Class Physics English Volume

) . That is, r V F F F = ∆ ∆ (iv) Thus the diode behaves as a conductor when it is forward biased. However, if the applied voltage is increased beyond a rated value, it will produce an extremely large current which may destroy the junction due to overheating. This is called as the breakdown of the diode and the voltage at which the diode breaks down is called the breakdown voltage.

Thus, it is safe to operate a diode between the threshold voltage and the breakdown voltage. ii) Reverse characteristics The circuit to study the reverse characteristics is shown in Figure . (a). In the reverse bias, the p -side of the diode is connected to the negative terminal and n -side to the positive terminal of the dc power supply.

A graph drawn between the reverse bias voltage and the current across the junction is called the reverse characteristics of a p-n junction diode. It is shown in Figure . (b). Under this bias, a very small µA ( –15V) – – – V R (a) D Reverse bias Reverse breakdown voltage (b) I F (mA) – I R ( µ A) – V R (V) Figure .

p-n junction diode (a) Diode under reverse bias (b) Reverse characteristics current in µA flows across the junction. This is due to the flow of the minority charge carriers and is called t he leakage current or reverse saturation current. This reverse current is independent of the voltage up to a certain voltage, known as breakdown voltage. Ideal diode: It acts like a conductor when it is forward biased.

When it is reverse biased, it acts like an insulator. For ideal diodes, the forward resistance is zero and barrier potential is considered negligible. Note The forward and reverse charactristics are given in one graph as shown in Figure . .

Unit electronics and Communication EXAMPLE . A silicon diode is connected with 1kΩ resistor as shown. Find the value of current flowing through AB . 1kΩ – .

V + . V Solution The P.D. between A and B is given by V = [ V A – V B ]– V b (Si) = [ . – (– .

)] – . = . – . = V The value of current flowing through AB can be obtained using Ohm’s law.

V R A mA . . Rectification The process in which alternating voltage or alternating current is converted into direct voltage or direct current is known as rectification . The device used for this process is called as rectifier.

In this section, we will discuss two types of rectifiers namely, half wave rectifier and full wave rectifier i) Half wave rectifier circuit The half wave rectifier circuit consists of a transformer, a p-n junction diode and a resistor (Figure . (a)). In a half wave rectifier circuit, either a positive half or the negative half of the AC input is passed through by the diode while the other half is blocked. Only one half of the input wave is rectified.

Therefore, it is called half wave rectifier. Here, a p-n junction diode acts as a rectifier diode. During the positive half cycle When the positive half cycle of the AC input signal passes through the circuit, EXAMPLE . An ideal diode and a Ω resistor are connected in series with a V power supply as shown in figure below.

Calculate the current that flows through the diode. 5Ω 15V – Solution The diode is forward biased and it is an ideal one. Hence, it acts like a closed switch with no barrier voltage. Therefore, current that flows through the diode can be calculated using Ohm’s law.

V IR V R 3A I F (mA) – I R ( µ A) Forward bias Reverse bias – V R (V) V F (V) Figure . Forward and reverse charactristics of a diode Unit electronics and Communication The output of the half wave rectifier is not a steady DC voltage but a pulsating wave. This pulsating voltage cannot be used for electronic equipments. A constant or a steady voltage is required which can be obtained with the help of filter circuits and voltage regulator circuits.

Efficiency (η) is the ratio of the output DC power to the AC input power supplied to the circuit . Its value for half wave rectifier is . %. If the direction of the diode is reversed, the negative half of the AC signal is passed through and the positive half is blocked.

Note ii) Full wave rectifier The positive and negative half cycles of the AC input signal are rectified in this circuit and hence it is called the full wave rectifier. The circuit is shown in Figure . (a). It consists of two p-n junction diodes, a centre tap transformer and a load resistor R L .

The centre is usually taken as the ground or zero voltage reference point. With the help of the centre tap transformer, each diode rectifies one half of the total secondary voltage. During positive half cycle When the positive half cycle of the AC input signal passes through the circuit, terminal M is positive, C is at zero potential and N is at negative potential. This forward biases diode D and reverse biases diode D .

Hence, being forward biased, diode D conducts and current flows along the path MD ABC During negative half cycle When the negative half cycle of the AC input signal passes through the circuit, terminal N becomes positive, C is at zero terminal A becomes positive with respect to terminal B . The diode is forward biased and hence it conducts. The current flows through the load resistor R L and the AC voltage developed across R L constitutes the output voltage V and the waveform of the output voltage is shown in Figure . (b).

During the negative half cycle When the negative half cycle of the AC input signal passes through the circuit, terminal A is negative with respect to terminal B . Now the diode is reverse biased and does not conduct. Hence no current passes through R L . The reverse saturation current in a diode is negligible.

Since there is no voltage drop across R L , the negative half cycle of AC supply is suppressed at the output. AC Input Diode R L V o – (a) (b) v i v o time time Figure . (a) Half wave rectifier circuit (b) Input and output waveforms Unit electronics and Communication Centre tap transformer : There is a facility to tap at halfway point in the secondary windings. This helps to measure the induced voltage from one end of the secondary to the centre point.

If the centre tap point is grounded, then the voltage across the secondary will be divided into two halves. For example, if the voltage across the secondary is V, then the voltage across one end and the centre tap point is + V and at the other end it is – V. Note . .

Breakdown mechanism The reverse current or the reverse saturation current due to the minority charge carriers is small. If the reverse bias applied to a p-n junction is increased beyond a point, the junction breaks down and the reverse current rises sharply. The voltage at which breakdown happens is called the breakdown voltage and it depends on the width of the depletion region, which in turn depends on the doping level. A normal p-n junction diode gets damaged at this point.

Specially designed diodes like Zener diode can be operated at this region and can be used for the purpose of voltage regulation in circuits. There are two mechanisms that are responsible for breakdown under increasing reverse voltage. i) Avalanche breakdown Avalanche breakdown occurs in a lightly doped junctions which have wide depletion region. When reverse bias voltage exceeds a certain value, the minority charge carriers are accelerated by reverse voltage and their kinetic energy increases.

These charge carriers collide potential and M is at negative potential. This forward biases diode D and reverse biases diode D . Hence, being forward biased, diode D conducts and current flows along the path ND ABC During both postive and negative half cycles of the input signal, the current flows through the load in the same direction. The output signal corresponding to the input signal is shown in Figure .

(b). Though both half cycles of AC input are rectified, the output is still pulsating in nature. The efficiency (η) of full wave rectifier is twice that of a half wave rectifier and is found to be . %.

It is because of power losses in the winding, the diode and the load resistance. M N R L – D D Current flow during positive half cycle Current flow during negative half cycle (a) C AC Input V o (b) v i v o time time Figure . (a) Full wave rectifier circuit (b) Input and output waveforms Unit electronics and Communication . .

Zener diode Zener diode is a heavily doped silicon diode used in reverse biased condition and is named after its inventor Clarence Melvin Zener. It is specially designed to be operated in the breakdown region. The doping level of the silicon diode can be varied to have a wide range of breakdown voltages from V to over V. As explained in the previous section, Zener breakdown occurs due to the breaking up of covalent bonds by the strong electric field set up in the depletion region by the reverse voltage.

It produces an extremely large number of electrons and holes which constitute the reverse saturation current. The current is limited by both external resistance and power dissipation of the diode. A Zener diode is shown in Figure . (a) and its circuit symbol is given in Figure .

(b). It looks like an ordinary p-n junction diode except that n -side lead resembles the shape of the letter ‘z’. The arrow head points the direction of conventional current. In Figure .

(a), black ring indicates the n -side lead. Figure . Zener diode (a) Commercial picture (b) Circuit symbol V-I Characteristics of Zener diode The circuit to study the forward and reverse characteristics of a Zener diode is shown in Figure . (a) and Figure .

(b). The V-I characteristics of a Zener diode is shown in Figure . (c). The forward characteristic of a Zener diode is similar to that of an ordinary p-n junction diode.

It starts conducting approximately around . V. However, the reverse characteristics is highly significant in Zener diode. The increase in reverse voltage with semiconductor atoms while passing through the depletion region.

This leads to the breaking up of covalent bonds and this results in the generation of electron - hole pairs. The newly generated charge carriers are also accelerated by the reverse voltage resulting in more collisions and further production of charge carriers. This cumulative process leads to an avalanche (uncontrollably large number) of charge carriers across the junction. This causes diode current to rise abruptly and breakdown takes place.

This breakdown is called avalanche breakdown. ii) Zener breakdown Heavily doped p-n junctions have narrow depletion layers whose width is of the order of < – m. When reverse voltage across this junction is increased to the breakdown limit, a very strong electric field of strength × V m – is set up across the narrow layer. This electric field is strong enough to break or rupture the covalent bonds in the lattice and thereby generating electron-hole pairs.

This effect is called Zener effect . Even a small further increase in reverse voltage produces a large number of charge carriers which move across the junction through the thin depletion region. This process gives rise to a large amount reverse current or breakdown current and this breakdown is called Zener breakdown. In Avalanche breakdown, the minority charge carriers gain sufficient energy from excessive reverse bias voltage to break covalent bond in order to produce new charge carriers.

But Zener breakdown occurs due to the direct rupture of covalent bonds because of the existence of the strong electric field. Since depletion region is thin, Zener breakdown occurs usually at lesser reverse bias voltage compared to Avalanche breakdown voltage. Note Unit electronics and Communication • Maximum current limited by maximum power dissipation, I Z max The Zener diode is operated in the reverse bias condition with the voltage greater than V Z and current less than I Z(max) . The reverse characteristic is not exactly vertical which means that the diode possesses some small resistance called Zener dynamic impedance.

Zener resistance is the inverse of the slope of the curve in the breakdown region. It means an increase in the Zener current produces only a very small increase in the reverse voltage. However this can be neglected. The voltage of an ideal Zener diode does not change once it goes into breakdown.

In other words, V Z remains almost constant even when I Z increases considerably . The maximum reverse bias that can be applied before entering into the Zener region is called the peak inverse voltage, commercially referred as PIV rating. Note Applications The zener diode can be used • as voltage regulator • for calibrating voltages • to provide fixed reference voltage in a network for biasing • to protect of any gadget against damage from accidental application of excessive voltage. Zener diode as a voltage regulator Zener diode working in the breakdown region can serve as a voltage regulator whose circuit diagram is given in Figure .

. A series resistance R s of suitable value is used to limit the Zener current to avoid any damage to the diode. This resistance also plays a role in voltage regulation. The fluctuating DC normally generates very small reverse current.

While in Zener diode, when the reverse voltage is increased to the breakdown voltage ( V Z ), the increase in current is very sharp. The voltage remains almost constant throughout the breakdown region. In Figure . (c), I Z(max) represents the maximum reverse current.

If the reverse current is increased further, the diode will be damaged. The important parameters of the reverse characteristics are • Zener breakdown voltage, V Z • Minimum current to sustain breakdown, I Z min I z(min) I z(max) – V Z Forward bias region Constant zener voltage Zener breakdown region Knee voltage (c) I F (mA) – I R ( µ A) – V R (V) V F (V) mA (a) ( –12V) V – – – – R D µ A ( –15V) V – – – – R D (b) Figure . Zener diode (a) Forward bias (b) Reverse bias (c) V-I characteristics Unit electronics and Communication EXAMPLE . Find the current through the Zener diode when the load resistance is kΩ.

Use diode approximation. V Z = V R L = 2kΩ R S = kΩ I L V Solution Voltage across AB, V Z = 9V Voltage drop across R S = – = 6V Therefore current through the resistor R S , I = × mA Voltage across the load resistor, V AB V = Current through load resistor, V R L AB L = × . mA The current through the Zener diode, Z L = − mA mA mA . .

Optoelectronic devices Optoelectronics deals with devices which convert electrical energy into light and light into electrical energy using semiconductors. Optoelectronic device is an electronic device which utilizes light for useful applications. We will discuss some important optoelectronic devices namely, light emitting diodes, photo diodes and solar cells. i) Light Emitting Diode (LED) LED is a p-n junction diode which emits visible or invisible light when it is input voltage is applied to the circuit and constant output voltage V o is taken across the load resistance R L which is connected in parallel with Zener diode.

The output voltage is maintained constant as long as the input voltage is greater than V Z . V i R s I L R L I z V V – o z Figure . Circuit to study voltage regulation by Zener diode If the input DC voltage is increased, the Zener current increases thereby increasing current through R s and the voltage drop across R s is also increased. The increased current flows through the diode without affecting the I L .

Since Zener diode is operated in the breakdown region, the Zener breakdown voltage across the diode is nearly constant even though the reverse bias current through the diode increases considerably. The increase in input voltage is dropped across R s and hence it is also called dropping resistance. Because of the parallel connection, the voltage across R L is also equal to Zener breakdown voltage which is taken as constant output voltage V . If the input DC voltage is decreased, the diode takes a smaller current and the voltage drop across R s is reduced.

Thus, the output voltage V remains constant. To sum up, if there is any change in input voltage, the voltage drop across R s changes accordingly. But the voltage across Zener diode or voltage across R L remains constant. Thus the Zener diode acts as a voltage regulator.

Unit electronics and Communication The colour of the light is determined by the energy band gap of the material. Therefore, LEDs are available in a wide range of colours such as blue (SiC), green (AlGaP) and red (GaAsP). Now a days, LED which emits white light (GaInN) is also available. Applications The light emitting diodes are used in · indicator lamps on the front panel of the scientific and laboratory equipments.

· seven-segment displays. · traffic signals, emergency vehicle lighting etc. · remote control of television, air­ conditioner etc. EXAMPLE .

Determine the wavelength of light emitted from LED which is made up of GaAsP semiconductor whose forbidden energy gap is . eV. Mention the colour of the light emitted (Take h = . × – Js).

Solution E hc hc E g g × × λ λ Therefore, . = nm The wavelength nm corresponds to red colour light. ii) Photodiodes A p-n junction diode which converts an optical signal into electric signal is known as photodiode. Thus, the operation forward biased.

Since electrical energy is converted into light energy, this process is also called electroluminescence. The circuit symbol of LED is shown in Figure . (a). The direction of arrows indicates that light is emitted from the diode.

(a) p – P-type N-type Hole Light Electron Conduction band E g Valence band Recombi- nation (b) Figure . (a) Circuit symbol of LED (b) Schematic diagram to explain recombination process When the p-n junction is forward biased, the conduction band electrons on n -side and valence band holes on p -side diffuse across the junction. When they cross the junction, they become excess minority carriers (electrons in p -side and holes in n -side). These excess minority carriers recombine with oppositely charged majority carriers in the respective regions, i.e.

the electrons in the conduction band recombine with holes in the valence band as shown in the Figure . (b). During recombination process, energy is released in the form of light (radiative) or heat (non-radiative). For radiative recombination, a photon of energy hv is emitted.

For non-radiative recombination, energy is liberated in the form of heat. Unit electronics and Communication This reverse current in the absence of any incident light is called dark current and is due to the thermally generated minority carriers. Applications The photodiodes are used in · alarm system · count items on a conveyor belt · photoconductors · compact disc players, smoke detectors · medical applications such as detectors for computed tomography etc. iii) Solar cell A solar cell, also known as photovoltaic cell, works on the principle of photovoltaic effect .

Accordingly, the p-n junction of the solar cell generates emf when solar radiation falls on it. The construction details and cross-sectional view are shown in Figure . . In a solar cell, electron–hole pairs are generated due to the absorption of light photons near the junction.

Then the charge carriers are separated due to the electric field of the depletion region. Electrons move towards n –type silicon layer and holes move towards p -type silicon layer. The electrons reaching the n -side are collected by the front contact (metal finger contact) and holes reaching p -side are collected by the back of photodiode is exactly inverse to that of an LED. Photodiode works in reverse bias condition.

Its circuit symbol is shown in Figure . (a). The direction of arrows indicates that the light is incident on the photodiode. The device consists of a p-n junction semiconductor made of photosensitive material kept safely inside a plastic case as shown in Figure .

(b). It has a small transparent window that allows light to be incident on the p-n junction. Photodiodes can generate current when the p-n junction is exposed to light and hence are called as light sensors. (a) p (b) Figure .

(a) Circuit symbol (b) Schematic view of photodiode When a photon of sufficient energy ( hv ) strikes the depletion region of the diode, some of the valence band electrons are elevated into conduction band, in turn holes are developed in the valence band. This creates electron-hole pairs. The amount of electron-hole pairs generated depends on the intensity of light incident on the p-n junction. These electrons and holes are swept across the p-n junction by the electric field created by reverse voltage before recombination takes place.

Thus, holes move towards the p -side and electrons towards the n -side. When the external circuit is made, the electrons flow through the external circuit and constitute the photocurrent. When there is no incident light, there exists a reverse current which is negligible. Figure .

Cross-sectional view of a solar cell Sunlight Front electrical contacts Anti-reflective coating n -side Back electrical contact p -side Depletion region – – – – – – – Unit electronics and Communication a p -type material sandwiched between two n -type materials (NPN transistor). To protect it against moisture, it is sealed inside a metal or a plastic case. The two types of transistors with their circuit symbols are shown in Figure . .