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Electronic Devices

Submitted by

Tauqeer Aslam 07010810-045

Submitted ToDr.Zafar-Ullah-Ghuman

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VARACTOR DIODESVaractor diodes are also known as

variable-capacitance diodes because the junction capacitance varies with theamount of reverse-bias voltage. Varactordiodes are specifically designed to takeadvantage of this variable-capacitancecharacteristic. These devices arecommonly used in electronic tuningcircuits used in communications systems.

A varactor is a diode that alwaysoperates in reverse-bias and is doped tomaximize the inherent capacitance of thedepletion region. The depletion region. widened by the reverse bias, acts as acapacitor dielectric because of its nonconductive characteristic. The p and n regionsare conductive and act as the capacitor plates, as illustrated in Figure 3-19.

Basic OperationAs the reverse-bias voltage increases, the depletion region widens, effectively

increasing the plate separation and the dielectric thicknessand thus decreasing the capacitance. When the reverse-biasvoltage decreases, the depletion region narrows, thusincreasing the capacitance. This action is shown in Figure 3-20(a) and (b). A graph of diode capacitance (C T ) versusreverse voltage for a certain varactor is shown in Figure 3-20(c). For this particular device. C T varies from 40 pF toslightly greater than 4 pF as V R varies from I V to 40 V.

The light-Emitting Diode (LED)

The symbol for an LED is shown in Figure 3-25.The basic operation of the light-emitting diode (LED) is as follows. When the device is forward-biased, electronscross the pn junction from the /Hype material and recombine with holes in the p-type material. Recall from Chapter I that these free electronsare in the conduction band and at a higher energy than theholes in the valence band. When recombination takes place, therecombining electrons release energy in the form of heat andlight. A large exposed surface area on one layer of the semi

conductive material permits the photons to be emitted asvisible light. This process, called electroluminescence, is illus-trated in Figure 3-26. Various impurities are added during the doping process toestablish the wavelength of the emitted light. The wavelength determines the colorof the light and if it is visible or infrared (IR).

LED Semiconductor Materials The semiconductor gallium arsenide (GaAs) wasused in early LEDs. The first visible red LEOs were produced using gallium arsenidephosphide (GaAsP) on a GaAs substrate. The efficiency was increased using agallium phosphide (GaP) substrate, resulting in brighter red LEOs and also allowing

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orange LEOs. GaAs LEDs emit infrared (IR) radiation, which is invisible. Later, GaPwas used as the light-emitter to achieve pale green light. By using a red and agreen chip, LEOs were able to produce yellow light. The first super-bright red.yellow, and green LEDs were produced using gallium aluminum arsenide phosphide(GaAIAsP). By the early 1990s ultrabright LEOs using indium gallium aluminumphosphide (InGaAIP) were available in red, orange, yellow, and green.

LED Biasing The forward voltage across an LED is considerably greater than fora silicon diode. Typically the maximum V p for LEDs is between 1.2 V and 3.2 V,depending on the device. Reverse breakdown for an LED is much less than for asilicon rectifier diode (3 V to 10 V is typical).The LED emits light in response to asufficient forward current, as shown in Figure 3-27(a). The amount of power outputtranslated into light is directly proportional to the for ward current, as indicated inFigure 3-27(b). An increase in IF corresponds proportionally to an increase in lightoutput.

The Photodiode

The photodiode is a device that operates in reverse bias. as shown in Figure 3-33(a), where I is the reverse current. The photodiode has a small transparentwindow that allows light to strike the pn junction. Some typical photodiodes areshown in Figure 3-33(b). An alter nate photodiode symbol is shown in Figure 3-33(c). Recall that when reverse-biased. a rectifier diode has a very small reverseleakage current. The same is true for a photodiode. The reverse-biased current isproduced by thermally generated electron-hole pairs in the depletion region. whichare swept across the pn junction by the electric field created by the reverse voltage.

Tn a rectifier diode, the reverse leakage current increases with temperature due toan increae in the number of electron-hole pairs.

A photodiode differs from a rectifier diode in that when its pn junction is exposedto light, the reverse current increases with the light intensity_ When there is noincident light. the reverse current, IA' is almost negligible and is called the darkcurrent. An increase in the amount of light intensity, expressed as irradiance(mW/cm 2 ), produces an increase in the reverse cunent,

The Schottky Diode

Schottky diodes are used primarily in high-frequency and fast-switchingapplications. They are also known as hot-carrier diodes. ASchottk}' diode symbol is shown in Figure 3-39. A Schottkydiode is fonned by joining a doped semiconductor region(usually IHype) with a metal such as gold, silver, or platinum.Rather than apn junction, there is a metal-to-semiconductor

junction, as shown in Figure 3-40. The forward voltage dropis typically around 0.3 V.Schottky diode symbol.

The Schottky diode operates only with majority carriers. There are no minority carriers and thus no reverse leakagecurrent as in other types of diodes. The metal region isheavily occupied with conduction-band electrons, and the IHype semiconductorregion is lightly doped.