Sep 24, 2005 a step pn junction diode is made in silicon with the n side having nd 2x1016 cm3 and the p side having a net doping of na 5x1015 cm3. Energy of an electron is on the vertical axis, position in the device is on the horizontal axis. These diagrams help to explain the operation of many kinds of semiconductor devices and to visualize how bands change with position band bending. Impurities donors andor acceptors can be added to each side, and the resulting change in the energy band diagram can be observed directly below each cube. When the pn junction is forward biased, the electrons in the ntype material which have been elevated to the conduction band and which have diffused across the junction find themselves at a higher energy than the holes in the ptype material. The full band diagram is a spaghetti of lines that includes the crystal momentum. Also, note that this flatband diagram is not an equilibrium diagram since both electrons and holes can lower their energy by crossing the junction. In effect, then, if one were to connect the two terminals of the pn junction to form a closed circuit, two currents would be present. The 3d diagram for converting mechanical energy into electrical energy by the dynamic pn junction is shown in figure 1f.
Draw the energy level diagram of a pn junction and explain how a junction barrier is developed across an unbiased junction. Draw the energy band diagram of an unbiased pn junction and mark the barrier potential and depletion region. In such case, there is a finite probability that electrons can tunnel from the conduction band of nregion to the valence band of pregion during the tunneling the particle energy does not change e. The loss of charge neutrality at the junction can be also expressed in terms of the potential, which is referred to as builtin voltage v bi. Therefore the valence band is completely full and the conduction band is completely empty. I attach a pdf file of one of my lectures where i explain this stepbystep. Electrons and holes can be localized trapped in a very small.
As a result, each atomic energy level occupied by these valence electrons expands into a wide energy band, which is composed of many discrete energy levels placed close together. Ms2018, school of materials science and engineering, 2015 wang junling, ph. Reducing depletion width can be inferred from the shrinking charge profile, as fewer dopants are exposed with increasing forward bias. By exploring the electrical properties of silicon, it is possible to acquire an understanding of the inner workings of the pn junction.
The metallurgical junction zone has been depleted of carriers compared with the bulk. Energy band diagram at thermal equilibrium at thermal equilibrium energy band diagram of a pn junction in thermal equilibrium while in thermal equilibrium no external voltage is applied between the ntype and ptype material, there is an internal potential, f, which is caused by the workfunction difference between the ntype and ptype pn. If somebody is able to understand pn junctions well, it becomes quite easy for him to understand all other. The energy band diagram of a ptype semiconductor is shown below. Semiconductor physics pn junction all about circuits. Therefore a hole in the ptype region of the pnjunction encounters an electrical potential barrier when trying to move into the ntype region. Unit i semiconductor diode ec8252 electronic devices notes. Energy band diagram of a pn junction a before and b after merging the ntype and ptype regions. Pn junction diode and characteristics of pn junction diode. Energy band diagram of a pn junction a before and b after merging the ntype and ptype regions note that this does not automatically align the fermi energies, ef,n and ef,p.
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. Energy diagram for pn junction in equilibrium the fermi energy must be the same everywhere, otherwise electrons could reduce the energy of the system by flowing to unoccupied states in a region of lower fermi energy. Energy band gap kit containing a pn junction diode placed. Directcurrent generator based on dynamic pn junctions. This applet shows device, charge profile, and energy band with carrier conc. Any applied voltage must therefore drop across the scl. An energy diagram for a pn junction at the instant of formation is shown in figure a. The band gap is the minimum amount of energy required for an electron to break free of its bound state. If the silicon crys tal is very pure, then the outer four electrons occupy all of the covalent. It does not correspond to any physical spatial bending.
Most semiconductor devices involve regions with two different kinds of doping, so a pn junction is present in all of them. 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. Pn junction introduction in this video i introduce the pn. Draw to scale the energy band diagram of the junction at equilibrium. Although the real situation is much more complicated, this simplified picture gives us an intuitive understanding of the energy band. Download file pdf semiconductor physics and devices neamen 4th edition. A w n a w n n d p a every electron moves across the. This is because the band diagram that you have been using has been momentumaveraged, so you are only mapping the energy band diagram as a function of distance of the material inside and outside.
Pn junction diode is one of the fundamental elements in electronics. C the band diagram and carrier dynamic process of the dynamic psingaas generator. Applet is similar to pn junction band diagram applet, but with an intrinsic layer inserted. Doped semiconductors ntype materials ptype materials diodes and transistors. A pn junction diode is a twoterminal electronics element, which can be classified as either step graded or linear graded. Electron tunneling in pn junction when the p and n region are highly doped, the depletion region becomes very thin 10nm.
First, a small current, called reverse saturation current, is, exists. Oct 05, 2017 in this lecture, i discussed about the energy band diagram of the pn junction in equilibrium. Formation of a pn junction diode and its band diagram. Energy band diagram of a pn junction in thermal equilibrium. Lecture 1 introduction to semiconductors and semiconductor. Energy band diagram spring 2003 ee lecture 10, slide 6 qualitative electrostatics band diagram. Band diagram electrostatic potential electric field charge density. The valence band and conduction band in ntype material are slightly lower than that of ptype material. The band diagram of a pn and metal semiconductor junctions. Berkeley 2 ee143 s06 semiconductor tutorial 2 the simplified electron energy band diagram. The builtin potential of an np junction is a little less than what. As you can see, the valence and conduction bands in the n region are at lower energy levels than those in the p region, but there is a significant amount of overlapping. The energy band diagram for each cube is displayed directly below it.
To return the semiconductor to separate pieces, click the separate button or drag one object away from th. A pn junction is a boundary or interface between two types of semiconductor materials, ptype and ntype, inside a single crystal of semiconductor. Introduction to semiconductors and semiconductor devices a background equalization lecture reading. Dec 27, 2017 this video provides knowledge on how to draw the energy band diagran for a pn junction. The student then brings the material together and can create a number of pn junction devices. Review, pn junctions, fermi levels, forward bias prof j. Draw the energy band diagram of a pn junction, the charge density, the electric field, and the electric potential for. The energy band diagram would provide us with i a qualitative variation of potential in the device ii boundary conditions for solution of poissons equation as usual, the energy band diagram of the pn junction will be obtained by combining the energy band diagrams of n and ptype semiconductors separately energy band. Chapter 5 spring 2003 ee lecture 10, slide 2 ntype ptype donors v i. This quick video will give you a complete analysis of energy band diagram of pn juction diode. As we increase the applied volatge the drop across the nuentral region increases such that the n side depletion region is always higher in potential to the p side depletion region. The upward direction in the diagram represents increasing electron energy.
A steadystate balance of carriers is achieved at the junction where diffusive flux of the carriers is balanced by the drift flux. When the band gap energy is met, the electron is excited into a free state, and can therefore participate in conduction. 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. Energy band diagrams for a pn junction under c reverse bias condition. Semiconductor physics and devices neamen 4th edition. It would be effective collecting the electrons arriving at this junction from the p type base to the metal. Energy band diagram of a pn junction a before and b after merging the ntype and ptype regions note that this does not automatically align the fermi energies, e f,n and e f,p. In this type of diode, we dope one side of a semiconductor piece with an acceptor impurity and another side with a donor impurity. V is the energy level of the valence band n v is the intrinsic density of states in the valence band cm3. That implies that you would have to supply energy to get an electron to go up on the diagram, and supply energy to get a hole to go down.
Find the builtin voltage, and compare with the value measured off. Realise the pn junction behaviour when applying a dcvoltage. Energy band diagrams for a pn junction under a open circuit, b forward bias and c reverse bias conditions. Ii professor mark lundstrom electrical and computer engineering. That implies that you would have to supply energy to get an electron to go up on the diagram, and supply energy to get a hole to. The energy diagram of the basecollector junction would be similar to fig. Which can be understood from the energy band diagram shown above. Semiconductors, diodes, transistors horst wahl, quarknet presentation, june 2001 electrical conductivity. Electrons and holes reach an equilibrium at the junction and form a depletion region. It is a region in which no electron can stay as there is no allowed energy state. They readily combine with those holes, making possible a continuous forward current through the junction. Step pn junction energy band diagram physics forums. The field at the schottky junction sweeps the electrons into the metal collector just as in the pn basecollector junction shown in fig.
Smith department of eecs university of california, berkeley eecs 105 spring 2004, lecture 19 prof. Junction coming to the equilibrium by balancing the fermi level ef ef the balance is achieved by electrons diffusing into a pside bringing an extra negative charge in there and by the holes diffusing into an nside bringing an extra negative charge in there pn. To determine the energy band gap of a semiconductor by using pn junction diode. A large number of holes or vacant space in the covalent bond is created in the crystal with the addition of the trivalent impurity. Draw and label a sketch of the energy band diagram for a silicon pn junction at room temperature. The band gap determines how much energy is needed from the sun for conduction, as well as how much energy is generated. Magnitude of forbidden energy gap in germanium and silicon is 0. However, since the carrier concentration must be continuous this is only required in a homojunction, the carrier density in the ntype region is. Apr 05, 2019 pn junction is formed in a single crystal by joining two ntype and ptype semiconductors.
Lets get a voltmeter and ammeter and connect it to the forward biased circuit of pn junction diode. Because of its electrical properties, silicon is called a semiconductor. Conversely, the energy for the positively charged hole increases when going downward in this band diagram. Energy band diagrams for a pn junction under a open circuit and b forward bias v i n p e oe e v ov ev e c e fn e v e v e c e fp b e c e v e c e fp m e fn ev o p n e o e v n p a band diagram of pn junction. Common diode packages and markings diodes often have their cathode end marked with a band.
In regions far away from the junction the band diagram looks like. A simple circuit diagram is shown below, which has a pn junction diode, a battery in picture it is not shown as variable. Band diagram of pn junction under a equilibrium and b forward. The depletion region extends mainly into the pside. Speaking of band diagrams in describing semiconductor devices i cant understand. Therefore a hole in the ptype region of the pn junction encounters an electrical potential barrier when trying to move into the ntype region.
The energy band diagram of the pnjunction unacademy live gate. Energy band diagram concepts pn junction diode no applied bias analog electronics. Electron energy band fermi levelelectrostatics of device charges. A step pn junction diode is made in silicon with the n side having nd 2x1016 cm3 and the p side having a net doping of na 5x1015 cm3. This file is licensed under the creative commons attributionshare alike 2. The backward diode has the unusual property that the socalled reverse bias direction actually has more current flow than the socalled forward bias. When pure silicon is at a temperature of 0k, all of the positions in the outer electron shells are occupied, due to the covalent bonds between atoms and there are no free electrons. Energy band diagram and carrier concentrations for a forwardbiased pn junction. In order to initiate the junction formation, click the formjunction button or mouse drag one object toward the other. Silicon a single silicon atom consists of fourteen negatively. I attach a pdf file of one of my lectures where i explain this stepby step. Spring 2003 ee lecture 10, slide 4 idealized junctions.
Jan 29, 2019 pn junction diode is one of the fundamental elements in electronics. The two diodes to the far left are different from the others in that the semiconductor they. In silicon, the bonding energy of the covalent bond is 1. There are a few basic concepts to be introduced, such as energy bands, drift and diffusion currents, etc. In solidstate physics of semiconductors, a band diagram is a diagram plotting various key electron energy levels fermi level and nearby energy band edges as a function of some spatial dimension, which is often denoted x. The panels show energy band diagram, electric field, and net charge density. Electronic devices ed ee8252 notes pdf free download. While in thermal equilibrium no external voltage is applied between the ntype and ptype material, there is an internal potential, f i, which is caused by the workfunction difference between the ntype and ptype semiconductors. The energy gap between the valence band and conduction band is known as forbidden energy gap. The pn junction is the fundamental building block of the electronic age. And not only that, you will be far ahead of your competitors. Electron energy band fermi level electrostatics of. Pn junction diode, current equations, energy band diagram, diffusion and drift current densities, forward and reverse bias characteristics, transition and diffusion capacitances, switching characteristics, breakdown in pn junction.
Pn junction in equilibriumbuiltin potential the difference in energy levels on the p and n side gives the builtin potential across the junction. Pn junction is formed in a single crystal by joining two ntype and ptype semiconductors. In this lecture, i discussed about the energy band diagram of the pn junction in equilibrium. Reverse bias this means you have to draw 12 graphs. A small or minute quantity of free electrons is also available in the conduction band. A diode is a simple device, which can be implemented by a pn junction, as well as other hetero junction structures. Pn junction diode is a two terminal device, the characteristics of diode depends on the polarity of the external potential applied to the pn junction diode. When looking at a band diagram, the electron energy states bands in a material can curve up or down near a junction. For a pn junction at equilibrium, the fermi levels match on the two sides of the junctions. However, there are many different packages for diodes and others are marked differently. The energy band diagram of the pn junction under open circuit conditions is shown in figure 6.
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