Chapter II Diodes and Transistors www infonics co

Chapter II Diodes and Transistors www. infonics. co. nr/electronics 1

Contents Intrinsic and extrinsic semiconductors. Principle of operation of PN Junction diode. V-I characteristics of PN Junction diode. Specifications of diode. Principle of working of Zener diode & LED. Principle of working of Photo diode & Solar cell. Bipolar Junction Transistors: PNP and NPN structures-Principle of operation. Input and output characteristics of common emitter configuration. Specifications of transistors. www. infonics. co. nr/electronics 2

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 3

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 conduction band is very large (15 e. V). Therefore a very high electric field is required to lift the valance electrons to the conduction band www. infonics. co. nr/electronics 4

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

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 e. V) 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 6

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 7

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 8

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 9

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 10

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 11

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 12

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

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 Ptype and negative terminal of the battery is connected to the Ntype. This arrangement is called forward biased. www. infonics. co. nr/electronics 14

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 15

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 16

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 17

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 18

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 19

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 Pside 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 20

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

Diode specifications The following are the different diode parameters. Semiconductor material Forward voltage drop (Vf) Peak Inverse Voltage (PIV) Maximum forward current Junction capacitance: www. infonics. co. nr/electronics 22

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 4 V and avalanche break down is predominant for voltages greater than 6 V. Between 4 V and 6 V, 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 23

Zener Diode Applications Voltage regulator Fixed reference voltage source Over voltage protection circuit. www. infonics. co. nr/electronics 24

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 25

LED- Principle of Operation Fig: The energy band diagram of a pn+ junction under unbiased and biased conditions www. infonics. co. nr/electronics 26

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 27

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 28

Photo diode-Working principle When light (photons) falls on the pn junction, 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 (photodiode) decreases. When no light is incident on the pn junction of photo-diode, the reverse current Ir is extremely small. This is called dark current. As the intensity of light increases, the reverse current IR goes on increasing till it becomes maximum. This is called saturation current. www. infonics. co. nr/electronics 29

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. This results in internal gain within the photodiode, which gradually increases the responsivity of the device. www. infonics. co. nr/electronics 30

Photodiode Applications Photodiodes find application in the following: Cameras Medical devices Optical communication devices Automotive devices www. infonics. co. nr/electronics 31

Solar Cell A solar cell is a solid-state electrical device (p-n junction) that converts the energy of light directly into electricity (DC) using the photovoltaic effect. www. infonics. co. nr/electronics 32

Illumination Characteristics www. infonics. co. nr/electronics 33

Numbering and coding schemes for diodes EIA/JEDEC A standardized 1 N-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: 1 N 4001 -1 N 4007 (Silicon 1 A 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: A = lowpower/signal, Y = Rectifier and Z = Voltage reference) e. g. : BY 127 www. infonics. co. nr/electronics 34

Transistor Three terminal active device which transforms current flow from low resistance path to high resistance path. This transfer of current through resistance path, given the name to the device ‘transfer resistor’ as transistor. Transistors consists of junctions within it, are called junction transistors. Current carries inside is by two opposite polarities of charge carriers (electrons and holes), hence the name bipolar junction transistor. www. infonics. co. nr/electronics 35

Transistor-Structure If a P-type material is sandwiched between two N-type materials , the resulting structure is called NPN transistor. Similarly when N-type material is sandwiched between the two P-type materials , the resulting structure is called PNP transistor. In both cases, the first layer where the emission or injection of the carriers starts is called emitter. The second layer through which carriers passes is called the base and the third layer which collects the injected carriers is called collector. www. infonics. co. nr/electronics 36

Transistor-Region of operation Emitter junction Collector junction Region of operation Reverse biased Cut-off region Forward biased Reverse biased Active region Forward biased Saturation region Reverse biased Forward biased Inverse action www. infonics. co. nr/electronics 37

Transistor-Operation (NPN) www. infonics. co. nr/electronics 38

Transistor Configurations: Common Base (CB). Common Emitter (CE). Common Collector (CC). www. infonics. co. nr/electronics 39

Common Emitter Configuration Input is given between base and emitter, while output is taken across the collector and emitter. Here emitter is common to both input and output. www. infonics. co. nr/electronics 40

CE Configuration Circuit Arrangement www. infonics. co. nr/electronics 41

CE Input Characteristics www. infonics. co. nr/electronics 42

CE Output Characteristics www. infonics. co. nr/electronics 43

Comparison between CB, CE & CC Properties Input Impedance Output Impedance Voltage Gain Current Gain Power Gain Common Base Low Common Emitter Medium Common Collector High Very High Low High Medium Low Medium High Low Very High Medium www. infonics. co. nr/electronics 44

Transistor Specifications Type number The type number of the device is a unique identifier given to each type of transistor. There are three international schemes that are widely used: European Pro-Electron scheme; US JEDEC (numbers start with 2 N for transistors); and the Japanese system (numbers start with 2 S). Polarity There are two types of transistor: NPN and PNP. It is important to choose the correct type otherwise all the circuit polarities will be wrong. Material The two main types of material used for transistors are germanium and silicon. Other materials are used, but in very specialised transistors. www. infonics. co. nr/electronics 45

Transistor Numbering Joint Electron Device Engineering Council (JEDEC) These part numbers take the form: digit, letter, sequential number, [suffix] The letter is always 'N', and the first digit is 1 for diodes, 2 for transistors, 3 for four-leaded devices, and so forth. The sequential numbers run from 100 to 9999 and indicate the approximate time the device was first made. If present, a suffix could indicate various things. For example, a 2 N 2222 A is an enhanced version of a 2 N 2222. It has higher gain, frequency, and voltage ratings. Always check the data sheet. Examples: 1 N 914 (diode), 2 N 2222 A, 2 N 904 (transistors). www. infonics. co. nr/electronics 46

Transistor Numbering Pro-Electron These part numbers take the form: two letters, [letter], sequential number, [suffix] The first letter indicates the material: A = Ge B = Si C = Ga. As The second letter indicates the device type and intended application: A: diode, RF C: transistor, AF, small signal D: transistor, AF, power F: transistor, HF, small signal L: Transistor, HF, power U: Transistor, power, switching Y: Rectifier Z: Zener, or voltage regulator diode The third letter indicates if the device is intended for industrial or commercial applications. It's usually a W, X, Y, or Z. The sequential numbers run from 100 -9999. Examples: BC 108 A, BAW 68, BF 239, BFY 51. www. infonics. co. nr/electronics 47