The pn junction is the fundamental building block of the electronic age. The current in the pn junction diode can only flow from one side to the other. Pn junction diode 12 spice model for pn junction diode r s. 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. T1 d nv v id is e biasing the junction diode pnjuntiondiode. Pdf accurate extraction of the diffusion current in. Zener diode is a pn junction diode specially designed to operate in the reverse biased mode. 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.
Current in a pn junction is due to injection of minority carriers in. Pn junctions are formed by joining ntype and ptype semiconductor materials, as shown below. The total current flowing through the depletion region under forward biasing is made up of mostly majority carrier diffusion. The diffusivity is a measure of how quickly a group of particles fill a space and it is described in more detail on the diffusion page the diffusivity is typically given in cm 2 s and the lifetime is in seconds so the above formula will give the diffusion length.
The doping of the diode is a nondegenerately doped step junction. 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. If the diffusion current dominates the saturation current, then x1. Temperature dependence of the saturation current of a junction diode 153 the temperature dependence of the saturation current can be written approximately in the form g i 0 constants exp. A pn junction diode is a twoterminal electronics element, which can be classified as either step graded or linear graded.
Note that the dominant current component flowing across the metallurgical junction is that associated with minority. Therefore, a drift current and a diffusion current is flowing. However, unlike a resistor, a diode does not behave linearly with respect to the applied voltage as the diode has an exponential currentvoltage iv relationship and therefore we can not. Current flow in a forward biased pn junction diode. Switching devices like diode, transistors etc are made up of sandwich.
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. Semiconductors are made up of two types of material i. Working of pn junction diode with biasing, characteristics. 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. Abrupt depletion layer approximation let apply bias and calculate current through the pn. Similarly, holes flow by diffusion from the ptype side to the ntype side. Theory of pn junction diodes theory of pn junction diode. What is the currentvoltage characteristic for the pn junction diode. Zener diodes, however, are designed to operate in the breakdown region. A pn junction is a boundary or interface between two types of semiconductor materials, ptype and ntype, inside a single crystal of semiconductor. The quasineutral regions are in lowlevel injection. When a forwardbias voltage is applied to a pn junction, a current will he.
The pn junction is also called as semiconductor diode. Silicon wafer cleaning rca process in the ic processing wafers it is necessary to maintain the purity and perfection of material. By exploring the electrical properties of silicon, it is possible to acquire an understanding of the inner workings of the pn junction. A forward bias corresponds to applying a positive voltage to the anode the ptype. Most semiconductor devices involve regions with two different kinds of doping, so a pn junction is present in all of them. Silicon a single silicon atom consists of fourteen negatively. There is a builtin voltage at the pn junction interface that prevents penetration of electrons into the pside and holes into the nside. Pn junction tutorial introduction to pn junction diodes. Now an electric field is created within in the depletion region. The diffusion forms a dipole charge layer at the pn junction interface. There will be a small amount of current called as leakage current due to minority charge carriers at the junction.
At equilibrium, the net current diffusion and drift current is zero for. Such junctions show a pronounced rectifying behavior. In this paper formation a pn junction diode is explained by vlsi fabrication steps which are stated below. There are a few basic concepts to be introduced, such as energy bands, drift and diffusion currents, etc. Pn junction diode and characteristics of pn junction diode. Why does the pn junction diode exhibit current rec tification. Capacitor c includes depletionlayer capacitance for the reversebias region and diffusion capacitance. The pn junction is a versatile element, which can be used as a. Shur introduction to electronic devices energy band diagram of a pnjunction silicon pnjunction at zero bias. 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. 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.
The boltzmann approximation is to say that the resulting current being small compared with the components of this current, we consider that we are still in quasi equilibrium and therefore that the currents equation is still valid by replacing v bi by v. Concept of drift current and diffusion current comes in the semiconductor material. This constitutes a forward current majority carrier movement diffusion current. Lecture 15 the pn junction diode i iv characteristics november. This allows electrical current to pass through the junction only in. Forward bias decreases depletion region and also increases diffusion current. Pn junctions consist of two semiconductor regions of opposite type. Diffusion capacitance minority carriers majority carriers ece 315 spring 2006 farhan rana cornell university pdoped ndoped. Read more pn junction diode and its working principle.
A pn junction is the metallurgical boundary between the n and pregions of a semiconductor crystal. A diode is a simple device, which can be implemented by a pnjunction, as well as other heterojunction structures. No generationrecombination in the depletion layer 6. A diode is a simple device, which can be implemented by a pn junction, as well as other hetero junction structures.
If a battery is connected to a diode, current will flow only when. 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. Driftdiffusion current diffusion in quasineutral regions 4. The width of the depletion region is incredibly thin, typically a few thousands of millimetres, current may not flow through the diode. Lecture 7 large and small signal modelling of pn junction. Once the electric field is strong enough to stop the diffusion currents the junction reaches. In thermal equilibrium there is a balance between drift and diffusion current. Minority carriers that reach the junction are swept across the depletion region due to drift. In this type of diode, we dope one side of a semiconductor piece with an acceptor impurity and another side with a donor impurity. The symbols for diodes and transistors use arrows to indicate the direction that conventional current can flow. This component is modeled by another capacitance, called the diffusion capacitance c d as a diode is turned off changes from forward biased to reverse biased for a short period of time a current will flow in the negative direction until the minority charge is removed. They are also called pn diodes in analogy with vacuum diodes.
Accurate extraction of the diffusion current in silicon pn junction diodes article pdf available in applied physics letters 729. The p positive side contains an excess of holes, while the n negative side contains an excess of electrons in the outer shells of the electrically neutral atoms there. Lecture 6 biased pn junction diodes and current flow. Questions you should be able to answer by the end of todays lecture. Department of eecs university of california, berkeley eecs 105 spring 2004, lecture 19 prof.
Large and small signal modelling of pn junction diodes. A pn junction aggregates the recombination, generation, diffusion and drift effects described in the previous pages into a single device. Diode junction capacitance transition capacitance and. Smith builtin field zin thermal equilibrium, the pn diode has.
Physical integrated structure p side intrinsic pn junction n b. Pn junction diode is one of the fundamental elements in electronics. Diode is a basic element in electronic and digital circuit. We now consider a pn diode with an applied bias voltage, va. Ee105 fall 2014 microelectronic devices and circuits. Fabrication of pnjunction diode by icfabrication process. Thus, electrons diffuse from the nregion to the pregion. An ideal pn junction diode is based on the following basic assumption.
When the diode is forward biased drift current is present, but because diffusion current grows exponentially, it dominates. Most semiconductor devices involve regions with two different kinds of doping, so a pnjunction is present in all of them. When the diode is reverse biased diffusion is negligible, but drift remains constant. Doping with acceptor creates p type semiconductor while doping with donors produces n type. Assuming current flowing through the diode to be very large, the diode can be approximated as short. Pn junction underbiasing 14 boltzmanns approximation. Minority carrier diffusion current electron diffusion hole diffusion. What is difference between drift current and diffusion. Forward bias operation the pn junction supports unidirectional current flow. Pn junction diode technology innovation and incubation. Nonideal pn junction diodes engineering libretexts. A pn junction diode is a two terminal single crystal semiconductor device whose one side is doped with acceptors and other side by donors. Electrons and holes diffuse from areas of high concentration toward areas of low concentration.
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