Category: basic Electronics

Basic Electronics – Transistors ”; Previous Next After having a good knowledge on the working of the diode, which is a single PN junction, let us try to connect two PN junctions which make a new component called Transistor. A Transistor is a three terminal semiconductor device that regulates current or voltage flow and acts as a switch or gate for signals. Why Do We Need Transistors? Suppose that you have a FM receiver which grabs the signal you want. The received signal will obviously be weak due to the disturbances it would face during its journey. Now if this signal is read as it is, you cannot get a fair output. Hence we need to amplify the signal. Amplification means increasing the signal strength. This is just an instance. Amplification is needed wherever the signal strength has to be increased. This is done by a transistor. A transistor also acts as a switch to choose between available options. It also regulates the incoming current and voltage of the signals. Constructional Details of a Transistor The Transistor is a three terminal solid state device which is formed by connecting two diodes back to back. Hence it has got two PN junctions. Three terminals are drawn out of the three semiconductor materials present in it. This type of connection offers two types of transistors. They are PNP and NPN which means an N-type material between two Ptypes and the other is a P-type material between two N-types respectively. The construction of transistors is as shown in the following figure which explains the idea discussed above. The three terminals drawn from the transistor indicate Emitter, Base and Collector terminals. They have their functionality as discussed below. Emitter The left hand side of the above shown structure can be understood as Emitter. This has a moderate size and is heavily doped as its main function is to supply a number of majority carriers, i.e. either electrons or holes. As this emits electrons, it is called as an Emitter. This is simply indicated with the letter E. Base The middle material in the above figure is the Base. This is thin and lightly doped. Its main function is to pass the majority carriers from the emitter to the collector. This is indicated by the letter B. Collector The right side material in the above figure can be understood as a Collector. Its name implies its function of collecting the carriers. This is a bit larger in size than emitter and base. It is moderately doped. This is indicated by the letter C. The symbols of PNP and NPN transistors are as shown below. The arrow-head in the above figures indicated the emitter of a transistor. As the collector of a transistor has to dissipate much greater power, it is made large. Due to the specific functions of emitter and collector, they are not interchangeable. Hence the terminals are always to be kept in mind while using a transistor. In a Practical transistor, there is a notch present near the emitter lead for identification. The PNP and NPN transistors can be differentiated using a Multimeter. The following figure shows how different practical transistors look like. We have so far discussed the constructional details of a transistor, but to understand the operation of a transistor, first we need to know about the biasing. Transistor Biasing As we know that a transistor is a combination of two diodes, we have two junctions here. As one junction is between the emitter and base, that is called as Emitter-Base junction and likewise, the other is Collector-Base junction. Biasing is controlling the operation of the circuit by providing power supply. The function of both the PN junctions is controlled by providing bias to the circuit through some dc supply. The figure below shows how a transistor is biased. By having a look at the above figure, it is understood that The N-type material is provided negative supply and P-type material is given positive supply to make the circuit Forward bias. The N-type material is provided positive supply and P-type material is given negative supply to make the circuit Reverse bias. By applying the power, the emitter base junction is always forward biased as the emitter resistance is very small. The collector base junction is reverse biased and its resistance is a bit higher. A small forward bias is sufficient at the emitter junction whereas a high reverse bias has to be applied at the collector junction. The direction of current indicated in the circuits above, also called as the Conventional Current, is the movement of hole current which is opposite to the electron current. Operation PNP Transistor The operation of a PNP transistor can be explained by having a look at the following figure, in which emitter-base junction is forward biased and collector-base junction is reverse biased. The voltage VEE provides a positive potential at the emitter which repels the holes in the P-type material and these holes cross the emitter-base junction, to reach the base region. There a very low percent of holes recombine with free electrons of N-region. This provides very low current which constitutes the base current IB. The remaining holes cross the collector-base junction, to constitute collector current IC, which is the hole current. As a hole reaches the collector terminal, an electron from the battery negative terminal fills the space in the collector. This flow slowly increases and the electron minority current flows through the emitter, where each electron entering the positive terminal of VEE, is replaced by a hole by moving towards the emitter junction. This constitutes emitter current IE. Hence we can understand that − The conduction in a PNP transistor takes place through holes. The collector current is slightly less than the emitter current. The increase or decrease in the emitter current affects the collector current. Operation NPN Transistor The operation of an NPN transistor can be explained by having a look at the following figure, in which

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Basic Electronics – Types of Transistors ”; Previous Next There are many types of transistors in use. Each transistor is specialized in its application. The main classification is as follows. The primary transistor is the BJT and FET is the modern version of transistor. Let us have a look at the BJTs. Bipolar Junction Transistor A Bipolar junction transistor, shortly termed as BJT is called so as it has two PN junctions for its function. This BJT is nothing but a normal transistor. It has got two types of configurations NPN and PNP. Usually NPN transistor is preferred for the sake of convenience. The following image shows how a practical BJT looks like. The types of BJT are NPN and PNP transistors. The NPN transistor is made by placing a ptype material between two n-type materials. The PNP transistor is made by placing an ntype material between two p-type materials. BJT is a current controlled device. A normal transistor which we had discussed in the previous chapters come under this category. The functionality, configurations and applications are all the same. Field Effect Transistor An FET is a three-terminal unipolar semiconductor device. It is a voltage controlled device unlike a bipolar junction transistor. The main advantage of FET is that it has a very high input impedance, which is in the order of Mega Ohms. It has many advantages like low power consumption, low heat dissipation and FETs are highly efficient devices. The following image shows how a practical FET looks like. The FET is a unipolar device, which means that it is made using either p-type or n-type material as main substrate. Hence the current conduction of a FET is done by either electrons or holes. Features of FET The following are the varied features of a Field Effect Transistor. Unipolar − It is unipolar as either holes or electrons are responsible for conduction. High input impedance − The input current in a FET flows due to the reverse bias. Hence it has high input impedance. Voltage controlled device − As the output voltage of a FET is controlled by the gate input voltage, FET is called as the voltage controlled device. Noise is low − There are no junctions present in the conduction path. Hence noise is lower than in BJTs. Gain is characterized as transconductance. Transconductance is the ratio of change in output current to the change in input voltage. The output impedance of a FET is low. Advantages of FET To prefer a FET over BJT, there should be few advantages of using FETs, rather than BJTs. Let us try to summarize the advantages of FET over BJT. JFET BJT It is an unipolar device It is a bipolar device Voltage driven device Current driven device High input impedance Low input impedance Low noise level High noise level Better thermal stability Less thermal stability Gain is characterized by transconductance Gain is characterized by voltage gain Applications of FET FET is used in circuits to reduce the loading effect. FETs are used in many circuits such as Buffer Amplifier, Phase shift Oscillators and Voltmeters. FET Terminals Though FET is a three terminal device, they are not the same as BJT terminals. The three terminals of FET are Gate, Source and Drain. The Source terminal in FET is analogous to the Emitter in BJT, while Gate is analogous to Base and Drain to Collector. The symbols of a FET for both NPN and PNP types are as shown below Source The Source terminal in a Field Effect Transistor is the one through which the carriers enter the channel. This is analogous to the emitter terminal in a Bipolar Junction Transistor. The Source terminal can be designated as S. The current entering the channel at Source terminal is indicated as IS. Gate The Gate terminal in a Field Effect Transistor plays a key role in the function of FET by controlling the current through the channel. By applying an external voltage at Gate terminal, the current through it can be controlled. Gate is a combination of two terminals connected internally that are heavily doped. The channel conductivity is said to be modulated by the Gate terminal. This is analogous to the base terminal in a Bipolar Junction Transistor. The Gate terminal can be designated as G. The current entering the channel at Gate terminal is indicated as IG. Drain The Drain terminal in a Field Effect Transistor is the one through which the carriers leave the channel. This is analogous to the collector terminal in a Bipolar Junction Transistor. The Drain to Source voltage is designated as VDS. The Drain terminal can be designated as D. The current leaving the channel at Drain terminal is indicated as ID. Types of FET There are two main types of FETS. They are JFET and MOSFET. The following figure gives further classification of FETs. In the subsequent chapters, we will have a detailed discussion on JFET and MOSFET. Print Page Previous Next Advertisements ”;

Basic Electronics – Diodes ”; Previous Next After having known about various components, let us focus on another important component in the field of electronics, known as a Diode. A semiconductor diode is a two terminal electronic component with a PN junction. This is also called as a Rectifier. The anode which is the positive terminal of a diode is represented with A and the cathode, which is the negative terminal is represented with K. To know the anode and cathode of a practical diode, a fine line is drawn on the diode which means cathode, while the other end represents anode. As we had already discussed about the P-type and N-type semiconductors, and the behavior of their carriers, let us now try to join these materials together to see what happens. Formation of a Diode If a P-type and an N-type material are brought close to each other, both of them join to form a junction, as shown in the figure below. A P-type material has holes as the majority carriers and an N-type material has electrons as the majority carriers. As opposite charges attract, few holes in P-type tend to go to n-side, whereas few electrons in N-type tend to go to P-side. As both of them travel towards the junction, holes and electrons recombine with each other to neutralize and forms ions. Now, in this junction, there exists a region where the positive and negative ions are formed, called as PN junction or junction barrier as shown in the figure. The formation of negative ions on P-side and positive ions on N-side results in the formation of a narrow charged region on either side of the PN junction. This region is now free from movable charge carriers. The ions present here have been stationary and maintain a region of space between them without any charge carriers. As this region acts as a barrier between P and N type materials, this is also called as Barrier junction. This has another name called as Depletion region meaning it depletes both the regions. There occurs a potential difference VD due to the formation of ions, across the junction called as Potential Barrier as it prevents further movement of holes and electrons through the junction. Biasing of a Diode When a diode or any two-terminal component is connected in a circuit, it has two biased conditions with the given supply. They are Forward biased condition and Reverse biased condition. Let us know them in detail. Forward Biased Condition When a diode is connected in a circuit, with its anode to the positive terminal and cathode to the negative terminal of the supply, then such a connection is said to be forward biased condition. This kind of connection makes the circuit more and more forward biased and helps in more conduction. A diode conducts well in forward biased condition. Reverse Biased Condition When a diode is connected in a circuit, with its anode to the negative terminal and cathode to the positive terminal of the supply, then such a connection is said to be Reverse biased condition. This kind of connection makes the circuit more and more reverse biased and helps in minimizing and preventing the conduction. A diode cannot conduct in reverse biased condition. Let us now try to know what happens if a diode is connected in forward biased and in reverse biased conditions. Working under Forward Biased When an external voltage is applied to a diode such that it cancels the potential barrier and permits the flow of current is called as forward bias. When anode and cathode are connected to positive and negative terminals respectively, the holes in P-type and electrons in N-type tend to move across the junction, breaking the barrier. There exists a free flow of current with this, almost eliminating the barrier. With the repulsive force provided by positive terminal to holes and by negative terminal to electrons, the recombination takes place in the junction. The supply voltage should be such high that it forces the movement of electrons and holes through the barrier and to cross it to provide forward current. Forward Current is the current produced by the diode when operating in forward biased condition and it is indicated by If. Working under Reverse Biased When an external voltage is applied to a diode such that it increases the potential barrier and restricts the flow of current is called as Reverse bias. When anode and cathode are connected to negative and positive terminals respectively, the electrons are attracted towards the positive terminal and holes are attracted towards the negative terminal. Hence both will be away from the potential barrier increasing the junction resistance and preventing any electron to cross the junction. The following figure explains this. The graph of conduction when no field is applied and when some external field is applied are also drawn. With the increasing reverse bias, the junction has few minority carriers to cross the junction. This current is normally negligible. This reverse current is almost constant when the temperature is constant. But when this reverse voltage increases further, then a point called reverse breakdown occurs, where an avalanche of current flows through the junction. This high reverse current damages the device. Reverse current is the current produced by the diode when operating in reverse biased condition and it is indicated by Ir. Hence a diode provides high resistance path in reverse biased condition and doesn’t conduct, where it provides a low resistance path in forward biased condition and conducts. Thus we can conclude that a diode is a one-way device which conducts in forward bias and acts as an insulator in reverse bias. This behavior makes it work as a rectifier, which converts AC to DC. Peak Inverse Voltage Peak Inverse Voltage is shortly called as PIV. It states the maximum voltage applied in reverse bias. The Peak Inverse Voltage can be defined as “The maximum reverse voltage that a diode can withstand without

Basic Electronics – JFET ”; Previous Next The JFET is abbreviated as Junction Field Effect Transistor. JFET is just like a normal FET. The types of JFET are n-channel FET and P-channel FET. A p-type material is added to the n-type substrate in n-channel FET, whereas an n-type material is added to the ptype substrate in p-channel FET. Hence it is enough to discuss one type of FET to understand both. N-Channel FET The N-channel FET is the mostly used Field Effect Transistor. For the fabrication of Nchannel FET, a narrow bar of N-type semiconductor is taken on which P-type material is formed by diffusion on the opposite sides. These two sides are joined to draw a single connection for gate terminal. This can be understood from the following figure. These two gate depositions (p-type materials) form two PN diodes. The area between gates is called as a channel. The majority carriers pass through this channel. Hence the cross sectional form of the FET is understood as the following figure. Ohmic contacts are made at the two ends of the n-type semiconductor bar, which form the source and the drain. The source and the drain terminals may be interchanged. Operation of N-channel FET Before going into the operation of the FET one should understand how the depletion layers are formed. For this, let us suppose that the voltage at gate terminal say VGG is reverse biased while the voltage at drain terminal say VDD is not applied. Let this be the case 1. In case 1, When VGG is reverse biased and VDD is not applied, the depletion regions between P and N layers tend to expand. This happens as the negative voltage applied, attracts the holes from the p-type layer towards the gate terminal. In case 2, When VDD is applied (positive terminal to drain and negative terminal to source) and VGG is not applied, the electrons flow from source to drain which constitute the drain current ID. Let us now consider the following figure, to understand what happens when both the supplies are given. The supply at gate terminal makes the depletion layer grow and the voltage at drain terminal allows the drain current from source to drain terminal. Suppose the point at source terminal is B and the point at drain terminal is A, then the resistance of the channel will be such that the voltage drop at the terminal A is greater than the voltage drop at the terminal B. Which means, VA>VB Hence the voltage drop is being progressive through the length of the channel. So, the reverse biasing effect is stronger at drain terminal than at the source terminal. This is why the depletion layer tends to penetrate more into the channel at point A than at point B, when both VGG and VDD are applied. The following figure explains this. Now that we have understood the behavior of FET, let us go through the real operation of FET. Depletion Mode of Operation As the width of depletion layer plays an important role in the operation of FET, the name depletion mode of operation implies. We have another mode called enhancement mode of operation, which will be discussed in the operation of MOSFETs. But JFETs have only depletion mode of operation. Let us consider that there is no potential applied between gate and source terminals and a potential VDD is applied between drain and source. Now, a current ID flows from drain to source terminal, at its maximum as the channel width is more. Let the voltage applied between gate and source terminal VGG is reverse biased. This increases the depletion width, as discussed above. As the layers grow, the cross-section of the channel decreases and hence the drain current ID also decreases. When this drain current is further increased, a stage occurs where both the depletion layers touch each other, and prevent the current ID flow. This is clearly shown in the following figure. The voltage at which both these depletion layers literally “touch” is called as “Pinch off voltage”. It is indicated as VP. The drain current is literally nil at this point. Hence the drain current is a function of reverse bias voltage at gate. Since gate voltage controls the drain current, FET is called as the voltage controlled device. This is more clearly understood from the drain characteristics curve. Drain Characteristics of JFET Let us try to summarize the function of FET through which we can obtain the characteristic curve for drain of FET. The circuit of FET to obtain these characteristics is given below. When the voltage between gate and source VGS is zero, or they are shorted, the current ID from source to drain is also nil as there is no VDS applied. As the voltage between drain and source VDS is increased, the current flow ID from source to drain increases. This increase in current is linear up to a certain point A, known as Knee Voltage. The gate terminals will be under reverse biased condition and as ID increases, the depletion regions tend to constrict. This constriction is unequal in length making these regions come closer at drain and farther at drain, which leads to pinch off voltage. The pinch off voltage is defined as the minimum drain to source voltage where the drain current approaches a constant value (saturation value). The point at which this pinch off voltage occurs is called as Pinch off point, denoted as B. As VDS is further increased, the channel resistance also increases in such a way that ID practically remains constant. The region BC is known as saturation region or amplifier region. All these along with the points A, B and C are plotted in the graph below. The drain characteristics are plotted for drain current ID against drain source voltage VDS for different values of gate source voltage VGS. The overall drain characteristics for such various input voltages is as given under. As the

Basic Electronics – Quick Guide ”; Previous Next Basic Electronics – Materials Matter is made up of molecules which consists of atoms. According to Bohr’s theory, “the atom consists of positively charged nucleus and a number of negatively charged electrons which revolve round the nucleus in various orbits”. When an electron is raised from a lower state to a higher state, it is said to be excited. While exciting, if the electron is completely removed from the nucleus, the atom is said to be ionized. So, the process of raising the atom from normal state to this ionized state is called as ionization. The following figure shows the structure of an atom. According to Bohr’s model, an electron is said to be moved in a particular Orbit, whereas according to quantum mechanics, an electron is said to be somewhere in free space of the atom, called as Orbital. This theory of quantum mechanics was proven to be right. Hence, a three dimensional boundary where an electron is probable to found is called as Atomic Orbital. Quantum Numbers Each orbital, where an electron moves, differs in its energy and shape. The energy levels of orbitals can be represented using discrete set of integrals and half-integrals known as quantum numbers. There are four quantum numbers used to define a wave function. Principal Quantum number The first quantum number that describes an electron is the Principal quantum number. Its symbol is n. It specifies the size or order (energy level) of the number. As the value of n increases, the average distance from electron to nucleus also increases, as well, the energy of the electron also increases. The main energy level can be understood as a shell. Angular Momentum Quantum number This quantum number has l as its symbol. This l indicates the shape of the orbital. It ranges from 0 to n-1. l = 0, 1, 2 …n-1 For the first shell, n = 1. i.e., for n-1, l = 0 is the only possible value of l as n = 1. So, when l = 0, it is called as S orbital. The shape of S is spherical. The following figure represents the shape of S. If n = 2, then l = 0, 1 as these are the two possible values for n = 2. We know that it is S orbital for l = 0, but if l = 1, it is P orbital. The P orbital where the electrons are more likely to find is in dumbbell shape. It is shown in the following figure. Magnetic Quantum number This quantum number is denoted by ml which represents the orientation of an orbital around the nucleus. The values of ml depend on l. $$m_{l}= int (-l::to:+l)$$ For l = 0, ml = 0 this represents S orbital. For l = 1, ml = -1, 0, +1 these are the three possible values and this represents P orbital. Hence we have three P orbitals as shown in the following figure. Spin Quantum number This is represented by ms and the electron here, spins on the axis. The movement of the spinning of electron could be either clockwise or anti-clockwise as shown here under. The possible values for this spin quantum number will be like, $$m_{s}= +frac{1}{2}::up$$ For a movement called spin up, the result is positive half. $$m_{s}= -frac{1}{2}::down$$ For a movement called spin down, the result is negative half. These are the four quantum numbers. Pauli Exclusion Principle According to Pauli Exclusion Principle, no two electrons in an atom can have the same set of four identical quantum numbers. It means, if any two electrons have same values of n, s, ml (as we just discussed above) then the l value would definitely be different in them. Hence, no two electrons will have same energy. Electronic shells If n = 1 is a shell, then l = 0 is a sub-shell. Likewise, n = 2 is a shell, and l = 0, 1 is a sub-shell. Shells of electrons corresponding to n = 1, 2, 3….. are represented by K, L, M, N respectively. The sub-shells or the orbitals corresponding to l = 0, 1, 2, 3 etc. are denoted by s, p, d, f etc. respectively. Let us have a look at the electronic configurations of carbon, silicon and germanium (Group IV – A). It is observed that the outermost p sub-shell in each case contains only two electrons. But the possible number of electrons is six. Hence, there are four valence electrons in each outer most shell. So, each electron in an atom has specific energy. The atomic arrangement inside the molecules in any type of substance is almost like this. But the spacing between the atoms differ from material to material. Basic Electronics – Energy Bands In gaseous substances, the arrangement of molecules is not close. In liquids, the molecular arrangement is moderate. But, in solids, the molecules are so closely arranged, that the electrons in the atoms of molecules tend to move into the orbitals of neighboring atoms. Hence the electron orbitals overlap when the atoms come together. Due to the intermixing of atoms in solids, instead of single energy levels, there will be bands of energy levels formed. These set of energy levels, which are closely packed are called as Energy bands. Valance Band The electrons move in the atoms in certain energy levels but the energy of the electrons in the innermost shell is higher than the outermost shell electrons. The electrons that are present in the outermost shell are called as Valance Electrons. These valance electrons, containing a series of energy levels, form an energy band which is called as Valence Band. The valence band is the band having the highest occupied energy. Conduction Band The valence electrons are so loosely attached to the nucleus that even at room temperature, few of the valence electrons leave the band to be free. These are called as free electrons as

Basic Electronics – Linear Resistors ”; Previous Next A Linear resistor is one whose resistance doesn’t vary with the flow of current through it. The current through it, will always be proportional to the voltage applied across it. Linear resistors are further classified as Fixed and Variable resistors. Variable Resistors Variable resistors are those whose values can be varied manually, according to the requirement. A particular value of resistance is chosen from a range of resistance values, with the help of a shaft connected. The symbol of a variable resistor is as shown below. These resistors are better understood with the help of the classification we have. Variable resistors are further divided into Potentiometers, Rheostats and Trimmers. Potentiometer A Potentiometer is simply called as a Pot. This is a three-terminal resistor having a shaft which slides or rotates. This shaft when operated forms an adjustable voltage divider. The following figure shows an image of a Potentiometer. A potentiometer also measures the potential difference (voltage) in a circuit. A path of resistive material with resistance of low to high value is laid internally and a wiper is placed so that it connects the resistive material to the circuit. This is mostly used as a volume controller in TV sets and Music systems. Rheostat A Rheostat can be simply called as a Wire wound resistor. A Resistive wire is wound around an insulating ceramic core tightly. A Wiper slides over these windings. One connection is made to one end of the resistive wire and the second connection is made to the wiper or the sliding contact, to obtain the desired resistance. The Rheostat is used to control current. These are mostly used in the speed control of heavy motors. The resistance obtained by these is in the order of kilo ohms. Rheostats are mostly available as single tube and double tube rheostats, as shown in the following figure. As a variable resistance they are often used for tuning and calibration in circuits. Now-a-days, the usage of rheostats was replaced by switching electronic devices, as rheostats have lower efficiency. Trimmer Trimmer is both a variable resistor and a potentiometer (measures potential difference). This Trimmer Potentiometer is, in short called as Trim Pot. If these are used as variable resistors, then they are called as Preset Resistors. These trim pots are of different types such as single turn or multi turn. These are small variable resistors used for tuning and calibration. Their life span is shorter than other variable resistors. Print Page Previous Next Advertisements ”;

Basic Electronics – MOSFET ”; Previous Next FETs have a few disadvantages like high drain resistance, moderate input impedance and slower operation. To overcome these disadvantages, the MOSFET which is an advanced FET is invented. MOSFET stands for Metal Oxide Silicon Field Effect Transistor or Metal Oxide Semiconductor Field Effect Transistor. This is also called as IGFET meaning Insulated Gate Field Effect Transistor. The FET is operated in both depletion and enhancement modes of operation. The following figure shows how a practical MOSFET looks like. Construction of a MOSFET The construction of a MOSFET is a bit similar to the FET. An oxide layer is deposited on the substrate to which the gate terminal is connected. This oxide layer acts as an insulator (sio2 insulates from the substrate), and hence the MOSFET has another name as IGFET. In the construction of MOSFET, a lightly doped substrate, is diffused with a heavily doped region. Depending upon the substrate used, they are called as P-type and N-type MOSFETs. The following figure shows the construction of a MOSFET. The voltage at gate controls the operation of the MOSFET. In this case, both positive and negative voltages can be applied on the gate as it is insulated from the channel. With negative gate bias voltage, it acts as depletion MOSFET while with positive gate bias voltage it acts as an Enhancement MOSFET. Classification of MOSFETs Depending upon the type of materials used in the construction, and the type of operation, the MOSFETs are classified as in the following figure. After the classification, let us go through the symbols of MOSFET. The N-channel MOSFETs are simply called as NMOS. The symbols for N-channel MOSFET are as given below. The P-channel MOSFETs are simply called as PMOS. The symbols for P-channel MOSFET are as given below. Now, let us go through the constructional details of an N-channel MOSFET. Usually an NChannel MOSFET is considered for explanation as this one is mostly used. Also, there is no need to mention that the study of one type explains the other too. Construction of N- Channel MOSFET Let us consider an N-channel MOSFET to understand its working. A lightly doped P-type substrate is taken into which two heavily doped N-type regions are diffused, which act as source and drain. Between these two N+ regions, there occurs diffusion to form an Nchannel, connecting drain and source. A thin layer of Silicon dioxide (SiO2) is grown over the entire surface and holes are made to draw ohmic contacts for drain and source terminals. A conducting layer of aluminum is laid over the entire channel, upon this SiO2 layer from source to drain which constitutes the gate. The SiO2 substrate is connected to the common or ground terminals. Because of its construction, the MOSFET has a very less chip area than BJT, which is 5% of the occupancy when compared to bipolar junction transistor. This device can be operated in modes. They are depletion and enhancement modes. Let us try to get into the details. Working of N – Channel (depletion mode) MOSFET For now, we have an idea that there is no PN junction present between gate and channel in this, unlike a FET. We can also observe that, the diffused channel N (between two N+ regions), the insulating dielectric SiO2 and the aluminum metal layer of the gate together form a parallel plate capacitor. If the NMOS has to be worked in depletion mode, the gate terminal should be at negative potential while drain is at positive potential, as shown in the following figure. When no voltage is applied between gate and source, some current flows due to the voltage between drain and source. Let some negative voltage is applied at VGG. Then the minority carriers i.e. holes, get attracted and settle near SiO2 layer. But the majority carriers, i.e., electrons get repelled. With some amount of negative potential at VGG a certain amount of drain current ID flows through source to drain. When this negative potential is further increased, the electrons get depleted and the current ID decreases. Hence the more negative the applied VGG, the lesser the value of drain current ID will be. The channel nearer to drain gets more depleted than at source (like in FET) and the current flow decreases due to this effect. Hence it is called as depletion mode MOSFET. Working of N-Channel MOSFET (Enhancement Mode) The same MOSFET can be worked in enhancement mode, if we can change the polarities of the voltage VGG. So, let us consider the MOSFET with gate source voltage VGG being positive as shown in the following figure. When no voltage is applied between gate and source, some current flows due to the voltage between drain and source. Let some positive voltage is applied at VGG. Then the minority carriers i.e. holes, get repelled and the majority carriers i.e. electrons gets attracted towards the SiO2 layer. With some amount of positive potential at VGG a certain amount of drain current ID flows through source to drain. When this positive potential is further increased, the current ID increases due to the flow of electrons from source and these are pushed further due to the voltage applied at VGG. Hence the more positive the applied VGG, the more the value of drain current ID will be. The current flow gets enhanced due to the increase in electron flow better than in depletion mode. Hence this mode is termed as Enhanced Mode MOSFET. P – Channel MOSFET The construction and working of a PMOS is same as NMOS. A lightly doped n-substrate is taken into which two heavily doped P+ regions are diffused. These two P+ regions act as source and drain. A thin layer of SiO2 is grown over the surface. Holes are cut through this layer to make contacts with P+ regions, as shown in the following figure. Working of PMOS When the gate terminal is given a negative potential at VGG than the

Basic Electronics Tutorial PDF Version Quick Guide Resources Job Search Discussion This tutorial supplies basic information on how to use electronic components and explains the logic behind solid state circuit design. Starting with an introduction to semiconductor physics, the tutorial moves on to cover topics such as resistors, capacitors, inductors, transformers, diodes, and transistors. Some of the topics and the circuits built with the components discussed in this tutorial are elaborately discussed in the ELECTRONIC CIRCUITS tutorial. Audience This tutorial should be useful for all readers who want to gain preliminary knowledge regarding the basic components used in electronic circuits. Prerequisites We don’t assume any prior knowledge of Electronics to understand this tutorial. The material is meant for beginners and it should be useful for most readers. Print Page Previous Next Advertisements ”;

Basic Electronics – RF Inductors ”; Previous Next RF inductors are the radio frequency inductors, which are used at high resonant frequencies. These can be multilayered coil inductor or a thin film coated ceramic inductor or some wire wound ceramic inductor. The following figure represents few RF inductors. These inductors are characterized by low current rating and high electrical resistance. But as the high frequencies are used here, the wire resistance increases. Also, few effects come into picture because of these high resonant radio frequencies. Let us have a look at them. Skin Effect At high frequencies, the alternating current has a tendency of unequal distribution of current through the conductor. The electric current flows highly at the surface of the conductor than at its center. It gets its energy concentrated in the skin of the conductor, leaving the deep core of the conductor, as shown in the following figure. As the energy gets concentrated at the skin of the conductor, this effect is called as the Skin Effect. Actually this skin effect is caused due to the eddy currents which are produced by the changing Magnetic field, resulting from alternating current. Now-a-days, the conductors carrying higher frequencies are made in the form of tube shape, in order to reduce the weight and cost of the conductors. Proximity Effect Along with the above one, this is another effect, which is observed here. Proximity effect is the one which increases the resistance of the wire at high frequencies. Proximity is the word which says that the effect will be on adjacent wires. The following figure shows the concentration of current on the edges of the adjacent cables. Each turn has some magnetic field which induces eddy currents in the wire that causes the current to be focused on the side of the adjacent wire. With this effect, the effective cross sectional area of the wire gets reduced and its resistance gets increased. Parasitic Capacitance Usually, an inductor internally contains a resistor in series (wire resistance) and a capacitor in shunt (parasitic capacitance). Each turn of winding has slightly different potential, in an inductor. The following figure shows the capacitance effect in an inductor. The two conductors that present in each turn, act as capacitor plates with air as dielectric. A capacitance called as Parasitic Capacitance exists here. In order to avoid this in certain applications, the windings are made far to each other. As the frequency increases, the impedance of the parasitic capacitance decreases and the impedance of inductor increases. Hence the inductor tends to behave like a capacitor. Dielectric losses The current through the conductor of an inductor makes the molecules of the insulators exert energy in the form of heat. The higher the frequency, the greater the heat dissipation will be. Chokes Inductors are also called as chokes. An Inductor blocks AC components and sends DC components through it. Hence as it chokes or stops AC, an inductor can simply be termed as a Choke. A coil of insulated wire is often wound on a magnetic core to form a choke. As the signal frequency increases, the impedance of the choke increases. Due to its reactance, it can limit the amount AC through it. Even though, practically some amount of AC passes through it due to its low electrical resistance. These are mostly used in tube lights and in transformers in electronic applications. Print Page Previous Next Advertisements ”;