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1 Introduction

Figure 1.40: Comparison between insulator, semiconductor, and conductor. In a conductor, the energy of

the electrons is high enough to be in the conduction band. In a semiconductor, energy has to be added be-

fore electrons conduct (i. e., overcome the band gap). In an insulator, the band gap is so high that electrons

will not reach the conduction band.

Figure 1.41: An example of a transistor. A transis-

tor combines n-type and p-type semiconductors.

The current will only flow when the gate-voltage

is high enough to provide more electrons than

there are “holes” in the p-type semiconductor.

transistor is the field-effect transistor (Figure 1.41). Basically, a secondary voltage turns

the circuit either off or on, which is why they act as switches. The secondary voltage adds

enough electrons to neutralize the positive charges of the p-type semiconductor, and

when more are added, they create a channel for negative charges, creating the primary

voltage. The secondary voltage can also be increased, adding to the primary voltage.

At that point, the transistor is not only a switch but also an amplifier. Transistors are

often named for the material that they are made of, e. g., MOSFET stands for metal-oxide

semiconductor field-effect transistor.

Transistors are often used as switches in logic gates (Figure 1.42). Logic gates add

two inputs together (the inputs are always either 0 or 1, as is the output); different logic

gates use different Boolean algebra operations. An “And” gate, for example, multiplies

the two inputs; the output of that logic gate will always be 0 unless the input consisted

of two 1s, in which case it will be 1.

Other important electronic components are resistors that modulate current, capac-

itors that store charges, and diodes. A resistor lowers the amount of current that passes

through (Figure 1.43). Resistance is measured in ohms. According to Ohm’s law, the resis-