You can find field effect transistors in many modern gadgets. A field effect transistor uses an electric field to control current flow. This technology has changed electronics a lot. In the last ten years, field effect transistors became more common. New improvements help control power in 5G networks and electric cars. You learn more about electronics when you study how these transistors work and why they are important.
Key Takeaways
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Field effect transistors, or FETs, use an electric field to control current. This makes them very important in today’s electronics.
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FETs have three main parts: source, gate, and drain. The gate voltage decides how many charge carriers move in the channel.
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Picking the right substrate for FETs can make them work better. It changes how fast charge carriers can move.
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JFETs are easier and cheaper to make. MOSFETs can do more things and can be made smaller for new uses.
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FETs are used in analog and digital circuits. They help devices get smaller, faster, and use less energy.
Field Effect Transistor Structure
FET Terminals
Every field effect transistor has three main terminals. These are called the source, gate, and drain. Each terminal does something special. The source gives charge carriers to the channel. These carriers can be electrons or holes. The drain takes these carriers after they move through the channel. The gate controls how many carriers go from the source to the drain. When you put voltage on the gate, it makes an electric field. This field changes how well the channel lets current pass. You can look at the table below to see what each part does:
| Component | Description | Function |
|---|---|---|
| Source | N-type semiconductor | Gives charge carriers (electrons) for conduction |
| Drain | N-type semiconductor | Takes charge carriers from the channel |
| Gate | P-type semiconductor with dielectric | Controls the flow of charge carriers by making an electric field |
How the source, gate, and drain are arranged matters. It affects how well the gate voltage can control the channel. This setup changes the electric field and how charge carriers move. You get different threshold voltages and current flows based on the design.
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The source puts charge carriers into the channel.
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The drain takes charge carriers from the channel.
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The gate changes how well the channel conducts by using voltage.
Channel and Substrate
The channel is between the source and drain. There are two main types: n-channel and p-channel. In an n-channel field effect transistor, electrons move from the source to the drain. In a p-channel device, holes move instead. The gate voltage decides how many carriers can move through the channel. If you raise the gate voltage, more electrons or holes can flow. This makes the current go up.
The substrate holds up the channel and other layers. Silicon is often used as the substrate. The type of substrate changes how well the field effect transistor works. Some substrates help carriers move faster. Others make them move slower. Thin films let you use many kinds of substrates, which can make the device work better. For example, using Si/SiO2 substrates in MoS2 field effect transistors changes how fast electrons move. Supported and suspended devices act differently because of the substrate.
Tip: You can make a field effect transistor work better by picking the right substrate. This choice changes how fast carriers move and how well the device works.
JFET vs MOSFET
There are two main types of field effect transistors: jfet and mosfet. Both have the same basic parts, but they are different in some ways.
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jfet has a doped semiconductor channel between two junctions. It has three terminals: source, drain, and gate.
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mosfet has a metal-oxide-semiconductor structure. It has four terminals: source, gate, drain, and body.
Here is a table that shows the main differences:
| Feature | JFET | MOSFET |
|---|---|---|
| Gate Structure | PN junction | Insulated gate (SiO2) |
| Operation Modes | Depletion-mode only | Depletion and enhancement modes |
| Input Impedance | Lower input impedance | Much higher input impedance |
| Terminal Count | Three terminals (source, drain, gate) | Four terminals (source, gate, drain, body) |
You use jfet when you want a simple design and lower cost. Making jfet is easier and costs less. mosfet costs more because it uses a metal oxide layer. But you can make mosfet smaller to fit more on a chip. This makes mosfet good for new electronics.
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jfet costs less to make because it is simpler.
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mosfet costs more because it is harder to make and uses metal oxide.
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You can make mosfet smaller, so it costs less per chip.
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jfet does not get smaller as easily, so it is not used as much in new devices.
You can pick n-channel or p-channel for both jfet and mosfet. n-channel devices use electrons. p-channel devices use holes. n-channel field effect transistors are usually faster and work better. p-channel devices are good for some circuits, especially when you need both types together.
Working Principle of Field Effect Transistors
Gate Voltage Control
You can control a field effect transistor by changing the gate voltage. The gate voltage decides how many charge carriers move in the channel. When you put voltage on the gate, it makes an electric field. This field can pull in or push away electrons and holes. In an n-channel field effect transistor, a positive gate voltage pulls in electrons. This makes the channel let more current through. In a p-channel device, a positive gate voltage pushes away holes. This makes a depletion zone and lowers how well current flows.
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The gate voltage changes how well the channel lets current pass by adding or taking away charge carriers.
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A positive gate voltage pushes holes away in p-channel channels, making a depletion zone, but pulls in electrons in n-channel channels, letting more current flow.
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The threshold voltage (VTH) is very important. It is the gate voltage needed to let current flow in the channel. n-channel and p-channel field effect transistors have different VTH values.
You must know the threshold voltage for your field effect transistor. If you do not reach this voltage, the channel stays closed and no current moves. When you go past the threshold, the channel opens and current goes from source to drain.
JFET Operation
A jfet works when you use the gate voltage to change the current in the channel. The jfet has a simple design with a channel between the source and drain. You can pick n-channel or p-channel jfet types. Here are the steps for how a jfet works:
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Zero Gate-Source Voltage (VGS = 0): The jfet lets the most current go through the channel. You see the highest current from drain to source.
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Applying Negative Gate-Source Voltage (VGS < 0): The depletion region gets bigger. The channel gets smaller, and less current flows.
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Pinch-Off Condition: If you make the gate voltage more negative, the channel gets so small it almost closes. The current almost stops.
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Saturation Region: After pinch-off, the current stays the same even if you raise the drain-source voltage. The jfet is now in saturation mode.
The pinch-off voltage is important. It is the point where the jfet changes from the linear region to the saturation region. You need to know this voltage to design good circuits. When the gate-source voltage goes past the pinch-off voltage, the depletion region gets wider, the channel gets smaller, and the current drops. In saturation, the jfet keeps the output steady, which helps make signals stronger.
The jfet is controlled by voltage. When the gate voltage is zero, you get the most current. If you make the gate voltage more negative, the channel gets smaller and less current flows. At pinch-off, more drain-source voltage does not make more current.
MOSFET Operation
A mosfet uses a metal-oxide gate to control the channel. You can find both n-channel and p-channel mosfet types. Here are the steps for how a mosfet works:
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When the gate-source voltage (VGS) is higher than the threshold voltage (VTH), an n-type inversion channel forms under the gate.
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This channel lets electrons move from source to drain. The drain-source voltage (VDS) controls how much current flows.
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A positive voltage at the gate makes an electric field. This field pushes holes away and pulls electrons to the surface.
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When VGS is above VTH, the surface bends and makes the n-type inversion channel. This channel links the drain and source.
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The drain current (ID) depends on both VGS and VDS. Higher values mean more current until the mosfet reaches saturation.
Most power mosfets work best with gate voltages between 4.5 and 5.5 volts. You often see a limit of 10 volts for gates rated at 20 volts. This keeps the mosfet from going into hard saturation and helps it switch faster.
The threshold voltage in mosfets changes how they switch. You see different switching actions based on the interface state density and the material used. The table below shows how threshold voltage changes switching:
| Evidence Description | Impact on Switching Behavior |
|---|---|
| Higher interface state density in SiC/SiO2 compared to Si/SiO2 | Makes threshold voltage less stable during switching and stress tests |
| Categories of Vth instability: BTI, GSI, Vth hysteresis | Each type changes how the mosfet switches and its output |
| Vth drift from BTI | Changes how the mosfet switches and its output, especially after long use and high heat |
FET Characteristics
Field effect transistors have many good points. The input impedance of a field effect transistor is much higher than a bipolar junction transistor. The table below compares the input impedance:
| Transistor Type | Input Impedance |
|---|---|
| FET | Several MΩ |
| BJT | A few kΩ |
Field effect transistors make less noise, but bipolar junction transistors have a better signal-to-noise ratio. This means BJTs are better for careful measurements. The signal-to-noise ratio for BJT sensors stays the same, but FET sensors have lower SNR, especially at low currents.
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Field effect transistors switch faster than bipolar junction transistors.
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You use FETs for high-frequency jobs like RF communication and fast data work.
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BJTs are good for analog circuits but switch slower, so they are not used for fast digital circuits.
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BJTs work up to a few hundred megahertz. You use them for radio amplifiers.
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Fast mosfets, like GaN or LDMOS, are used in radar and communication devices. They switch quickly and save energy.
Field effect transistors, including jfet and mosfet, have high input impedance, low noise, and fast switching. You can pick n-channel or p-channel types for different uses. The way a field effect transistor works lets you control current with gate voltage, so it is useful in many electronic circuits.
Applications of FETs
Transistor Uses
Field effect transistors are in almost every electronic device today. They help make gadgets smaller and faster. You can find them in smartphones. They work inside processors, memory chips, and power chips. FETs help your phone use less power. This makes the battery last longer and keeps the device cool.
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Field effect transistors help make electronics small and fast.
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You use them in both analog and digital circuits.
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In digital circuits, the fet acts like a switch. It turns the channel on or off to control signals.
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In analog circuits, the fet acts like a resistor. It changes the channel resistance to control current.
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NMOS and CMOS use n-channel and p-channel fets. These make up about 75% of all integrated circuits.
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Both jfet and mosfet designs use n-channel and p-channel fets.
Field effect transistors help your devices get faster, smaller, and use less energy.
FinFETs are a special kind of field effect transistor. They use a 3D channel. This design gives better control of current and stops leaks. It helps make devices smaller and stronger.
Advantages of FETs
Field effect transistors have many good points. FETs use less power than bipolar junction transistors. This makes them great for things like phones and tablets. Power is only needed when charging or discharging the gate. This keeps the fet cool and efficient.
| Feature | Field Effect Transistors (FETs) | Bipolar Junction Transistors (BJTs) |
|---|---|---|
| Power Consumption | Low | Higher |
| Efficiency | High | Moderate |
| Heat Generation | Minimal | Significant |
| Suitability for Portable Use | Excellent | Limited |
You can make fets very small. This lets you put more on a chip. This follows Moore’s law, so you get better performance. New designs like tri-gate and gate-all-around fets help control the channel as devices get smaller.
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FETs use less power, so batteries last longer.
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Less heat means your gadgets stay cool.
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You can use high-κ materials in the gate to lower voltage. This saves even more power.
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Good insulators in fets, like hBN, make them last longer and work well.
When you pick a field effect transistor, you get high efficiency, low power use, and good performance for both n-channel and p-channel types. This is why fets are in almost every modern circuit, from jfet amplifiers to mosfet switches.
You now know how field effect transistors work. FETs are important in electronics. They have high input impedance. They also make little noise. This helps you build stable circuits. It also helps make sensitive circuits. You can look at the table below to see how FETs work in different regions:
| Operational Region | Description |
|---|---|
| Ohmic Region | The channel acts like a resistor. Current goes up when VDS gets higher. |
| Saturation Region | The current stays the same. You use this region to make signals stronger. |
| Cutoff Region | The FET is off. No current moves through it. |
| Breakdown Region | Too much voltage makes the biggest current flow. |
FETs are used in new technology. You find them in IoT and AI hardware. They are small, quick, and use little power.
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Transistors help with digital data for AI.
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They let devices work faster and save energy.
FETs will help power future electronics.
Written by Jack Elliott from AIChipLink.
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