Working principle of an Field Effect Transistor
FET is a transistor in which current is controlled by varying the resistance of the channel from where the current is passing. The resistance is controlled by electric field.
Type of Field Effect Transistor
Depending upon the construction. FETs are divided into two categories
- Junction Field Effect Transistor (JFET)
- Metal-Oxide-Semiconductor Field Effect Transistor (MOSFET)
MOSFET is very popular type of transistor which is a sub-category of field effect transistor. MOSFET full form is “ Metal Oxide Semiconductor Field Effect Transistor”. Just like JFET, it has three terminals gate (G), source (S) and drain (D). There is an insulating layer of silicon dioxide (SiO2) between the gate and the channel.
It has become a backbone of today’s modern technology products. Advanced microcontroller is based on MOSFET technology. One reason is that switching speed of mosfet transistors is fast as compared to BJT. The small size is another reason. These features are very important that is why its has extensive applications in electrical and electronics industry.
Mosfet transistor has further two types
- Enhancement MOSFET (E-MOSFET)
- Depletion MOSFET (D-MOSFET)
You can also watch this comprehensive tutorial to understand the basics
If You want to know more about these two types continue reading this article.
Enhancement MOSFET (E-MOSFET) Operation
Figure-1 explains the structure of enhancement MOSFET. For the sake of simplicity we are considering n-channel. The working of p-channel enhancement MOSFET is opposite to it.
The body of the E-MOSFET is made of p-type material which is also known as substrate. The body terminal and the source are usually connected internally. The n-type source and drain are diffused into the body of MOSFET.
Fig-1: MOSFET Construction
For the flow of current we apply positive voltage between the G and S. Please note If we apply the potential difference between the D and the S without applying any potential at the gate. The depletion region will increase subject to which terminal is at a higher potential (See Fig 2). For example If we connect the positive terminal of the battery with the drain and negative terminal with source. Depletion region between n-type drain and p-type substrate will be reverse biased and the junction between n-type source and p-type substrate will be forward biased. So no current will flow in that case. Similar is the case if we reverse the polarity.
Fig-2: Voltage applied between drain and source with gate voltage = 0
This is why we apply positive voltage at G. 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 (See Figure-3).
Fig-3: Electrons gather near gate
This creates a channel between the D and S within substrate. 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. As there is no direct or physical connection between the G and the substrate but the electric field between them is the cause of this channel. This is why it is also called induced channel of MOSFET. (See Fig-4)
Fig-4: Induced Channel MOSFET
Enhancement MOSFET Symbol
Depletion MOSFET (D-MOSFET) Operation
The main difference between depletion MOSFET and enhancement MOSFET is that in D-MOSFET the channel is already diffused during the manufacturing process. So channel is already there we don’t need to create it by applying potential difference at the gate.
Modes of operation of depletion type MOSFET
This works in following two modes
1. Depletion Mode
To operate in this mode we apply negative voltage at the gate in case of n-channel. The negative charge at the gate evacuate the free electrons in the channel. So more holes are created near the gate. You can compare this situation with parallel plate capacitor. Gate is one plate of the capacitor and 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 time will come when channel will be completely off and drain current will be zero. Figure-5 expresses this mode.
Fig-5: Depletion mode of n-channel D-MOSFET
2. Enhancement Mode
In this mode we apply positive voltage at the gate. This attracts electrons towards the gate. As a result, the conductivity of the MOS increases. Refer the figure-6 to understand this mode
Fig-6: Enhancement mode of n-channel D-MOSFET
Note: The working of p channel D-MOSFET in both modes is the same except the polarities are reversed.
Depletion MOSFET Symbol
MOSFET Characteristics and Parameters
Enhancement MOSFET Characteristics
You can observe in figure-7 there is no drain current below a threshold voltage (Vth). Below Vth the resistance is infinite. So we get no parameter in this area. After Vth the current increases significantly. The equation that represent E-MOSFET transfer characteristic curve is given below
Where gk is the gradient which can be found by using the following formula
ID(on) is provided on the data sheet depending upon the type of MOSFET.
Fig-7: N-channel E-MOSFET transfer curve
For the p-channel you give negative voltage at the gate so the curve is a reflection of the above curve i.e -VGS and -Vth.
Depletion MOSFET Characteristics
As we discussed above that D-MOSFET can be operated by applying positive as well as negative voltage at the gate. Figure-8 shows the transfer curve for n-channel D-MOSFET. Recall this curve is just a replica of transconductance curve of JFET. So the same square law equation is applicable here.
Fig-8: N-channel D-MOSFET transfer curve
MOSFET Biasing Techniques
Biasing means basically the way or ways with which we can put an electronic device into working. The biasing of MOSFET is done by the following ways
- Voltage-Divider Bias
- Drain-Feedback Bias
Enhancement MOSFET Biasing
It is clear from the fig-7 that E-MOSFET will not work before threshold voltage. So instead of zero bias we will use two other methods to bias E-MOSFET.
Voltage Divider Bias
E-MOSFET (n-channel) circuit diagram as voltage divider bias is given below
Fig-9: N-channel E-MOSFET Voltage Divider Circuit
We applied positive voltage at the drain because this is n-channel MOSFET for the p-channel we will apply negative voltage as we discussed above. This positive voltage is greater than Vth.
The voltage at the gate is
And drain to source voltage can be calculated as
The ID can be calculated from equation (1)
In this biasing state, feedback is taken from the drain and given to the gate. The circuit is given below
Fig-10: Common Source Drain Feedback Biasing
Depletion MOSFET Biasing
A simple method of biasing D-MOSFET is used also known as self-bias method. If you observe in transfer curve of D-MOSFET in figure-8 above. You will see that max current ( IDSS ) passes between drain and source when VGS = 0. (See Fig-11)
Fig-11: D-MOSFET Biasing
Operating regions of MOSFET
Similar to JFET, MOSFET operations can be explained in three regions given below.
1. Cutoff Region
In the cutoff region there is no current flows through the MOS device. In case of n-channel mosfet. When VGS is less than Vth then no channel forms or in other words negligible amount of current flows from drain to source
But in the case of p-channel we give negative voltage at the gate. So cut off region is above the Vth.
2. Linear Region
This region also known as Ohmic region. When VGS is greater than or equal to Vth, channel forms. The current starts flowing if drain voltage is greater the source. The VDS must be lesser than VGS– Vth.
For the p-channel everything explained in the above paragraph should be reversed to achieve linear region.
3. Saturation Region
If you increase VDS such that VDS>VGS – Vth, then n-channel MOSFET goes into saturation. Means further increment on VDS doesn’t increase ID.
MOSFET Handling Techniques
The gate the insulated so high input resistance and capacitance can accommodate very high static charge. This charge can damage the MOSFET. That is why one should be aware that how to handle MOSFET when used in a circuit.
- Usually a wire ring is rounded on the MOSFET leads. Don’t remove them until you are just going to install it in the circuit. Otherwise if leads touched each other, leakage current from the gate can damage the device
- Don’t remove MOSFET device when the circuit is ON
- All testing instruments should be grounded to earth ground.
MOSFET vs BJT
There are many reasons we can compare between both types of transistors
- BJT contains both types of currents i.e hole and electrons. In other words you can say the current contains both majority and minority carriers. Whereas in MOSFET most of the current consist of majority carriers.
- The input resistance of MOSFET is very high as compared to BJT. You can observe from the above discussion on MOSFET, the resistance is almost infinite because gate is insulated from the body of the MOSFET. That is why MOSFETs are good amplifiers as compared to BJTs.
- MOSFET are smaller in size as compared to BJT. So one BJT can be replaced by many MOSFETs. Small size is one of the major reasons software speed.
- MOSFETs consume less power as compared to BJTs. BJTs have constant voltage drop equal to their barrier potential.
Some Common Questions and Their Answers
Why MOSFET is called IGFET?
IGFET means “Insulated Gate Field Effect Transistor”. As you can see in figure-1 above the gate is insulated from the main channel with SiO2 layer. Thats is why it is called IGFET.
Why MOSFET is a majority carrier device?
If you remember BJT current flows from emitter to base and then to collector. In the case of NPN transistor electrons are majority carriers in emitter and they are the minority carriers in the base. So the current in BJT contains both minority and majority carriers. But if you observe above figures from fig-3 to fig-6. Either it is enhancement MOSFET (P or n- channel) or depletion MOSFET (p or n-channel) the channel is always made by one type of carriers (holes or electrons) which are in majority in the channel. That is why MOSFET is called a majority carrier device.
Why MOSFETis better than JFET?
Let’s compare n-channel MOSFET and JFET to understand this question. In case of n-channel JFET if we apply positive pulse at the gate the junction will be forward biased. This is undesirable because JFET is designed to work by reverse biasing the PN junction. In MOSFET the gate is insulated from the channel. But MOSFET is also sensitive to electrostatic charge.
Why MOSFET is called unipolar?
Because only one type of current passes through the channel i.e hole current or electronic current
Who invented MOSFET?
Muhammad Atalia and Dawon Kahng invnted MOSFET in Nov 1959.
When does MOSFET turn on?
Every MOSFET has threshold value provided on the data sheet. Providing voltage greater than threshold voltage turns ON the MOS device.