Transcript BTEC-Electronics

Chapter 1
Chapter 1
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1.9
Semiconductor diodes
Semiconductor diodes
Types of material
Semiconductor materials
Conduction in semiconductor materials
The p-n junction
Forward and reverse bias
Semiconductor diodes
Character and maximum ratings
Rectification
Zener diodes
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Semiconductor diodes
1.1 Types of material
◆
Materials may be classified as conductors, semiconduc-cond
uctor, insulators.
◆ The classification depends on the value of resistivity of the
material.
◆ Good conductors are usually metals and have resistivities in
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the order of 10 to 10 Ωm,
◆ semiconductors have resistivities in the order of 10-3 to 103
Ωm,
◆ the resistivities of insulators are 104 to 1014 Ωm,
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Semiconductor diodes
1.1 Types of material
Conductors:
Aluminium 2.7× 10-8 Ωm
Copper (pure annealed) 1.7× 10-8 Ωm
Semiconductors: (at 27oC)
Silicon 2.3× 103 Ωm
Germanium 0.45 Ωm
Insulators:
Glass > 1010 Ωm
Figure 1.1
PVC > 1013 Ωm
variation for a small increase in temperature
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1.2 Semiconductor materials
◆ An atom contains both negative charge carriers (electrons) and positive
charge carriers (protons).
◆ Electrons each carry a single unit of negative electric charge while
protons each exhibit a single unit of positive charge.
◆ Atoms normally contain an equal number of electrons and protons, the
net charge present will be zero.
◆ most semiconductor chips and transistors are created with silicon and
germanium. You may have heard expressions like “Silicon Valley” and the
“silicon economy”.
◆ If you look "silicon" up in the periodic table, you will find that it sits next
to aluminum, below carbon and above germanium.
Figure 1.2
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1.2 Semiconductor materials
价电子
惯性核
Figure 1.3 Atomic structure model of silicon and germanium
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1.3 Conduction in semiconductor materials
◆ In its pure state, silicon is an insulator because the covalent bonding rigidly
holds all of the electrons leaving no free (easily loosened) electrons to
conduct current. See figure 1.4.
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Figure 1.4 Covalent bond of semiconductor
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1.3 Conduction in semiconductor materials
◆ You can change the behavior of silicon
and turn it into a conductor by doping it.
In doping, you mix a small amount of an
impurity into the silicon crystal.
◆ There are 2 types of doping “N” and
“P”
◆ In N-type doping, phosphorus or
arsenic is added to the silicon in small
quantities. Phosphorus and arsenic each
have five outer electrons, so they're out
of place when they get into the silicon
lattice. The fifth electron has nothing to
bond to, so it's free to move around.
Electrons have a negative charge, hence
the name N-type.
Figure 1.5 N-type doping semiconductor
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Semiconductor diodes
1.3 Conduction in semiconductor materials
◆ In
P-type doping, boron or gallium is
the dopant. Boron and gallium each
have only three outer electrons. When
mixed into the silicon lattice, they form
"holes" in the lattice where a silicon
electron has nothing to bond to. The
absence of an electron creates the effect
of a positive charge, hence the name Ptype. Holes can conduct current. A hole
happily accepts an electron from a
neighbour, moving the hole over a space.
◆ A minute amount of either N-type or
P-type doping turns a silicon crystal
from a good insulator into a viable (but
not great) conductor -- hence the name Figure 1.6 P-type doping semiconductor
"semiconductor."
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Semiconductor diodes
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Semiconductor diodes
1.4 The p-n junction
◆ A p-n junction is a piece of semiconductor material in which part of the
material is p-type and part is n-type.
Figure 1.7 The fomation of p-n junction
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1.5 Forward and reverse bias
◆ When an external voltage is applied to a p-n junction making the p-type
material positive with respect to the n-type material, the p-n junction is
forward biased.
Figure 1.8 forward biased and graphs of the current-voltage relationship
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1.5 Forward and reverse bias
◆ When an external voltage is applied to a p-n junction making the p-type
material negative with respect to the n-type material, the p-n junction is
reverse biased.
Figure 1.9 reverse biased and graphs of the current-voltage relationship
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1.5 Forward and reverse bias
◆ When an external voltage is applied to a p-n junction making the p-type
material negative with respect to the n-type material, the p-n junction is
reverse biased.
Figure 1.9 reverse biased and graphs of the current-voltage relationship
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1.5 Forward and reverse bias
Figure 11.11 Problem analysis
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Semiconductor diodes
(a) From Figure 11.11, when V =0.4V, current flowing,
I =1.9mA
(b) When I =9 mA, the voltage dropped across the
diode, V =0.67V
(c) From the graph, when V =0.6V, I =6 mA.
Thus, resistance of the diode
(d) Form the graph the current start at 0.2-0.3V
So it is Germanium
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Semiconductor diodes
1.5 Forward and reverse bias
Figure 11.13
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Problem analysis
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Semiconductor diodes
1.6 Semiconductor diodes
◆ A semiconductor diode is an encapsulated p-n junction fitted with
connecting leads or tags for connection to external circuitry.
Figure 1.12
Diodes
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Semiconductor diodes
1.6 Semiconductor diodes
Figure 1.13
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Diodes
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Semiconductor diodes
1.7 Characteristics and maximum ratings
Table 1.1
Characteristicsof some typical signaland rectifier diodes
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Semiconductor diodes
1.8 Rectification
◆ The process of obtaining unidirectional currents and voltages from
alternating currents and voltages is called rectification.
◆ Semiconductor diodes are commonly used to convert alternating current
(a.c.) to direct current (d.c.), in which case they are referred to as rectifiers.
half-wave rectifier
full-wave rectifier
Figure 1.14
Rectifier
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Semiconductor diodes
1.9 Zener diodes
◆ Zener diodes are heavily doped
silicon diodes that, unlike normal
diodes, exhibit an abrupt reverse
break down at relatively low
voltages (typically less than 6V).
◆ Zener diodes are available in
various families (according to their
general characteristics,
encapsulations and power ratings)
with reverse breakdown (Zener)
voltages in the range 2.4V to 91V.
Figure 1.15 graphs of Zener diodes
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1.9 Zener diodes
Figure 1.16 Program analysis
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Semiconductor diodes
(a)When V =−30V, the current flowing in the diode,
I=−32.5mA
(b) When I =−5 mA, the voltage dropped across the
diode, V =−27.5V
(c) The characteristic shows the onset of Zener action
at 27V; this would suggest a Zener voltage rating
of 27V
(d) Power, P=V ×I, from which, power dissipated
when the reverse voltage is 30V,
P = 30 × (32.5 × 10−3) = 0.975W = 975mW
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