By definition, ntype semiconductor has an excess of free electrons compared to the ptyp. 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. Whereas when a forward bias is applied depletion region width of the pn junction diode decreases so the amount of the uncovered space charge decreases as well. Doped atoms near the metallurgical junction lose their free carriers by diffusion. If the boundary condition imposes a zero electric field outside the depletion region, then, for electrical neutrality reasons, the total positive charge is equal to the total negative charge, according to the charge conservation theorem. The behaviour of the pn junction with regards to the potential barriers width produces an asymmetrical conducting two terminal device, better known as. Both the processes of drift and diffusion occur in the depletion region in opposite directions.
The concentration of these donors and acceptors are selected to give the pn junction desired device properties pnjuntiondiode. Pn junction in equilibrium no bias applied diffusion across the junction causes a lot of electrons and holes to recombine leaving a depletion region in proximity of the junction the fixed ions left behind create an electric field the electric field opposes the diffusion of holes in the n region and free. Silicon a single silicon atom consists of fourteen negatively. Pn junction forward bias reverse bias zener diodes duration. Neither free mobile electrons nor holes your comments. 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. In other words, why voc of schottky contact is smaller than the pn junction. I explain what happens when you put together a piece of n and ptype material. Substituting with boron boron, with five protons and five electrons, can also substitute in a silicon crystal where. The decrease in the energy level of the conduction band in the n region is due to the loss of the higher nergy electrons that have diffused across the junction to the p region. We can assume no free carriers since the electric field sweeps them out of the depletion region quickly. Physics of semiconductor devices pn junction depletion width. Thus a small region is formed in the vicinity of the junction which is depleted of free charge carrier and has only immobile ions.
If, we apply forward bias voltage to the pn junction diode. Pn junction capacitor under thermal equilibrium, the pn junction does not draw any much current but notice that a pn junction stores charge in the space charge region transition region since the device is storing charge, it s acting like a capacitor positive charge is stored in the nregion, and negative charge is in the pregion. Jan 29, 2019 the electric filed opposes further diffusion of free electrons from the ntype side and holes from the ptype side of the pn junction diode. The width of depletion region is depends on the amount of impurities added to the semiconductor. Pn junction in equilibrium no bias applied diffusion across the junction causes a lot of electrons and holes to recombine leaving a depletion region in proximity of the junction the fixed ions left behind create an electric field the electric field opposes the diffusion of holes in the n region and free electrons in the p region. The depletion region is so named because it is formed from a conducting region by removal of all free charge carriers, leaving none to carry a current. What are the depletion width in the nregion, the depletion width in the pregion, and the builtin potential at 300k. We call this region across the junction where the uncovered charges ions exist, as depletion region. The pside or the positive side of the semiconductor has an excess of holes and the nside or the negative side has an excess of electrons. The main problemp that needs to be tackled is he effect of the bandgap discontinuities and the different materialt parameters, which make the actual calculations more complex even though the n diode p concepts need almost no changing. The p region is called as the anode and is connected to a positive terminal of a battery and it has holes in majority carrier and electrons in minority carrier.
Atoms in the depletion region are depleted of majority charge carriers and are ionized. Space charge region an overview sciencedirect topics. As these fixed atoms lose their free carriers, they build up an electric field, which. D the width of the depleted ntype region which is left positively charged. Neither free mobile electrons nor holes view answer. Pn junction diode multiple choice questions answers. So the pn junction diode behaves as a device in which the amount of charge in depletion region depends on the voltage across the device. Pn junction article about pn junction by the free dictionary. The pn junction is the fundamental building block of the electronic age. This recombination of electrons and holes produces a narrow region at junction called depletion layer. Questions you should be able to answer by the end of todays lecture. Pn junction diode plays a vital role in our electronic fields, because of their unique property current flows in only one direction they are used in many electronic or electrical circuits like rectifiers, switches, clippers, clampers, voltage multipliers in this article, we will learn about what is a pn junction diode and how it works and also effect on pn junction diode with different. Depletion region spacecharge in the pn junction total charge density in the semiconductor. The concentration of these donors and acceptors are selected to give the pn junction desired device properties pn juntiondiode.
As the depletion layer built up a different of potential appears across the junction this difference of potential at junction is called potential barrier. Width of depletion region physics and radioelectronics. Calculate the total width of the depletion region if the applied voltage v a equals 0, 0. Depletion region has no electrons or holes, but has fixed immobile charges from donor and acceptor ions the fixed charges establish an electric field, and create a potential difference between p. The junction plays an important role in the development of the diode which is the building block of most of the semiconductor devices like transistors, bjts, solar cells and integrated circuits. The positive potential applied to the ptype material repels the holes, while the negative potential applied to the ntype material repels the electrons. Poissons equation xn0, xp0 the depletion region width on the n and psides. The boundaries separating these three regions are charge neutral planes at x. The hetero pn junction the heterojunction n diode is in principle very similar to a homojunction. If a free electron in the p region or a hole in the n region somehow reach the edge of the depletion region get swept by the electric field drift pm mn kvl at equilibrium. Pn junction tutorial introduction to pn junction diodes.
The region in a backilluminated ccd between the edge of the depletion region and the back surface is called the field free region. For some devices the depletion region where electrons are swept to the potential well minima does not extend throughout the entire device. In addition to reading the questions and answers on my site, i would suggest you to check the following, on amazon, as well. At this coupling point, free electrons ntype and holes ptype cancel each other and form a depletion zone that acts as a nonconductive barrier. The width of depletion layer adecreases in forward bias bincreases in reverse bias. The depletion region acts like a wall between ptype and ntype semiconductor and prevents further flow of free electrons and holes. Depletion region is a region near the pn junction where flow of charge carriers free electrons and holes is reduced over a given period and finally results in zero charge carriers.
The depletion layer width, builtin voltage, maximum field, and depletion capacitance will be displayed in the appropriate boxes. Therefore, at the junction there is an inclination for the free electrons to diffuse into the ptype region and holes to the ntype region and this process is named as diffusion. There is an energy gradiant across the depletion region which acts as an energy hill that an nregion electron must climb to get to the p region. A depletion region forms instantaneously across a pn junction. Pn junction diode and characteristics of pn junction diode. Physics of semiconductor devices pn junction depletion. This current is fed by carriers in the p region which happen to diffuse into the depletion zone. Impurities are the atoms pentavalent and trivalent. What happens when we join ptype and ntype semiconductors. Oct 10, 2019 a pn junction diode is a semiconductor device with two opposite region such as ptype region and ntype region.
Shockley model from sze, 1981 vv at 1 j j e tot s n n p p p n s l d n q l d p j. What is the width of the depletion region in a pn junction. How is a depletion layer formed in a pn junction diode. The width of the depleted ptype region which is left negatively charged. The depletion region is also called as depletion zone, depletion layer, space charge region, or space charge layer. When the light illuminates the pn junction, the photons energy releases free electrons and holes. For an abrupt pn junction, if we only consider a 1d case, we have. Depletion region when a pn junction is formed, some of the free electrons in the nregion diffuse across the junction and combine with holes to form negative ions. The effect of adding this additional energy source results in the free electrons being able to cross the depletion region from one side to the other.
The electric filed opposes further diffusion of free electrons from the ntype side and holes from the ptype side of the pn junction diode. Pn junction diode, characteristics and applications 2020. In so doing they leave behind positive ions at the donor impurity sites. Pn junction in thermal equilibrium if no external stimulus is applied zero applied voltage, no. With a battery connected this way, the holes in the ptype region and the electrons in the ntype region are pushed toward the junction and start to neutralize the depletion zone, reducing its width. Because of the structure of a semiconductor junction, the effective width of this depletion layer is not constant. Depletion region definition depletion region is a region near the boundary of a pn junction region. Any semiconductor junction will contain a depletion region of nonconducting semiconductor between the two conducting regions. By exploring the electrical properties of silicon, it is possible to acquire an understanding of the inner workings of the pn junction. But notice that a pn junction stores charge in the space charge region transition region. In the immediate vicinity of the junction of the p and n layers, excess electrons from the n layer nearest the junction will recombine with holes from the p layer also nearest the junction, forming a volume about the junction of these layers in which no free carriers exist.
Pn junctions consist of a spacecharge region sandwiched by two quasineutral bulk regions. Calculate the builtin potential of this pn junction. It is most easily described when the junction is in thermal equilibrium or in a steady state. 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. Integrating from the depletion region edge to an arbitrary point x. Regarding the formation of depletion region,it forms because free electrons diffuse into p region from n region. Multiple choice questions and answers on semiconductor theory.
What are the depletion width in the n region, the depletion width in the p region, and the builtin potential at 300k. A depletion region is a natural feature of doping a semiconductor ntype and right next to it ptype. The skinny layer sandwiched between these two regions is depleted of majority carriers are referred to as the depletion region. Understanding the depletion region is key to explaining modern semiconductor electronics. Pn junction properties calculator select a semiconductor substrate and a doping profile below. Introduction to pn junction the engineering projects. The pn junction is a junction formed together by ptype semiconductor and ntype semiconductor material. They are referred to as photoelectrons and photoholes the applied voltage separates the photocarriers attracting electrons toward plus and holes toward minus as long as the light is on, there is a current flowing through the pn. The voltage drop across the depletion region is equal to vv q x bb. Pn junction the interface at which ptype silicon and ntype silicon make contact with each other. If the effective densities of states are the same the expression reduces to. Ee105 fall 2015 microelectronic devices and circuits. Electrons and holes diffuse into regions with lower concentrations of electrons and holes, much as ink diffuses into water until it is uniformly distributed. Department of eecs university of california, berkeley.
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