The difference between an insulator and a semiconductor lies in the size of the bandgap. Video created by university of colorado boulder for the course diode pn junction and metal semiconductor contact. Band structure of open circuted pn junction study buddy. The holes remaining conduct in the opposite direction in the valence band, as described for the bond model. The left side in blue contains ptype dopant which provides excess holes brown circles as th.
Chapter 1 semiconductor pn junction theory and applications. The breakdown voltage of the diode of 400 m diameter with the fp structure were. This builtin potential vbi can be found as follows. The electron movement in a lattice is different from the movement in free space. In the ptype region there are holes from the acceptor impurities and in the ntype region there are extra electrons when a pn junction is formed, some of the electrons from the nregion which have reached the conduction band are free to diffuse across the junction and combine with holes filling a hole makes a negative ion and leaves behind a positive ion on the nside. Alan doolittle definitions vacuum level, e 0corresponds to energy of free electrons in vacuum. Therefore a hole in the ptype region of the pnjunction encounters an electrical potential barrier when trying to move into the ntype region. A pn junction is a boundary or interface between two types of semiconductor materials, ptype and ntype, inside a single crystal of semiconductor.
Pn junction diode is one of the fundamental elements in electronics. 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. These homojunctions are formed from two blocks of the same type but of different doping to form a pn junction. Doped semiconductors ntype materials ptype materials diodes and transistors. Volume ii 2nd edition modular series on solid state dev. The region on the left is ptype with an acceptor density n a, while the region on the right is ntype with a donor density n d.
Such junctions show a pronounced rectifying behavior. Quantum well in a pn junction consider a pn junction with a quantum well located between the n and p region as shown in the figure below. The upward direction in the diagram represents increasing electron energy. Pn junctions consist of a spacecharge region sandwiched by two quasineutral bulk regions. The boundaries separating these three regions are charge neutral planes at x. The pn junction the pn junction is a homojunction between a ptype and an ntype semiconductor. The frontier electronic structure of solids is characterized by valence and conduction bands, which are analogous to the homo and lumo.
The valence band and conduction band in ntype material are slightly lower than that of ptype material. A steadystate balance of carriers is achieved at the junction where diffusive flux of the carriers is balanced by the drift flux. The structure and energy band model of a pn junction in equilibrium when the junction reaches equilibrium, further diffusion is inhibited by the coulomb force between the newly created negative ions and the electrons. Two separate bits of semiconductor, one is an ntype, the other is a ptype ef 2. At t 00, semiconductor materials have basically the same structure as insulators. The band structure of a semiconductor gives the energy of the electrons on the yaxis and is called a band diagram. Pn junction diode and characteristics of pn junction diode.
The electric field results from the deviation from charge neutrality in the vicinity of the junction. The term pn junction refers to the joint interface and the immediate surrounding area of the two semiconductors. The result is that some of the free electrons from the donor impurity atoms. The area of the ptype near the junction becomes more negative due to the excess electrons while the ntype. A pnjunction is formed when an ntype material is fused together with a. Introduction to 1 semiconductors chapter outline 11 the atom 12 materials used in electronic devices current in semiconductors 14 ntype and ptype semiconductors 15 the pn junction chapter objectives describe the structure of an atom discuss insulators, conductors, and semiconductors and how they differ describe how current is produced in a. A pn junction is formed when two types of semiconductors, n type excess electrons and p type excess holes, come into contact. In a crystal the electron will pass through energy band full of electrons and some with very few.
If you find these terms foreign, just read the chapter about. Homojunctionbased optoelectronic components have been developed since the 1960s, which represent the birth of light emissions on semiconductors. The term band gap refers to the energy difference between the top of the valence outer electron band and the bottom of the. Energy band diagram of the pn junction in equilibrium. Working of pn junction diode with biasing, characteristics. This means that eventually in vicinity of the junction all free carriers will be depleted leaving stripped ions. For a pn junction at equilibrium, the fermi levels match on the two sides of the junctions. By exploring the electrical properties of silicon, it is possible to acquire an understanding of the inner workings of the pn junction.
This potential difference can alter the flow of majority carriers, so that the pn junction can be used as an opportunity for the diffusion of electrons and holes. Device structure for pn junction, energy band diagram at. They are also called pn diodes in analogy with vacuum diodes. We have also learned about diffusion current, depletion region, drift current and barrier potential. 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. Bandgap engineering is a powerful technique for the design of new semiconductor materials and devices. In the band diagrams for the metal contacts with n and ptype semiconductors, you see from the bending of the vacuum potential energy that in the ntype case, an electrical potential barrier exists for. The pn junction diode the negative terminal of the biasvoltage source pushes the conductionband electrons in the n region toward the pn junction, while the positive terminal pushes the holes in the p region toward the pn junction when it overcomes the barrier potential v b, the external voltage source provides the n region electrons with enough energy to penetrate the. In this module, we look at pn junction in equilibrium. A pn junction diode is a twoterminal electronics element, which can be classified as either step graded or linear graded. Equilibrium band diagrams appear below the semiconductor. Energy band diagram2, fermi energy level electronic devices2 by sahav singh yadav duration.
The dopants are assumed to be shallow, so that the electron hole density in the ntype ptype region is approximately equal to the donor acceptor density. If an external potential is applied to the terminals of pn junction, it will alter the potential between the p and nregions. The pn junction is a versatile element, which can be used as a. In order to prevent outer electric discharge at a high voltage, the diodes were measured in insulating oil with a high dielectric breakdown. Draw the energy level diagram of a pn junction and explain how a junction barrier is developed across an unbiased junction. Smith department of eecs university of california, berkeley eecs 105 spring 2004, lecture 19 prof.
Heterojunction pn junction at equilibrium coursera. Pn junctions are types of semiconductors with a unique band structure and are used in variety of applications from solar cells to leds to transistors. What is the currentvoltage characteristic for the pn junction diode. In this video, i go over the band diagram of the pn junction, which will allow us to find the electric field and the pn junction width through what is called the builtin potential vbi. Band structure of open circuted pn junction youtube. The energy band diagram of a pn junction showing the location of carriers under zero voltage bias 1. The band gap of a semiconductor is the minimum energy required to excite an electron that is stuck in its bound state into a free state where it can participate in conduction. Review, pn junctions, fermi levels, forward bias prof j. Band bending implies an electric field and, therefore, a potential difference across the junction. Space charge region an overview sciencedirect topics. Insulators at t0, with a small density of electrons excited at finite temperatures. Therefore, the region around the junction will contain no free carriers and is considered depleted. It acts as a diode, which can serve in electronics as a rectifier, logic gate, voltage regulator zener diode, switching or tuner varactor diode.
These diagrams help to explain the operation of many kinds of semiconductor devices and to visualize how bands change with position band bending. This allows electrical current to pass through the junction only. In this type of diode, we dope one side of a semiconductor piece with an acceptor impurity and another side with a donor impurity. Pn junctions consist of two semiconductor regions of opposite type. A gatefree monolayer wse 2 pn diode nature communications. This means that jn is constant across the depletion. We will refer to this structure as an abrupt pn junction. Here dec is the change in the energy of the conduction band across the junction. Valence band p n forming a pn junction electrons continue to diffuse across the junction. Semiconductors, diodes, transistors horst wahl, quarknet presentation, june 2001 electrical conductivity. Depletion region approximation states that approximately no free carriers exist in. The band diagram of the pn junction in equilibrium is shown in figure 8.
The difference between vacuum level and fermilevel is called workfunction, of materials. No free carriers means 1 transport equations drop out and 2 no recombination or generation, so the continuity equation becomes. A pn junction is the metallurgical boundary between the n and pregions of a semiconductor crystal. A pnjunction diode is formed when a ptype semiconductor is fused to an ntype semiconductor creating a potential barrier voltage across the diode junction the effect described in the previous tutorial is achieved without any external voltage being applied to the actual pn junction resulting in the junction being in a state of equilibrium. Group iv semiconductor produces ntype or p type semiconductors. In this lecture, i discussed about the energy band diagram of the pn junction in equilibrium. A pn junction consists of two semiconductor regions with opposite doping type as shown in figure 4. Semiconductors semiconductors are materials with a relatively small band gap typically 1ev between a filled valence band and an empty conduction band. The forbidden gap corresponds to the minimum energy needed to release an electron from a covalent bond to the conducting band where it can conduct a current. 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. We will assume, unless stated otherwise, that the doped.
Frequently we will deal with pn junctions in which one side is distinctly higherdoped than the other. This demonstration shows the electronic band structure above of a pn junction as well as the physical junction below for a generic semiconductor. 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. Semiconductors and devices based on pn junctions chapter 7 wednesday, october 28, 2015. The pn junction is the fundamental building block of the electronic age. Electrons and holes reach an equilibrium at the junction and form a depletion region. Band structure of pn junction semiconductor wolfram. The dopants are assumed to be shallow, so that the electron hole density in the ntype ptype region is approximately equal to the donor. We can assume no free carriers since the electric field sweeps them out of the depletion region quickly. The depletion region extends mainly into the pside. In regions far away from the junction the band diagram looks like.