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Types of Transistors

Classification Based on their Structure

Point Contact Transistors

These were the very-first Germanium transistors, which worked based on the basis of a difficult and unreliable electrical forming process, causing them fail quite often. Further, they had a common-base current gain α greater than 1 and exhibited negative resistance.

Bipolar Junction Transistor (BJT)

BJTs are the transistors with three terminals (Emitter, Base and Collector) and hence possess two-junctions viz., Base-Emitter junction and Collector-Base junction. These are current-controlled devices whose conduction current relies on both majority as well as minority charge carriers (hence bipolar).

Bipolar Junction Transistor BJT Further these can be either (i) npn with majority charge carriers as electrons or (ii) pnp with majority charge carriers as holes, depending on their doping nature. Apart from these, many other types of BJTs found are

Field Effect Transistor (FET)

These are the voltage-controlled transistors which are tri-terminal wherein the Gate terminal controls the flow of current between the Source and the Drain terminals. These are also called unipolar devices as their conduction current is only due to the majority charge carriers according to which they can be either n-channel (the majority charge carriers are electrons) FETs or p-channel (the majority charge carriers are holes) FETs. Further, FETs can be classified as Field Effect Transistor FET

Classification Based on their Function

  1. Small Signal Transistors: These type of transistors are particularly used to amplify the low level signals (rarely for switching) and can be either npn or pnp in nature.
  2. Small Switching Transistors: These are extensively used for switching purposes although they can be employed for amplification process. These transistors are available in both npn as well as pnp forms.
  3. Power Transistors: These are used as power amplifiers in high power applications and can be npn or pnp or Darlington transistors.
  4. High Frequency Transistors: These are also known as RF Transistors and are used in high speed switching devices wherein the small signals operate at high frequencies.
  5. Photo-Transistor: These are two-terminal light-sensitive devices are nothing but the standard transistors which have photosensitive region as a substitute for the base region.
  6. Unijunction Transistors: These transistors are exclusively used as switches and are not suitable for amplification.
  7. Biomedical and Environmental Transistors: These are used in the field of biomedicine and environmental sensing.
In addition to these, there also exist an Insulated-Gate Bipolar Transistor (IGBT) which combines the features of both BJTs as well as FETs as it uses a insulated gate to control a bipolar power transistor acting as a switch. There are also devices which comprise of two tunneling junctions enclosing a gate-controlled island called Single Electron Transistors (SETs). Certain transistors like those of junction less Nano wire Transistor (JNT) even lack the presence of gate junction which lead to denser and cheaper microchips. Lastly, it is to be noted that these are just a countable number of transistor types amongst the ample types present in the market.



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