electronic properties
DESCRIPTION
A set of slides created to teach Electronic Properties to learners at Bishops Diocesan College in Cape Town.TRANSCRIPT
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Band Theory
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Band Theory
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Band Theory
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Doping
• Impurities that donate electrons called – DONORS• Impurities that ACCEPT electrons called – ACCEPTORS
• Semi conductors can be - DOPED – with an impurity which changes the conduction properties.
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Lattice
• The semi-conductor forms a LATTICE structure around the impurity.
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N-type semiconductors
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Forward Bias• Electric field of
battery overwhelms interface electric field allowing holes and electrons to be pushed acrossinterface.
Such a connection is called a "forward bias", because it allows current to go forward.
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Reverse bias
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Graph
• pn junction diode "turns on" at about 0.6 V, but that variesaccording to the doping
concentration.
Notice the sharp rise in currentnear the turn-on voltage. This behavior will be exploited laterin the construction of the transistoramplifier.
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Graph
This is the "characteristic" curveof a pn junction diode. It showsthe slow, then abrupt, rise ofcurrent as the voltage is raised.
Under reverse bias, even verylarge voltages will cause onlyvery small currents, essentiallyconstant reverse bias currents.
The reverse bias current dependsmainly on the rate of thermal generation of electron-hole pairs
Reverse current exaggerated; typicalreverse current: 10 microamperes.
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Black band corresponds to the "point" in thediode symbol on the right.
It points as "point" is spelled, with "p" first, and"n" later.
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Rectification of an AC Signal
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Smoothing
Capacitor charges up during the positive half-cycle, then releases its charge through the resistor during the negative half-cycle, causing current through the resistor that otherwise wouldn't exist.
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Bipolar Junction Transistors
Bipolar junction transistors (BJT).
Two types of charge carriers are involved.
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The PNP Transistor
The junction on the left side ofthe base is forward-biased;the junction on the right sideof the base is reverse-biased.
Base is very narrow, and not drawn to scale.
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Amplification
Emitter-base junction is biased at the turn-on voltage, so even very small changes in voltage there will lead to very large changes in the emitter-base current (see detail).
Electric field at the base-collector junction sweeps up almost all holes which enter the base. Weak input signal is amplified by the transistor (details follow).