffusion. Application of voltage to pn junction results in disruption of balance between drift and diffusion in scr in forward bias, minority carriers are injected into quasineutral regions in reverse bias, minority carriers are extracted from the quasineutral regions.
A pn junction with open terminals including minority carriers thermallygenerated minority carriers on the n side holes move toward the depletion region, and are swept into the p side by the potential where the combine with electrons. There are a few basic concepts to be introduced, such as energy bands, drift and diffusion currents, etc. In the pn junction diode if it is doped nonuniformly then there exists a movement of charge carriers from higher concentration to lower concentration. It is necessary to consider the part of diffusion current when describing many semiconductor devices. Apr 05, 2019 the current that flows in a pn junction diode is the small leakage current, due to minority carriers generated at the depletion layer or minority carriers which drift across the pn junction. Such junctions show a pronounced rectifying behavior. An abrupt silicon pn junction n a 10 16 cm3 and n d 4 x 10 16 cm3 is biased with v a 0. There is a builtin voltage at the pn junction interface that prevents penetration of electrons into the pside and holes into the nside.
Working of pn junction this simple pn junction animation video clearly explains the topics pn junction semi conductor or diode, what is forward bias and reverse bias, what is diffusion current. Pn junction diodes form the basis not only of solar cells, but of many other electronic devices such as leds, lasers, photodiodes and bipolar junction transistors bjts. Lecture 6 biased pn junction diodes and current flow. Diffusion current occurs when you have a pn junction diode created. The applied voltage upsets the steadystate balance between drift and diffusion, which unleashes the flow of diffusion current. Diode under thermal equilibrium zdiffusion small since few carriers have enough energy to penetrate barrier zdrift current is small since minority carriers are few and far between. Finally, the result is that the growth in the width of the depletion layer presents a high impedance path which acts as an insulator. In semiconductors, this flow of carriers from one region of higher concentration to lower concentration results in a diffusion current. Diffusion ficks law describes diffusion as the flux, f, of particles in our case is proportional to the gradient in concentration. Alan doolittle ways carriers electrons and holes can change concentrations current flow. Theory of pn junction diode, diode current equation s. Inside the depletion region, both diffusion current and drift current are present. The basic pn junction diode circuit symbol, and conventional current direction and voltage polarity.
Difference between drift current and diffusion current. Mar 02, 2018 working of pn junction this simple pn junction animation video clearly explains the topics pn junction semi conductor or diode, what is forward bias and reverse bias, what is diffusion current. As diffusion continues, the spacecharge regions on either side of the junction start extending. A pn junction consists of two semiconductor regions with opposite doping type as shown in figure. A diode is a simple device, which can be implemented by a pnjunction, as well as other heterojunction structures. Driftdiffusion current diffusion in quasineutral regions 4.
When the diffusion starts, the diffusion current is large and the drift current is very small. In this video, lets explore the famous pn junction. I diffusion i drift, ii d0, very large currents if v is sufficiently large v i d v pn junction diode electrostatics a depletion region in which nand pare each much smaller than the net dopantconcentration is formed at the junction between p and ntype regions a built in potential barrier voltage drop exists across the depletion region, opposing carrier diffusion due to a concentration gradient across the junction. Kasap, 1990 2001 an ebooklet5 to calculate the forward current when v 0. Animation p n junction semiconductor works forward. Semiconductors are made up of two types of material i. What is difference between drift current and diffusion current.
We will see what happens when we have a single crystal with a ptype semiconductor on one side and an ntype on the other. Morris 2 majority carrier diffusion barrier lowered balanced by diffusion reduced diffusion increased minority carrier drift drift enhanced. Only minority carriers generated within a diffusion length can contribute current zimportant point. A pn junction is the metallurgical boundary between the n and pregions of a semiconductor crystal. Note that the dominant current component flowing across the metallurgical junction is that associated with minority. Pdf diffusionlimited current in organic metalinsulator. It is likely that the diffusion component will exceed the.
Drift current in a pn junction diode in a pn junction diode, electrons and holes are the minority charge carriers in the pregion and the nregion, respectively. Biased pn junction diodes forward biased and reverse biased pn diodes depletion capacitance of pn junction diodes minority and majority carrier distributions in a biased pn junction diodes carrier transport and current flow in biased pn junction diodes. Diffusion current is a current in a semiconductor caused by the diffusion of charge carriers holes and or electrons. Chapter 1 semiconductor pn junction theory and applications. Diode junction capacitance transition capacitance and. A very small drift current flows, limited by the rate at which minority carriers diffuse from the quasi. Lets consider a pn junction which is at zero biased voltage. The carrier distribution, pnx, in a long diode is illustrated by figure 4. The behaviour of the pn junction with regards to the potential barriers width produces an asymmetrical conducting two terminal device, better known as the pn junction diode. Electron diffusion electron drift current current hole diffusion current hole drift current current flow is proportional to evavref due to the exponential decay of carriers into the majority carrier bands pn junction iv characteristics current flow is dominated by majority carriers flowing across the junction and becoming minority carriers. I believe you mistakenly called this drift in your question. This strengthens the electric field and eventually the drift current. A diode is a simple device, which can be implemented by a pn junction, as well as other hetero junction structures.
The region on the left is ptype with an acceptor density na, while the region on the right is ntype with a. As we know, there is a difference in the concentration of holes and electrons at the two sides of a junction, the holes from the pside diffuse to the nside and the electrons from the nside diffuse to the pside. We cover the basic differences between drift current and diffusion current below with a comparison table. Direction of the drift current is always in the direction of the electric field. Most semiconductor devices involve regions with two different kinds of doping, so a pnjunction is present in all of them. You say that the pn junction only starts to conduct current after the voltage aplied on the diode vd reaches 0.
Diffusion if a teacher places a few drops of perfume at the front of a classroom it wont be long until students at the back of the room can detect its odour. Total current p n n n drift n diffusion n n p p drift p diffusion p p j j j and. The depletionlayer, energyband diagram, and concentrations on a. Working of pn junction diode with biasing, characteristics. For example, the current near the depletion region of a pn junction is dominated by the diffusion current. The basic device formed because of the above process is the pn junction diode. The current that flows in a pn junction diode is the small leakage current, due to minority carriers generated at the depletion layer or minority carriers which drift across the pn junction. Abrupt depletion layer approximation let apply bias and calculate current through the pn junction. But due to the created electric field minority carriers can drift across the depletion layer creating a drift current. How diffusion and drift current in a pn junction get equal. Show their direction and magnitude in the dark and under illumination. The diffusion forms a dipole charge layer at the pn junction interface. No generationrecombination in the depletion layer 6.
Forward bias balance between diffusion and drift currents is broken. Drift current is caused by external electric field while diffusion current is caused by a carrier concentration gradient. The depletionlayer, energyband diagram, and concentrations on a logarithmic scale of the mobile electrons. Describe the nature of drift, diffusion, and illumination currents in a diode. The processes that follow after the formation of a pn junction are of two types diffusion and drift. Calculate minority carrier diffusion current at scr. Once the junction is formed charges are diffused across the junction and form the depletion region and stops further diffusion. The current is very nearly zero in the reversebias region. The total current flowing through the depletion region under forward biasing is made up of mostly majority carrier diffusion.
Direction of the diffusion current depends on the gradient of the carrier concentration. A pn junction diode is one of the simplest semiconductor devices around, and which has the characteristic of passing current in only one direction only. Applying a potential to the ends of a diode does not increase current through drift rather it. When the diode is reverse biased diffusion is negligible, but drift remains constant. The capacitance at the depletion region changes with the change in applied voltage. Since the conduction through the junction happens via.
Calculate the ideal diode current assuming that the ntype region is much smaller than the diffusion length with w n 1 mm and assuming a long ptype region. We will assume, unless stated otherwise, that the doped. They are also called pn diodes in analogy with vacuum diodes. Applying a potential to the ends of a diode does not increase current through drift rather it lowers the potential barrier to diffusion. Switching devices like diode, transistors etc are made up of sandwich. As an example, drift current occurs in metal when you apply a potential across a say, a metal wire. A pn junction aggregates the recombination, generation, diffusion and drift effects described in the previous pages into a single device. We now distinguish between the two limiting cases as discussed when calculating the ideal diode current, namely the long diode and a short diode. Most semiconductor devices involve regions with two different kinds of doping, so a pn junction is present in all of them. The applied electric field in a forward biased pn junction cancels the electric field in the depletion region, eliminating the potential barrier and allowing diffusion current to flow. The diffusion current and drift current together are described by the driftdiffusion equation. Now an electric field is created within in the depletion region and creates a drift current that opposes the. Pn junction diode i i drift diffusion with bias voltage, v0. The diffusion process leaves a depletion region near the such that as the charges build up, a net positive charge and net negative charge are created at the p and the nside respectively.
Finally, the result is that the growth in the width of the depletion layer presents a. This is the current which is due to the transport of charges occurring because of nonuniform concentration of charged particles in a semiconductor. Diffusion is the process by which things move from a region of high concentration to a region of low concentration. Now an electric field is created within in the depletion region. Current flow in a forward biased pn junction diode.