Learning Objectives:
By the end of this topic the learner should be able to:
a) state the differences between conductors and insulators;
b) define intrinsic and extrinsic semi-conductors;
c) explain doping in semi-conductors;
d) explain the working of a p-n junction diode;
e) sketch current-voltage characteristics for a diode;
f) explain the application of diodes in rectification.
Introduction
Electronics deals with electric circuits and their wide applications.
Energy Band Theory
Electrons revolve around the nucleus in orbits called energy levels. The nature of electrons determines the associated magnetic and electric field of the atom. According to the band theory, when two atoms are brought closer, their fields interact, splitting up each into energy levels called bands. There are two important bands in electronics;valence and conduction bands.
The energy levels and bands
We use energy band theory to differentiate the three. Conduction band contains conduction electrons while valence band contains valence electrons. The gap between these bands is called forbidden energy gap. The forbidden energy gap distinguishes insulators, conductors and semiconductors.
INSULATORS
Has a large energy gap of approximately 3eV between empty conduction(no electrons) band and completely filled valence band. Has a high resistance to the flow of electric current. Tightly bound electrons in the valence band requires large energy to dislodge them and make them cross energy gap to conduction band. Increase in temperature and addition of impurities has no effect on the conductivity of insulators.
Conductors
Have many electrons in the conduction band. Valence and conduction bands overlap, electrons move freely from partly filled valence band to conduction band. Increase in temperatures increases resistance of conductors as it causes atom vibration that interferes with movement of electrons
The conductor
Semiconductors
Their conductivity lies between those of conductors and insulators.eg. Germanium, selenium and silicon. The forbidden energy gap is smaller as compared to that of insulators.
Semiconductor
Conduction band has no electrons while valence band if filled completely. Increase in temperature reduces the forbidden energy gap; for electron to cross over the energy gap to reach the conduction band. A hole(absence of electron) is created. A hole is an electron behaving as if it’s a positive charge. A hole moves generating a hole current. Conventional current due to flow of electrons in conduction band is in the same direction as the hole current. Total current is due to electrons and hole current. Electrical resistance of semiconductors reduces with increase in temperature.
Types of Semiconductor
There are two types:
a)Intrinsic semiconductor.
b)Extrinsic semiconductor
Intrinsic Semiconductor
This is pure semiconductor.eg. Silicon and germanium. The outermost shell has 4 electrons.
At zero kelvin, the crystal is an insulator. At room temperature some electrons in valence band gain energy and jump into conduction band leaving behind holes. The material becomes a conductor. High temperature causes more conductivity of the material. The electrons and holes are called charge carriers.
Extrinsic Semiconductor
These are obtained by chemically adding impurities in the intrinsic semiconductors. This process is called Doping. There are two types of extrinsic semi-conductors.
a)The n- type semiconductor.
b)The p-type semiconductor.
The n-type semiconductor :
Obtained by doping an intrinsic semiconductor with pentavalent atom(group -5 element)i.e. phosphorous
The dopant has five electrons, four of which contribute in forming covalent bonds with tetravalent atoms. The remaining electrons is for electrical conductivity. The pentavalent atom(donor or n-type impurities). The electrons are the majority charge i.e. n-type.
The P-type semiconductor :
Holes are the majority charge carriers. Obtained by doping an intrinsic semiconductor with a trivalent atom e.g. Boron. Boron has three electrons in the valence band. Therefore Boron will have one electron less to complete bonding.
The p-type semiconductor.
The vacant place due to a missing electron called a hole. Holes are majority charge carriers(positive charge). These group three atoms creates holes which can accept electrons; they are acceptor atoms.
The p- n junction
This is doping of an intrinsic semiconductor such that half of it becomes an n-type and the other half an p-type . The junction is called p-n junction between the layers.
P-n junction
The free electrons and holes near the junction diffuses across i.e. electrons enter the p-zone as holes into the n-zone.
Recombination of mobile charge carriers takes place on either side of the junction; thus depleting mobile charge carriers within a region (10^-4 to 10^-6 ).
The formation of uncovered fixed ions on either side of the junction stops further diffusion of mobile charge. The region occupied by fixed ions called depletion layer(0.3V for germanium and 0.7V for silicon). Fixed ions set up the potential barrier(VB )
This hill stops holes and electrons to cross.
Biasing the p-n junction
A p-n junction is said to be biased when the potential difference is applied across it. There are two types of biasing:
a) forward biasing
b )reverse biasing
Forward biasing
The p-type is connected to the positive and n- type is connected to the negative terminal of the external cell. Before biasing, the potential barrier Vb exists which opposes diffusion of holes and electrons across the junction. The voltage Ve connected overcomes potential barrier V.
This repels holes from p- type and electrons from n-type across the junction. This reduces the depletion layer enabling more charges to flow across the junction.
Reverse Biasing
The p-region is connected to the negative terminal of the cell and n-region is connected to the positive terminal. The electrons and holes are attracted away from the junction by Ve. This increases the depletion layer, which increases the potential barrier and blocks the flow of current.
Although small current called leakage current flows across the junction due minority charge carriers.
The p-n Junction DiodeOne-way device consisting of a p-n junction and having two terminals i.e. anode and cathode.
The arrow indicates the direction of conventional current when diode is forward biased.
Diode Characteristics The set-up below can be used to investigate the characteristics of a p-n junction. Objective: To study the forward and reverse characteristics of p-n junction diode.
Apparatus: Diode, potential divider, ammeter, voltmeter, cell.
The graph of I(mA) against VB (v) is shown below.
The diode is non-Ohmic i.e. is not linear. As voltage is increased from zero , a very small current flows through. This is because the potential barrier opposes the forward bias voltage. The current increases rapidly when potential barrier is overcome at bias voltage called the cut-in/ threshold /break-point voltage.There is a sharp increase in the forward current because of increased kinetic energy of the electrons and holes i.e. rapturing of the charge carrier (covalent bonds) producing more electron- hole pairs.
REVERSE CURRENT CHARACTERISTICS
When voltage is zero, leakage current Ia flows due to minority charge. There is no significant change in the current when the voltage is increased. At a reverse bias voltage called zener or breakdown voltage current suddenly flows. In this case resistance is zero and further increases in reverse bias voltage blow out the diode.
Zener Diode
It is designed to operate within a breakdown region. It is correctly placed when it is reversed biased with a voltage greater than the zener and current less than the maximum value it can safely accommodate. The symbol for the zener diode is shown below.
The combined characteristic of the diode is as shown below.
APPLICATIONS OF JUNCTION DIODES
Diode as a rectifier. A rectifier is a device used to transform an a.c voltage into d.c (unidirectional) voltage.
Types Of Rectification
There are two types:
a) Half – wave rectification.
b) Full -wave rectification.
Half- Wave Rectification
Only one diode is used in series with the load across which a dc voltage is required.
During the 1st cycle(positive) cycle, the diode is forward biased, so it conducts. Current flows through RL . During the 2nd cycle(negative) the diode is reversed biased, so it does not conduct. This process repeats itself so long as the input voltage is being supplied. The CRO display for alternating voltage and half- wave rectification as shown below.
It is half-wave because half of the input sinusoidal wave is phased out in the output. There is power loss due to elimination of one cycle and also the output is not smooth.
Full – wave Rectification
Achieved using either:
a) 2 diodes and centre- tap transformer or
b) 4 diodes (bridge rectifiers).
Using two Diode
The two diodes should conduct a current through the load resistor during a particular half- cycle.
During the 1st half-cycle D1 is forward biased while D2 is reversed biased. Path of current AD1BCA. During the 2nd half-cycle D2 is forward biased while D1 is reversed biased. The path of current DD2BCD.
The output signal is as shown below.
Bridge Rectification
Has four diodes.
During the 1st half cycle, A is positive w.r.t to C i.e. D1 and D3 are forward biased and conducts. Path of current ABDCA. During the 2nd cycle, point A becomes negative w.r.t to C. Diode D2 and D4 forward biased and conduct. Path of the conventional current CBDAC.
ADVANTAGES OF BRIDGE RECTIFIER
a) Smaller transformer can be used because there is no need for centre tapping.
b) Suitable for high voltage regulation(gives a stronger output than half-wave rectification)
If the capacitor is connected across the resistor as shown, the rectified output is smoothed.
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