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The most comprehensive introduction to bipolar transistors

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The most comprehensive introduction to bipolar transistors

June 09
16:54 2022
A bipolar junction transistor (BJT) is a type of transistor that uses both electrons and electron holes as charge carriers.

Bipolar transistor, the full name of bipolar junction transistor (BJT), commonly known as a triode, is an electronic components device with three terminals, made of three parts of semiconductors with different degrees of doping. As a result, the charge flow in the transistor is mainly due to the diffusion and drift motion of carriers at the PN junction.

The work of this kind of transistor involves the flow of both electrons and holes, so it is called bipolar, and it is also called bipolar carrier transistor. This mode of operation differs from unipolar transistors such as field-effect transistors, which only involve the drift of a single kind of carrier. The boundary between the two different dopant concentration regions is formed by a PN junction.

Bipolar Transistor Types

NPN transistor

An NPN transistor is one of two types of bipolar transistors, consisting of two layers of N-type doped regions and a layer of P-type doped semiconductor (base) in between. The tiny current input to the base will be amplified, resulting in a larger collector-emitter current. When the base voltage of the NPN transistor is higher than the emitter voltage, and the collector voltage is higher than the base voltage, the transistor is in a forward amplifying state. In this state, there is a current flow between the collector and emitter of the transistor. The amplified current is the result of the electrons (minority carriers in the base region) injected by the emitter into the base region and drifting to the collector under the impetus of the electric field. Since electron mobility is higher than hole mobility, most bipolar transistors in use today are of the NPN type.

The electrical symbol for an NPN bipolar transistor with the arrow between the base and emitter pointing towards the emitter.

PNP transistor

Another type of bipolar transistor is the PNP transistor, which consists of two layers of P-type doped regions and a layer of N-type doped semiconductors in between. The tiny current flowing through the base can be amplified at the emitter. That is, when the base voltage of the PNP transistor is lower than the emitter, the collector voltage is lower than the base, and the transistor is in the forward amplification region.

In the bipolar transistor electrical symbol, the arrow between the base and the emitter points in the direction of current flow, which is the opposite direction of electron flow. Contrary to the NPN type, the arrow of the PNP type transistor points from the emitter to the base.

Bipolar Transistor Working Principle

An NPN type bipolar transistor can be seen as two diodes that share an anode joined together. In the normal operating state of a bipolar transistor, the base-emitter junction (call this PN junction “emitter junction”) is forward biased, while the base-collector (call this PN junction “collector”) is in a forward-biased state. junction”) is reverse biased. In the absence of an applied voltage, the electron concentration in the N region of the emitting junction (the majority carrier in this region) is greater than that in the P region, and part of the electrons will diffuse into the P region. Similarly, part of the holes in the P region will also diffuse to the N region. In this way, a space charge region (also known as a depletion layer) will be formed on the emitter junction, generating an internal electric field whose direction is from the N region to the P region. This electric field will hinder the further occurrence of the above-mentioned diffusion process, thereby achieving dynamic balance. At this time, if a forward voltage is applied to the emitter junction, the dynamic balance between the above-mentioned carrier diffusion motion and the intrinsic electric field in the depletion layer will be broken, which will cause thermally excited electrons to be injected into the base region. In NPN transistors, the base region is P-type doped, where holes are the majority dopant, so electrons in this region are called “minority carriers”.

The electrons injected from the emitter to the base region recombine with the majority of carrier holes here on the one hand; In the biased state, most of the electrons will reach the collector region through drift motion, forming a collector current. To minimize the recombination of electrons before reaching the collector junction, the base region of the transistor must be made thin enough that the time required for carrier diffusion is shorter than the lifetime of the semiconductor minority carriers, and the base The thickness must be much smaller than the diffusion length of the electrons (see Fick’s law). In modern bipolar transistors, the thickness of the base region is typically a few tenths of a micrometer. It should be noted that although both the collector and the emitter are N-type doped, their doping levels and physical properties are not the same. Therefore, it is necessary to distinguish the bipolar transistor and the two opposite-direction diodes in series. Come on.

Difference Between Bipolar Transistors and Field-Effect Transistors

field-effect transistor (FET) is a voltage-controlled semiconductor device with an input resistance between 108Ω and 109Ω (high input resistance), low noise, low power consumption, large dynamic range, easy integration, no secondary breakdown, and safe operating range. At present, FETs have become the strongest competitors of bipolar transistors and power transistors. Field-effect transistors are mainly divided into large categories: junction field-effect transistors and insulated gate field-effect transistors.

FET is not equal to MOS tube, MOS tube is just a type of FET; and FET is just a type of transistor, which is divided into unipolar and bipolar transistors; For how to distinguish it, please see below ‘s introduction.

Compared with bipolar transistors, FETs:

(1) In the field-effect transistor, the conduction process is the drift motion of the majority carriers, so it is called a unipolar transistor; in a bipolar transistor, there are both the diffusion motion of the majority carrier and the drift motion of the minority carrier.

(2) The FET controls the drain current iD through the gate voltage uGS, which is called a voltage control device; the bipolar transistor uses the base current IB (or the emitter current iE) to control the collector current iC, called for the current control device.

(3) The input resistance of the FET is large; the input resistance of the bipolar transistor is small.

(4) The value of the transconductance gm of the FET is small, and the β value of the bipolar transistor is large. Under the same conditions, the amplification capability of the FET is not as high as that of the transistor.

(5) The drain and source of the junction field-effect transistor can be used interchangeably. If the substrate of the MOS tube is not connected to the source, the d and s electrodes can also be used interchangeably; The e-pole swap is called the inverted working state, and β will become very small at this time.

(6) FETs can be used as voltage-controlled resistors.

(7) The FET relies on multi-sub conduction, so it has better temperature stability, radiation resistance, and lower noise.

The FET also has some disadvantages: such as low power and slow speed. But because of its simple process and easy integration, it is widely used in integrated circuits.

Differences between bipolar and unipolar transistors

The structure and function of these two kinds of pipes are different, as shown below:

1. Bipolar transistor, the full name of bipolar junction transistor (BJT), commonly known as a triode, is an electronic device with three terminals. Bipolar transistors are made of three parts of semiconductors with different degrees of doping. The charge flow in the transistor is mainly due to the diffusion and drift motion of carriers at the PN junction. Taking NPN transistors as an example, according to the design, electrons in the highly doped emitter region move to the base through diffusion. In the base region, holes are the majority carriers and electrons are the minority carriers. Bipolar transistors are classified as minority carrier devices because the base region is thin, and these electrons travel to the collector through drift motion, resulting in a collector current.

2. Unijunction transistor (UJT for short) is also called a base diode. A three-terminal semiconductor device with only one PN junction as the emitter and two bases was called a double-base diode in the early days. Its typical structure uses an N-type single crystal semiconductor with uniform light doping and high resistivity as the base, two bases with ohmic contact at both ends, and shallow diffusion in the center of the base or a position biased to one of the poles. The method is heavily doped to make a PN junction as an emitter. It is especially suitable for relaxation oscillators in switching systems and can be used in timing circuits, control circuits, and readout circuits.

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