Forward Bias And Reverse Bias Diode Pdf

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It is widely used in different applications like a mixer, in radio frequency applications, and as a rectifier in power applications. This page of application notes section covers PN junction diode applications.

If this external voltage becomes greater than the value of the potential barrier, approx. One of the junctions of a transistor must be forward biased and other must be reverse biased when it operates. The forward and reverse biasing is differentiated below in the comparison chart. Forward Biased … If the reverse-biasing voltage is sufficiently large the diode is in reverse-breakdown region and large current flows though it. In reverse bias, the connections are interchanged.

Diode operation

This article provides a more detailed explanation of p—n diode behavior than is found in the articles p—n junction or diode. A p—n diode is a type of semiconductor diode based upon the p—n junction. The diode conducts current in only one direction, and it is made by joining a p -type semiconducting layer to an n -type semiconducting layer.

Semiconductor diodes have multiple uses including rectification of alternating current to direct current, detection of radio signals, emitting light and detecting light. The figure shows two of the many possible structures used for p—n semiconductor diodes, both adapted to increase the voltage the devices can withstand in reverse bias.

The ideal diode has zero resistance for the forward bias polarity , and infinite resistance conducts zero current for the reverse voltage polarity ; if connected in an alternating current circuit, the semiconductor diode acts as an electrical rectifier.

The semiconductor diode is not ideal. As shown in the figure, the diode does not conduct appreciably until a nonzero knee voltage also called the turn-on voltage or the cut-in voltage is reached. Above this voltage the slope of the current-voltage curve is not infinite on-resistance is not zero. In the reverse direction the diode conducts a nonzero leakage current exaggerated by a smaller scale in the figure and at a sufficiently large reverse voltage below the breakdown voltage the current increases very rapidly with more negative reverse voltages.

As shown in the figure, the on and off resistances are the reciprocal slopes of the current-voltage characteristic at a selected bias point:.

Here, the operation of the abrupt p—n diode is considered. By "abrupt" is meant that the p- and n-type doping exhibit a step function discontinuity at the plane where they encounter each other. The objective is to explain the various bias regimes in the figure displaying current-voltage characteristics. Operation is described using band-bending diagrams that show how the lowest conduction band energy and the highest valence band energy vary with position inside the diode under various bias conditions.

For additional discussion, see the articles Semiconductor and Band diagram. The figure shows a band bending diagram for a p—n diode; that is, the band edges for the conduction band upper line and the valence band lower line are shown as a function of position on both sides of the junction between the p -type material left side and the n -type material right side. When a p -type and an n -type region of the same semiconductor are brought together and the two diode contacts are short-circuited, the Fermi half-occupancy level dashed horizontal straight line is situated at a constant level.

This level ensures that in the field-free bulk on both sides of the junction the hole and electron occupancies are correct. So, for example, it is not necessary for an electron to leave the n -side and travel to the p -side through the short circuit to adjust the occupancies.

Similarly, hole density on the n -side is a Boltzmann factor smaller than on the p -side. This reciprocal reduction in minority carrier density across the junction forces the pn -product of carrier densities to be. As a result of this step in band edges, a depletion region near the junction becomes depleted of both holes and electrons, forming an insulating region with almost no mobile charges.

There are, however, fixed, immobile charges due to dopant ions. The near absence of mobile charge in the depletion layer means that the mobile charges present are insufficient to balance the immobile charge contributed by the dopant ions: a negative charge on the p -type side due to acceptor dopant and as a positive charge on the n -type side due to donor dopant. Because of this charge there is an electric field in this region, as determined by Poisson's equation.

The width of the depletion region adjusts so the negative acceptor charge on the p -side exactly balances the positive donor charge on the n -side, so there is no electric field outside the depletion region on either side.

In this band configuration no voltage is applied and no current flows through the diode. To force current through the diode a forward bias must be applied, as described next. In forward bias, the positive terminal of the battery is connected to the p -type material and the negative terminal is connected to the n -type material so that holes are injected into the p -type material and electrons into the n -type material.

The electrons in the n -type material are called majority carriers on that side, but electrons that make it to the p -type side are called minority carriers. The same descriptors apply to holes: they are majority carriers on the p -type side, and minority carriers on the n -type side. A forward bias separates the two bulk half-occupancy levels by the amount of the applied voltage, which lowers the separation of the p -type bulk band edges to be closer in energy to those of the n -type. The band bending diagram is made in units of volts, so no electron charge appears to convert v D to energy.

Under forward bias, a diffusion current flows that is a current driven by a concentration gradient of holes from the p -side into the n- side, and of electrons in the opposite direction from the n -side to the p- side.

Within the junction, the pn- product is increased above the equilibrium value to: [1]. The gradient driving the diffusion is then the difference between the large excess minority carrier densities at the barrier and the low densities in the bulk, and that gradient drives diffusion of minority carriers from the interface into the bulk.

The injected minority carriers are reduced in number as they travel into the bulk by recombination mechanisms that drive the excess concentrations toward the bulk values. Recombination can occur by direct encounter with a majority carrier, annihilating both carriers, or through a recombination-generation center , a defect that alternately traps holes and electrons, assisting recombination.

The minority carriers have a limited lifetime , and this lifetime in turn limits how far they can diffuse from the majority carrier side into the minority carrier side, the so-called diffusion length. In the LED recombination of electrons and holes is accompanied by emission of light of a wavelength related to the energy gap between valence and conduction bands, so the diode converts a portion of the forward current into light.

Under forward bias, the half-occupancy lines for holes and electrons cannot remain flat throughout the device as they are when in equilibrium, but become quasi-Fermi levels that vary with position.

As shown in the figure, the electron quasi-Fermi level shifts with position, from the half-occupancy equilibrium Fermi level in the n- bulk, to the half-occupancy equilibrium level for holes deep in the p- bulk. The hole quasi-Fermi level does the reverse. The two quasi-Fermi levels do not coincide except deep in the bulk materials. Because this barrier is located in the oppositely doped material, the injected carriers at the barrier position are now minority carriers.

At this point the quasi-Fermi levels rejoin the bulk Fermi level positions. The reduced step in band edges also means that under forward bias the depletion region narrows as holes are pushed into it from the p -side and electrons from the n -side. In the simple p—n diode the forward current increases exponentially with forward bias voltage due to the exponential increase in carrier densities, so there is always some current at even very small values of applied voltage.

However, if one is interested in some particular current level, it will require a "knee" voltage before that current level is reached. Some special diodes, such as some varactors, are designed deliberately to maintain a low current level up to some knee voltage in the forward direction.

In reverse bias the occupancy level for holes again tends to stay at the level of the bulk p -type semiconductor while the occupancy level for electrons follows that for the bulk n -type.

In this case, the p -type bulk band edges are raised relative to the n -type bulk by the reverse bias v R , so the two bulk occupancy levels are separated again by an energy determined by the applied voltage.

When the reverse bias is applied, the electric field in the depletion region is increased, pulling the electrons and holes further apart than in the zero bias case. Thus, any current that flows is due to the very weak process of carrier generation inside the depletion region due to generation-recombination defects in this region.

That very small current is the source of the leakage current under reverse bias. In the photodiode , reverse current is introduced using creation of holes and electrons in the depletion region by incident light, thus converting a portion of the incident light into an electric current.

When the reverse bias becomes very large, reaching the breakdown voltage, the generation process in the depletion region accelerates leading to an avalanche condition which can cause runaway and destroy the diode. The DC current-voltage behavior of the ideal p—n diode is governed by the Shockley diode equation : [3]. This equation does not model the non-ideal behavior such as excess reverse leakage or breakdown phenomena.

In many practical diodes this equation must be modified to read. Using this equation, the diode on- resistance is. The depletion layer between the n - and p -sides of a p—n -diode serves as an insulating region that separates the two diode contacts. Thus, the diode in reverse bias exhibits a depletion-layer capacitance , sometimes more vaguely called a junction capacitance , analogous to a parallel plate capacitor with a dielectric spacer between the contacts.

In reverse bias the width of the depletion layer is widened with increasing reverse bias v R , and the capacitance is accordingly decreased. Thus, the junction serves as a voltage-controllable capacitor.

In a simplified one-dimensional model, the junction capacitance is:. In forward bias, besides the above depletion-layer capacitance, minority carrier charge injection and diffusion occurs. A diffusion capacitance exists expressing the change in minority carrier charge that occurs with a change in forward bias.

In terms of the stored minority carrier charge, the diode current i D is:. Typical values for transit time are 0. Generally speaking, for usual current levels in forward bias, this capacitance far exceeds the depletion-layer capacitance. The diode is a highly non-linear device, but for small-signal variations its response can be analyzed using a small-signal circuit based upon the DC bias about which the signal is imagined to vary.

The equivalent circuit is shown at the right for a diode driven by a Norton source. Using Kirchhoff's current law at the output node:. The output voltage provided by this circuit is then:. This transresistance amplifier exhibits a corner frequency , denoted f C :. For diodes operated in reverse bias, C D is zero and the term corner frequency often is replaced by cutoff frequency. In any event, in reverse bias the diode resistance becomes quite large, although not infinite as the ideal diode law suggests, and the assumption that it is less than the Norton resistance of the driver may not be accurate.

The junction capacitance is small and depends upon the reverse bias v R. The cutoff frequency is then:. This article incorporates material from the Citizendium article " Semiconductor diode ", which is licensed under the Creative Commons Attribution-ShareAlike 3. From Wikipedia, the free encyclopedia. Semiconductor diode based upon the p—n junction. See also: Zener diode and Photodiode. See also: Varactor. CRC Press.

This voltage for the p—n diode is taken variously as 0. Oxford University Press. RF and Microwave Transmitter Design. Mosfet modeling for VLSI simulation: theory and practice. World Scientific. Jean-Pierre Colinge, Cynthia A.

Colinge See, for example, V. Bagad Technical Publications Pune. Categories : Diodes Semiconductor devices. Hidden categories: Articles with short description Short description matches Wikidata Wikipedia articles incorporating text from Citizendium.

PCB Design & Analysis

Since the day my mother surprised me with the first home computer for Christmas back in, well, let's just say a long time ago, I've been intrigued by the technology. Anyway, at the time, I was the envy of every fellow geek, nerd, and teacher at my school. There I was with an impressive 64, wait for it, kilobytes of raw processing power. Now, fast forward to the present day, and my laptop utilizes , times that amount in RAM alone. So, it is safe to say that computer technology has evolved. However, there is one thing that has not and that is the competitiveness of the computer manufacturers.

In chapter 1 — Understanding the PN junction , we have seen how a PN junction is formed from a p-type and n-type semiconductor. We have also learned about diffusion current, depletion region, drift current and barrier potential. Lets just make some questions. What is the use of a PN junction? Why have scientists created a pn junction device?

PN Junction Diode Characteristics – Explained in Detail with Graphs

A diode is an electrical device allowing current to move through it in one direction with far greater ease than in the other. The most common kind of diode in modern circuit design is the semiconductor diode, although other diode technologies exist. Semiconductor diodes are symbolized in schematic diagrams such as the figure below. When placed in a simple battery-lamp circuit, the diode will either allow or prevent current through the lamp, depending on the polarity of the applied voltage. Diode operation: a Current flow is permitted; the diode is forward biased.

This article provides a more detailed explanation of p—n diode behavior than is found in the articles p—n junction or diode. A p—n diode is a type of semiconductor diode based upon the p—n junction. The diode conducts current in only one direction, and it is made by joining a p -type semiconducting layer to an n -type semiconducting layer.

Difference Between Forward & Reverse Biasing

Diode Switching Times

Diode is a two terminal PN junction that can be used in various applications. One of such applications is an electrical switch. The PN junction, when forward biased acts as close circuited and when reverse biased acts as open circuited. Hence the change of forward and reverse biased states makes the diode work as a switch, the forward being ON and the reverse being OFF state. Whenever a specified voltage is exceeded, the diode resistance gets increased, making the diode reverse biased and it acts as an open switch.

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4 Response
  1. Mandy R.

    1. Zero Bias – No external voltage potential is applied to the PN junction diode. · 2. Reverse Bias · 3. Forward Bias.

  2. Josephine A.

    One of the major difference between the forward and the reverse biasing is that in forward biasing the positive terminal of the battery is connected to the p-type semiconductor material and the negative terminal is connected to the n-type semiconductor material.

  3. Karen C.

    Voltage drop across the diode when forward biased: V. The current though the diode when reversed biased: ~ 1nA (A). Temperature dependence.

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