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Sign in. Log into your account. Password recovery. Thursday, January 13, About Contact Privacy Policy Disclaimer. The barrier potential so formed prevents any further flow of electrons. When a forward bias is applied, the conduction is solely by electrons as there are no holes in the metal. This means less power dissipation in a Schottky diode. Further, the recovery time upon switch-off is very small, about 10—20 ns, as there is no recombination of electron-holes.

The symbol of Schottky diode and equivalent circuit are shown in Fig. When the depletion layer width becomes 1 nm or less, low-energy electrons are able to cross the barrier by a process called tunnelling.

So the conduction begins at much lower values of bias voltage, where there is no injunction current. The IV characteristic of a tunnel diode is sketched in Fig. As the bias voltage increases, the current reaches its maximum value Ip at Vp, beyond which no more electrons are available and holes have decreased.

The current now begins to reduce reaching the minimum value of Iv Fig. In this region, the diode offers negative resistance; beyond Vv the diode behaves like a normal diode.

Its value is 10 for germanium and 20 for gallium arsenide. Therefore, it possesses a capacitance. As the width of the depletion region varies with reverse voltage VR , the transition capacitance varies accordingly. Equation 2. The temperature coefficient of C 0 is provided by manufacturers. Being a reverse biased diode the capacitance is highly temperature dependent. Here, we will study two kinds of devices—one in which light controls diode current and the other in which diode emits light when carrying current.

Note that light also behaves as a travelling wave. As light is made to impinge on the junction, the light photons impart energy to the valence electrons causing more electron-hole pairs to be released. The symbol of a photodiode is drawn in Fig. The IV characteristics for various values of light intensity fc are drawn in Fig. By examining the characteristics it is found that at a certain Vl say 20 V , Il increases almost linearly with fc.

It has been found that Ge photodiode has more overlaps compared to Si, which is in the range of light frequencies to which the human eye is sensitive. Ge is, therefore, more suitable for infra-red IR light sources like laser. Upon capture of a free electron by a hole, the electron goes into a new state and its kinetic energy is given off as heat and as light photons. In a silicon diode, most of this energy is given off as heat but in other materials such as gallium arsenide GaAs or gallium phosphide GaP , sufficient number of photons light are generated so as to create a visible source.

This process of light emission in PN-junctions of such materials is illustrated in Fig. The voltage levels of LEDs are 1. LEDs find several display applications, particularly Fig. The LED is forward biased and the photodiode is reverse biased. The output is available across R2. The key advantage of the photocoupler is the electrical isolation between two circuits.

It is employed to couple circuits whose voltage level may differ by several thousand volts. Also determine the values of K in Eq. Read from Fig. Solution Light intensity in fc 3. Explain what is depletion region in a PN-junction diode? What is reverse saturation current in a diode? Does it exist in both reverse-biased and forward-biased diodes.

What is threshold voltage of a diode? What is its value for Si and Gi diodes? Write the diode conduction equation. Explain the meaning of each symbol. Draw the circuit equivalent of a forward-biased diode.

Explain the operation of a zener diode and draw its circuit equivalent. A zener diode acts as a voltage regulator. Explain the meaning of the statement. Draw the circuit of a bridge rectifier. What is the input and output waveform? Write the expression of dc voltage of half-wave and full-wave rectifiers. What is the ripple factor of a diode rectifier? Derive its expression for a full-wave rectifier. Will the ripple factor be more or less than this value for a half-wave rectifier?

What is the conversion efficiency of a rectifier circuit? Will the value be more or less than this in a half-wave rectifier? For the diode circuit of Fig.

For the diode OR gate logic of Fig. For the diode AND logic of Fig. In the zener diode circuit of Fig. Calculate the values of Vz, RS and power rating of the zener. For the circuit with zener diode Fig. VL RL. If the source voltage VS varies from 20 V to 30 V, find the maximum and minimum current in the diode? For the diode bridge of Fig. Sketch vo. Find Vo dc and PIV of each diode. For the diode network of Fig. A CT transformer full-wave rectifier has ac voltage of each half-secondary of 20 V rms.

The resistance of each half is 1 W. The load resistance is Determine Vdc and Idc of the load. A bridge rectifier has four identical diodes of forward resistance of 5 W each.

It is supplied from a transformer with output voltage of 20 V rms and secondary winding resistance of 10 W. Calculate the a dc output voltage at a dc load current of mA b rms value of output voltage at a dc load current of mA c rms value of the ac component of the voltage in part b.

For the circuit of Fig. Draw the output voltage of Fig. In Fig. The cut in knee voltage of a germanium diode is a 0. Junction breakdown of a PN-junction diode occurs a with forward bias b with reverse bias c because of improper design d All of these 3. A zener diode a is useful as an amplifier b has a negative resistance c has a high forward voltage d has a sharp breakdown at low reverse voltage 4. Which of the following diodes is best suited as a switching diode for very high frequencies?

The ripple factor of a half-wave rectifier is a 0. Which of the following is a type of clipping circuit? Level shifter circuits are also known as a clipper circuits b clamper circuits c diodes d transistors 9.

In a zener diode, the value of Izm, if power dissipation rating is mW and zener voltage rating is 6. Basically, a transistor is a combination of two back-to-back diodes, provided crystal continuity is maintained. Addition of another layer results in a three-layer two junctions device which has npn or pnp form and is called a transistor.

Such a transistor is known as Bipolar Junction Transistor BJT which acts as a current-controlled device with the output current being controlled by the input current, such that the input-current waveform is replicated at the output. This mode of operation of a BJT finds wide applications in high-speed digital electronics. In a pnp transistor, a thin N-type layer is sandwiched between two P-type layers, while in an npn transistor, a thin layer of P-type is sandwiched between two N-type layers.

A transistor is like two diodes. The type of transistor can be recognised from the direction of arrow of E emitter , Fig. These constitute the emitter current IE. This is why the base width is kept very small. Recombination rate is small as electron concentration is very light in the base. To replenish the recombining electrons, a small electron current IB flows out of the base. Note that direction of IB is that of conventional current. Basic Electronics 3.

This current flow can be ignored [not shown in the figure]. Carrier flow in an npn transistor is shown in Fig. At the CB junction, there is a depletion region as the CB junction is reversed biased, while there is no such region at the EB junction as it is forward biased.

As VCB increases, the depletion region, and so effective base width, reduces. This base-width modulation is known as early effect. This phenomenon is known as punch through.

The EB diode is forward biased and the CB diode is reverse biased. It can, therefore, be modelled as two diodes shown in Fig. This will result in equations already presented. For circuit operations, we require two terminals for input as well as output. So, one terminal of the transistor is grounded. Common base 2. Common emitter 3. Common collector To describe the behaviour of any configuration, two characteristics are required.

Unless otherwise mentioned the transistor is npn. A typical characteristic to scale is presented Fig. It is found that it is practically independent of VCB. It can be approximated as a diode characteristic.

Typical output or collector characteristics are drawn in Fig. The characteristics can be divided into three regions. All the carriers that are injected into the emitter are swept away through the base to the collector. This is easily seen from Fig. This is the linear region in which amplifying action of the circuit takes place. The input current is transferred by the transistor to output.

Voltage amplification will result from low driving-point resistance 10 — W [see Fig. The configuration is not used for amplification but serves certain special purposes. Its circuit is drawn in Fig.

The collector characteristics are drawn in Fig. These are related by b. The middle of this region is linear w. IB and VCE. From Eq. The b: bdc and bac lie in the middle region of Fig. We observe here that b is the common-emitter forward-current gain. This causes, the output to be in phase with input signal. It offers a high input resistance and low output resistance. It is, therefore, employed for impedance matching. In this configuration, aR factor will exist which shows the amplification of input at output.

As shown in Fig. The frequency range has now been extended to 50 kHz, which are employed in high-frequency applications like induction heating and ultrasonic cleaning. Its four layers are arranged as pnpn shown in Fig. The outer layers are connected to terminals to form anode positive terminal and cathode negative terminal. The P-layer closer to the cathode is connected to the gate terminal.

The SCR symbol is drawn in Fig. It is similar to that of a diode, the difference being the indication of the gate terminal.

If a positive pulse is applied at the gate, such that a current of magnitude equal to more than IG turn-on flows into the gate, the processes in the device cause it to go into conduction.

The forward current anode to cathode is offered a resistance as low as 0. However, because of regenerative action, removing the gate current does not cause the device to turn off. The dynamic reverse resistance of an SCR is as high as kW or more.

The middle n and p layers can be imagined to be subdivided into two halves, as shown by the dotted line. The corresponding two-transistor equivalent circuit is drawn in Fig. This circuit will now be used to explain the action of the gate pulse IG. This in turn, increases IB2 causing a regenerative action to set in this is indeed a positive internal feedback.

The result is that the SCR is turned on, that is, the switch between the anode E1 and cathode E2 is closed turn-on. The current IA must be limited by the external circuit, say a series resistance between the source and E1.

The turn-on time of an SCR is typically 0. The turn-off mechanism is called commutation and it can be achieved in two ways explained below. Natural Commutation When the source that feeds the current to anode of SCR is such that it naturally passes through zero, the SCR turns off at the current zero.

This is the case when the SCR is fed from the ac source. In this situation, the commutation is also known as line commutation. Forced Commutation In this method of commutation, the current through the SCR is forced to become zero by passing a current through it in opposite direction from an independent circuit. One basic turn-off circuit which illustrates the principle is drawn in Fig. A transistor and dc battery source in series are connected to the SCR.

To turn off the SCR, a positive IB pulse of magnitude large enough to drive the transistor into saturation is applied at the transistor base. The transistor acts almost like a short circuit. This causes flow of very large Ioff through the SCR in the opposite direction to its conduction current. The total SCR current reduces to zero in a very short time causing it to turn off. The transistor has to withstand a large current but for a very short time.

Various voltages and currents which provide important information for SCR applications are described below. As is seen from Fig. Holding current IH is the value of the current below which SCR switches from conduction state to forward blocking regions of specified conditions. Forward and reverse blocking regions are those regions in which the SCR is open circuited and no current flows from anode to cathode.

Reverse breakdown voltage corresponds to Zener or avalanche region of a diode. As for applications in power and drives, these form the subject matter of a separate course for which several excellent books are available. We shall give here a single application for illustrative purpose. This action is the same as that of a diode. On application of alternating voltage, it causes rectified ac to flow but it needs to be triggerd for each positive half cycle of ac.

It then produces constant dc average value current through load and dc voltage across load. Adjusting the triggering time on positive half cycle of ac voltage would yield variable dc output. This method is known as phase control. A variable-resistance phase-control circuit is provided in Fig. As R1 is reduced, IG rises to turn-on value at a particular angle time of vi.

If R1 is adjusted for firing at a [see Fig. So the operation of this circuit is known as half-wave, variable-resistance phase control. It may be noted that a diode is provided in the firing circuit to prevent the flow of reverse gate current. The characteristics of the device, presented in Fig. This possibility of an on condition in either direction can be used to its fullest advantage in ac applications. Note that neither terminal is referred to as the cathode.

Instead, there is an anode 1 or electrode 1 and an anode 2 or electrode 2. When the anode 1 is positive with respect to the anode 2, the semiconductor layers of particular interest are p1n2p2 and n3. For the anode 2 to be positive with respect to the anode 1, the applicable layers are p2n2p1 and n1. In other words, for either direction, the gate current can control the action of the device in a manner very similar to that demonstrated for an SCR.

Note the holding current in each direction not present in the characteristics of the DIAC. The graphical symbol for the device and the distribution of the semiconductor layers are provided in Fig. For each possible direction of conduction, there is a combination of semiconductor layers whose state will be controlled by the signal applied to the gate terminal. A UJT is a three-terminal device basic construction shown in Fig. Two base contacts are made at each end of one side of the slab, while an aluminium rod is fused on the other side to form a single pn-junction, and hence the name unijunction.

The rod is located closer to the base terminal 2 which is made positive with respect to the base terminal 1 by VBB. The symbol and biasing of a UJT is shown in Fig. The circuit equivalent to the UJT is drawn in Fig. Here the input diode represents the pn-junction operation; RB2 is a fixed resistance and RB1 is a variable resistance, which reduces with increase in emitter current IE. This ratio h is controlled by the location of the aluminium rod Fig.

As VE crosses VP, the emitter fires and holes are injected into the slab from the P-type aluminium rod. This causes increase in the hole content of the N-type slab with consequent increase in the number of free electrons in it, and so, increased conductivity.

Thus, VE drops off while IE increases. The device is designed with large base and collector regions as compared with ordinary BJTs with photosensitive material being used for base. Figure 3. The symbol of npn phototransistor and physical phototransistor are shown in Fig.

The lines pointing towards the base represent the light input to base for the required voltage generation. The metal casing is usually used to improve the exposure of base of transistor to light. The biasing circuit and output characteristics of phototransistor are shown in Fig. At the initial stage, when no exposure of light is there, a minute reverse saturation current flows owing to the presence of minority charge carriers.

When exposed to light, transistor starts conducting through reverse biasing. The response of transistor is different for different intensities of light. The number of free electrons generated in each material is proportional to the intensity of incident light.

The material used for construction of phototransistor includes silicon, germanium with non-identical material like gallium arsenide on either side of p-n junction. The different applications of phototransistor include optocoupling, switching and controlling, optical isolation, optical sensor, etc. This process is called biasing the BJT. The biasing locates an operating point, also called quiescent point Q , on the characteristics, about which signal-caused variation takes place [ac input causing ac output suitably modified amplified ]; dc biasing and ac analysing can be carried out separately results superimposed if necessary.

In order to isolate the ac signals from dc sources, isolating capacitors are used, which act as short circuits for ac signals. From the signal point of view, these are coupling capacitors. We shall now proceed to analyse and design the dc biasing which determines the location of the Q-point appropriately. KVL equation for BE loop.

VCC - 0. Along VCE axis. It intersects the characteristics for IBQ at the operating point Q. If IB is increased, the Q-point moves up along the load line. The fixed bias cannot counter the thermal effect on the BJT characteristics.

Correspondingly, the Q-point moves up on the load line. The circuit also shows coupling capacitors Cc which are open circuit for dc. The emitter includes voltage drop RE IE, which acts as negative feedback to stabilize the bias Q-point. So there is no significant shift in Q-point, which has been stabilised by inclusion of RE. It can be shown by exact analysis that the sensitivity of the Q-point to changes in b is quite small by proper design of circuit parameters.

If b changes, the level of IBQ will change because of the negative feedback effect of RE but the collector characteristics also change accordingly. Voltage feedback bias is somewhat better than emitter self-bias because of additional voltage feedback. But the voltage-divider bias is the best of all the schemes of BJT bias and is universally adopted. Remark: In order that emitter resistance RE does not affect the ac performance, it should be shunted by a capacitor called bypass capacitor.

Determine IC and IB. Solution From Fig. Solution a We find from Fig. Solution a The operating point is located at Q in Fig. ICBO 0. Solution Assume active mode. Solution 18 - 0. Solution 20 - 0. Solution a and b Q-point is located on Fig. Review Questions 1. Comment on the doping levels of the three components of a BJT. Why is the base layer of a BJT made very thin compared to emitter and collector layer? What are the three regions of operation of a BJT. Explain with help of CE configuration collector characteristics.

In the three regions of operation how are the BJT junctions biased? Define a of a BJT. Define b of a BJT. How are a and b related? Distinguish between adc and aac 9. Distinguish between bdc and bac. What is reverse saturation current in a BJT? How can this be observed independently?

Draw the symbols for pnp and npn BJT. What distinguishes one from the other? What is meant by biasing a transistor? What is self-bias?

How does it help in stabilising the Q-point? Draw the circuit of a voltage-divider bias. What is voltage feedback bias? Draw the circuit. Is this type of bias almost independent of b? What is meant by b insensitive bias? Problems 1. What is the value of IC? Does it depend on VCB in the active region. From the collector characteristics of Fig. Calculate adc and the corresponding IE.

Check from the value of VCB if it is so. Hint: CB junction should be reverse biased. For the fixed-bias configuration of Fig. For the emitter-stabilised bias circuit of Fig. For the voltage-divider bias circuit of Fig. For the voltage feedback network of Fig. Multiple-Choice Questions 1. Current gain of BJT in common base is a a b b c g d none of these 2.

An SCR device has a four layers b three layers c two layers d one layer 3. When the collector junction in transistors is biased in the reverse direction and the emitter junction in forward directions, the transistor is said to be in the a cut-off region b saturation region c active region d none of these 6.

Which of the following acts like a diode and two transistors? Early effect in BJT refers to a avalanche breakdown b zener breakdown c base narrowing d none of these 8. In a BJT, largest current flows a in the base b in the emitter c in the base and emitter d in the collector 9. These differ from BJTs in two respects.

Basic Electronics 4. It is embedded on both sides with P-type material as shown in Fig. The two p-type materials are joined together through ohmic contacts and connected to the terminal gate G. The N and P materials form PN-junctions on the two sides of the channel. The top and bottom of the N-channel make ohmic contact with the Drain D and source S terminals, respectively. Around both the PN-junctions there is the depletion region like in a diode.

The reverse biasing reduces towards the S terminal because of voltage drop in the channel from D to S. As a result, the depletion region widens at the D-end of the channel. The width of depletion region reduces along the channel towards the S-end. The depletion region is therefore non-uniform as shown in Fig. The initial behaviour of the channel is that of voltage-controlled resistor.

This negative VGS reverse biases both the junctions uniformly, reducing the channel width throughout. This is over and above the effect of VDS.

The complete characteristics typical are drawn in Fig. ID mA Locus of pinch-off values. It is further observed that the slope decreases as VGS increases. That is why JFET is a voltage- controlled device in which input voltage controls the output current. The current flow is due to the movement of these holes.

It carries two n-type silicon terminals positioned on both sides and connected to the gate G terminal. The drain and source leads are connected to both sides of p-type channel. When VGS is zero, the current due to holes as majority carriers flows freely. When positive voltage is applied to the gate terminal, the drain source current starts decreasing till it reaches its cutoff and transistor enters into OFF state.

For zero voltage at the gate, the drain current attains its maximum value. Saturation Region This is the fully operational region and maximum current flows during this region. During this region transistor is in ON state. Breakdown Region It is the region when the voltage supplied to the source exceeds the required voltage. The transistor loses its ability to resist current and breaks down. During this the current flows from source to drain.

What is the nature of their relationship? Small signal modeling analysis is done by replacing all signal sources with their ideal internal resistance, leaving the ac voltages and currents in the circuit. Small signal models are different for low and high frequencies.

Though the resistance of input terminal is very high but gate source voltage affects the value of drain current which is represented by voltage controlled current source gm , VGS with value proportional to gate source voltage.

The output resistance or the drain resistance is represented by rd. Drain resistance may have a typical value from kW. From Fig. Due to internal capacitances, the feedback exits between the input and output terminals. With increase in frequency, Fig. Find mid-band ac gain, output and input impedance.

Its thermal stability and other general features makes it suitable for IC design fabrication because of smaller silicon-chip area. On a p-type substrate, an n-channel is formed which is connected to D and S terminals through heavily doped n-regions.

The N-channel is insulated from the gate G terminal by an SiO2 layer, which extends over the complete device. The holes recombine with electrons being repelled by the gate, thereby reducing the concentration of electrons in the channel as shown in Fig.

The result is reduction in saturation current ID sat. The drain characteristics and transfer characteristics are similar to those of JFET, as shown in Fig. Depletion mode and enhancement mode are both shown in Fig. In its ON state, the current carrier holes move through the channel. Sign in.

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