Effect tubes and their types
Field Eeffect Transistor (FET) is a semiconductor device that uses the electric field effect of the input circuit to control the output circuit current. Because it only relies on the majority carriers in the semiconductor to conduct electricity, it is also called a unipolar transistor.
According to different structures, they can be divided into two categories: Junction Field Effect Transistor (JFET) and Insulated Gate Field Effect Transistor (IGFET). The latter uses silicon dioxide as an insulating layer between the gate source and gate drain, and is also known as a metal-oxide-semiconductor field effect transistor (MOSFET, metal-oxide-semiconductor, referred to as MOS tube).
Each category has two types: N-channel and Р-channel. Among them, MOS tubes can be divided into two types: enhancement type and depletion type.
4 junction field effect tube
4.1 Working principle of junction field effect transistor
A negative voltage ( ) is applied between the gate and the source to ensure that the depletion layer withstands the reverse voltage.
A forward voltage is applied between the drain and the source to form a drain current
.
, which not only ensures the high internal resistance between gate and source, but also achieves
control of the channel current.
We often say that gate-source voltage and drain-source voltage
have an impact on the conductive channel. (
(1) When (that is, when d and s are short-circuited),
it has a controlling effect on the conductive channel. At this time
, the depletion layer is very narrow and the conductive channel is very wide. With
the increase of , the depletion layer widens, the channel becomes narrower, and the channel resistance becomes larger. This is called I will take the stage after you sing. When
it increases to a certain value, the depletion layer is closed, the channel disappears, and the channel resistance tends to infinity.
The value at this time is called the pinch-off voltage
.
(2) When it is
a certain fixed value, it has an impact
on the drain current
.
(3) At that time ,
the person
showed control.
4.2 N-channel enhanced MOS tube structure
N-channel enhancement mode MOSFET is basically a left-right symmetrical topology. It generates a SiOz thin film insulating layer on the Р-type semiconductor, and then uses a photolithography process to diffuse two highly doped N-type regions. The type area leads to two electrodes, the drain D and the source S. A layer of metallic aluminum is plated on the insulating layer between the source and drain electrodes as the gate electrode G. The Р-type semiconductor is called the substrate and is represented by the symbol B. Because this kind of MOS tube =0 when Vcs=0V; the drain current will only appear when Us>Ucs(th), so it is called an enhancement MOS tube.
4.3 Working principle of N-channel enhancement MOS tube
(1) When the working condition of the pinch-off area is
Ucs=0, there are two PN junctions connected in reverse series between D and S, and there is no conductive channel. No matter what polarity of voltage is added between D and S, the drain current is close to at zero; when 0<Ucs <Ucs(uhs. The electric field directed from the gate to the direction of the substrate causes holes to move downward and electrons to move upward, forming a depletion layer on the upper surface of the Р-shaped silicon substrate, and there is still no drain current.
(2) When the working condition of the variable resistance region
is Ucs>Ucsth, an N-type conductive channel (inversion layer) is formed on the surface of the Р-type semiconductor under the gate. If a forward voltage is applied between D and S, a drain current stone can be generated. . If ups<1cs-Usth), the channel is not pinched off, and corresponding to different ucs and ds, they are equivalent to resistors with different resistance values. At this time, the FET is equivalent to a voltage-controlled resistor.
(3) Working conditions in the constant current area (or saturation area)
. When uIps=lcs-Ucsah, the channel is pre-pinched off; if ups>ucS-UGs(h), the channel has been pinched off, and ip is only determined by ucs, and has nothing to do with ulps. At this time, i, is approximately regarded as a current source controlled by ucs, and FET is equivalent to a voltage-controlled current source.
It can be seen that for N-channel enhancement mode MOS transistors, the gate-source voltage Vcs has a controlling effect on the conductive channel, that is, when Ucs>Ucsct, a conductive channel can be formed to communicate between the drain and the source. If the drain-source voltage is applied at this time, the drain current I can be formed.
When the field effect transistor operates in the constant current region, the electric field generated by the applied voltage ucs between the gate and the source is used to change the width of the conductive channel, thereby controlling the drain current I generated by the multi-sub drift motion. At this time, l can be regarded as a current source controlled by voltage ucs.
4.4 N-channel depletion mode MOSFET
N-channel depletion-mode MOSFET has a large number of positive metal ions doped into the SOz insulating layer below the gate, so when Ucs=0, these positive ions have induced the inversion layer and formed a channel. As shown in Figure 1.48 on P45. Therefore, as long as there is a drain-source voltage, there is a drain current. When Uos>0, it will increase further. When Ucs<0, the drain current gradually decreases as Ues decreases until I=0. Ucs corresponding to stone = 0 is called the pinch-off voltage, represented by the symbol UGs(off).
4.5 P-channel enhancement mode and depletion mode MOSFET
The working principle of Р-channel MOSFET is exactly the same as that of N-channel MOSFET, except that the conductive carriers are different and the polarity of the supply voltage is different. This is just like the bipolar transistor has NPN type and PNP type.
4.6 Volt-ampere characteristics of field effect tube
There are many types of characteristic curves of field effect transistors. There are four transfer characteristic curves and output characteristic curves depending on the conductive channel and whether it is enhancement type or depletion type. Its voltage and current directions are also different.
Taking the enhancement type N-channel MOSFET as an example,
the output characteristics: ip=f(ups)│ Us=-constant reflects the influence of Uos on I when UGs>Ucsth) and is fixed to a certain value; transfer characteristics: iD-8ucs) l Ups=constant reflects the control relationship of Ucs on the drain current;
the output characteristics and transfer characteristics reflect the same physical process of field effect transistor operation. Therefore, the transfer characteristics can be obtained from the output characteristics using a diagram method in one-to-one correspondence.
The output characteristics of the field effect transistor can be divided into four areas: pinch-off area, variable resistance area, saturation area (or constant current area) and breakdown area. In amplifier circuits, field effect transistors work in the saturation region.
4.7 Main parameters of field effect tube
(1) DC parameters
① Turn-on voltage (Uosuh): Turn-on voltage is a parameter of the MOS enhancement tube. If the gate-source voltage is less than the absolute value of the turn-on voltage, the field effect transistor
cannot be turned on.
②Pinch-off voltage Uas(ofl): Pinch-off voltage is a parameter of depletion mode FET. When Ucs=Ucsom, the drain current is zero.
③Saturation drain current Ioss: Ioss is a parameter of the depletion mode FET, the corresponding drain current when Uas-0.
④DC input resistance Rgs Dc: Gate-source input resistance of FET. For JFET, Rcs is approximately greater than 107Q when reverse biased; for MOSFET, RGs is approximately 109~10159.
(2) Communication parameters
① Low-frequency transconductance gm: Low-frequency transconductance reflects the control effect of gate voltage on drain current, which is very similar to the control effect of electron tubes. m can be found on the transfer characteristic curve, and the unit is mS (milliSiemens).
②Inter-stage capacitance: There is inter-electrode capacitance between the three electrodes of FET. Usually Cs and Cga are about 1~3pF, while Ca is about 0.1~lpF. In high-frequency circuits, the influence of interelectrode capacitance should be considered.
(3) Limit parameters
①Maximum drain current IDM: It is the upper limit of the drain current when the FET is operating normally.
②Drain-source breakdown voltage UBR)Ds: After the FET enters the constant current region, the ups value that causes ip to suddenly increase is called the drain-source breakdown voltage. Uips exceeding this value will cause the tube to burn out.
③Maximum power dissipation PoM: It can be determined by PoM=VDsIo, which is equivalent to the Pcm of a bipolar transistor.
4.8 Comparison of field effect transistor FET and transistor BJT
FET is another type of semiconductor device. In FET, only multiple carriers participate in conduction, so it is called unipolar transistor. In ordinary transistors, both majority carriers and minority carriers participate in conduction, so it is called bipolar transistor. Transistor (BJT). Since the concentration of minority carriers is easily affected by temperature, FET is superior to BJT in terms of temperature stability and low noise.
BJT is a current control device that controls the output current by controlling the base current. Therefore, there is always a certain current in the base, so the input resistance of BJT is low; FET is a voltage-controlled device, and its output current depends on the voltage between the gate and source. The gate takes almost no current, so the input resistance of FET is very low. High, it can reach 10~104Q. High input resistance is a prominent advantage of FETs.
The drain and source of the FET can be used interchangeably, and the gate voltage of the depletion-mode MOS tube can be positive or negative. Therefore, the structure of the FET amplifier circuit is more flexible than the BJT amplifier circuit.
Both FETs and BJTs can be used for amplification or controllable switching. But FET can also be used as a voltage-controlled resistor, can work under micro-current and low-voltage conditions, and is easy to integrate. It is widely used in large-scale and very large-scale integrated circuits.