Lesson 5 - University of Nairobi
Transcript of Lesson 5 - University of Nairobi
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Lesson 5 Electronics:
• Semiconductors
• Doping
• p-n Junction Diode
• Half Wave and Full Wave Rectification
• Introduction to Transistors- Types and Connections
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Semiconductors
• If there are many free electrons to carry current, the semiconductor acts more like a conductor.
• If there are few electrons, the semiconductor acts like an insulator.
• Silicon is the most commonly used semiconductor.
• Atoms of silicon have 14 electrons.
• Ten of the electrons are bound tightly inside the atom.
• Four electrons are near the outside of the atom and only loosely bound.
• The relative ease at which electric current flows through a material is known as conductivity.
• Conductors (like copper) have very high conductivity.
• Insulators (like rubber) have very low conductivity.
• The conductivity of a semiconductor depends on its conditions.
• For example, at low temperatures and low voltages a semiconductor acts like an insulator.
• When the temperature and/or the voltage is increased, the conductivity increases and the material acts more like a conductor.
Semiconductors
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Semiconductors
• Pure semiconductors – thermal vibration results in some bonds being broken generating free electrons
which move about – these leave behind holes which accept electrons from adjacent atoms and
therefore also move about – electrons are negative charge carriers – holes are positive charge carriers – At room temperatures there are few charge carriers – pure semiconductors are poor conductors – this is intrinsic conduction
• Doping – the addition of small amounts of impurities drastically affects its properties – an excess of electrons produce an n-type semiconductor – an excess of holes produce a p-type semiconductor – both n-type and p-type materials have much greater conductivity than pure
semiconductors – this is extrinsic conduction
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Semiconductors
• The dominant charge carriers in a doped semiconductor (e.g. electrons in n-type material) are called majority charge carriers. Other type are minority charge carriers
• The overall doped material is electrically neutral
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p-n Junction Diode
• Potential Barrier – the barrier opposes the flow of
majority charge carriers and only a small number have enough energy to surmount it • this generates a small diffusion
current
– the barrier encourages the flow of minority carriers and any that come close to it will be swept over • this generates a small drift current
– for an isolated junction these two currents must balance each other and the net current is zero
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p-n Junction Diode
• The diffusion of positive charge in one direction and negative charge in the other produces a charge imbalance – this results in a potential barrier across the junction
• When p-type and n-type materials are joined this forms a p-n junction – majority charge carriers on each side diffuse across the junction where they
combine with (and remove) charge carriers of the opposite polarity
– hence around the junction there are few free charge carriers and we have a depletion layer (also called a space-charge layer)
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p-n Junction Diode • Forward bias
– if the p-type side is made positive with respect to the n-type side the height of the barrier is reduced
– more majority charge carriers have sufficient energy to surmount it
– the diffusion current therefore increases while the drift current remains the same
– there is thus a net current flow across the junction which increases with the applied voltage
1.5V
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p-n Junction Diode
• Reverse bias – if the p-type side is made negative with respect to the
n-type side the height of the barrier is increased – the number of majority charge carriers that have
sufficient energy to surmount it rapidly decreases – the diffusion current therefore vanishes while the drift
current remains the same – thus the only current is a small leakage current caused by
the (approximately constant) drift current – the leakage current is usually negligible (a few nA)
1.5V
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Forward I–V Diode Characteristics
The load line plots all possible combinations of diode current (ID) and voltage (VD) for a
given circuit. The maximum ID equals E/R, and the maximum VD equals E.
The point where the load line and the characteristic curve intersect is the Q-point, which
identifies ID and VD for a particular diode in a given circuit.
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p-n Junction Diode
• Forward and Reverse Currents – p-n junction current is given approximately by
– where I is the current, e is the electronic charge, V is the applied voltage, k is Boltzmann’s constant, T is the absolute temperature and (Greek letter eta) is a constant in the range 1 to 2 determined by the junction material
– for most purposes we can assume = 1
1exp
ηkT
eVII s The Shockley Equation
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Different types of Diodes
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Different types of Diodes
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Different types of Diodes
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Diode Applications - The Half-Wave Rectifier
A Typical Battery Charging Circuit
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Diode Applications - The Half-Wave Rectifier
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Diode Applications - The Full-Wave Rectifier
The full-wave rectifier
Voltage across each half of the transformer secondary
Full-wave load voltage
Diode Applications - The Bridge Rectifier
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Transistors Transistors
BJT Transistors
FET Transistors
JFET Transistors
MOSFET Transistors
npn pnp
n channel p channel Depletion MOSFET
Enhancement MOSFET
n channel p channel n channel
(NMOS)
p channel (PMOS)
3
2
1
3
2
1
NMOS +PMOS=CMOS
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BJT Transistors
The BJT is a nonlinear, 3-Terminal device based on the junction diode. A representative structure sandwiches one semiconductor type between layers of the opposite type. We first examine the npn BJT
A pnp BJT and its schematic symbol.
A npn BJT and its schematic symbol.
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BJT Transistors The npn Transistor
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BJT Transistors
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BJT Transistors
the constant β is called the common-emitter current gain
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BJT Transistors
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BJT Transistors
Input Characteristics
Circuit for measuring BJT characteristics.
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BJT Transistor
Output Characteristics
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BJT Transistor
(a) Conceptual circuit for measuring the iC –vCE characteristics of the BJT. (b) The iC –vCE characteristics of a practical BJT.
Output Characteristics
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BJT Applications
The Common-Emitter Amplifier
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The Field-Effect Transistor (FET)
• The field-effect transistor, or FET, is also a 3-terminal device, but it is constructed, and functions, somewhat differently than the BJT.
• There are several types. We begin with the junction FET (JFET), specifically, the n-channel JFET
• The p-n junction is a typical diode. Holes move from p-type into n-type .
• Electrons move from n-type into p-type . • Region near the p-n junction is left without any
available carriers -depletion region • Carriers are still present in the n-type channel . • Current could flow between drain and source
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FET Transistors
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FET Transistors
N-channel JFET P-channel JFET
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FET Transistors
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n-channel JFET
Typical output characteristics, n-channel JFET.
Transfer characteristics, n-channel JFET.
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MOSFET Transistors
N-channel MOSFET P-channel MOSFET
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MOSFET Transistors
Depletion mode N-channel MOSFET Enhancement mode N-channel MOSFET
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MOSFET Transistors
P-channel MOSFET N-channel MOSFET
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MOSFET Transistors Static characteristics of a Depletion and Enhancement mode N-channel MOSFET
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MOSFET Transistors Enhancement only N-channel MOSFET (NMOS)
NMOS can never operate with a negative gate voltage
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MOSFET Transistors Enhancement only N-channel MOSFET (NMOS) Enhancement only P-channel MOSFET (PMOS)
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MOSFET Transistors Enhancement only N-channel MOSFET (NMOS) symbol, drain and transfer characteristics
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CMOS
Q1PMOSFET
Q2NMOSFET
+10v
01
NMOS
PMOS