Field effect transistor or bipolar transistor? Are you confused?

Field effect transistor or bipolar transistor? Are you confused?

Field effect transistor includes insulated gate type field effect transistor (metal oxide semiconductor field effect transistor, MOSFET) and junction type field effect transistor (JFET).

Field effect transistor has three electrodes: gate, drain and source.

Field effect transistor conducts electricity only by the majority carriers in the semiconductor, also called unipolar transistor.

Field effect transistor controls the output current by controlling the voltage of the gate to produce an electric field effect.

Field effect transistor has the advantages of high input impedance, low noise, low power consumption, stable performance, simple manufacturing, etc.
It can be used as amplifier, electronic switch, impedance conversion, variable resistance, constant current source, etc., and is widely used in the circuit.

According to where the carriers move, there is N channel and P channel.

Depletion type and enhancement type
JFET is depletion type, and MOSFET is depletion type or enhanced type.

Depletion layer
The concentration of free electrons in the N region is high and the concentration of free electrons in the P region is low.
When an N-type semiconductor contacts with a P-type, free electrons move from the N-type to the P-type.
At this time, the N-region will form the positive ion region, and the P-region will form the negative ion region.
These two ion regions are called depletion layer.
Depletion layer does not conduct electricity and acts as an insulator.
By applying reverse bias to N-type and P-type semiconductors, the depletion layer width will change and the current can be controlled.

JFET N-channel type is to make two P-type semiconductors on either side of an N-type semiconductor.
The two P-regions are connected to the gate, and the N-region ends are connected to the source and drain respectively.
When the gate has no voltage, the free electrons in N-region can move freely and the channel is conductive.
When a reverse bias is applied to the gate, there will be a depletion layer between the P region and the N region , making the channel narrow.
When the reverse bias of the gate reaches a certain value, the depletion layer closes the channel.

Enhancement type
N-channel MOSFET are made of two N-type semiconductors on one P-type semiconductor.
The N-regions are connected to the source and drain respectively.
A very thin layer of silica insulation is added to the P-region between the N-regions, which is covered with a metal plate that acts as the gate.
When there is no gate voltage, the N-region does not conduct electricity and the channel is closed.
When a voltage is applied to the gate, the free electrons in the P-region are drawn between the N-regions, forming a channel.
When the gate voltage reaches a certain value, the channel current does not increase any more.

Depletion type
Depletion MOSFET adds positively charged sodium or potassium ions to an insulating layer to generate an electric field.
There are channels in the gate without voltage.
When the gate has a voltage, it attracts more free electrons into the channel and the current increases.
When a reverse bias is applied to the gate, the electrons in the channel are reduced until the channel current disappears, forming an depletion layer.

Characteristics of MOSFET
Because of the insulating layer, the input impedance of the gate is very high.
However, the insulating layer is easily broken down by static electricity, causing damage to MOSFET.

MOSFET capacitance
The MOSFET is equivalent to a capacitor when the channel is formed, using an insulating layer of the gate.

V-channel MOSFET
Also known as VMOS or power field effect transistor.

VMOS is a new power transistor with high current and high magnification, which is improved based on MOSFET.
The gate of VMOS adopts V-shaped groove structure, and the current is vertical flow, while the general MOSFET current is horizontal flow.
VMOS are enhancement type.

Other field effect transistors

Comparison of field effect transistor and bipolar transistor
Field effect transistor is a voltage control device, and bipolar transistor is a current control device. In the case of minimal rated current, the field effect transistor should be selected.
Field effect transistor conducts majority carriers, while bipolar transistor conduct two carriers.
Because minority carriers are sensitive to temperature, radiation and other external conditions, and easily affected by the environment, so the field effect transistor performance is more stable.
In addition to being used as amplifying device and controllable switch, field effect transistor can also be used as voltage-controlled variable linear resistor.
The source and drain of the field effect transistor can be used interchangeably, and the gate voltage of the depletion type MOSFET can be positive or negative, which is more flexible than the bipolar transistor.
The fabrication process of field effect transistor can easily integrate many unites onto a single silicon wafer, so field effect transistor have been widely used in large-scale integrated circuits.

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