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Chapter 1 Semiconductor diodes

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1.1 Types of material

1.2 Semiconductor materials

1.3 Conduction in semiconductor materials

1.4 The p-n junction

1.5 Forward and reverse bias

1.6 Semiconductor diodes

1.7 Character and maximum ratings

1.8 Rectification

1.9 Zener diodes

Chapter 1 Semiconductor diodes

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◆ Materials may be classified as conductors, semiconduc-conductor, insulators. ◆ The classification depends on the value of resistivity of the material. ◆ Good conductors are usually metals and have resistivities in the order of 10-7 to 10-8 Ωm, ◆ semiconductors have resistivities in the order of 10-3 to 103 Ωm, ◆ the resistivities of insulators are 104 to 1014 Ωm,

1.1 Types of material

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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

PVC > 1013 Ωm variation for a small increase in temperature

1.1 Types of material

Figure 1.1

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◆ 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.

1.2 Semiconductor materials

Figure 1.2

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价电子

惯性核

1.2 Semiconductor materials

Figure 1.3 Atomic structure model of silicon and germanium

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◆ 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.

1.3 Conduction in semiconductor materials

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Figure 1.4 Covalent bond of semiconductor

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◆ 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.

1.3 Conduction in semiconductor materials

Figure 1.5 N-type doping semiconductor

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◆ 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 P-type. 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 "semiconductor."

1.3 Conduction in semiconductor materials

Figure 1.6 P-type doping semiconductor

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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 m

aterial 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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(a) From Figure 11.11, when V =0.4V, current flowing,I =1.9mA

(b) When I =9 mA, the voltage dropped across thediode, 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 0.2-0.3VSo it is Germanium

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1.5 Forward and reverse bias

Figure 11.13 Problem analysis

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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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1.6 Semiconductor diodes

Figure 1.13 Diodes

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1.7 Characteristics and maximum ratings

Table 1.1 Characteristicsof some typical signaland rectifier diodes

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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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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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(a)When V =−30V, the current flowing in the diode,I=−32.5mA

(b) When I =−5 mA, the voltage dropped across thediode, V =−27.5V

(c) The characteristic shows the onset of Zener actionat 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