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Introduction to Electronics

Section 1.3

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Analog vs. Digital

• Analog Signal– Can take on any value– Most signals around us are like this

• Digital Signal– Sequence of numbers– Each number is an approximation of an analog

signal at an instant of time– Usually represented using binary number system

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Figure 1.7 Sampling the continuous-time analog signal in (a) results in the discrete-time signal in (b).

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Figure 1.8 Variation of a particular binary digital signal with time.

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Figure 1.9 Block-diagram representation of the analog-to-digital converter (ADC).

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Diodes

Sections 3.7.1

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Semiconductor Materials

• silicon, germanium, gallium arsenide• conductivity => number of free charge carriers per

unit volume• metal --- high conductivity• insulator --- low conductivity• semiconductor --- variable conductivity changed by

the addition of dopants• crystalline structure with covalent bonds• at T = 0 insulator (i.e no free charge carriers)

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Figure 3.39 Simplified physical structure of the junction diode. (Actual geometries are given in Appendix A.)

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Figure 3.40 Two-dimensional representation of the silicon crystal. The circles represent the inner core of silicon atoms, with +4 indicating its positive charge of +4q, which is neutralized by the charge of the four valence electrons. Observe how the covalent bonds are formed by sharing of the valence electrons. At 0 K, all bonds are intact and no free electrons are available for current conduction.

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Figure 3.41 At room temperature, some of the covalent bonds are broken by thermal ionization. Each broken bond gives rise to a free electron and a hole, both of which become available for current conduction.

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• generated hole - electron pairs• electron --- negative• hole --- positive• holes and electrons can contribute to current• equal number of holes and electrons called

“intrinsic” semiconductor

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Diffusion and Drift

• Diffusion– Random motion due to thermal agitation– Carriers move from high to low concentration– Results in diffusion current

• Drift– Movement of carriers due to electric field– Drift velocity depends on mobility of carrier and

magnitude of electric field– Results in drift current

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Diffusion

Figure 3.42 A bar of intrinsic silicon (a) in which the hole concentration profile shown in (b) has been created along the x-axis by some unspecified mechanism.

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• Drift– A hole migrates to the right as different electrons fill in the hole’s

vacancy by moving one site to the left. – The motion of the individual electrons is toward the left, against the

electric field, net hole motion is toward the right (i.e.: in the direction of the electric field).

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Dopants

• hole/electron concentrations can be changed by the additions of small amounts of elements (dopants) to create “extrinsic” semiconductor

• Dopants– acceptors - boron, indium, aluminum – p type– donors - phosphorous, arsenic, antimony (have extra

electron) – n type• maintain charge neutrality• minimum amount of energy to move unbound

electrons/holes (i.e. easy to jump from one site to another)

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Figure 3.43 A silicon crystal doped by a pentavalent element. Each dopant atom donates a free electron and is thus called a donor. The doped semiconductor becomes n type.

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Figure 3.44 A silicon crystal doped with a trivalent impurity. Each dopant atom gives rise to a hole, and the semiconductor becomes p type.