ELECTRONIC DEVICES - Class XII (Boys Wing) Page 01 Electronics is the fast developing branch of Physics. Before the discovery of transistors in 1948, vacuum tubes (thermionic valves) were used as the building blocks of electronic circuits. The field of electronics gained a large momentum after the introduction of semi-conducting devices. Advantages of semi-conducting devices over vacuum tubes. Small in size, consume less power, no need of a warm-up time, have long life and high reliability BAND THEORY OF SOLIDS According to the Bohr atomic model, in an isolated atom the energy of any of its electrons is decided by the orbit in which it revolves. But when the atoms come together to form a solid they are close to each other. So the outer orbits of electrons from neighbouring atoms would come very close or could even overlap. Hence the energy possessed by electrons in the outermost orbits of atoms in a solid may be different rather than discrete. This range of energies possessed by the electrons in the outermost orbit of atoms in a solid is called energy band. The energy band which includes the energy levels of the valence electrons is called the valence band. The energy band above the valence band is called the conduction band. The energy required to jump from valence and conduction bands is called Forbidden energy gap (E g ). DISTINGUISH BETWEEN CONDUCTORS, INSULATORS AND SEMI CONDUCTORS (A). Conductors:- Conduction and valence bands overlap. No energy gap. (B) Insulators:- Conduction band is empty. Large energy gap. Hence no free electrons. (C) Semiconductors:- Small energy gap. Easy for electrons to get transferred to conduction band, even at room temperature. (A) Conductors (B) Insulators (C) Semiconductors INTRINSIC SEMICONDUCTORS A semiconductor in its pure form is called an intrinsic semiconductor. Examples: Germanium or Silicon. In the structure of an intrinsic semiconductor, each atom makes co-valent bond with the neighbouring atoms so that no free electrons are available in the structure at absolute zero (figure1). But even at room temperature, due to thermal agitations, some co-valent bonds may be broken and hence a few free electrons may be released in the structure. This creates vacancies of electrons in some of the co-valent bonds and are called holes. (figure 2) Next page dept. of physics, shrajah indian school, sharjah dept.of physics. sharjah indian school sharjah dept.of physics. sharjah indian sch..
ELECTRONIC DEVICES - Class XII (Boys Wing) Page 02 Figure 1 Figure 2 The presence of holes and free electrons contribute to the conductivity of a semiconductor. As temperature increases, more electron- hole pairs are released, increasing the conductivity. Thus the conductivity of a semiconductor increases with temperature. In an intrinsic semiconductor, the number of free electrons is equal to the number of holes. i.e., n e = n h = n i, called intrinsic carrier concentration. The following diagrams represent the band diagrams of an intrinsic semi conductor at 0K and at T>0K. At T= 0 K At T > 0 K EXTRINSIC SEMICONDUCTORS The conductivity of a semiconductor can be increased by the addition of certain impurities to the structure of a pure semiconductor. The process is called doping and the impurities are called dopants. (i) n-type semiconductors. If pentavalent impurities such as, Arsenic, Antimony, Bismuth, Phosphorous are added to Ge or Si, Some atoms are displaced by the impurity atoms. Out of the five valence electrons, four of them make co-valent bond with the neighbouring Ge atoms whereas the fifth electron remains weakly bound to the parent atom. This increases the conductivity the semiconductor. (figure 4) dept. of physics, shrajah indian school, sharjah dept.of physics. sharjah indian school sharjah dept.of physics. sharjah indian sch..
ELECTRONIC DEVICES - Class XII (Boys Wing) Page 03 Figure 4 Conduction of electricity through such a semiconductor is due to the presence of such free electrons, which are negatively charged. Thus such a semiconductor is called negative-type or n- type semiconductor. It shows that a pentavalent dopant is donating one extra electron for conduction and hence it is called a donor impurity. The energy band diagram of n-type semiconductor is called donor level diagram. (figure 5) In n-type semiconductors, electrons are the majority charge carriers and holes, the minority carriers. i.e., n e >>n h. Figure 5 (ii) p-type semiconductors. When trivalent impurities such as Ga, In, B,. are added to the structure of a pure semiconductor, each impurity atom can make co-valent bonds with the three neighbouring Ge atoms whereas, the forth bond has a vacancy or a hole. (Figure 6) This increases the conductivity of the semiconductor. The charge carriers are Conduction Band holes, which behave as positive charges. Hence such a semiconductor is called positive type or p-type. oooooooooooooooooooo The trivalent impurities are Acceptor Level called acceptor impurities and the energy band diagram Figure 07 Valence Band is called acceptor level diagram. (Figure 7) Figure 06 Conduction Band Donor Level Valence Band P.N.Junction When a pure semiconductor (e.g: Si) is doped with penta-valent and trivalent impurities on both the sides equally, we get a p-n junction. Two important processes during the formation of p-n junction are diffusion and drift. Holes diffuse from p-side to n-side (p n) and electrons diffuse from n-side to p-side (n p). This motion of charge carriers gives rise to diffusion current across the junction. This leaves behind an ionised donor (positive charge) on the n-side and an ionised acceptor (negative charge) on the p-side. These are immobile. As the process continues a layer of positive charge called positive space-charge region is formed at the n-side of the junction and a layer of negative charge called negative space-charge region on the p-side of the junction. Hence a neutral layer called depletion layer is developed at the junction. dept. of physics, shrajah indian school, sharjah dept.of physics. sharjah indian school sharjah dept.of physics. sharjah indian sch..
ELECTRONIC DEVICES - Class XII (Boys Wing) Page 04 This further gives rise to an electric field (junction field) directed from positive charge towards negative charge. Due to this field, electrons move from p-side to n-side and a hole from n- side to p-side. The motion of charge carriers due to the electric field is called drift. This causes a drift current, opposite to the diffusion current. (Fig.6). Figure 06 The process of increasing the junction field and hence the diffusion current continues until both the currents become the same. So, no net current is there at the p-n junction under equilibrium. Due to the loss of electrons from the n-side and gain of electrons from the p-side, positive and negative potentials are developed respectively. This potential prevents further movement of charge carriers across the junction. This is called barrier potential. SEMICONDUCTOR DIODE. It is a p-n junction diode with metallic contacts to apply external voltage. The construction and the circuit symbol of a semiconductor diode are shown below: Figure 07 Forward Bias When the p-side of the diode is connected to the positive and n-side to the negative terminal of a battery, the diode is said to be forward biased. During forward bias, the holes from p-side and the electrons from the n-side are pushed respectively, by positive and negative terminals of the battery. Thus the width of the depletion layer decreases. Under sufficient voltage, more charge carriers gain sufficient energy to cross the junction and hence more electron-hole Figure 8 combinations take place. This gives rise to a current, called forward bias current. Reverse Bias When the p-side of the diode is connected to the negative and n-side to the positive terminal of a battery, the diode is said to be reverse biased. During reverse bias, the holes from p-side and the electrons from the n-side are attracted respectively, by negative and positive terminals of the battery. Thus the width of the depletion layer increases. So, practically, no current flows through the circuit. But a feeble current (in µa) flows through Fig 9 the junction due to the movement of the minority charge carriers. This current is called reverse saturation current. It is independent of the applied voltage. As the voltage increases, the minority charge carriers gain sufficient energy to break a large number of co-valent bonds at the junction, releasing a large number of electron-hole pairs. This happens at a critical reverse bias voltage, called breakdown voltage or Zener voltage and this phenomenon is called avalanche breakdown. At this stage, the reverse current increases abruptly. Ordinary p-n diodes may get destroyed after this voltage due to overheating.
ELECTRONIC DEVICES - Class XII (Boys Wing) Page 05 V-I characteristics of a diode. The circuit diagram to study the V-I characteristics of a p-n diode is shown below: Forward bias Reverse bias Figure 10 For different values of voltages, corresponding values of current are measured under forward bias and reverse bias and a graph is plotted between V and I as given in the figure 11. In forward bias, the diode current increases significantly after a given minimum voltage called threshold voltage or cut-in voltage or knee voltage. (0.2V for Ge and 0.7V for Si) In reverse bias, only reverse saturation current (in µa) is present. This is negligible and hence practically no current flows in the circuit. But beyond a given reverse voltage called break down voltage or Zener voltage, the current suddenly increases. General purpose diodes get destroyed when it crosses this voltage. [thus an ideal diode offers zero resistance at forward bias and infinite resistance at reverse bias.] Dynamic resistance (R d ) is the ratio of the change in voltage to the corresponding change in current. i.e., R d = V/ I. RECTIFICATION It is the process of converting a.c into d.c. a.c Rectifier 1. Half-Wave Rectifier. The following circuit diagram shows the use of a diode (D) as a half-wave rectifier. (figure 12) The role of transformer: (a) It reduces the voltage of a.c. (b) It isolates output from the input. During the positive half cycles of the input a.c at A, the diode is forward biased and hence it conducts. But during the negative half cycles at A, the diode is reverse biased and it doesn t conduct. Thus we get a pulsating d.c across the load resistor (R L ), as shown below: (figure 13) d.c The following points may be noted: To protect diode from reverse break down, the peak ac voltage at the secondary should not exceed the Zener voltage of the diode. The efficiency of a half-wave rectifier is 40% The frequency of the pulsating output dc is the same as that of the input ac.
ELECTRONIC DEVICES - Class XII (Boys Wing) Page 05 2. Full-Wave Rectifier. Two diodes D 1 and D 2 and a centre-tap transformer can be used as a full-wave rectifier, as shown below: (Figure 14) Notes: (a) the efficiency is 80% (b) the frequency of the pulsating dc output is twice that of the input. The output from this rectifier is further given to a filter circuit consisting of a capacitor and a resistor ( Figure 16). This gives out a smoothened dc, similar to that from a battery. (fig 17) The role of centre-tap transformer: It acts as a potential divider It isolates the output from the input It reduces the voltage of a.c During the positive cycles at A, the point B is negative and vice-versa. Thus for positive half cycles at A, D 1 is forward biased and D 2, reverse biased. So, D 1 conducts and D 2 doesn t. But D 2 conducts during the other half cycles. Hence the for both the half-cycles, the current flows through R L in the same direction. i.e, we get a pulsating dc, as shown: (Figure 15) Special Purpose Diodes ZENER DIODE It is a reverse p-n junction with a sharp breakdown voltage. (Figure 18) The depletion layer of a Zener Diode is very thin and the junction field is extremely high, due to heavy doping. At the reverse bias voltage, the feeble current is due to the minority charge carriers. When the voltage becomes equal to Vz, the junction field becomes sufficient to break co-valent bonds, releasing electron-hole pairs. This causes the reverse current to increase to a high value almost insignificant with the change in voltage. The emission of electrons from the host atoms due to the high electric field is known as internal field emission or field ionisation. The circuit symbol of Zener Diode is: Zener Diode as a voltage regulator. A Zener Diode can be used as a voltage regulator using the circuit shown in figure 19 below: Any increase or decrease in the input voltage results in the increase or decrease in the voltage drop across the series resistor R s, without any change in the voltage across the Zener diode. Hence we get a constant output voltage (equal to Zener voltage) across the load resistor R L. The Zener diode and the series resistor Rs should be selected according to the required output voltage.
ELECTRONIC DEVICES - Class XII (Boys Wing) Page 06 Optoelectronic junction devices. The devices in which charge carriers are generated by photons are called optoelectronic devices. (i) Photodiode. It is reverse biased p-n junction used to convert light energy into electrical energy. When light (photons) with energy h ν >Eg fall on a photodiode, then electron-hole pairs are generated due to the absorption of photons (Figure 20). The junction field separates these electrons and holes. This gives rise to an emf. When an external load is connected, current flows. Thus we get an output. This photocurrent is directly proportional to incident light intensity. The V-I graph of a photodiode for different intensities I1, I2, I3 and I4 are given in figure 21. Photo diode is used to detect optical signals. It is in reverse bias because, the fractional change in minority charged carriers is greater than that in majority charged carriers. So, reverse saturation current is easily affected by the photons. So it is easier to observe in the change in current with change in intensity of light. (Refer example.14.6) (ii) Light Emitting Diode (LED) It is a forward biased p-n junction used to convert electrical energy into light energy. In forward bias, during electron-hole combinations, energy is released. In ordinary diodes, this energy is in the invisible region. LED s, made from specially semi-conducting compounds such as Gallium Arsenide-Phosphide, releasing energy in the visible region during forward bias. The energy of photons emitted is nearly equal to the energy gap (E g ) of the diode. In LED s the frequency of emitted photons depends on the energy gap (Eg) of the diode The intensity of the light emitted depends on the forward bias current. LED s are used in remote controls, burglar alarms, optical communication systems etc. Symbol of LED (iii) Solar Cell It is unbiased p-n junction used to convert light energy into electrical energy. When light photons of energy, h > E g, electron-hole pairs are generated. They are further separated by the junction field. Holes are collected at the p-side and electrons at the n-side. This gives rise to a voltage. Hence a photocurrent I L flows in the circuit. (Fig 22) The V-I graph of a solar cell is in the fourth quadrant (Fig 23). This is because the cell does not draw any current, but it supplies current to the load. Advantages of LED s over conventional lamps Can be switched on and off rapidly. Low operational voltage No warm-up time needed Long life ruggedness Notes: (i) Semiconductors with Eg nearly 1.5eV are ideal to make solar cells. It has more solar conversion efficiency (refer Example 14.7). (ii) The important criteria for the selection of a material for solar cell fabrication are (a) band gap (b) high optical absorption (c) electrical conductivity, (d) availability of the raw material and (v) cost. (iii) Solar cells are used to power electronic devices in satellites and also in some calculators.
ELECTRONIC DEVICES - Class XII (Boys Wing) Page 07 TRANSISTORS These are another group of electronic components made from semiconductors. They are of two types:- npn and pnp. The three segments of a transistor are emitter, base and collector. Schematic representation and their symbols are given in figure 24. The essential requirements for the proper functioning of a transistor are: (i) Size:- Collector should be largest. (ii) Doping level:- Emitter should be heavily doped. (iii) Biasing:-The collector-base junction should be reverse biased and the emitter-base junction should be forward biased. Action of a transistor An n-p-n transistor is connected in proper biasing as shown in figure 25: The electrons from the n-side and the holes from the p- side are repelled, respectively, by the negative and positive terminals of the battery. Thus majority of the electrons reach the collector. These excess electrons are attracted by the positive near the collector. This creates collector current (Ic). The deficiency so created at the emitter is compensated by absorbing electrons from the negative of the battery at the emitter side. This leads to emitter current (I E ). A few electron-hole combinations taking place at the base leads to a feeble current from the base, called base current (I B ). Applying Kirchoff s junction rule at O, we get I E = I B + I C. But I B << I E. So, I E I C. Configurations of a transistor. 1. Common Base (CB) configuration 2. Common Emitter (CE) configuration Current gain = I C /I E A.C.Current gain ac = I C / I E Current gain = I C /I B A.C.Current gain ac = I C / I B 3. Common Collector (CC) Configuration Current gain = I B / I E A.C.Current gain = I B / I E It shows that > > Due to the high value of current gain, the CE configuration is widely preferred. Due negligible gain, CC configuration is seldom used in practice. [ ac is also referred as current amplification factor.]
ELECTRONIC DEVICES - Class XII (Boys Wing) Page 08 Characteristics of a transistor. The following circuit diagram (Figure 26) is used to study the characteristics of an n-p-n transistor connected at the common emitter mode. There are two types of characteristics: (i) Input characteristics It is the graph between input current I B and input voltage V BE at a constant value of the output voltage V CE. To plot the graph, different values of I B and the corresponding values of V BE are measured with the help of the rheostat at the input side, keeping V CE the same using the rheostat at the output side. The experiment can be repeated, using different values of V CE. The graph obtained is shown in figure 27 below. Input resistance is the ratio of the change in V BE to the corresponding change in the base current I B. r i = [ V BE / I B ] (i) Output characteristics It is the graph between output current I C and output voltage V CE at a constant value of the base current I B. To plot the graph, different values of I C and the corresponding values of V CE are measured with the help of the rheostat at the output side, keeping I B the same using the rheostat at the input side. The experiment can be repeated, using different values of I B. The graph obtained is shown in figure 28 below. I C (ma) Output resistance is the ratio of the change in V CE to the corresponding change in the collector current I C. r o = V CE / I C V CE {Trans-conductance of a transistor is defined as the ratio of the change in collector current to the corresponding change in base-emitter voltage. i.e. g m = I c / V BE } Transistor as an amplifier Amplification is the process of increasing the strength of weak signals. Amplifier An n-p-n transistor in the common emitter mode can be used as an amplifier as shown in Fig29. The small signal voltage is superposed on the dc bias voltage. This causes variation in the base current. As a result, Ic also varies accordingly, producing corresponding output voltage v o. We can measure the ac variations across the input and output terminals by blocking the dc voltages by large capacitors.
ELECTRONIC DEVICES - Class XII (Boys Wing) Page 09 Assume the case when there is no signal voltage (v i = 0). Applying Kirchoff s rule, to the output loop, Current gain ac = I C / I B V CC = V CE + I C R L..(1) Voltage gain Av = V CE / V BE When there is a signal vi, it can be written as, I V CC = V CE + I C R C RL L = But V CC = 0, as Vcc is a constant (e.m.f) I B Ri Thus we get, V CE = - I C R L (2) = ac R L / Ri We know, ac = I C / I B Power gain = voltage gain x current So, V CE = - ac I B R L gain The input signal causes a variation in I B, which gets = ac R L / Ri x ac multiplied many times and obtained across R L as = 2 ac R L / Ri the values of ac and R L are very high. Further, the negative sign indicates that the output is out of phase ( i.e. phase diff 180 o ) with the input Why is there a phase difference of 180 o between the input and output in the case of a common emitter amplifier? We have, V CE = V CC - I C R L During the positive half cycles of the input signals, the input voltage increases. Thus the input current I B and the output current I C also increases. This will reduce the value of V CE, as V CC is a constant. So the out becomes negative. During the negative half cycles of the input, the input voltage decreases and thereby Ic also decreases. This will increase V CE. So, the output becomes positive. This shows that there is a phase difference of 180 o ( ) exists between the input and the output. Saturation State: We have, V CE = V CC - I C R L For large values of I C, I C R L can be greater than V CC. So V CE may become negative. Thus the output side of the transistor becomes improperly biased. Hence it stops working. This state is called saturation state. Cut-off state When the negative part of the input signals is dominating at the input voltage, the input of the transistor becomes improperly biased. Again the transistor becomes improperly biased. At this stage, the transistor stops working and is called saturation state. The cut-off region and saturation region of a transistor are represented in the output characteristics as shown below (Figure 31). I c The region between saturation and cut-off is the region where the transistor actually works. This region is called active region. V CE
ELECTRONIC DEVICES - Class XII (Boys Wing) Page10 Transfer characteristics In the given circuit of a transistor in the common emitter mode (Fig32), V CE = V CC - I C R L i.e. Vo = V CC - I C R L...(1) Let Vi be the input dc voltage. The transfer characteristics of the transistor is shown in figure 33 When Vi is low, Ic = 0. So Vo is high. i.e.vo = Vcc So transistor enters cut-off state. When Vi is high, Ic is large and hence it enters saturation state. We define low and high states to the cut-off and saturation states respectively. LOGIC GATES These are the circuits that follow definite logic relationship between the inputs and outputs of the circuits. Fundamental gates 1. OR gate Meaning Logic Symbol Truth Table Boolean Expression A + B = Y Realisation 2. AND gate Meaning Logic Symbol Truth Table Boolean Expression A. B = Y 3. NOT gate It produces an inverted version of the input at its output. This is why it is also known as an inverter. Truth Table Boolean Expression Realisation Logic Symbol A = Y
ELECTRONIC DEVICES - Class XII (Boys Wing) Page12 Combination of gates 1. NOR gate In this, the output of the OR gate is fed to the input of the NOT gate. Logic Symbol Truth Table Boolean Expression A + B = Y 2. NAND gate In this, the output of the AND gate is fed to the input of the NOT gate. Logic Symbol Truth Table Boolean Expression A. B = Y NAND A a universal gate NAND gates can be used to make even the fundamental gates as shown below. Hence any required input-output combinations are possible with NAND gates. Hence NAND gates are called universal gates. NAND as NOT NAND as AND NAND as OR Note: Similarly NOR gates can also be called universal gates, as they can also be used to make fundamental gates. (Refer Exercises 14.18 & 14.19) A question from the CBSE sample paper-2008 Input signals A and B are applied to the input terminals of the dotted box set-up shown here. Let Y be the final output signal from the box. Draw the wave forms of the signals labelled as C1 and C2 within the box, giving (in brief) the reasons for getting these wave forms. Hence draw the wave form of the final output signal Y. Give reasons for your choice. What can we state (in words) as the relation between the final output signal Y and the input signals A and B?