For a pn junction with na1017cm3 and nd 1016cm3, find, at t 300k, the built in voltage, the width of the depletion region, and the distance it extends in the p side and the n side of the junction. A pn junction is formed by diffusing a ptype material to an ntype wafer and there are several other ways as well, see your text. Electrostatics of pn junction in equilibrium a space charge region surrounded by two quasineutral regions formed. These carriers create in turn other carriers before leaving the depletion region critical field in. Physics of semiconductor devices pn junction depletion width problem a silicon pn diode has a doping of n d 81015 1 cm3 and n a 210 16 1 cm3. A pn junction is a boundary or interface between two types of semiconductor materials, ptype and ntype, inside a single crystal of semiconductor. Wikimedia because of the accumulation of electrons on the p side of. The figure below shows the condition after the formation of depletion region. The regions near the pn interface lose their neutrality and most of their mobile carriers, forming the space charge region or depletion layer see figure a.
When analyzing the pn junction, we are most interested in the depletion region, or space charge region scr. May 08, 2017 formation of depletion region and pn junction electronics physics and spirituality. The thicknesses of the depletion region are not the same with x. When the diode is in forward bias, or v d0 v, then the voltage drop. A metalsemiconductor schottky junction can be considered an extreme case of a heavily doped junction where the depletion region lies entirely in the semiconductor side. K0 is a constant depending on the pn junction geometry and v0 is the built. Lecture 050 pn junction and cmos transistors 43010 page 0507 cmos analog circuit design p. The pn junction is the fundamental building block of the electronic age. Jan 21, 2019 the term depletion is due to the fact that near the junction, the region is depleted i. Think of it like this, with a low dopant level id have to take 3mm of space to free 20 charges, but if they were a higher density, id.
Mobile charges in the depletion region are neglected, i. Lecture 050 pn junction and cmos transistors 43010 page 0501 cmos analog circuit design p. There is initially a small reverse saturation current due to thermally generated electron and holes in the depletion region. By definition, ntype semiconductor has an excess of free electrons compared to the p. The electrons which had migrated across from the n to the p region in the forming of the depletion layer have now reached equilibrium. B the total depletion layer depth, w, from pierret, eqn. You are right that a depletion region forms at the pn junction due to recombination, but the change in charge concentration near the junction also has another effect.
Doped semiconductors ntype materials ptype materials diodes and transistors. The width of the depletion region with a reverse bias v. For this case, the depletion region is only due to the pn junction as shown in yellow in the figure. Pn junction, depletion region, diode equation duration. Physical electronicsece3540 the pn junction diodes. Due to the abundance presence of holes or electrons, they become much better conductors than intrinsic semiconductors. Now the reason that the depletion region gets smaller with higher dopants is that there is more static charges exposed in the depletion region. The full depletion approximation assumes that the depletion region around the metallurgical junction has welldefined edges with an abrupt transition between the fully depleted region where no carriers are present and the quasineutral region, a neutral region where the carrier density is close to the doping density. Now an electric field is created within in the depletion region and creates a drift current that opposes the diffusion current. Within the depletion region, there are very few mobile electrons and holes. What are the depletion width in the n region, the depletion width in the p region, and the builtin potential at 300k. Basic structure of the pn junction the two regions are referred to as the space charge region or depletion region. Realise the pn junction behaviour when applying a dcvoltage. This allows electrical current to pass through the junction only in.
As soon as you connect a p type region with an n type region, carriers will begin diffusing from regions of high concentration to regions of lower. D 1019 the semiconductor is on the edge of degeneracy, but we can assume. By applying a large enough forward voltage that overcomes the built in. A is the crosssectional area of the pn junction xp is the distance.
Qualitatively explain the formation of the depletion region and the. This pn junction separates ntype semiconductor from ptype semiconductor fig. The doping profile is our starting point to analyze pn junction electrostatics, since it combines the information given for n d and n a. Formation of the schottky barrier, a metal and semiconductor in their. Formation of depletion region and pn junction youtube. Physics of semiconductor devices pn junction depletion. This is more than 8 orders of magnitude higher than the. 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 region of the ptype material near the junction takes on a net negative charge because of the electrons attracted. Other electrons from the n region cannot migrate because they are repelled by the negative ions in the p region and attracted by the positive ions in the n region. At the pn junction the border of the two types, the mobile electrons in the ntype this is due to the excess electron provided by the ndopan. Pn junction diodes the depletion approximation 9it is very hard to solve p n n d n a d q. When we talked about band bending, one of the reasons that it occurred was because. Pdf simulating the processes in the depletion region of a schottky.
Since the voltage drop across the channel increases from source to drain, the reverse bias of the pn junction also increases from source to drain. A silicon pn junction diode is doped with na 10 16 cm3 and n d 5x10 14 cm3. In semiconductor physics, the depletion region, also called depletion layer, depletion zone. This region behaves as the dielectric material used for making capacitors. The solid circles on the right of the junction represent the available electrons from the ntype dopant. The barrier increase reduces the majority carrier diffusion to a negligible level. Since the voltage drop across the channel increases from source to drain, the reverse bias. As v dd increases, the increasing potential at the drain reverse biases the pn junctions.
Pdf abstract this paper along with the attached maple files takes us through the. Reverse bias causes majority carriers to move away from the junction, thereby creating more ions. Semiconductors, diodes, transistors horst wahl, quarknet presentation, june 2001 electrical conductivity. The pside electrons and nside holes can wander into the depletion region and. Indicate conduction and valence bands, intrinsic fermi level and fermi level in your sketch. Depletion region when a ptype and ntype semiconductor region are brought together, carriers to diffuse across the junction to recombine with dopants, leaving behind ionized cores. The thin layer of the crystal lattice between these charges has been depleted of majority carriers, thus, is known as the depletion region. The holes and electrons are carriers of electricity when they move about. A depletion region is a natural feature of doping a semiconductor ntype and right next to it ptype.
At a critical field, the carriers have enough kinetic energy to create electronhole pairs by collisions with the lattice silicon atoms. There is no knee in an exponentially increasing function like the ideal pnjunction iv characteristic. By applying a large enough forward voltage that overcomes the built in voltage, you can make the depletion zone disappear. Depletion layer scl width 3 where v o ktelnn a n dn i 2 is the builtin voltage. The fulldepletion approximation assumes that the depletion region around the metallurgical junction has welldefined edges with an abrupt transition between the fully depleted region where no carriers are present and the quasineutral region, a neutral region where the carrier density is close to the doping density. Near the junction, electrons diffuse across to combine with holes, creating a depletion region. Depletion region when the p and ntype materials are placed in contact forming a junction, two things happen in the contact region. Only a few carriers at a time being in the space charge region depletion approximation.
Density gradients still exist in the majority carrier concentrations at each edge of. In the following, v d implies the diode biasing voltage. Most semiconductor devices contains one or more pn junctions. The energy level sketch above right is a way to visualize the equilibrium condition of the pn junction. Space charge is primarily composed of uncompensated donors and acceptors.
If ptype semiconductor is joined with ntype semiconductor, a pn junction is formed. These two layers of positive and negative ions form the depletion region. The width of the resulting depletion region is related to the barrier potential and is a function of applied bias. Allen 2010 breakdown voltage as a function of doping it can. The region around the junction from which the mobile charge carriers electrons and holes are depleted is called as depletion region. The term depletion refers to the fact that the region near the junction is depleted of their respective majority charge carriers. Iv characteristics uthe barrier increase reduces the majority carrier diffusion to a negligible level uthe pside electrons and nside holes can wander into the depletion region and be swept to the other side a reverse inap ureverse bias situation aan increase of the potential hill ubeing associated with. What is a pn junction when a ptype semiconductor is suitably joined to an ntype semiconductor, the contact surface is called pn junction. An electric field is set up by the charge distribution, opposing the process until an equilibrium is reached hyperphysics diffusion recombination leaves ion cores. This current is fed by carriers in the pregion which happen to diffuse into the depletion zone. Physics of semiconductor devices pn junction depletion width. There is no knee in an exponentially increasing function like the ideal pn junction iv characteristic. The knee voltage is a rather arbitrary concept sometimes taken as the voltage where the current is 1ma.
Electronic materials, devices, and fabrication here, p p0 is the equilibrium concentration of holes in the pside equal to n a and p n0 is the excess hole concentration at the interface between the depletion region and nside. By exploring the electrical properties of silicon, it is possible to acquire an understanding of the inner workings of the pn junction. A reverse bias will increase the depletion region width while a forward bias reduces the width and can lead to the disappearance of the depletion region when large currents. Well, an electric field can result because of nonuniform doping, as in a pn junction, and that is why the energy band diagram for a pn junction has band bending even when it is in equilibrium. Allen 2010 lecture 050 pn junctions and cmos transistors. The pn junction is of great importance as it is the main control element for the semiconductor devices. What links here related changes upload file special pages permanent link page.
Silicon a single silicon atom consists of fourteen negatively. The result is a region near the junction that acts to repel the mobile charges away from the junction through the electric field that these charged regions create. Pdf simulating the processes in the depletion region of. Essentially all electrons and holes are swept out of the space charge region by the electric field. Since electrons departed the ntype region, it takes on a localized positive charge.
To first order, carrier concentrations in space charge region are much smaller than the doping level. The region in which the ptype and ntype semiconductors are joined is called pn junction. A silicon pn junction in thermal equilibrium at t 300 k is doped such that e fe fi 0. Formation of pn junction in actual practice, a pn junction will not be formed if. If the length of the depletion region in the p region is xp then the total charge contained in it is. This current is called drift current, since this is due to movement of the thermally generated. The ptype and ntype conducting on each side of dielectric act as the plate. The depletion layer is formed very quickly and is very thin as compared to the n region and the p region. At the point of the junction, which we call the metallurgical junction, is where n d n a 0.
The depletion region becomes wider and the current ow becomes smaller. What are the depletion width in the nregion, the depletion width in the pregion, and the builtin potential at 300k. Formation of depletion region and pn junction electronics physics and spirituality. Pdf simulating the processes in the depletion region of a. Suppose that the voltage across the pn junction is now v. The charge on the left of the junction must be balanced by the charge on the right side of the junction. A pnjunction of two di erent materials in the open circuit case. The pn junction solidstate device theory electronics. This result into the formation of a charged depletion region formed at the pn junction.
Explain the formation of depletion layer in pn junction electrons and holes diffuse into regions with lower concentrations of electrons and holes, much as ink diffuses into water until it is uniformly distributed. Ece 255, pn junction and diode college of engineering. Coulomb force from ions prevents further migration across the pn junction. Why doesnt the whole pn junction become depletion region. 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. Abstract this paper along with the attached maple files takes us through the study and process of simulating the processes in the depletion region of a schottky diode. Think of it like this, with a low dopant level id have to take 3mm of space to free 20 charges, but if they were a higher density, id only have to take 3um to free 20 charges. The width of this depletion region can be calculated.