Introduction to MOSFET
- MOSFET is a type of FET which is made by the oxidation of silicon.
- It is a voltage controlled device which comes with three terminals named as drain, source and gate. The voltage at the gate terminal mostly controls the conductivity between drain and source.
- MOSFET is a unipolar device i.e. conduction of current is either carried out by the movement of electrons or holes. The conduction path between source and drain is normally called as channel.
- Voltage at the gate side is used to control the width of the channel. In MOSFET gate is practically separated from the entire body by insulating layer.
- The conductivity between source and drain is highly dependent on the input applied voltage at the gate terminal, this is the main process used for amplification and switching purposes.
- MOSFET is also termed as IGFET (Insulated Gate Field Effect Transistor). MOSFET has advantage over normal BJT as it requires no input current to control the large amount of current.
- MOSFET are classified into two types in terms of their modes of operation
1: Enhancement ModeIf the conductivity between source and drain increases with the addition of applied voltage at the gate terminal, then MOSFET is said to have an enhancement mode. Enhanced Mode MOSFET are also termed as off devices. They will only conduct in the addition of positive voltage at the gate terminal.
2: Depletion ModeIf conductivity of the device decreases with the addition of applied voltage at the gate terminals, then MOSFET is said to have a depletion mode. Depletion mode is normally termed as ON device, means they will conduct when applied voltage at the gate is zero.MOSFETs are commonly used in many electronic applications. Number of MOSFETs are added in tiny memory chip or microprocessor that are widely used in cell phones and laptops.MOSFET comes with very high impedance that makes them easy to bias and a remarkable choice for linear amplification of small signals.MOSFET are mainly classified into two types in terms of construction
- N-Channel MOSFET
- P-Channel MOSFET
1: N-Channel MOSFET
- In N-Channel MOSFET conduction between source and drain is carried out by the movement of electrons.
- In N-Type MOSFET, source and drain are made up of n-type material while its body and substrate is made up of p-type material.
- In this transistor, gate is biased by applying the positive input voltage that will attract the electrons available in the p-type semiconductor substrate.
- In N-Channel MOSFET the drain and source are composed of N type material while body and substrate is composed of P type material.
- The gate of this transistor is used composed of layer of poly-silicon.
- When we add Silicon dioxide on the layer of substrate it gives the typical metal oxide semiconductor construction.
- Silicon dioxide is a dielectric material so it will act as a capacitor where one of its electrodes will be replaced by the semiconductor.
- As we apply positive voltage at the MOS structure, it will alter the charge distribution in the semiconductor. With the addition of positive voltage, the holes present under the oxide layer will encounter a force and allow the holes to move downward. The depletion region will be accumulated by the bound negative charges which are connected with acceptors atoms.
- The over supply of free electrons available in the p-type substrate increases the overall conductivity of channel, and constantly inverts the electrical properties of p-type substrate, allowing the substrate to change into n-type material.
- The movement of electron is controlled by the positive voltage applied on the gate terminal. As we increase the positive change at the gate terminal the more it will attract the electron, hence resulting in widening the channel path between source and drain terminals. Thus, increasing the positive voltage at the gate terminal will increase the overall conductivity of the transistor.
- In P-Channel MOSFET, conduction of current is carried out by the movement of holes.
- In P-Channel, the drain and source are composed p-type material and body and substrate is composed of n-type material.
- As we apply negative voltage at the gate terminal, it allow the electron in the oxide layer to move downward in the substrate layer with strong repulsive force.
- Hence, positive holes will be accumulated around the gate region.
- The negative voltage applied at the gate terminal will also attracts the holes, hence n-type substrate will produce the p-type conducting channel.
Enhancement MOSFET Amplifiers:
- In the composition of enhancement MOSFET, there must be minimum input gate-source voltage is applied to the gate before it starts conducting, this minimum voltage is called threshold voltage.
- In order to conduct for these enhancement amplifiers, the gate source voltage Vgs must be greater than threshold voltage.
- Drain current Id will increase by increasing the forward biasing of MOSFET, making them suitable for efficient amplifier circuits.
- When we apply fixed voltage between drain and source Vds, we can plot the values of drain current Id for different values of the voltage across gate and source Vgs.
- These VI characteristics show the transconductance of the the MOSFET. This transconductance is the ratio between the output drain current to the input gate-source voltage.
- For fixed value of Vds, the slope of transconductance can be found as
- This ratio is termed as transconductance which is a short form of "transfer conductance". The SI unit of transconductance is Siemens which is ampere per volt.
- Voltage gain of this MOSFET increases with the increase in transconductance and value of drain resister.
- At Vgs=0, N-Type enhancement MOSFET acts like an open switch or normally off, because field effect won't be able to open the N-Type channel around the gate.
- Thus transistor will fall under "cut-off" region at this point. The OFF condition of the eMOSFET is represented by the dotted line unlike depletion region which shows continuous line, showing conduction region of the transistor.
- As we apply gate-source voltage Vgs at the gate terminal, it will start to conduct in the region between source and drain.
- The voltage at which transistors starts conducting is known as threshold voltage and is represented by Vth.
- As we increase the gate-source voltage it will allow the conducting channel go wider and ultimately increases drain current Id.
- It is important to note that gate never conducts as it is practically isolated from the conducting channel between source and drain. MOSFET encompasses high impedance which is useful in many electrical amplifying circuits.
- If the threshold voltage is greater than gate-source voltage, then the channel will not conduct, it will only conduct when threshold voltage will be less than gate-source voltage Vgs.
- In the conduction or saturation region drain current can be calculated as
- Id= K(Vgs-Vth²)
- It is important to note that value of threshold voltage Vth and K(conduction parameter) are different for different eMOSFET, these values don't vary physically as they come by default during the composition of the material from which transistor are made.
- It is clear from the figure above that graph on the right side starts as a parabola and then it becomes linear, and it gives the slope of characteristic curve that increases with the increase in drain current for fixed value of drain-source voltage Vds.
- In order to put the MOSFET in ON state, gate of the transistor must be biased from its given threshold level.
- Biasing of gate terminal can be achieved using two different method i.e. zener diode biasing, and drain feedback biasing. Before biasing you must take one point into consideration that gate voltage must be greater than source by a value greater than threshold voltage.
Comparision between MOSFET and BJT
- Major difference between BJT and MOSFET is that BJT are bipolar devices in which conductivity is carried out by the movement of both electrons and holes while MOSFET are uni-polar devices in which conduction is carried out by movement of electrons or holes.
- The three terminals in BJT called emitter base collector are the analogous of MOSFET three terminals called as source gate and drain respectively.
- Another area where MOSFET differs with BJT is that there is no direct connection between gate and conducting channel of source and drain, unlike BJT where small current at the base side is used to control the large current at the emitter and collector side. That's the reason MOSFET are also named as IGFET (Insulated Gate Field Effect Transistor)
- MOSFET is a type of FET that is a unipolar device i.e. single charge carriers are responsible for the conduction between source and drain.
- Voltage applied at the gate side is used to control the current flowing through conducting channel between source and drain.
- MOSFET is a voltage controlled device unlike BJT which are current controlled device.
- Practically gate of the MOSFET draws no current. However, small amount of initial current is needed to charge the capacitance of the gate terminal.
- MOSFET is mostly used as a swtich
- It is widely used in many applications where high amplification is required.