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