SURFACE MOUNT TRANSISTOR IN MOBILE PHONE & THEIR FUNCTION

Surface Mount Transistor in Mobile Phone and Their Function
Surface Mount Transistor or SMT Transistor is an SMD electronic component made up of semiconductor material like silicon or germanium. There are 2 types of Surface Mount Transistors:


  1. NPN Type
  2. PNP Type

Terminals of a Transistor

There are three terminals of a surface mount transistor or thru-hole transistor:
  1. Ammeter (E) – Flows current on receipt of forward bias. Electrons are emitted in NPN transistors whereas PNP transistors emit ‘holes’.
  2. Collector (C) – The terminal of the transistor which receives the emitted electrons or holes. Collector always works or reverse bias mode.
  3. Base (B) – The layer between ammeter and collector is called base. Base displays the property of showing low resistance in ammeter junction forward bias and high in collector junction reverse bias.

Facts about Transistor

  1. Indicating Character: Q or V, TR
  2. Function: Switching, Amplification, Regulating Voltage.
  3. Unit: Transistors are identified according to the code.

Digital Surface Mount Transistor

In digital transistor, resistance in built in the base and ammeter. This transistor is also called RET (Resistance Equipped Transistor). This type of transistor is used in mobile phones for reducing the current consumption.

Field Effect Transistor (FET)

This type of transistor is controlled by Voltage instead of current. Flow of working current through a semiconductor channel is switched and regulated by the effect of electrical charge in the area near channel which is called gate. This is called unipolar transistor.  FETcan be P-Channel type or N-Channel Type.

Metal Oxide Semiconductor (MOSFET)

MOSFET is active semiconductor components. MOSFET has 3 terminals – source, drain and gate. There are 2 types of MOSFET:
  1. P-Channel MOSFET (PMOS)
  2. N-Channel MOSFET (NMOS)

How to Read SMD Transistor Code

All SMD Transistors are Marked with Codes to denote the type of semiconductor used and use of transistor.
Here I explain How to Read SMD Transistor Code:
First Alphabet:
  • A= Germanium
  • B = Silicon
  • C = Gallium Arsenide
  • D = Indium Antimide
Second Alphabet:
  • C = Audio Frequency Amplifier
  • D = Audio Frequency Power Amplifier
  • F = Low Power Radio Frequency Amplifier
  • P = High Power Radio Frequency Amplifier
Therefore, Identification of a Transistor Marked with the Code – BC486 will be:
  • B = Silicon
  • C = Audio Frequency Amplifier
  • The Transistor = Silicon Audio Frequency Amplifier
More Examples:
  • BD 187 = B for Silicon, D for Audio Frequency Power Amplifier
  • AD 486 = A for Germanium, D for Audio Frequency Power Amplifier
  • AC 140 = A for Germanium, C for Audio Frequency Amplifier

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