As an example, drift current occurs in metal when you apply a potential across a say, a metal wire. An example is the very famous shockley pn junction diode equation and the famous shockley theory for transistors. When the diode is reverse biased diffusion is negligible. 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. A diode is a simple device, which can be implemented by a pnjunction, as well as other heterojunction structures. Current voltage characteristic reverse biased junction. The drift current, by contrast, is due to the motion of charge carriers due to the force. What is difference between drift current and diffusion. Drift current is caused by external electric field while diffusion current is caused by a carrier concentration gradient. But due to the created electric field minority carriers can drift across the depletion layer creating a drift current.
The total current flowing through the depletion region under forward biasing is made up of mostly majority carrier diffusion. The junction can be formed by the implantation or diffusion of dopants in an wafer. There will be a small amount of current called as leakage current due to minority charge carriers at the junction. Diffusion and drift currents in a pn junction stack exchange. Difference between drift current and diffusion current. We have also learned about diffusion current, depletion region, drift current and barrier potential.
Lets consider a pn junction which is at zero biased voltage. Because of this, the minority charge carriers are attracted and they constitute drift current opposite to diffusion current. Chapter 1 semiconductor pn junction theory and applications. Unfortunately, this requires the knowledge of the quasifermi energies, which is only known if the currents are known. Review, pn junctions, fermi levels, forward bias prof j. Diffusion ficks law describes diffusion as the flux, f, of particles in our case is proportional to the gradient in concentration.
A pn junction aggregates the recombination, generation, diffusion and drift effects described in the previous pages into a single device. There is a builtin voltage at the pn junction interface that prevents penetration of electrons into. Applying a potential to the ends of a diode does not increase current through drift rather it. Basic diode electronics introduction to diodes the pn junction. Since the ntype region has a high electron concentration and the ptype a high hole concentration, electrons diffuse from the ntype side to the ptype side. Driftdiffusion theory of symmetrical doublejunction diodes. Negative charge stops electrons from further diffusion positive charge stops holes from further diffusion the diffusion forms a dipole charge layer at the pn junction interface. Diffusion current occurs when you have a pn junction diode created. If you find these terms foreign, just read the chapter about. 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. In the pn junction diode if it is doped nonuniformly then there exists a movement of charge carriers from higher concentration to lower concentration. Constant quasifermi level implies that the electrostatic force equals the diffusion force. Pn junction diode and its characteristics in chapter 1 understanding the pn junction, we have seen how a pn junction is formed from a ptype and ntype semiconductor.
Given current density j ij x area flowing in a semiconductor block with. There are a few basic concepts to be introduced, such as energy bands, drift and diffusion currents, etc. If the pn junction is forward biased, the diffusion would increase and drift current is negligible. The capacitance at the depletion region changes with the change in applied voltage.
Both the processes reach equilibrium when idriftidiff. The pn junction is the fundamental building block of the electronic age. Diode junction capacitance transition capacitance and. To derive the diffusion current in a semiconductor diode, the depletion layer must be large. However, pn junction diodes are considered a minority carrier device due to the current control coming from the diffusion of minority carriers. So on pside, the current carried by electrons which is diffusion current due to minority carriers, decreases exponentially with respect to distance measured from the junction. The drift velocity v is dependant upon the mobility of the holes and electrons in the junction as well as the electric field placed across the junction.
When calculating the current in a pn diode one needs to know the carrier density and the electric field throughout the pn diode which can then be used to obtain the drift and diffusion current. Nonideal pn junction diodes engineering libretexts. 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. Abrupt depletion layer approximation let apply bias and calculate current through the pn. In semiconductors, this flow of carriers from one region of higher concentration to lower concentration results in a diffusion current. When the diode is reverse biased diffusion is negligible, but drift remains constant. Drift transport is the type of transport that occurs not only in semiconductor material, but also in metals.
I pn x current due to holes in nside as a function of x. What is the physical significance of poisson, drift. Since the conduction through the junction happens via. No generationrecombination in the depletion layer 6. Lecture 6 biased pn junction diodes and current flow. What is the concept of diffusion current semiconductors edc. 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. Pn junctions are formed by joining ntype and ptype semiconductor materials, as shown below. The current in the pn junction diode can only flow from one side to the other. When the junction diode is biased in the reverse direction, the majority charge carriers are attracted by the respective terminals away from the pn junction, thus avoiding the diffusion of electrons and holes at the junction. Transport due to the movement of carriers due to the presence of an electric field is called drift transport. 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. Concept of drift current and diffusion current comes in the semiconductor material.
The drift current and the diffusion current describes the flow rates of the. In this lecture, well discuss one of the foundational pieces of nearly all modern semiconductor devices. 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 current. Switching devices like diode, transistors etc are made up of sandwich. The basic device formed because of the above process is the pn junction diode. 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. Lets look at the different currents in a pn junction. Example determine the current in a pn junction diode. If the pn junction is reverse biased, the diffusion current would be negligible and the drift current would increase drastically. If the diode is sufficiently far in the forwardbias region, then the 1 term.
Current flow in a forward biased pn junction diode. Similarly, holes flow by diffusion from the ptype side to the ntype side. This strengthens the electric field and eventually the drift current. Drift current occurs whenever there an some electric field that causes the carriers to exhibit a net flow. Semiconductors are made up of two types of material i. Theory of pn junction diode, diode current equation diode resistance static and dynamic resistance. Theory of pn junction diode, diode current equation o0yigenf4z8. Now an electric field is created within in the depletion region. Once the junction is formed charges are diffused across the junction and form the depletion region and stops further diffusion.
The drift current of a pn junction is dependent upon the bias placed across the junction. Most semiconductor devices involve regions with two different kinds of doping, so a pnjunction is present in all of them. How diffusion and drift current in a pn junction get equal. We cover the basic differences between drift current and diffusion current below with a comparison table. Current voltage characteristic, forward bias junctions. This lecture will complete our threelecture sequence on chemistry. Quasifermi level can be related to the drift diffusion current density. The unbiased pn junction is in an equilibrium where the diffusion current is cancelled by the drift current.
Smith department of eecs university of california, berkeley eecs 105 spring 2004, lecture 19 prof. In the pn junction diode if it is doped nonuniformly then there exists a movement of charge carriers from higher concentration to lower. L4 drift, diffusion, pn junction free download as powerpoint presentation. Both majority and minority carriers can participate in drift current, not just minority carriers. Why does the pn junction diode exhibit current rec. 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 currents associated with the diffusion and builtin field drift current cancel for both the electrons and holes. Lecture 15 the pn junction diode i iv characteristics november. Current generated by the movement of charged particles. Drift current and diffusion currents in semiconductors. 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. It is found that the iv characteristics at low currents and voltages depend greatly on the doping concentration of the base region. 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.