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Two Terminal Special Purpose Devices First Year www.uptunotes.com Content: - Special Purpose two terminal Devices: Light-Emitting Diodes, Varactor (Varicap)Diodes, Tunnel Diodes, Liquid-Crystal Displays. 1- Light Emitting Diode (LED) Light Emitting Diode is a photo electronic device which converts electrical energy to light energy. It is heavily doped P-N junction diode which under forward biased emits spontaneous radiation. The diode is covered with a transparent cover so that the emitted light may come out. Construction- In an LED, the upper layer of p-type semiconductor is deposited by diffusion on n-type layer of semiconductor. The metalized contacts are provided for applying the forward bias to the P-N junction diode from battery B through resistance R which controls the brightness of light emitted. Fig: LED construction Working of LED The LED works on the principle of electroluminescence. When a P-N junction is forward biases, the electrons in N- region cross the junction & recombines with holes in P-region. When electrons recombine with the holes, they move from conduction band to valence band which is at lower energy level. While moving, the additional energy is released by the free electrons which appear in the form of light due to the special material used in the LED. The energy released depends on the forbidden gap energy which determines the wavelength & the colour of the emitted light. Materials of LED The materials which are the mixtures of Gallium, Arsenic & Phosphorus are used in LED to obtain different coliour of light. S.No. Material Symbol Colour 1 Gallium Arsenide GaAs Infrared(Invisible) 2 Gallium Phosphide GaP Red or Green 3 Gallium Arsenide Phosphide GaAsP Red or Yellow By: Navneet Pal Email:[email protected] Page 1 www.uptunotes.com Output Characteristics- The amount of power output translated into light is directly propotional to the forward current If , more forward current If , the greater the output light. LED Symbol Fig: Typical output characteristics of LED Advantages of LEDs Applications of LEDs LED has less power & low operational voltage. In Burglar- Alarm System. LED has fast action & requires no warm up In calculators & Digital watches. time. LED is cheap & easy to handle In the field of communication. The LEDs are light in weight. In computers & remote control. The LEDs are available in various colours. In picture phones & video displays. The LEDs have long life. In traffic light. 2- Varactor Diode (VARICAP) Varactor Diode can be used as variable capacitors in microwave circuits. In reverse bias , the P-region & N-region act like the plates of capacitor while the depletion region acts like dielectric. Thus, there exists a capacitance, space charge capacitance or depletion region capacitance. Mathematically ε= permittivity of semiconductor CT= A= area of cross section W= width of depletion region. As the reverse biased applied to the diode increases, the width of the depletion region (W) increases. Thus the transition capacitance CT decreases. In short, the capacitance can be controlled by the applied voltage. Fig: Variation of CT according to VR By: Navneet Pal Email:[email protected] Page 2 www.uptunotes.com Reverse Biased P-N Junction Expression for varactor diode-The diode capacitance (CT) as a function of applied reverse-bias potential described as. Where, CT= diode capacitance C= diode capacitance at unbiased condition V= applied potential Vb=barrier potential m= constant that is material dependent K= a constant (often 1) Equivalent Circuit Fig: Equivalent Circuit of tunnel diode The series resistance & diode capacitance determine the maximum operating frequency according to the relationship. F= Where K= cut-ff freq in hertz Rs= series resistance in Ώ CT= diode capacitance in farads Applications of Varactor Diode A varactor diode is used as an automatic control device in a receiver. Varactor diodes are also used as frequency modulator. Varactor diodes can also be used as frequency multiplier. It can be used as phase shifter. 3- Tunnel Diode A tunnel diode is a semiconductor P-N junction device which is extremely heavy doping on both the side of junction & abrupt transition from p-side to n-side .i.e. the depletion width is very small. Hence the carriers are tunnel through the potential barriers, even if they do not have sufficient kinetic energy to cross it. By: Navneet Pal Email:[email protected] Page 3 www.uptunotes.com Due to the depletion region, an electron penetrates through the barrier. This is called tunneling & hence such high impurity density P-N junction devices are called tunnel diodes. Characteristics of tunnel diode Characteristics shows that at first forward current rises sharply as voltage is applied up to point P. The voltage associated with this point P is called as peak voltage VP & current associated with this point is called as peak current IP. As the forward bias is increased further, the forward current drops continuously up to point V. The voltage & current associated with this point is called as valley voltage(VV) & Valley Current (IV) respectively. If voltage is increased further, the current through diode increases at shown in figure. When tunnel diode is reverse biased. It acts as a good conductor. Fig: Tunnel Diode Characteristics Tunnel Diode Construction-Semiconductor material used in tunnel diode is germanium (Ge), gallium antimonide (GaSb) or Gallium arsenide (GaAs). Fig: Tunnel Diode A very small tin dot is soldered to a heavily doped pellet of N-type Ge, GaSb or GaAs. The pellet is soldered on Kovar anode contact, The cathode contact is also Kovar, which is connected to the tin dot via a mesh screen to eliminate inductance. The diode has a ceramic body & is hermetically sealed. By: Navneet Pal Email:[email protected] Page 4 www.uptunotes.com Tunnel Diode Equivalent Circuit Where Rs= Series resistance. Is a combination of ohmic contacts & spreading resistance in wafer. Ls= Series inductance being a function of conduction of diode & surrounding circuit geometry. Cj= Junction capacitance corresponding to space charge depletion width & varies with applied bias. Rj= Non- linear differential resistance of tunnel diode. Application of Tunnel Diode As a high speed switch In negative resistance & high frequency (microwave) oscillator. In pulse & digital circuits. Symbol In switching network. In timing & computer logic circuitry. Design of pulse generators & amplifiers. 4- Liquid crstal display(LCD) The liquid crystals are one of the most fascinating material systems in nature, having properties of liquids as well as of a solid crystal. The term liquid crystal refers to the fact that there compounds have a crystalline arrangement of molecules, yet they flow like a liquid. Liquid crystal display do not emit or generate light, but rather alter extremely generated illumination. Their ability to modulate light when electrical signal is applied has made them very useful is flat panel display technology. The crystal is made up of organic molecules which are rod-like molecule defines the “director” of the liquid crystal. The different arrangements of these rod like molecules leads to three main categories of liquid crystals 1- Smectic 2- Nematic 3- Cholesteric Smetic- In this structure the rod-like molecules are arranged in layers & within each layer there is orientational order over a long range. Nematic- In this structure, the potential order between layer of molecules is lost, but the orientation order is maintained. Cholesteric- In these crystals the rod-like molecules in each layer are oriented at a different angle within each layer. By: Navneet Pal Email:[email protected] Page 5 www.uptunotes.com Types of LEDs 1- Dynamic scattering 2- Field effect 1-Dynamic Scattering- It consists of two glass plates with a liquid crystal fluid in between. The back plate is coated with thin transparent layer of conductive material, when as front plate has a photo etched conductive coating with seven segment pattern. Fig: Dynamic scattering Type LCD In the absence of the electrical signal , orientation order is maintained in the crystal allowing light to transmit. This makes LCD display clear. 2-Field Effect Display- In these nematic liquid crystals are used. It consists of two glass plates, a liquid crystal fluid, polarizers & transparent conductors. The liquid crystal fluids sandwiched between two glass plates. Each glass plate is associated with light polarizer; the light polarizers are placed at right angle to each other. Advantages of LCDs Disadvantages of LCDs Less power consumption. Poor reliability. Low cost. Limited temperature range. Uniform brightness with good contrast. Poor visibility in low ambient temperature. Low operating voltage & current. Slow speed. Requires an A.C. drive. By: Navneet Pal Email:[email protected] Page 6 .
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