About the MOSFET
If you look at the structure of the MOSFET...
Metal -> Oxide -> Semiconductor structure as above, right?
Here, there is something called Oxide, which applies voltage to the gate, but current does not enter the device.
Because Oxide is an insulator!
If so, the gate becomes the input of the signal. The fact that the gate current does not flow through the device means that the input impedance is high.
Also, since the current consumption is reduced that much, it will be suitable for low-power circuits.
That's why MOSFETs are used a lot in low power circuits.. These days, when low power is in the spotlight, it has become more popular than BJTs and JFETs.
So, shall we look at the circuit symbol of the MOSFET?
MOSFET can be classified into 4 types as above.
I think you know well that it is divided into N channels and P channels..
In addition, each channel type can be divided into Depletion type (depletion type) and Enhancement type (incremental type).
As you can see from only the circuit symbol, P-type and N-type are separated in the direction of the arrow.
It is possible to distinguish between the depletion type and the increase type by whether the middle line is broken..
But, is this wire broken? Is it stuck? I think this alone is the best way to distinguish between the depletion type and the increase type MOSFET! You'll find out why.
First, let's take a look at the depletion MOSFET.
The basic structure and operation of the Depletion type MOSFET is shown in the figure above. First, the structure is an N-channel type~
Originally, the depletion MOSFET has a non-zero drain current Id even when Vgs=0. In other words, the drain current flows.
Because a channel through which electrons can pass (the path through which carriers travel in a JFET is called a channel) is formed
Because there is.
However, if you apply reverse voltage between Gate and Source as shown on the left in the picture above...
In the gate, negative charges with -polarity accumulate..(Because of oxide, it cannot enter the device!!)
By this negative charge, the holes of the P-type substrate will be attracted toward the gate.
Then, the P-type area is expanded and the middle N-channel area is gradually reduced.
In the end, there will be times when it will completely block it.. If that happens, the drain current will no longer flow because the channel is gone.
However, if you apply a + voltage to the gate in the opposite direction as shown on the right...
Since the P-type holes are pushed out by the repulsive force, the N channel is further expanded.. As a result, the movement of electrons becomes easier, so the drain current will increase.
However, if you think about it, when we normally use it, it is used to pass or block the current between the drain and the source through the gate.
There will be many times...
In that case, since the drain current can flow even when Vgs=0, the Deplation type MOSFET should be designed with an emphasis on blocking the reverse. So, it should be designed to apply a reverse voltage to the gate to use it properly.
Then the purpose of the gate is to use it to reduce the width of the channel...
Compared to the dam, the sluice gate becomes the gate.. Water is electrons. In other words, when the sluice gate is gradually closed, the flowing water (electrons) decreases.
Due to the characteristics of such a depletion MOSFET, the I-V curve as above is drawn. As in the case of a JFET, if something goes wrong,
Id does not increase any more and enters the saturation region. The reason for going to saturation is the same as for JFET!!
Then, let’s continue looking at the enhancement type MOSFET~
Unlike depletion type MOSFETs, when Vgs=0, no channel is formed, so the drain current Id=0.
In other words, if a voltage is not applied to the gate differently from the depletion type, the channel is not formed and the carrier cannot move, so current cannot flow!
Operation can be said to be a principle similar to the depletion type.
When + voltage is applied to the gate as shown in Figure 2 above, the holes in the P-type substrate are pushed only at the gate side...
With such a space, the electrons in the source and drain regions can move around quickly. Of course, this space is a path for electrons.
So it will be an N-channel.. It is also called an Invertion Layer because it is opposite to the P-Substrate due to its characteristics.
However, no current flows until this channel is formed. In other words, the voltage must be applied to the gate so that a channel is formed.
It means everything... It's a similar principle as the current rapidly increases from a voltage of 0.6~0.7V in a diode~
The gate voltage at the point where the current rapidly increases due to the formation of the channel is called Threshold Volatege and is called the threshold voltage.
That is why this threshold voltage becomes a very important factor in an incremental MOSFET, so it is important to remember it.
In the above picture, if you look closely at the shape of the channels in Figs. 3 and 4, you can see that they are focused to one side.
As we said that the + voltage is applied to the drain side of the JFET, so the depletion layer tends to one side.
In the MOSFET as well, as Vds increases, the depletion layer on the drain side develops more than the depletion layer on the source side.
uh? But after that, the drain channel becomes too narrow and eventually the channel disappears..
But~~~~~~ Drain current doesn't decrease or increase as we summarized in JFET~
Again, it is because there is an electric field force between the drain and the source. However, it enters the saturation region as in the JFET.
Take a look at the graph below~
In other words, at Vds above a certain level as above, the drain current does not increase any more and becomes saturation.
Also note that the above graphs are all + voltages, unlike depletion MOSFETs... Also, as I said before, there is a threshold voltage!