what is transistor?

Added Date:2007-06-15

A transistor is a semiconductor device, commonly used as an amplifier or an electrically controlled switch. The transistor is the fundamental building block of the circuitry that governs the operation of computers, cellular phones, and all other modern electronics.

Because of its fast response and accuracy, the transistor may be used in a wide variety of digital and analog functions, including amplification, switching, voltage regulation, signal modulation, and oscillators. Transistors may be packaged individually or as part of an integrated circuit, which may hold millions of transistors in a very small area.

Introduction

Modern transistors are divided into two main categories: bipolar junction transistors (BJTs) and field effect transistors (FETs). Application of current in BJTs and voltage in FETs between the input and common terminals increases the conductivity between the common and output terminals, thereby controlling current flow between them. The transistor characteristics depend on their type. See Transistor models.

The term "transistor" originally referred to the point contact type, but these only saw very limited commercial application, being replaced by the much more practical bipolar junction types in the early 1950s. Ironically both the term "transistor" itself and the schematic symbol most widely used for it today are the ones that specifically referred to these long-obsolete devices.[1] For a short time in the early 1960s, some manufacturers and publishers of electronics magazines started to replace these with symbols that more accurately depicted the different construction of the bipolar transistor, but this idea was soon abandoned.

In analog circuits, transistors are used in amplifiers, (direct current amplifiers, audio amplifiers, radio frequency amplifiers), and linear regulated power supplies. Transistors are also used in digital circuits where they function as electronic switches, but rarely as discrete devices, almost always being incorporated in monolithic Integrated Circuits. Digital circuits include logic gates, random access memory (RAM), microprocessors, and digital signal processors (DSPs).

Advantages of transistors over vacuum tubes

Prior to the development of transistors, vacuum (electron) tubes (or in the UK thermionic valves or just valves) were the main active components in electronic equipment. The key advantages that have allowed transistors to replace their vacuum tube predecessors in most applications are:

* Small size and minimum weight, allowing the development of miniaturized electronic devices..
* Highly automated manufacturing processes, resulting in low per-unit cost.
* Lower possible operating voltages, making transistors suitable for small, battery-powered applications.
* No warm-up period required after power application.
* Lower power dissipation and generally greater energy efficiency.
* Higher reliability and greater physical ruggedness.
* Extremely long life. Transistorized devices produced more than 30 years ago are still in service.
* Complementary devices available, facilitating the design of complementary-symmetry circuits, something not possible with vacuum tubes.
* Ability to control very large currents, as much as several hundred amperes.
* Insensitivity to mechanical shock and vibration, thus avoiding the problem of microphonics in audio applications.

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