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Transistor is an important electronic component in PCBA board

Transistor is an important electronic component in PCBA board
     Transistor is a semiconductor device used to amplify or switch electronic signals and power. It is composed of semiconductor materials that are usually connected to external circuits by at least three pins. They are important electronic components in PCBA boards. Transistors are mainly divided into two categories: bipolar junction transistors and field effect transistors.

     1. Bipolar Junction Transistor (BJT)
Bipolar junction transistor, also called semiconductor triode. Because the transistor is performed by using majority and minority carriers, the bipolar junction transistor is the first type of transistor to be mass-produced. It is a combination of two junction diodes and is sandwiched between two n-type semiconductors. Between the p-type semiconductor film (np-n-type transistor), or sandwiched between two p-type semiconductors (pnp transistor) between the n-type semiconductor thin film. This structure creates two p-n junctions: base, emitter junction, and collector junction, separated by a thin semiconductor region in the base region in the middle.


    BJT has three terminals corresponding to three layers of semiconductors (emitter, base and collector). They are useful in amplifiers because the current at the emitter and collector can be controlled by a relatively small base current. In an n-p-n transistor operating in the active region, the emitter and base junctions are forward biased (electrons and holes recombine at the junction), and electrons are injected into the base region. Due to the narrow base, most of these electrons will diffuse to the reverse-biased base and collector and be swept into the collector; maybe one percent of the electrons will recombine in the base, which is part of the base current Leading mechanism. By controlling the number of electrons that can leave the base, the number of electrons entering the collector is controlled. The collector current is approximately β of the reference current (common emitter current gain). It is usually greater than 100 for small-signal transistors, but can be smaller in transistors designed for high-power applications.

    Unlike field effect transistors, BJT is a low input impedance device. In addition, as the base and emitter voltage (VBE) increases, the base, emitter current and collector and emitter current (ICE) increase exponentially according to the Shockley diode model and the Ebers-Moll model. Due to this exponential relationship, BJT has a higher transconductance of FET.

     Bipolar transistors can conduct electricity by exposure to light, because the absorption of photons in the base region produces a photocurrent as the base current; the collector current is about β times the photocurrent. The device designed for this has a transparent window in the package, called a phototransistor.

2. Field Effect Transistor (FET)
Field effect transistors are mainly used to conduct conduction on electrons (in n-channel FETs) or holes (p-channel FETs). The four terminals of the FET are called source, gate, drain and body (substrate). In most FETs, the body is connected to a source in the package.

    In a FET, drain and source currents flow through a conductive channel connecting the source region to the drain region. The conductivity is changed by the electric field generated when a voltage is applied between the gate and the source terminal; therefore, the current flowing between the drain and the source is controlled by the voltage applied between the gate and the source. As the gate voltage (VGS) increases, the drain and source currents (IDS) increase exponentially when VGS is lower than the threshold, and then the "space charge limited" area above the threshold at a roughly quadratic rate. No secondaryness is observed in modern devices, such as at the 65-nanometer technology node. For narrow bandwidth and low noise, the higher input resistance of the FET is advantageous.

    The commonly used electronic components of transistors in circuits include diodes, triodes, field effect tubes, etc. Clearly identifying the type of transistor is a basic skill in the PCBA processing process.