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Transistor Solid state transistor is ubiquitous in modern life since the 1950's when its usefulness was discovered at Bell Labs in 1947. As shown in Figure 13-03c(a), this device consists of all the three types of solid in the Band Theory. The semiconductor is usually a piece of doped silicon, which allows some free electrons (N-type) or holes (P-type) to move around. The transistor

Figure 13-03c Transistor
[view large image]

is off admitting at most a weak current before a certain threshold gate voltage (usually a few volts) is attained at the base. Once the gate voltage is over the threshold, the transistor is turned on (acting as a switch) and the current increases exponentially (acting as an amplifier).
Such behavior is the result of the attraction of electrons by the positive charge at the base as shown in Figure 13-03c(b). This type is called field-effect transistor (FET) and uses mostly in integrated circuits. There are other types of transistor with a thin layer of other type in between such as NPN or PNP called bipolar junction transistor (BJT). They are especially useful as amplifier in analog circuits. Figure 13-03c also displays the various forms of transistor.
Electronic Circiut Logic Gate
    Figure 13-03d shows a very primitive electronic circuit (characterized by non-linear relationship between current and voltage) for testing a BC547 transistor with many components including :
  • Diode - A semiconductor diode, is a P type semiconductor connected to a N type one with two electrical terminals. It allows current to pass through in only one direct (a rectifier). LED (light emitting diode) is a diode emitting light at the junction.
  • Resistors - They are used for protecting the transistor and diode from damage by current overload.
  • Pot - The potentiometer is an instrument to measure the voltage of a seat without drawing any current from the seat.

Figure 13-03d Electronic Circuit [view large image]

Figure 13-03e Logic Gate
[view large image]

Beside acting as switches and signal amplifiers, transistors are also involved in power regulators and equipment controllers. They are the building blocks of integrated circuits and most modern electronics. Microprocessors often include more than a billion of them in each chip. They are in fact used in almost everything, from stoves to computers and pacemakers to aircraft. Figure 13-03e illustrates just one of the applications as a logic gate. This one performs the function of the AND gate, which produces an output signal only when both inputs are turned on.

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