Transition and diffusion capacitance in pn junction diode. A pn junction is the metallurgical boundary between the n and pregions of a semiconductor crystal. Electrostatics of pn junction in thermal equilibrium 3. Before going to varactor diode, lets first take a look at the capacitor. Denoting the positions ofthe edges onthepand on the nside of the classical depletion region byxp andxn, respectively, the total capacitance can be split into36 3 with. In a normal capacitor, the two parallel conducting plates are electrodes which allow the conduction. Therefore, the applied voltage is inversely proportional to junction capacitance formed in a pn junction diode under reverse bias condition. How to measure pn junction diode junction capacitance. For a forward biased diode the junction capacitance is roughly approximated by. The pand ntype regions are referred to as anode and cathode respectively.
The junction current is due to minority carriers only. Depiction layer figure 14 pn junction figure 15 cv characteristics of a pn junction 1. Lecture 6 biased pn junction diodes and current flow. Thus, pn junction diode can be considered as a parallel plate capacitor. This allows electrical current to pass through the junction only in. Electrostatic analysis of a pn diode pn juntiondiode. Such junctions show a pronounced rectifying behavior. The barrier capacitance is the dominant source of capacitance.
The junction current at equilibrium is zero as equal but opposite carriers are crossing the junction. They are also called pn diodes in analogy with vacuum diodes. 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. Lecture 103 depletion capacitance cj k vo vd m vo is the builtin voltage applied only for negative external voltage, v d.
First time calculation of the depletion region width and. The approximation is not critical since the diffusion capacitance is typically much larger than than the depletion capacitance ctotal. Forward biased junction stores an excessive charge. The junction current at equilibrium is zero as charges do not cross the junction. When reverse bias voltage applied to the pn junction diode is increased, a large number of holes majority carriers from pside and electrons majority carriers from nside are moved away from the pn junction. Junction capacitance is due to the dipole in the transition region under reverse bias. When pn junction is reverse biased the depletion region act as an insulator or as a dielectric medium and the ptype an ntype. Pn junction underbiasing 14 boltzmanns approximation. In the case of a slightly forwardbiased junction, vr is to be replaced by vf.
By exploring the electrical properties of silicon, it is possible to acquire an understanding of the inner workings of the pn junction. Pn junction capacitances,part lthe depletion capacitance in fig. Pn junction capacitanceincreasing the reverse bias voltage, vj, across a pn junction leads to the redistribution of charge away from the interface, leaving a depleted region or layer, w, in figure 1. Any variation of the charge within a pn diode with an applied. When the pn junction is forwardbiased, the potential barrier is reduced and significant current begins to flow across the junction. The objective of this lab activity is to measure the voltagedependent capacitance of a reversebiased pn junction. The amount of capacitance changed with increase in voltage is called transition capacitance.
Plotting one over the capacitance squared one expects a linear dependence as expressed by. The pn junction diode with narrow depletion width and large p and n regions can store more electric charge. Supposing that the junction is reverse biased, and that the semiconductor material is homogenous e. Lecture 16 the pn junction diode ii equivalent circuit model november 3, 2005 contents.
Pn junction capacitance changing the voltage at the pn junction leads to a redistribution of charge on it, which means that it has capacitance. Diode junction capacitance transition capacitance and diffusion. When one side of an intrinsic semiconductor is doped with acceptor i. Varactor diode definition, symbol, construction and working. Two types of capacitance associated with a pn junction are. The example is a germanium pn junction with na 1019 cm3 and nd 1014 cm3. A capacitance versus voltage measurement can be used to obtain the builtin voltage and the doping density of a onesided pn diode. The equation describing the distribution of the diffused profile of impurities in fig. The total diffusion capacitance is obtained by adding the diffusion capacitance of the ntype quasineutral region to that of the ptype quasineutral region. Pn junctions consist of two semiconductor regions of opposite type. The pn junction capacitance is divided into two components, the barrier capacitance and the diffusion. Pn junction properties calculator select a semiconductor substrate and a doping profile below. If you want a small value for the capacitance, you should apply a larger reverse bias more negative v d to the pn junction.
Varactor diode is a pn junction diode whose capacitance is varied by varying the reverse voltage. Lecture 6 pn junction and mos electrostatics iii electrostatics of pn junction under bias september 27, 2005 contents. I also discuss one of my favorite algebraic tricks. Pn junction diode in the reversebiased configuration is sensitive to light from a range between 400nm to nm, which includes visible light. Analysis of semiconductor capacitance characteristics. Silicon a single silicon atom consists of fourteen negatively. The analysis is very similar to that of a metalsemiconductor junction. May 02, 2012 pn junction capacitance changing the voltage at the pn junction leads to a redistribution of charge on it, which means that it has capacitance.
A key difference is that a pn diode contains two depletion regions of opposite type. In a pn junction diode, two types of capacitance take place. In a pn junction diode under reverse bias, the magnitude of electric field is maximum at gate 2015 a the edge of the depletion region on the p side b the edge of the depletion region on the n side c the centre of the depletion region on the n side d the pn junction 6. The diode with broad depletion width and small p and n regions can store less electric charge. Junction capacitance is the capacitance which forms in a pn junction diode under reverse bias. Printed in great britain the capacitance of pn junctions y. The pn junction is the fundamental building block of the electronic age. The total capacitance of the junction equals the sum of the junction capacitance, discussed in section 4.
Use m n cm 2 vs, mp 300 cm 2 vs, w p 1 mm and w n 1 mm. The metallurgical junction between p and n regions is at x 0. A pn junction is a boundary or interface between two types of semiconductor materials, ptype and ntype, inside a single crystal of semiconductor. In this type of diode, we dope one side of a semiconductor piece with an acceptor impurity and another side with a donor impurity. 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 0 0 j j x j x. The pn junction is a versatile element, which can be used as a. Here the depletion region is the dielectric and two regions ptype and ntype at both ends act as the charged plates of a capacitor. Octavian florescu 2 the pn junction diode schematic diagram. The voltage dependent capacitance of the pn junction. Pdf a set of capacitance measurements is proposed to identify the different contributions to the junction capacitance diffusion capacitance and. The pn junction capacitance is divided into two components, the barrier capacitance and the diffusion capacitance.
The charge in the depletion region per cross sectional area is the depletion width times the concentration of ionic charge qn. Pn junction diode 12 spice model for pn junction diode r s. The ratios qj vd or q j v d are not equal to the capacitance whereas the ratio qj vd is equal to the capacitance. The boltzmann approximation is to say that the resulting current being small compared with the components of. Device physics i pn junctions junction capacitance martin p.
Calculate the diffusion capacitance of the diode described in example 4. If an ac signal is superposed on the reverse biased dc voltage, the small signal model of a diode is simply a capacitance, which is often called junction capacitance c j. Pn junction diode and characteristics of pn junction diode. This leads to a capacitance that we can calculate from the chargevoltage dependence. Capacitance in pn junction diode, diffusion and transition. Ee105 fall 2014 microelectronic devices and circuits. Diode junction capacitance transition capacitance and. V 22 shows that the exact solution contains two extra terms involving the carrier concentrations at the neutral contact. Uncompensated acceptor atoms on the pn a junction between p and n regions is at x 0. Chang battelle memorial institute, columbus laboratories, 505 king avenue, columbus, ohio 43201, u. Minimize the area of the source and drain to reduce bulksourcedrain capacitance.
It is also required to obtain the capacitance voltage characteristics of the diode. Pn junction and mos electrostaticsii pn junction in thermal equilibrium outline 1. The junction current reduces with rise in temperature. A pn junction diode in thermal equilibrium you have already seen a pn junction. Pn junction diode is the most fundamental and the simplest electronics device. C j0 is the zero bias junction capacitance per unit area. P n junction theory and diodes part i gate problems. Jan 29, 2019 pn junction diode is one of the fundamental elements in electronics.
Lecture 05 pn junction and cmos transistors 8514 page 051. A pn junction 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. Charge capacitance is due to charge storage effect when the junction is forward biased. They are transition capacitance ct and diffusion capacitance cd. Temperature coefficient of silicon junction transition capacity the transition capacity of a pn junction is a useful nonlinear capacitor because it. The capacitance at the depletion region changes with the change in applied voltage. A pn junction diode is a twoterminal electronics element, which can be classified as either. The boltzmann approximation is to say that the resulting current being small compared with the components of this current, we consider that we are still in quasi equilibrium and therefore that the currents equation is still valid by replacing v bi by v.
Pn junction diode multiple choice questions answers. The barrier capacitance the specific capacitance per area unit is given by 2. The charge in the depletion region depends on the reverse bias. Now apply the definition of the capacitance junction capacitance.
Overview of silicon pn junctions west virginia university. The transition capacitance is also known as depletion region capacitance, junction capacitance or barrier capacitance. Total capacitance of forward biased diode it is the sum of the diffusion capacitance cd and the depletion capacitance cj for a forward biased diode the junction capacitance is roughly approximated by. The depletion layer width, builtin voltage, maximum field, and depletion capacitance will be displayed in the appropriate boxes.
A comparison with the classical equation vqnde cdeplcl vb. Capacitor c includes depletionlayer capacitance for the reversebias region and diffusion capacitance associated with the junction under forward bias. Pn junction diode baising and its vi characteristics. Junction capacitance when combine the equations result in the charge on either side of the dipole total charge. This presentation is a very informative piece on pn junction diode an important part of electronics education. Under the conditions of reversebias, free carrier injection does not occur. When depletion region exist in the common junction around, the diode acts as a capacitor. Whereas, the medium between two parallel conducting plates is purely insulating dielectric material which does not allow conduction. For the same diode, find the voltage for which the junction capacitance equals the diffusion capacitance. A silicon pn junction is forward biased with a constant current at room temperature. In this video, i derive the expression for junction capacitance of a pn junction under forward or reverse bias.