Chapter II Semiconductors & Diodes

1. Chapter IISemiconductors & Diodes www.infonics.co.nr/electronics

2. Contents • Diodes: Intrinsic and extrinsic semiconductors • PN junction diode, barrier potential, V-I characteristics, Effect of temperature. • Equivalent circuit of a diode. Piece wise linear model. • Specification parameters of diodes and numbering. • Zener diode, Varactor diodes, characteristics, working principle of LED, photo diode, solar cell. www.infonics.co.nr/electronics

3. Conductor,Insulator,Semiconductor • Conductor: • If the number of valance electrons is less than 4, the material is generally called conductor. Instead of accepting electrons, it is easier to donate electrons to fill the outer sub shell as 8. • Insulator: • If the number of valance electrons is more than 4, the material is generally called insulator. Instead of donating electrons, it is easier to accept lesser electrons to fill the outer sub shell. • Semiconductor: • If the number of valance electrons is equal to 4, the material is generally called semi conductor. Here the probability of donating and accepting electrons is equal. www.infonics.co.nr/electronics

4. Energy Band of Insulators The electrical behavior of solid can be explained with the help of energy bands. • Insulators • Here the valance band is full while the conduction band is empty. The energy gap between valance band and conduction band is very large (15 eV).Therefore a very high electric field is required to lift the valance electrons to the conduction band www.infonics.co.nr/electronics

5. Energy Band of Conductors • Conductors • In the energy band diagram, there is no forbidden energy gap between the valance band and the conduction band .The two bands actually overlap as shown in fig. www.infonics.co.nr/electronics

6. Energy Band of Semi Conductors • In the case of semi conductors, the valance band is almost filled and conduction band is empty. But the forbidden energy gap is very small (1 eV) as shown in fig. There fore comparatively a smaller electric field is required to lift the valance electrons to the conduction band. Thus the conductivity of semiconductor lies between a conductor and insulator. www.infonics.co.nr/electronics

7. Intrinsic Semi Conductor: • A semi conductor in its purest form is known as intrinsic semi conductor. • Eg: Ge or Si crystal www.infonics.co.nr/electronics

8. Extrinsic Semi Conductor: • The conductivity of the intrinsic semiconductor can be increased by adding small amount of impurities. The process of adding impurities to the intrinsic (pure) semiconductor is called doping. The doped semiconductor is then called extrinsic (impure) semi conductor. • Depending on the dopant (impurity) used, extrinsic semi conductor can be divided in to two classes. • N-type Semi conductor. • P-type Semi conductor. www.infonics.co.nr/electronics

9. N-type Semi conductor • N-type semi conductor is an extrinsic semi conductor doped with a pentavalent impurity like Antimony, Phosphorus and Arsenic etc. www.infonics.co.nr/electronics

10. N-type semi conductor • The N-type semi conductor can be represented as shown in fig. • It consists of • Free electrons (Majority carriers). • Holes (Minority Carriers). • Immobile positive ions. www.infonics.co.nr/electronics

11. P-type Semi conductor • P-type semi conductor is an extrinsic semi conductor doped with a trivalent impurity like Gallium, indium and Boron etc. www.infonics.co.nr/electronics

12. P-type semi conductor • The P-type semi conductor can be represented as shown in fig. • It consists of • Holes (Majority carriers). • Free electrons (Minority Carriers). • Immobile negative ions. www.infonics.co.nr/electronics

13. PN Junction www.infonics.co.nr/electronics

14. PN Junction with Forward Bias • When an external voltage is applied to the PN junction in such a way that positive terminal of the battery is connected to the P-type and negative terminal of the battery is connected to the N-type. This arrangement is called forward biased. www.infonics.co.nr/electronics

15. PN Junction with Reverse Bias • When an external voltage is applied to the PN junction in such a way that positive terminal of the battery is connected to the N-type and negative terminal of the battery is connected to the P-type. This arrangement is called reverse biased. www.infonics.co.nr/electronics

16. Break down in PN Junction • If the reverse bias voltage is increased beyond a certain limit, a new phenomenon called break down occurs. In this region high current may be passed through the junction. This high current may generate large amount of heat to destroy the junction. The two processes are responsible for junction break down in reverse biased condition namely, • Avalanche break down • Zener break down www.infonics.co.nr/electronics

17. Avalanche Break Down • The increased reverse voltage increases the amount of energy impaled to minority carriers. As the reverse voltage is increased further, the minority carriers acquire a large amount of energy. When these carriers collide with atoms, within the crystal structure they impact sufficient energy to break a covalent bond and generate additional carriers (electron hole pairs). These additional carriers pick up energy from the applied voltage and generate more carriers, and reverse current increased rapidly. This cumulative process of carrier generation (or multiplication) is known as Avalanche breakdown. www.infonics.co.nr/electronics

18. Zener Break Down: • It occurs when diode is heavily doped. Due to heavy doping, depletion layer is narrow. When the reverse voltage across the diode is increased, electric field is developed across depletion layer. Electric field is strong enough to generate large number of electron-hole pair by breaking covalent bonds. Because of large number of these carriers reverse current increases sharply and breakdown occurs which is known as Zener Breakdown. www.infonics.co.nr/electronics

19. Zener Breakdown Vs Avalanche Breakdown • Diode junctions that breakdown below 5 V are caused by Zener effect whereas Junctions that experience breakdown above 5 V are caused by Avalanche effect. • The Zener breakdown occurs in heavily doped junctions, which produce Narrow depletion layers, whereas Avalanche breakdown occurs in lightly doped junctions, which produce wide depletion layers. • With the increase in junction temperature, Zener breakdown voltage is reduced while the Avalanche breakdown voltage is increases. • The zener diodes have a negative temperature coefficient while Avalanche diodes have a positive temperature coefficient. www.infonics.co.nr/electronics

20. Semi conductor Diodes: • Diode is a two terminal device consisting of a PN junction formed either in Ge or Si crystal. Here the terminal on the P-side is called the anode and the terminal on the N-side is called the cathode. The PN junction conducts the current only when it is in forward biased and no current flows through it when it is in reverse biased(i.e. ,current flows in only one direction). Thus the diode is called uni directional device. www.infonics.co.nr/electronics

21. VI characteristics of junction Diode www.infonics.co.nr/electronics

22. Ideal Diode • An ideal diode is a diode that acts like a perfect conductor when voltage is applied forward biased and like a perfect insulator when voltage is applied reverse biased. www.infonics.co.nr/electronics

23. Diode Equation • The ideal diode characteristic equation is known as the Shockley equation, or simply the diode equation. It gives an expression for the current through a diode as a function of voltage.  • Where,V is the applied voltage across the terminals of the diode. • VT is called thermal voltage and is given by VT = kT/q. • Therefore the equation become, • k = Boltzmann’s constant = 1.38 x 10-23 J/K • T= Temperature in Kelvin and • q = Magnitude of charge on an electron = 1.6 x 10-19 C www.infonics.co.nr/electronics

24. Sample Problem A silicon diode has reverse saturation current of 2.5 µA at 300K.Find forwardvoltage for a forward current of 10mA. • Soln: Given, reverse saturation current is = 2.5×10-6 A Temperature in Kelvin,T= 300K Diode forward current, i = 10×10-3A Diode equation is given by Where, k = Boltzmann’s constant = 1.38 x 10-23 J/K T= Temperature in Kelvin and q = magnitude of charge on an electron = 1.6 x 10-19 C After simplifying, we get; Forward voltage, v = ln (1+) v= .071 volt= 71mv www.infonics.co.nr/electronics

25. Piecewise-Linear Equivalent Circuit www.infonics.co.nr/electronics

26. Effect of Temperature www.infonics.co.nr/electronics

27. Effect of Temperature Forward Biased Condition • As the temperature increases the forward characteristics are actually becoming more “ideal” . • cut-in voltage decreases as the temperature increases. The diode conducts at smaller voltage at large temperature. Reverse Biased Condition • Avalanche Diodes (PTC): The break down voltage and reverse saturation current increases as temperature increases. • Zener Diodes (NTC): The breakdown voltage and reverse saturation current decreases as temperature increases. www.infonics.co.nr/electronics

28. Diode specifications • The following are the different diodeparameters. • Semiconductor material • Forward voltage drop (Vf) • Peak Inverse Voltage (PIV) • Maximum forward current • Junction capacitance www.infonics.co.nr/electronics

29. Zener Diode • Zener diodes are also called breakdown diodes. • Specially doped PN junction diodes to produce controlled break down characteristics without damage and are operated in the break down region. • Break down in zener diode is influenced by two phenomenon, zener effect and avalanche effect. • Zener effect is predominant for break down voltages less than about 4V and avalanche break down is predominant for voltages greater than 6V.Between 4V and 6V, both effects are present. • Because of high temperature and current capability, Silicon is usually preferred for the manufacture of zener diodes. www.infonics.co.nr/electronics

30. Zener Diode • Applications • Voltage regulator • Fixed reference voltage source • Over voltage protection circuit. www.infonics.co.nr/electronics

31. Zener Diode Specifications • Zener voltage (VZ): Zener voltage (VZ) is the reverse voltage at which breakdown occurs in a Zener diode. • Maximum zener Current (IZmax): It is the maximum current that can flow through a Zener diode at its rated voltage, VZ. • Power rating:   It defines the maximum power that can be dissipated during it’s operation and it is the product of the voltage across the diode multiplied by the current flowing through it. www.infonics.co.nr/electronics

32. Varactor diode (Tuning Diode) • Also known as varicap diode that has a variable capacitance which is a function of the voltage applied across its terminals. • Varactor diodes are operated reverse-biased and therefore no current flows. However, since the width of the depletion region varies with the applied bias voltage, the capacitance of the diode can be made to vary. • Usually, the capacitance is inversely proportional to the width of the depletion region. www.infonics.co.nr/electronics

33. Variation of Capacitance with Variation in Depletion www.infonics.co.nr/electronics

34. Varactor Diode characteristics www.infonics.co.nr/electronics

35. Varactor Diode Applications • Voltage-controlled capacitors. • Voltage-controlled oscillators. • Frequency multipliers. • FM transmitters • Phase locked loops in radio, television sets and cellular telephone. www.infonics.co.nr/electronics

36. Light Emitting Diode (LED) • Light emitting diode is a PN junction that emits optical radiation generated by the recombination of electrons and holes, when the junction is forward biased. Most of the commercial LEDs are realized using a highly doped N and a P Junction. www.infonics.co.nr/electronics

37. LED- Principle of Operation • Fig:The energy band diagram of a pn+junction under unbiased and biased conditions • Note: Most commonly used materials for constructing LEDs are GaAs,GaP,GaAsp www.infonics.co.nr/electronics

38. LED-Advantages,Disadvantages and Applications • Advantages: • High reliability • Fast response • Low cost • Low power consumption • Disadvantages: • Temperature dependence of radiation • Sensitivity to over voltage damage • Applications: • Indicator lamp and displays in equipments such as digital watches, calculators etc. • Optical communication system www.infonics.co.nr/electronics

39. Photo-diode • A photo-diode is a reverse-biased silicon or germanium pn junction in which reverse current increases when the junction is exposed to light. The reverse current in a photo-diode is directly proportional to the intensity of light falling on its pn junction. This means that greater the intensity of light falling on the pn junction of photo-diode, the greater will be the reverse current. www.infonics.co.nr/electronics

40. Photo diode-Working principle • When light (photons) falls on the pnjunction, the energy is imparted by the photons to the atoms in the junction. This will create more free electrons (and more holes). These additional free electrons will increase the reverse current. As the intensity of light incident on the pn junction increases, the reverse current also increases. In other words, as the incident light intensity increases, the resistance of the device (photo-diode) decreases. When no light is incident on the pn junction of photo-diode, the reverse current Iris extremely small. This is called dark current. As the intensity of light increases, the reverse current IRgoes on increasing till it becomes maximum. This is called saturation current. www.infonics.co.nr/electronics

41. Modes of Operation • Photodiodes can be operated in different modes, which are as follows: • Photovoltaic mode – It is also known as zero bias mode, in which a voltage is generated by the illuminated photodiode. • Photoconductive mode - The diode used in this mode is more commonly reverse biased. • Avalanche diode mode - Avalanche photodiodes are operated in a high reverse bias condition, which allow multiplication of an avalanche breakdown to each photo-generated electron-hole pair. www.infonics.co.nr/electronics

42. Photodiode Applications • Photodiodes find application in the following: • Cameras • Medical devices • Optical communication devices • Automotive devices www.infonics.co.nr/electronics

43. Solar Cell • A solar cell is a p-n junction device that converts the energy of light directly into electricity using the photovoltaic effect. • When a p-n junction is exposed to light, the some of the photons of light ray are absorbed by the semiconductor crystal and these photons in turn excite some of the electrons of covalent bonds. These excited electrons then get sufficient energy to migrate from valence band to conduction band. Electrons leave from the covalent bond leaving holes in the bond. which causes significant number of free electrons and holes in the crystal. As the negative charge (light generated electrons) in one side and positive charge (light generated holes) in opposite side of a cell there will be a potential difference between these two sides of the cell. www.infonics.co.nr/electronics

44. Illumination Characteristics www.infonics.co.nr/electronics

45. Numbering and coding schemes for diodes • EIA/JEDEC • A standardized 1N-series numbering system was introduced in the US by EIA/JEDEC (Joint Electron Device Engineering Council) about 1960. Among the most popular in this series were: 1N4001-1N4007 (Silicon 1A power rectifier) • Pro Electron • The European Pro Electron coding system for active components was introduced in 1966 and comprises two letters followed by the part code. The first letter represents the semiconductor material used for the component (A = Germanium and B = Silicon) and the second letter represents the general function of the part (for diodes: C= AF, A = low-power/signal,Y = Rectifier and Z = Voltage reference) • e.g.: BC 107 www.infonics.co.nr/electronics