1. Electronics
A device whose functioning is based on controlled movement of electrons through it is called an electronic device. Some of the present-day most common such devices include a semiconductor junction diode, a transistor and integrated circuits. The related branch in which we study the functioning and use of such devices is called Electronics.
2. Energy Bands in Solids
An isolated atom has well defined energy levels. However, when large number of such atoms get together to form a real solid, these individual energy levels overlap and get completely modified. Instead of discrete value of energy of electrons, the energy values lie in a certain range. The collection of these closely packed energy levels are said to form an energy band. Two types of such bands formed in solids are called Valence Band and Conduction Band. The band formed by filled energy levels is known as Valence Band whereas partially filled or unfilled band is known as Conduction Band. The two bands are generally separated by a gap called energy gap or forbidden gap. Depending upon the size of this energy gap, different materials behave as conductors, semi-conductors or insulators. The insulators have generally large energy gap whereas the conductors do not have any such gap. Semi-conductors have small energy gap.
3. Types of Semi-conductors—Intrinsic and Extrinsic
Common Semiconductors are of two types—intrinsic and extrinsic. Germanium and silicon are two most commonly used semiconductor material.
Intrinsic Semiconductor: Pure semiconductors is in which the conductivity is caused due to charge carriers made available from within the material are called intrinsic semiconductors. There are no free charge carriers available under normal conditions. However, when the temperature is raised slightly, some of the covalent bonds in the material get broken due to thermal agitation and few electrons become free. In order to fill the vacancy created by absence of electron at a particular location, electron from other position move to this location and create a vacancy (absence of electron) at another place called hole. The movement/shifting of electrons and holes within the material results in conduction.
An intrinsic semiconductors behaves as a perfect insulator at temperature 0 K.
Extrinsic semiconductors: The semiconductors in which the conductivity is caused due to charge carriers made available from external source by adding impurity from outside are called extensive extrinsic semiconductor. The process of adding impurity is called doping. The impurity added is generally from third group or fifth group. There are two types of extrinsic semiconductors:
(a) n–Type or (b) p–Type.
If ni is the density of intrinsic charge carriers, ne and nh are densities of electrons and hole in extrinsic semiconductors, then the selection among them is
(a) n-type semiconductors: When a pentavalent impurity like
Phosphorus, Antimony, Arsenic is doped in pure-Germanium (or Silicon), then the conductivity of crystal increases due to surplus electrons and such a crystal is said to be n-type semiconductor, while the impurity atoms are called donors atoms. Thus, in n-type semiconductors the charge carriers are negatively charged electrons and the donor level lies near the bottom of the conduction band.
(b) p-type semiconductors: When a trivalent impurity like Aluminium, Indium, Boron, Gallium, etc., is doped in pure Germanium (or silicon), then the conductivity of the crystal increases due to deficiency of electrons i.e., holes and such a crystal is said to be p–type semiconductor while the impurity atoms are called acceptors. Thus in p–type semiconductors the charge carriers are holes. Acceptor level lies near the top of the valence band.
4. Semiconductor Diode: p-n Junction Diode
A semiconductor having p-type impurity at one end and n-type impurity at the other end is known as p – n junction diode. The junction at which p-type and n-type semiconductors combine is called p-n junction.
In p-type region there is majority of holes and in n-type region there is majority of electrons.
Formation of Depletion Layer and Potential Barrier
At the junction, there is diffusion of charge carriers due to thermal agitation; therefore some of electrons of n-region diffuse to p-region while some of holes of p-region diffuse into n-region. Some charge carriers combine with opposite charges to neutralise each other.
Thus, near the junction there is an excess of positively charged ions in n-region and an excess of negatively charged ions in p-region. This sets up a potential difference called potential barrier and hence an internal electric field Ei across the junction. The potential barrier is usually of the order of µV. The field Ei is directed from n-region to p-region. This field stops the further diffusion of charge carriers. Thus the layers (≈10–4 cm to 10–6 cm) on either side of the junction becomes free from mobile charge carriers and hence is called the depletion layer. The symbol of p-n junction diode is shown in figure.
Forward and Reverse Bias
The external battery is connected across the junction in the following two ways:
(i) Forward Bias: In this arrangement the positive terminal of battery is connected to p-end and negative terminal to n-end of the crystal, so that an external electric field E is established directed from p to n-end to oppose the internal field Ei. Thus, the junction is said to conduct.
Under this arrangement the holes move along the field E from p-region to n-region and electrons move opposite to field E from n-region to p-region; eliminating the depletion layer. A current is thus set up in the junction diode. The following are the basic features of forward biasing:
(a) Within the junction diode the current is due to both types of majority charge carriers but in external circuit it is due to electrons only.
(b) The current is due to diffusion of majority charge carriers through the junction and is of the order of milliamperes.
(ii) Reverse Bias: In this arrangement the positive terminal of battery is connected to n-end and negative terminal to p-end of the crystal, so that the external field is established to support the internal field Ei as shown in fig. Under the biasing the holes in p-region and the electrons in n-region are pushed away from the junction to widen the depletion layer and hence increases the size of the potential barrier, therefore, the junction does not conduct.
When the potential difference across the junction is increased in steps, a very small reverse current of the order to micro-amperes flows. The reason is that due to thermal agitation some covalent bonds of pure semi-conductor break releasing a few holes in n-region and a few electrons in p-region called the minority charge carriers. The reverse bias opposes the majority charge carriers but aids the minority charge carriers to move across the junction. Hence a very small current flows.
The basic features of reverse bias are:
(a) Within the junction diode the current is due to both types of minority charge carriers but in external circuit it is due to electrons only.
(b) The current is due to leakage of minority charge carriers through the junction and is very small of the order of µA.
Characteristics of a p–n junction diode:
The graph of voltage V versus current I in forward bias and reverse bias of a p–n junction is shown in the figure.
Avalanche Break Down:
If the reverse bias is made sufficiently high, the covalent bonds near the junction break down releasing free electrons and holes. These electrons and holes gain sufficient energy to break other covalent bonds. Thus a large number of electrons and holes get free. The reverse current increases abruptly to high value. This is called avalanche break down and may damage the junction.
5. p-n Junction Diode as a Half-wave Rectifier
The conversion of ac into dc is called the rectification.
Half Wave Rectifier: The circuit diagram for junction diode as half wave rectifier is shown in
fig. (a)
During first half of the input cycle, the secondary terminal S1 of transformer be positive relative to S2 then the junction diode is forward biased. Therefore, the current flows and its direction of current in load resistance RL is from A to B. In next half cycle, the terminal S1 becomes negative relative to S2, then the diode is in reverse bias, therefore no current flows in diode and hence there is no potential difference across load RL. The cycle repeats. The output current in load flows only when S1 is positive relative to S2 That is during first half cycles of input ac signal there is a current in circuit and hence a potential difference across load resistance RL while no current flows, for next half cycle. The direction of current in load is always from A to B which is direct current. Thus, a single p-n junction diode acts as a half wave rectifier.
The input and output waveforms of half wave rectifier are shown in fig. (b).
Full Wave Rectifier: For full wave rectifier, we use two junction diodes. The circuit diagram for full wave rectifier using two junction diodes is shown in figure.
During first half cycle of input ac signal the terminal S1 is positive relative to S and S2 is negative relative to S, then diode D1 is forward biased and diode D2 is reverse biased. Therefore current flows in diode D1 and not in diode D2. The direction of current i1 due to diode D1 in load resistance RL is directed from A to B. In next half cycle, the terminal S1 is negative relative to S and S2 is positive relative to S. Then diode D1 is reverse biased and diode D2 is forward biased. Therefore, current flows in diode D2 and there is no current in diode D1. The direction of current i2 due to diode D2 in load resistance is again from A to B Thus, for input ac signal the output current is a continuous series of unidirectional pulses. The input and output sequels are shown in the figure. This output current can be converted into steady current by the use of suitable filters.
Remark: In full wave rectifier if the fundamental frequency of input ac signal is 50 Hz, then the fundamental frequency of output is 100 Hz.
6. Light Emitting Diode (LED)
The light emitting diode, represented by either of the two symbols shown here, is basically the same as a conventional p-n junction diode. Its actual shape is also shown here. The shorter, of its two leads, corresponds to its n (or cathode side) while the longer lead corresponds to its p (or anode side).
The general shape of the I-V characteristics of a LED, is similar to that of a conventional p-n junction diode as shown in the figure. However, the &aposbarrier potential&apos changes slightly with the colour.
The colour of the light emitted, by a given LED, depends in its band-gap energy. The energy of the photons emitted is equal to or slightly less than this band gap energy. The other main characteristic of the emitted light, its intensity, is determined by the forward current conducted by the junction.
7. Photodiode
A photodiode is a junction diode fabricated by using a photo sensitive semiconductor material. When light of suitable frequency is made to fall on the junction, it starts conducting.
(i) A photodiode is used in reverse bias, although in forward bias current is more than current in reverse bias because in reverse bias it is easier to observe change in current with change in light intensity.
(ii) Photodiode is used to measure light intensity because reverse current increases with increase of intensity of light. The characteristic curves of a photodiode for two different illuminations I1 and I2 (I2 >I1) are shown in fig. (c).
8. Solar Cell
A solar cell is a junction diode which converts light energy into electrical energy. It is based on photovoltaic effect. The surface layer of p-region is made very thin so that the incident photons may easily penetrate to reach the junction which is the active region. In an operation in the photovoltaic mode (i.e., generation of voltage due to bombardment of optical photons); the materials suitable for photocells are silicon (Si), gallium arsenide (GaAs), cadmium sulphide (CdS) and cadmium selenide (CdSe).
Working: When photons of energy greater than band gap energy (hν>Eg) are made to incident on the junction, electron-hole pairs are created which move in opposite directions due to junction field. These are collected at two sides of junction, thus producing photo-voltage; this gives rise to photocurrent. The characteristic curve of solar cell is shown above. Solar cells are used in satellites to recharge their batteries.
9. Zener Diode
A zener diode is a specially designed heavily doped p-n junction, having a very thin depletion layer and having a very sharp breakdown voltage. It is always operated in reverse breakdown region. Its breakdown voltage VZ is less than 6 V. The symbol of Zener diode is
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