Although in the previous lectures we discussed briefly the working of JFET (Junction Field Effect Transistor) and MOSFET (Metal Oxide Semiconductor Field Effect Transistor). But many times my students ask me what is the main difference between jfet and mosfet? So in this lecture we will mainly focus on the difference between JFET and MOSFET.
As you already know JFET and MOSFET both belong to the family of field effect transistors (FETs). They have plenty of advantages and applications in electronics. JFETs have two categories: n-channel JFET and p-channel JFET.
The MOSFETs also have two categories enhancement MOSFET (E-MOSFET) and depletion MOSFET (D-MOSFET). These two categories are further divided into n-channel and p-channel types.
Following are some differences between MOSFET and JFET
1. Gate is insulated in MOSFET
Apart from the symbols, the structure of a JFET transistor is different from the structure of a mosfet. We have taken examples of n-channel JFET as well as n-channel MOSFET.
You can see the structure of JFET in figure-1, the body of jfet is made of n type semiconductor material whereas the body of mosfet is made of p type semiconductor material the body is also known as p-type substrate. Both of these devices have three terminals, i.e drain, source and gate.
Fig-1: Construction of JFET and MOSFET
The mosfet has one additional terminal i.e body terminal. This terminal is not important because it is internally connected with the source. Whereas drain, source and gate are important. We use these three terminals to put mosfet into work.
In case of jfet we do doping of p-type materials on opposite sides and then we connect these two p-type regions with each other via a common terminal known as gate. This terminal is not isolated from the body of jfet.
If we give higher potential at the gate and lower potential at the drain or source the pn junction will be forward biased. This situation is undesirable and we don’t do this. In fact we control the flow of current through the JFET by using this pn junction in the reverse bias condition.
The one basic difference between both of these is the gate terminal. In case of a mosfet, the gate is not directly connected to the body. An insulating layer of silicon dioxide (SiO2) is used to isolate the gate from the body of the mosfet. Due to this it shows more resistance to current leakage and is a bit expensive too in comparison to JFET.
2. JFET Works in Depletion Mode
The second major difference between the jfets and mosfets is JFET only works in depletion mode whereas mosfet operates in both depletion and enhancement modes. Let me explain to you the difference between depletion mode and enhancement mode.
First we take the example of jfet. As JFET is a voltage controlled device. If we apply zero voltage (Vgs) between gate and source and some potential difference (Vds) between the source and drain, the current will start flowing, because the channel is already there which we made during the manufacturing process. The channel exists between two p-type regions as shown in figure-2.
In this situation a maximum current (IDSS) will flow through the jfet. We cannot enhance or increase the current. If we apply some -Vgs, the depletion region will be reverse biased and increase. So the flow of current will decrease. In other words we have depleted the flow of current through the jfet.
Watch the following lecture to understand different parameters of JFET including IDSS.
In case of a mosfet only the depletion type mosfet has a built-in channel We don’t need to create a channel as we do in e-mosfet. Because in e mosfet there is no physical channel between the drain and source until we apply some voltage at the gate.
Let’s take an example of n-channel d-mosfet. Because the channel is already built during the manufacturing process. So if we don’t apply any potential at the gate but we apply some potential difference between the drain and the source. The current will start flowing.
Depletion Mode of D-MOSFET
To operate n-channel d-mosfet in depletion mode, we apply negative voltage at the gate. The negative charge evacuates the free electrons in the channel. So more holes are created near the gate.
You can compare this situation with a parallel plate capacitor. Gate is one plate of the capacitor and the channel is the other plate whereas SiO2 layer acts as dielectric. More negative voltage results in more depletion of electrons from the channel. This reduces the channel width and also the conductivity. You keep on decreasing the voltage a voltage level will come when the channel will be completely off and drain current will be zero. Figure-3 expresses this mode.
Fig-3: N-channel D-MOSFET in depletion mode
Enhancement Mode of D-MOSFET
For the flow of current we apply positive voltage between the gate (G) and source (S). The positive voltage at G pulls free electrons of the substrate towards the gate but due to insulating SiO2 layer no electron can go into the gate. But the electrons start gathering near the gate. The width of the channel depends upon the positive voltage applied at the G. More positive voltage will create more electric field so more electrons will accumulate near the gate. This is how we enhance conductivity.
From the above discussion now I can say JFET only works in depletion mode whereas mosfet works both modes.
Remember e-mosfet will not work if we don’t apply to any potential at the gate. To understand this point you can watch my full lecture on the working of mosfet below:
3. MOSFET has More Input Impedance than JFET
The third difference is also very important. The mosfet has higher input impedance at the gate typically of the order of 10^14 Ohms. On the other side if it has input impedance of the order of 10^9 ohms.
MOSFET achieved this higher input impedance due to the insulating layer of silicon dioxide. Due to this silicon dioxide layer, the gate is completely isolated from the body or the channel. No current will flow between the gate and channel under normal operating conditions. But in the case of JFET some current leakage takes place.
4. MOSFET is more Susceptible to Damage at Higher Voltage
Although the gate is completely isolated from the body in case of a mosfet. But this advantage makes it less sensitive at high voltage.
Consider, mosfet is working in enhancement mode and you are continuously increasing the positive voltage at the gate. This will pull more and more electrons towards the gate. Not only the depletion region is increasing (i.e depletion of +ve charges/holes) but more and more electrons are also coming near the gate.
By doing this you have created a channel between the drain and the source but these positive charges have also started attracting electrons. See figure-4, red lines show force of attraction between charges.
Fig-4: Increase in depletion region in MOSFET
So at high voltage there would be higher electrostatic force of attraction between the positive charges and electrons take place. Due to this, the electron can break the insulating layer and start entering into the gate.
In case of jfet at higher voltages the depletion region will be large enough so there are less chances that electrons can break the depletion region and start entering into the drain.