A pn junction in forward bias mode, the depletion width decreases. Pn junction tutorial introduction to pn junction diodes. Within the depletion region, there are very few mobile electrons and holes. When the diode is in forward bias, or v d0 v, then the voltage drop. Ee105 fall 2015 microelectronic devices and circuits. Approximate solution using the full depletion approximation a abrupt pn junction an easy way to derive the depletion layer widths in a pn diode is to treat it as a combination of two schottky diodes, one with an ntype semiconductor and an other with a ptype. The pn junction is the fundamental building block of the electronic age. Because of the structure of a semiconductor junction, the effective width of this depletion layer is not constant. A pn junction is a boundary or interface between two types of semiconductor materials, ptype and ntype, inside a single crystal of semiconductor. Here the depletion region is the dielectric and two regions ptype and ntype at both ends act as the charged plates of a capacitor. The voltage drop across the depletion region is equal to vv q x bb. Hence, in absence of potential barrier across the junction in the forward bias condition free electrons enter into the ptype region and holes enter into the ntype region, where they recombine and. This allows electrical current to pass through the junction only in.
Any semiconductor junction will contain a depletion region of nonconducting semiconductor between the two conducting regions. Depletion capacitance university of california, berkeley. 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. 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 concentration of these donors and acceptors are selected to give the pn junction desired device properties pnjuntiondiode. 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 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. Pn junction diode multiple choice questions answers. When we use the expression for the junction capacitance and the depletion region width, we can obtain a familiar form for the capacitance. A depletion region is a natural feature of doping a semiconductor ntype and right next to it ptype. Pn junction in equilibrium pn junctions are important for the following reasons. Physics of semiconductor devices pn junction depletion width.
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. Lecture 6 biased pn junction diodes and current flow. 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. Pn junction diode and characteristics of pn junction diode. This region is called the depletion region since the electric field quickly sweeps free carriers out, hence the region is depleted of free carriers.
If ptype semiconductor is joined with ntype semiconductor, a pn junction is formed. This shall mean that majority charge carriers of the ntype and ptype should at least attain energy of qvb electron volts ev before it will surmount the barrier and diffuse into either ptype or n. First time calculation of the depletion region width and. Pn junction properties calculator select a semiconductor substrate and a doping profile below. Physics of semiconductor devices pn junction depletion. The width of depletion region is depends on the amount of impurities added to the semiconductor.
The depletion layer width, builtin voltage, maximum field, and depletion capacitance will be displayed in the appropriate boxes. When analyzing the pn junction, we are most interested in the depletion region, or space charge region scr. A depletion region forms instantaneously across a pn junction. How is a depletion layer formed in a pn junction diode. A is the crosssectional area of the pn junction xp is the distance. When depletion region exist in the common junction around, the diode acts as a capacitor. After pn in collisions by the primary electrons, we have the primary plus the. Pn junction, depletion region, diode equation duration. Avalanche noise is created when a pn junction is operated in the reverse breakdown mode. Depletion region has no electrons or holes, but has fixed. The region in which the ptype and ntype semiconductors are joined is called pn junction. Mobile electrons of pside of the pn junction diode constitute. A builtin potential v bi is formed at the junction due to e. The diffusion of majority charge carriers have to cross the potential barrier vb of the pn junction formed as the effect of the depletion region.
A pn junction diode is a semiconductor device with two opposite region such as ptype region and ntype region. Width of depletion region is increased for reverse bias and increased for forward. In other words, why voc of schottky contact is smaller than the pn junction. So, on an application of a forward bias voltage the width of the depletion region i. Under the influence of a strong reverse electric field within the junctions depletion region, electrons have enough kinetic energy that, when they collide with the atoms of the crystal lattice, additional electronhole pairs are formed figure 12. The depletion region becomes wider and the current ow becomes smaller. From reverse bias capacitancefrequency measurements, at a reverse bias voltage of. The capacitance associated with the charge variation in the depletion layer is called the junction capacitance, while the capacitance associated with the excess carriers in the quasineutral region is called the diffusion capacitance. Electrons and holes diffuse into regions with lower concentrations of electrons and holes, much as ink diffuses into water until it is uniformly distributed. 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. In the following, v d implies the diode biasing voltage.
The term depletion refers to the fact that the region near the pn junction is depleted of charge carriers electrons and holes due to diffusion across the junction. In the case of a reversebiased junction, vf is replaced by vr. To first order, carrier concentrations in spacecharge region are much smaller than the doping level. In semiconductor physics, the depletion region, also called depletion layer, depletion zone. This is called the depletion region or the space charge layer. Width of depletion region physics and radioelectronics. Diffusion across the junction causes a lot of electrons and holes to recombine leaving a depletion region in. Movement of electrons and holes when forming the junction depletion region approximation. When electronhole pairs approach the junction they are pulled apart by. The depletion region of a pn junction is one that is depleted of. Step junction solution bi a a d s o a d p n bi a a a d s o d bi a p d a d s o a n v v n n n n q k w x x v v n n n n q k v v and x n n n n q k x. What are the depletion width in the nregion, the depletion width in the pregion, and the builtin potential at 300k.
Ece 255, pn junction and diode college of engineering. The capacitance related to the charge in the depletion layer is called junction capacitance. Pn junction diode, characteristics and applications 2020. Both types of capacitances are nonlinear so that we will derive the smallsignal capacitance in each case. 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. The behaviour of the pn junction with regards to the potential barriers width produces an asymmetrical conducting two terminal device, better known as. What is the currentvoltage characteristic for the pn junction diode. It is most easily described when the junction is in thermal equilibrium or in a steady state. When we talked about band bending, one of the reasons that it occurred was because. This pn junction separates ntype semiconductor from ptype semiconductor fig. As the depletion layer decreases the capacitance value goes down. By definition, ntype semiconductor has an excess of free electrons compared to the ptyp. A region depleted of mobile charge carriers exists in the vicinity of the metallurgical junction, due to mobile charge carrier.
The region in the vicinity of the junction, which has been depleted of the free charge. The spacecharge region is extended mainly to the lowdoped side of the pn. Silicon a single silicon atom consists of fourteen negatively. Electrostatics of pn junction in equilibrium a spacecharge region surrounded by two quasineutral regions formed. The animation below shows the formation of the e at the junction between n and ptype material. The pregion 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. By exploring the electrical properties of silicon, it is possible to acquire an understanding of the inner workings of the pn junction.