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Sunday, 26 June 2011

Introduction to the transistor

Introduction to the transistor

The purpose of this text is to give a basic understanding of the behaviour of a transistor. It is made for readers with some knowledge about electricity: voltage, current, resistors, batteries, Ohm's law. Technical problems like non-linearity and behaviour under high frequencies are not talked about.



Description of the transistor

The transistor is a component with 3 electric wires coming out of it. They are named B (base), C (collector), and E (emitter).

This is a drawing of the BC 547 transistor, four times bigger:



transistor



Such a transistor costs $0.3 in electric components stores.

Here is a classic drawing for a transistor inside electronic diagrams:


transistor




How it is used

  • If one connects a tension source between the wires C and E, the transistor will not let any current trough (fig. 1).

  • But between B and E there is a shortcut. If one wants to make a given current go trough B and E, one must use a tension source and a resistor (fig. 2).

  • If one sends a current of IB amperes between B and E, then the resistor will allow a current of IC = ß . IB amperes pass between C et E (fig. 3). In this case, ß is about 100.


electronic circuit



The electronic diagrams corresponding to figures 1, 2 and 3 are figures 4, 5 and 6:


electronic circuit



Note: For those who would like to try out these diagrams, one sole battery of 9 Volts can replace the two batteries (fig. 7 and 8):


electronic circuit



Be careful for the polarity: put the positive wire and the negative wire of the battery on the right place. The direction of the current is very important for a transistor.

The BC 547 is a somewhat weak transistor to make a lamp light up. Perhaps you will get better results using a stronger transistor, for example the BD 649. Here is a drawing of it, two times bigger:


transistor



As a beginner, by making wiring errors or making the transistor dissipate too much heat, you will probably burn a few of them. That's normal.

The reason why one subtracts systematically 0.7 Volts from the UBE tension is that bipolar transistors contain sort of "parasite" diode. The tension that must be subtracted depends on the sort of semiconductor: 0.7 Volts for silicon, 0.2 Volts for germanium.


History

History

A replica of the first working transistor.
Physicist Julius Edgar Lilienfeld filed the first patent for a transistor in Canada in 1925, describing a device similar to a field-effect transistor or "FET".[1] However, Lilienfeld did not publish any research articles about his devices,[citation needed] nor did his patent cite any examples of devices actually constructed. In 1934, German inventor Oskar Heil patented a similar device.[2]
From 1942 Herbert Mataré experimented with so-called duodiodes while working on a detector for a Doppler RADAR system. The duodiodes he built had two separate but very close metal contacts on the semiconductor substrate. He discovered effects that could not be explained by two independently operating diodes and thus formed the basic idea for the later point contact transistor.
In 1947, John Bardeen and Walter Brattain at AT&T's Bell Labs in the United States observed that when electrical contacts were applied to a crystal of germanium, the output power was larger than the input. Solid State Physics Group leader William Shockley saw the potential in this, and over the next few months worked to greatly expand the knowledge of semiconductors. The term transistor was coined by John R. Pierce as a portmanteau of the term "transfer resistor".[3][4] According to physicist/historian Robert Arns, legal papers from the Bell Labs patent show that William Shockley and Gerald Pearson had built operational versions from Lilienfeld's patents, yet they never referenced this work in any of their later research papers or historical articles.[5]

Transistor as a switch

Transistor as a switch

BJT used as an electronic switch, in grounded-emitter configuration.
Transistors are commonly used as electronic switches, both for high-power applications such as switched-mode power supplies and for low-power applications such as logic gates.
In a grounded-emitter transistor circuit, such as the light-switch circuit shown, as the base voltage rises the base and collector current rise exponentially, and the collector voltage drops because of the collector load resistor. The relevant equations:
VRC = ICE × RC, the voltage across the load (the lamp with resistance RC)
VRC + VCE = VCC, the supply voltage shown as 6V
If VCE could fall to 0 (perfect closed switch) then Ic could go no higher than VCC / RC, even with higher base voltage and current. The transistor is then said to be saturated. Hence, values of input voltage can be chosen such that the output is either completely off,[13] or completely on. The transistor is acting as a switch, and this type of operation is common in digital circuits where only "on" and "off" values are relevant.

Part numbers

Part numbers

The types of some transistors can be parsed from the part number. There are three major semiconductor naming standards; in each the alphanumeric prefix provides clues to type of the device:
Japanese Industrial Standard (JIS) has a standard for transistor part numbers. They begin with "2S",[20] e.g. 2SD965, but sometimes the "2S" prefix is not marked on the package – a 2SD965 might only be marked "D965"; a 2SC1815 might be listed by a supplier as simply "C1815". This series sometimes has suffixes (such as "R", "O", "BL"... standing for "Red", "Orange", "Blue" etc.) to denote variants, such as tighter hFE (gain) groupings.
Beginning of Part Number Type of Transistor
2SA high frequency PNP BJTs
2SB audio frequency PNP BJTs
2SC high frequency NPN BJTs
2SD audio frequency NPN BJTs
2SJ P-channel FETs (both JFETs and MOSFETs)
2SK N-channel FETs (both JFETs and MOSFETs)
The Pro Electron part numbers begin with two letters: the first gives the semiconductor type (A for Germanium, B for Silicon, and C for materials like GaAs); the second letter denotes the intended use (A for diode, C for general-purpose transistor, etc.). A 3-digit sequence number (or one letter then 2 digits, for industrial types) follows (and, with early devices, indicated the case type – just as the older system for vacuum tubes used the last digit or two to indicate the number of pins, and the first digit or two for the filament voltage). Suffixes may be used, such as a letter (e.g. "C" often means high hFE, such as in: BC549C[21]) or other codes may follow to show gain (e.g. BC327-25) or voltage rating (e.g. BUK854-800A[22]). The more common prefixes are:
Prefix class Usage Example
AC Germanium small signal transistor AC126
AF Germanium RF transistor AF117
BC Silicon, small signal transistor ("allround") BC548B
BD Silicon, power transistor BD139
BF Silicon, RF (high frequency) BJT or FET BF245
BS Silicon, switching transistor (BJT or MOSFET) BS170
BL Silicon, high frequency, high power (for transmitters) BLW34
BU Silicon, high voltage (for CRT horizontal deflection circuits) BU508

Types

Types

BJT PNP symbol.svg PNP JFET P-Channel Labelled.svg P-channel
BJT NPN symbol.svg NPN JFET N-Channel Labelled.svg N-channel
BJT
JFET
BJT and JFET symbols
JFET P-Channel Labelled.svg IGFET P-Ch Enh Labelled.svg IGFET P-Ch Enh Labelled simplified.svg IGFET P-Ch Dep Labelled.svg P-channel
JFET N-Channel Labelled.svg IGFET N-Ch Enh Labelled.svg IGFET N-Ch Enh Labelled simplified.svg IGFET N-Ch Dep Labelled.svg N-channel
JFET MOSFET enh MOSFET dep
JFET and IGFET symbols
Transistors are categorized by
  • Semiconductor material: graphene, germanium, silicon, gallium arsenide, silicon carbide, etc.
  • Structure: BJT, JFET, IGFET (MOSFET), IGBT, "other types"
  • Polarity: NPN, PNP (BJTs); N-channel, P-channel (FETs)
  • Maximum power rating: low, medium, high
  • Maximum operating frequency: low, medium, high, radio frequency (RF), microwave (The maximum effective frequency of a transistor is denoted by the term fT, an abbreviation for "frequency of transition". The frequency of transition is the frequency at which the transistor yields unity gain).
  • Application: switch, general purpose, audio, high voltage, super-beta, matched pair
  • Physical packaging: through hole metal, through hole plastic, surface mount, ball grid array, power modules
  • Amplification factor hfe (transistor beta)[14]
Thus, a particular transistor may be described as silicon, surface mount, BJT, NPN, low power, high frequency switch.

Comparison with vacuum tubes

Comparison with 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.

Advantages

The key advantages that have allowed transistors to replace their vacuum tube predecessors in most applications are
  • Small size and minimal 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 for cathode heaters required after power application.
  • Lower power dissipation and generally greater energy efficiency.
  • Higher reliability and greater physical ruggedness.
  • Extremely long life. Some transistorized devices have been in service for more than 50 years.
  • Complementary devices available, facilitating the design of complementary-symmetry circuits, something not possible with vacuum tubes.
  • Insensitivity to mechanical shock and vibration, thus avoiding the problem of microphonics in audio applications