MOSFET or Metal-Oxide-Silicon Field-Effect Transistor is a type of field-effect transistor (FET) that is commonly fabricated using silicon. It features an insulated gate, the voltage of which determines the conductivity of the device. MOSFETs are used for amplifying or switching electronic signals and are known for their high input impedance, which results in high switching speed.
In this article, we will discuss MOSFET in detail including its types, working as well as application.
What Is a MOSFET?
MOSFET is the abbreviation of Metal Oxide Field Effect Transistor. MOSFET was developed to improve the disadvantages present in FETs such as high drain resistance, moderate input impedance, and lower operation speed. Therefore, a MOSFET is the improved version of FET.
In some cases, MOSFETs are also called IGFET (Insulated Gate Field Effect Transistors). They have three primary terminals—the source, drain, and gate—and can operate in either enhancement mode (requiring a nonzero gate-to-source voltage to turn on) or depletion mode (turning off when a certain gate-to-source voltage is reached).
MOSFET Symbol
Symbol for MOSFET in an electric circuit is:
Structure of MOSFET
MOSFETs are constructed using a semiconductor material, typically silicon, with a thin layer of insulating oxide separating the gate from the semiconductor channel. MOSFET is a four-terminal device consisting of the following components:
- Gate: An insulated gate made of polysilicon or metal that controls the flow of current between the source and drain.
- Drain: A heavily doped p+ region or n+region that receives the majority charge carriers when the transistor is turned ON.
- Source: A lightly doped p+ region or n+ region that supplies the majority charge carriers when the transistor is turned ON.
- Body (substrate): A p-type or n-type semiconductor material that serves as the foundation for the transistor and provides electrical connection to the source.
Other than these parts there are some more components in MOSFET:
Channel: The oxide layer, which forms between the source and drain, is the insulator. The gating of this channel is determined by the electric field, which is produced by the gate voltage.
Oxide Layer: An insulating layer (normally silicon dioxide) is put between the gate and the semiconductor, so that the gate cannot make a direct electrical contact with the semiconductor.
MOSFET Diagram
The diagram of a MOSFET in a circuit as a switch is given as follows:
Working of MOSFET
Working of MOSFET can be understand as follows:
- Formation of Channel: The application of voltage to the gate produces an electric field which, in its turn, attracts or repels carriers of charges in the semiconductor. This alters the carrier density in a region known as the channel, creating a conduction path for current flow.
- Controlling the Flow: For an N-channel MOSFET, a positive supply to the gate lures electrons, therefore, it becomes a conductive channel between the source and drain terminals. In a P-channel MOSFET, a negative voltage applied on its gate brings holes to experience attraction.
- Operation Modes: MOSFETs can function in three modes - cut-off, saturation, and triode. As there is no current in the cutoff mode since the path is blocked. When the channel is fully opened, it is in saturation and maximum current can flow. Triode has a channel that is partly open and hence allows only a controlled current flow.
- Voltage Control: The essential advantage of MOSFET is its voltage controlled conductance operation that allows the current to be modulated precisely according to the input signal voltage. This feature is important for them being able to act as components in digital circuits.
Operation Region of MOSFET
The functioning of a MOSFET can be classified into different regions depending on the voltages applied at its terminals. The three main operation regions are:The three main operation regions are:
- Cut-off Region (VGS < Vt): In this area, the gate-to-source voltage (VGS) is lower than the threshold voltage (Vt) and the MOSFET is in the off state. The channel is not conducting and the current from the drain to the source is very small. MOSFET is an open switch in this region.
- Triode Region (VGS > Vt, VDS < VGS - Vt): In the triode region the gate-to-source voltage (VGS) exceeds the threshold voltage (Vt), and the drain-to-source voltage (VDS) is also significant but less than the difference between VGS and Vt. In this region, the MOSFET behaves as a variable resistor. Both VGS and VDS control the drain current (ID) and the MOSFET is not fully on.
- Saturation Region (VGS > Vt, VDS ≥ VGS - Vt): In the saturation region, both VGS and VDS voltages are significant. The MOSFET is completely on, and there is a linear relationship between ID and VDS.The MOSFET acts as an voltage-controlled current source in this region. The saturation is the region of interest for many applications like amplifiers and digital logic circuits.
Types of MOSFET
MOSFET can be categorized based on different parameters i.e.,
Based on the polarity of the channel, MOSFET can be classified as:
Based on the gate voltage, MOSFET can be classified as:
- Depletion Mode
- Enhancement Mode
Note: In each mode, depletion and enhancement both n and p MOSFET can be possible.
Lets Discuss these types in detail as follows:
n - MOSFET
An n-channel MOSFET (also known as n-MOSFET) is a type of Metal Oxide Semiconductor Field Effect Transistor (MOSFET) wherein the channel consists primarily of negatively charged electrons as current carriers i.e., n-type semiconductor.
When a positive voltage is applied to the gate relative to the source, an electric field is created in the channel, controlling the flow of electrons from the source to the drain. Typically, the threshold voltage for an n-MOSFET is negative, meaning a positive voltage must be applied to the gate to enable conduction.
p-MOSFET
A p-MOSFET (p-type Metal-Oxide-Semiconductor Field-Effect Transistor) is a type of MOSFET where the semiconductor channel is made of p-type material. It operates by applying a negative voltage to the gate terminal, creating an electric field in the channel. When the gate voltage is less than the threshold voltage, the MOSFET conducts current from the drain to the source.
Depletion Mode
A Depletion Mode MOSFET is a type of MOSFET that is normally on at zero gate-source voltage. Depletion Mode MOSFETs are used as load "resistors" in logic circuits and can be turned off by applying a negative gate voltage. These MOSFETs are ideal for applications like power supply startup power, over-voltage protection, and in-rush-current limiting.
They provide a constant current source and can function as voltage regulators. Infineon Technologies offers a range of N-channel Depletion Mode MOSFETs with voltages from 60 V to 600 V, providing higher efficiency and ruggedness for various applications.
Enhancement Mode
An Enhancement Mode MOSFET is a type of MOSFET that is normally off at zero gate-source voltage. When a positive voltage is applied to the N-channel gate terminal, the channel conducts and the drain current flows through the channel. If this bias voltage increases to more positive, then the channel width and drain current through the channel increases. If the bias voltage is zero or negative, then the transistor may switch off and the channel is in non-conductive mode.
Enhancement Mode MOSFETs are commonly used as switches in electronic circuits because of their low ON resistance and are used to make logic gates and in power switching circuits, such as CMOS gates, which have both NMOS and PMOS Transistors.
Applications of MOSFET
Some common applications of MOSFETs are:
- Amplification: MOSFETs in amplifiers are used to intensify electronic signals, making them the key parts of the audio amplifiers, RF amplifiers and operational amplifiers.
- Switching Circuits: MOSFETs are used to guide the current flow as electronic switches, which is important in digital circuits. These are used in logic gates, memory cells, and microprocessor designs.
- Voltage Regulators: MOSFETs are used in the voltage regulation and voltage regulation circuits, which are responsible for providing stable and regulated output voltages in power supplies.
- Power Amplifiers: High-power MOSFETs are employed in power amplifiers of audio systems, RF transmitters and power inverters.
- Radio Frequency (RF) Devices: MOSFETs are critical to RF amplifiers, oscillators and mixers for the wireless communication systems such as cell phones, Wi-Fi routers and satellites.
- Analog Signal Processing: MOSFETs are widely adopted in many analog signal processing circuits like filters, oscillators, and analog-to-digital converters.
MOSFET as Switch
Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET) can be utilized as a switch in electronic circuits. The MOSFET has three terminals: the gate (G), the source (S), and the drain (D). Depending on the voltage applied to the gate terminal, the MOSFET can either be in an "on" state (conducting) or an "off" state (non-conducting), effectively acting as a switch to control the flow of current between the source and drain terminals.
MOSFET as Capacitor
MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) can be utilized as a component in a circuit that behaves like a capacitor, although it's not typically used for this purpose due to its primary function as a voltage-controlled switch. However, under certain conditions, the gate-source capacitance (Cgs) of a MOSFET can exhibit capacitor-like behavior.
Difference between MOSFET and BJT
The key differences between MOSFET and BJT are:
| Feature | MOSFET | BJT |
|---|
| Full Form | Metal-Oxide-Semiconductor Field-Effect Transistor | Bipolar Junction Transistor |
|---|
| Structure | Three-terminal device: Source, Drain, Gate | Three-terminal device: Collector, Base, Emitter |
|---|
| Principle of Operation | Majority carrier device: controlled by gate voltage | Minority carrier device: controlled by base current |
|---|
| Current Carrying Mechanism | Majority carrier (electrons or holes) | Minority carrier (holes or electrons) |
|---|
| Input Impedance | High (insulated gate) | Low (input current controls output current) |
|---|
| Output Impedance | Moderate to High (depends on circuit) | Low (depends on circuit) |
|---|
| Switching Speed | Faster | Slower |
|---|
| Gain | Voltage-controlled | Current-controlled |
|---|
| Saturation Voltage | Typically lower (near 0V) | Typically higher (around 0.2V to 0.7V) |
|---|
| Temperature Sensitivity | Low | Moderate to high |
|---|
| Thermal Stability | Generally more stable | Less stable |
|---|
JFET and MOSFET
Common differences between JFET and MOSFET are:
| Feature | JFET | MOSFET |
|---|
| Full Form | Junction Field-Effect Transistor | Metal-Oxide-Semiconductor Field-Effect Transistor |
|---|
| Structure | Three-terminal device: Source, Drain, Gate | Three-terminal device: Source, Drain, Gate |
|---|
| Channel Type | Majority carrier device: controlled by gate-source voltage | Majority carrier device: controlled by gate-source voltage |
|---|
| Channel Composition | Semiconductor channel formed by reverse-biased pn-junction | Semiconductor channel formed by insulated gate |
|---|
| Gate Construction | No insulating layer between gate and channel | Insulating layer (oxide) between gate and channel |
|---|
| Voltage Applied | Gate-source voltage controls channel conductivity | Gate-source voltage controls channel conductivity |
|---|
| Conductivity Control | Voltage-controlled resistor | Voltage-controlled resistor |
|---|
| Input Impedance | High (insulated gate) | High (insulated gate) |
|---|
| Output Impedance | Moderate to High (depends on circuit) | Moderate to High (depends on circuit) |
|---|
| Switching Speed | Faster than MOSFETs | Generally slower than JFETs |
|---|
| Voltage Ratings | Low to medium voltage applications | High voltage applications |
|---|
| Temperature Sensitivity | Moderate | Low |
|---|
| Thermal Stability | Generally more stable | Generally more stable |
|---|
Advantanges and Disadvantages of MOSFETs
Metal Oxide Semiconductor Field Effect Transistors (MOSFETs) offer several advantages over other transistors, making them widely used in various electronic applications. Some of the common advantages of MOSFETs are listed below:
- Power Efficiency: MOSFETs provide exceptional power efficiency due to their low on-resistance and negligible static power consumption.
- High Input Impedance: MOSFETs have a very high input impedance due to the insulation layer between the channel and the gate electrode, allowing them to operate without any gate current.
- High Drain Resistance: They exhibit high drain resistance due to the lower resistance of the channel, contributing to their efficiency.
Despite their numerous advantages, MOSFETs also come with some drawbacks that need to be considered:
- Fragile Gate-Channel Layer: The layer between the gate and channel in MOSFETs is delicate and susceptible to electro-static damage during installation, requiring well-designed circuits to prevent issues.
- Susceptibility to Overload Voltages: MOSFETs are very sensitive to overload voltages, necessitating special handling during installation to avoid damage.
- Sensitivity to Static Electricity: MOSFETs are highly sensitive to static electricity, which can lead to damage if not handled carefully during assembly and installation.
- Cost: MOSFETs can be more expensive than other transistor types, impacting their suitability for cost-sensitive applications.
Conclusion
In summary, MOSFETs are like tiny electronic switches that help control the flow of electricity in gadgets we use every day. They're super important because they make our devices work better and smarter. As technology gets even more advanced, MOSFETs will keep playing a big role in making our gadgets smaller, faster, and more powerful.
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