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Dustin Hodges  

Understanding MOSFETs: 12 Essential Questions Answered

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Introduction

The Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET) plays a pivotal role in countless applications in electronics ranging from power management to digital circuits. Unlike traditional transistors, MOSFETs are voltage-controlled devices, offering advantages such as high input impedance, fast switching speeds, and low power consumption. Their operation is based on the voltage applied to the gate terminal, which creates an electric field to control the conductivity of the channel between the source and drain.

This topic on MOSFETs goes through a series of 12 common questions on MOSFETs and the answers to those questions. These questions were chose based on the common questions most people have when first learning about MOSFETs.

These questions and their answers are meant to supplement our post on MOSFETs titled “Understanding MOSFETs: Key Concepts and Practical Examples”. We encourage you to go check that post out.

Whether you’re a student, a professional, or simply curious about this essential component, these questions provide a comprehensive understanding of MOSFETs and their significance in the field of electronics.

12 Essential Questions Answered About MOSFETs

Q1.) What is a MOSFET and what are they used for?

MOSFETs are a type of transistor used for switching and amplifying electronic signals. They are crucial components in modern electronics, found in everything from power supplies to microprocessors. They are particularly useful in digital circuits, analog applications, and power electronics due to their efficiency and versatility.

Looking at the image of a representation of the structure of a MOSFET above, we can observe that it consists of a p-type semiconductor that forms its base. We also see that there are two types of the base that are highly doped with an n-type impurity (n+). These heavily doped regions of the base are where the source (S) and drain (D) terminals are located.

Structure of MOSFETs:

  • Gate (G): The gate is separated from the channel by a thin layer of insulating material (usually silicon dioxide (SiO2)). The gate is the terminal that controls the channel of the MOSFET.
  • Source (S) and Drain (D): The source and drain are heavily doped regions that form the channel through which current flows when the MOSFET is turned on. The source is the terminal where current enters the MOSFET, and the drain is the terminal where current exits the MOSFET.
  • Channel: Depending on the type of MOSFET, the channel can either be n-type (allowing electrons to flow) or p-type (allowing holes to flow).

Working Principle of MOSFETs:

  • When a voltage is applied to the gate terminal, it creates an electric field that either enhances or depletes the charge carriers in the channel.
  • For an n-channel MOSFET, applying a positive voltage to the gate allows electrons to flow from the source to the drain.
  • For an p-channel MOSFET, applying a negative voltage to a p-channel MOSFET enables holes to flow from the source to the drain.

Applications of MOSFETs:

  1. Switching Circuits: MOSFETs are commonly used as electronic switches. They can turn on and off the flow of current in circuits, making them ideal for applications like power management, where they control the supply to devices based on demand.
  2. Amplification: In analog circuits, MOSFETs can amplify weak signals, making them useful in audio and radio frequency applications.
  3. Digital Logic Circuits: MOSFETs form the basis of CMOS (Complementary Metal-Oxide-Semiconductor) technology, which is widely used in microprocessors and memory devices due to its low power consumption and high density of logic functions.
  4. Power Electronics: MOSFETs can handle high voltages and currents, making them suitable for managing power in various applications such as electric vehicles and renewable energy systems.
  5. Signal Processing: In telecommunications and signal processing applications, MOSFETs help in modulating and demodulating signals, ensuring efficient transmission and reception.

MOSFETs are important components in modern electronics due to their ability to function as efficient switches and amplifiers. Their structure allows them to operate with high speed and low power consumption, making them suitable for a wide range of applications.

Q2.) What’s the difference between N-Channel and P-Channel MOSFETs?

N-Channel and P-Channel MOSFETs are two types of field-effect transistors that differ in their structure, operation, and applications. The following table explains their differences:

N-Channel MOSFETP-Channel MOSFET
Charge CarriersUse electrons as the majority charge carriers. Electrons have higher mobility, which makes N-channel MOSFETs more efficient and faster in operation.Use holes as the majority charge carriers. Holes have lower mobility compared to electrons, which makes P-channel MOSFETs less efficient and slower.
Gate Voltage (VGS) RequirementsRequire a positive gate-to-source voltage (VGS) to turn on. This makes them suitable for low-side switching, where the MOSFET is placed between the load and ground.Require a negative gate-to-source voltage (VGS) to turn on. This makes them ideal for high-side switching, where the MOSFET is placed between the load and the positive supply voltage (VCC).
Placement in CircuitsTypically connected between the load and ground (low-side switching). This configuration is more common and efficient.Typically connected between the load and the positive supply (high-side switching). They are often used when a simple circuit design is needed without additional components like charge pumps.
Efficiency and SizeMore efficient due to the higher mobility of electrons. They also have a higher packing density, meaning they can be smaller for the same current rating.Less efficient because of the lower mobility of holes. They require a larger area for the same current rating, which makes them bulkier and less space-efficient.
ApplicationsPreferred in most applications due to their efficiency, speed, and cost-effectiveness. They are commonly used in switching circuits, motor drivers, and power supplies.Used in high-side switching applications where simplicity is prioritized, such as battery-powered devices or circuits where a negative gate voltage is easier to generate.

Q3.) What are the advantages and disadvantages of using MOSFETs compared to BJTs?

Both BJTs and MOSFETs are crucial components in electronic circuits, but they have different characteristics, advantages, disadvantages, and applications. Here’s a couple of simple, visual comparisons of the two:

BJT vs. MOSFET:

AdvantagesDisadvantages
BJT☑ More efficient in low-frequency applications where switching speed is not critical.☒ Current-controlled devices, meaning they require a continuous base current to operate, which increases power consumption.
☑ Provide higher current gain, making them suitable for applications requiring strong signal amplification.☒ Provide higher current gain, making them suitable for applications requiring strong signal amplification.
☑ Generally cheaper than MOSFETs, especially for low-power applications.☒ Have a negative temperature coefficient, meaning their current increases with temperature, which can lead to thermal runaway if not properly managed.
☑ More robust against electrostatic discharge and voltage spikes compared to MOSFETs.☒ Tend to generate more noise than MOSFETs, which can be a disadvantage in low-noise signal processing applications
AdvantagesDisadvantages
MOSFET☑ Have extremely high input impedance because the gate is insulated from the rest of the device. This means they draw virtually no current at the gate, making them more power-efficient in control circuits.☒ Are more sensitive to electrostatic discharge (ESD) and voltage spikes due to their thin gate oxide layer, which can be easily damaged.
☑ Are voltage-controlled devices (as opposed to BJTs, which are current-controlled), they consume less power in the control circuit.☒ Are generally more expensive than BJTs, especially for low-power applications where BJTs can suffice.
☑ Excel in high-speed switching applications due to their lower switching times. This makes them ideal for high-frequency circuits like switching power supplies and motor drivers.☒ Are efficient, but their on-resistance (RDS(on)) can still lead to power losses, especially in low-voltage, high-current applications.
☑ Have lower conduction losses compared to BJTs, especially at higher currents, which reduces heat generation and improves efficiency.☒ Require specific gate drive voltages to operate efficiently. For example, N-channel MOSFETs often need a higher gate voltage than the source voltage in high-side switching, which may require additional circuitry.
☑ Can be made smaller and are easier to integrate into ICs (Integrated Circuits), making them suitable for modern compact electronics.
☑ Are better suited for high-frequency applications because of their faster switching capabilities and lower switching losses.
☑ Have a positive temperature coefficient, meaning their resistance increases with temperature. This helps prevent thermal runaway, a common issue with BJTs.

Q4.) What is the difference between power MOSFETs and logic level MOSFETs?

Power MOSFETs and Logic Level MOSFETs are designed for different applications and have distinct characteristics:

Power MOSFETs:

  • Purpose: Designed to handle high voltages and currents.
  • Gate Voltage: Typically requires a higher gate voltage (10V or more) to fully turn on.
  • On-Resistance (RDS(on)): Generally lower compared to logic level MOSFETs, which reduces heat generation under load.
  • Applications: Used in power electronics, motor drives, and high-power switching applications.

Logic Level MOSFETs:

  • Purpose: Designed for low voltage, low current applications.
  • Gate Voltage: Can be fully turned on with lower gate voltages (usually around 5V or even less).
  • On-Resistance (RDS(on)): Higher compared to power MOSFETs for the same package size; intended for less demanding applications.
  • Applications: Commonly used in microcontroller interfacing, low-power switching, and signal processing.

Features Between Power vs. Logic Level MOSFETs:

FeaturePower MOSFETsLogic Level MOSFETs
High Current HandlingExcellent for high-power applicationsSuitable for low-power applications
Low On-ResistanceReduces heat and power lossReasonably low on-resistance for low voltage
RobustnessCan operate at high voltages and currentsWorks well with low-voltage control signals
Thermal PerformanceBetter thermal management under high loadsGenerally requires less thermal management
Higher Gate VoltageRequires more complex drive circuitsCan be driven directly from microcontrollers
SizeTypically larger, which may limit design flexibilitySmaller, allowing for compact designs
CostGenerally more expensive due to robust constructionUsually cheaper, making them cost-effective

Q5.) Does the gate of a MOSFET act as a capacitor?

Yes, the gate of a MOSFET behaves like a capacitor. This is due to the physical structure of the MOSFET, where the gate terminal is separated from the channel (the conductive path between the source and drain) by a thin insulating layer, typically made of silicon dioxide (SiO2). This structure creates a capacitive effect between the gate and the channel.

Observing the structure of the MOSFET in the image above, we can see that the layer of the substrate is coated with a layer of silicon dioxide (SiO2), the insulator, and that a thin insulated metallic plate is placed at the top of the SiO2. These sandwiched materials in this region of the structure act as a capacitor. The gate (G) terminal of the MOSFET connects to the metallic plate atop the SiO2 layer.

How the Gate Acts as a Capacitor

Capacitive Structure:

  • The gate capacitance is a combination of several parasitic capacitances:
  • Gate-to-source capacitance (CGS): Between the gate and source terminals.
  • Gate-to-drain capacitance (CGD): Between the gate and drain terminals, which is influenced by the Miller effect.
  • These capacitances are small but significant, especially in high-speed switching applications, where they affect the MOSFET’s switching speed and power dissipation.

Impact on Circuit Design:

  • The capacitive nature of the gate means that charging and discharging the gate capacitance requires current. This is why MOSFET gate drivers are often used to provide the necessary current for fast switching.
  • The gate capacitance also affects the MOSFET’s response to high-frequency signals, as it can act as a low-pass filter.

Practical Implications:

  • The capacitive behavior of the gate is a key factor in designing circuits with MOSFETs, especially in switching applications like power supplies or motor drivers. Engineers must account for the gate capacitance to ensure proper operation and avoid issues like slow switching or excessive power loss.

Q6.) Does a fully charged gate of a MOSFET “block” current flow?

Not exactly. The gate of a MOSFET does not directly “block” current flow; instead, it controls the flow of current between the drain and source terminals by creating an electric field. Let’s briefly go over how this works:

How the Gate Works:

  1. The gate of a MOSFET acts as a capacitor, as we discussed earlier. When a voltage is applied to the gate relative to the source (VGS), the gate “charges” by accumulating an electric field across the insulating layer (typically silicon dioxide). For an N-channel enhancement-mode MOSFET like the 2N7000, the MOSFET remains off (non-conducting) until the gate-source voltage (VGS) exceeds a certain threshold voltage (Vth). This threshold voltage is typically around 2V to 4V for the 2N7000. Once VGS exceeds Vth, the electric field created by the charged gate capacitance attracts electrons into the channel, forming a conductive path between the drain and source. This allows current to flow.
  2. If the gate is “fully charged” (i.e., VGS is high enough), the MOSFET is turned ON, and current flows freely between the drain and source (depending on the load and circuit design). If the gate is not charged (i.e., VGS is below the threshold voltage), the MOSFET is turned OFF, and the drain-source path is non-conductive, effectively “blocking” current flow.

Why the Gate Doesn’t Directly Block Current:

The gate itself does not directly block current because it is electrically isolated from the rest of the MOSFET by the insulating layer. Instead, the gate voltage controls the conductivity of the channel between the drain and source. The “blocking” or “allowing” of current is a result of whether the channel is formed (on) or not (off).

Q7.) Does current flow from the gate to the source of a MOSFET?

No, there is no continuous current flow from the gate to the source of a MOSFET under normal operating conditions. The gate of a MOSFET is electrically isolated from the source and drain by a thin insulating layer (usually silicon dioxide), which makes the gate behave like a capacitor. Here’s a detailed explanation:

  1. Charging the Gate Capacitance:
    • When a voltage is applied between the gate and source (VGS), the gate capacitance charges. During this charging process, a small current flows into the gate momentarily to charge the gate capacitance.
    • Once the gate is fully charged (i.e., the gate-source voltage reaches the applied voltage), the current flow into the gate stops. At this point, the gate acts as an open circuit, and no continuous current flows from the gate to the source.
  2. Gate Leakage Current:
    • In an ideal MOSFET, the gate is perfectly insulated, and no current flows through the insulating layer. However, in real-world MOSFETs, there is a very tiny leakage current, called the gate-to-source leakage current (IGSS). This current is typically in the range of nanoamps (nA) or even picoamps (pA) and is negligible in most applications.
  3. Why the Gate Doesn’t Conduct Current:
    • The gate is insulated from the source and drain by the oxide layer, so it cannot conduct current like a resistor or diode. Instead, the gate voltage creates an electric field that controls the conductivity of the channel between the drain and source. This is why MOSFETs are considered voltage-controlled devices.

Q8.) Does the gate of a MOSFET isolate current flow from gate-to-source?

Yes, the gate of a MOSFET isolates the flow of current from gate-to-source under normal operating conditions. This isolation is due to the physical structure of the MOSFET, where the gate is separated from the source (and the drain) by a thin insulating layer, typically made of silicon dioxide. Here’s a detailed explanation:

  1. Gate Insulation and Isolation:
    • The gate of a MOSFET is electrically insulated from the source and drain by the oxide layer. This insulation prevents any direct current flow between the gate and source (or gate and drain).
    • Because of this, the gate behaves like the plate of a capacitor, where applying a voltage to the gate creates an electric field that controls the flow of current through the channel between the drain and source.
  2. Capacitive Charging of the Gate:
    • When a voltage is applied to the gate, a small current flows momentarily to charge the gate capacitance. This is not a continuous current but rather a transient one that occurs only during the charging or discharging of the gate.
    • Once the gate is fully charged, no further current flows into or out of the gate, and the gate effectively isolates itself from the source and drain.
  3. High Input Resistance:
    • MOSFETs have an extremely high input resistance at the gate, often in the range of megaohms or higher. This high resistance ensures that the gate does not draw significant current during operation, further reinforcing the isolation between the gate and source.
  4. Leakage Current:
    • In real-world MOSFETs, there is a very small leakage current (called gate-to-source leakage current, or IGSS) that flows through the insulating layer. However, this current is typically negligible (in the range of nanoamps or picoamps) and does not affect the gate’s ability to isolate current flow.

Q9.) Is a 2N7000 a MOSFET?

Yes, the 2N7000 is a MOSFET. The 2N7000 is a specific type of N-channel MOSFET. Here are some key details about it:

Q10.) Why do some MOSFET circuit diagrams show two battery sources?

For example, a circuit diagram shows a 2N7000 MOSFET with a 9V battery source (V1) with its positive supply tied to the gate (through a push-button switch (S1) and resistor (R1)) and its negative supply tied to common ground. Another 9V battery source (V2) has its positive terminal tied to the drain (through an LED) and its negative terminal tied to common ground. The negative supply from each battery are tied together to a common ground, as shown in the image below:

Why Some MOSFET Circuits Use Two Battery Sources:

In MOSFET circuit diagrams, the use of two separate battery sources is often necessary to properly bias the MOSFET and power the load. The reason why this happens in the specific example above, is as follows:

  1. Gate Drive Voltage (Control Signal):
    • The gate-source voltage (VGS) determines whether the MOSFET is turned on or off. For an N-channel MOSFET like the 2N7000, the gate voltage must be higher than the source voltage by at least the threshold voltage (Vth), typically around 2-4V for the 2N7000.
    • In this example, one 9V battery (V1) is connected to the gate and ground to provide the control signal. This ensures the gate voltage is high enough relative to the source to turn the MOSFET on.
  2. Powering the Load:
    • The second 9V battery (V2) is connected between the drain and source to power the load. This is because the MOSFET itself does not generate power; it only acts as a switch to control the flow of current from the drain to the source.
    • The load (e.g., a motor, some other device, or an LED in this case) is powered by this second battery, and the MOSFET controls whether current flows through the load.
  3. Shared Ground:
    • The negative terminals of both batteries are tied together to create a common ground. This is essential because the MOSFET’s operation depends on the voltage difference between its terminals (e.g., VGS and VDS). Without a shared ground, the circuit would not have a proper reference point for these voltages.

Why Not Use a Single Battery?

In some cases, a single battery can be used if the gate voltage can be derived from the same power source (e.g., using a voltage divider or a microcontroller). However, in circuits where the load requires a different voltage or the gate drive voltage needs to be independent, two separate power sources are used.

The two-battery setup in the example above ensures:

  • The gate voltage is high enough to turn the MOSFET on.
  • The load is powered by a separate source, which can be at a different voltage level.
  • A common ground is established for proper operation of the MOSFET.

Q11.) Can you use one 9V battery instead of two?

Let’s say we have a simple switch application circuit that uses the 2N7000 N-channel, enhancement mode MOSFET, as described below:

Referring to the image below, we can see that there’s a 9V battery supply (V1) that has its positive terminal that goes to a normally-open push-button switch (S1). From the switch, there’s a 1kΩ resistor (R1) that is inline to the gate of the MOSFET. From the gate of the MOSFET is another resistor (R2) with a value of 2kΩ resistor that goes to a common ground; this is the pull-down resistor for the gate.

A second 9V battery supply (V2) has its positive terminal going to the anode of an LED, and the cathode of the LED is inline with the drain of the MOSFET. At the source of the MOSFET, there’s a 200Ω resistor (R3) that’s tied to the common ground. The common ground includes both of the negative terminals of the two battery supplies, V1 and V2.

Do we need to actually use two 9V batteries if setting up this circuit on a breadboard, or can we just use one 9V battery?

Yes, you can use one 9V battery instead of two in this circuit, provided you make a small adjustment to the wiring. Here’s why and how:

Explanation of the Circuit in the Schematic Above:

  1. Two Batteries in the Original Design:
    • In the original circuit, V1 powers the gate control circuit (via the push-button switch and resistors), while V2 powers the LED and load circuit.
    • The two batteries share a common ground, which ensures proper voltage references for the MOSFET’s operation.
  2. Using a Single Battery:
    • Since both batteries are 9V and share a common ground, you can combine them into a single 9V battery. The single battery will simultaneously power the gate control circuit and the LED/load circuit.
    • The positive terminal of the single battery will connect to both the push-button switch (for the gate control) and the LED (for the load). The negative terminal will remain connected to the common ground.

Adjusted Circuit with One Battery:

The image above is how the circuit would look with a single 9V battery:

  • The positive terminal of the 9V battery connects to the push-button switch (S1), which is in series with the 1kΩ resistor (R1) to the gate of the MOSFET. The positive terminal is also connected to the anode of the LED (which is in series with the drain of the MOSFET).
  • The negative terminal of the 9V battery connects to the common ground, which ties together the source of the MOSFET, the cathode of the LED (via the MOSFET and R3), and the pull-down resistor (R2).

Why This Works:

  • The MOSFET operates based on the voltage difference between the gate and source (VGS). With a single battery, the gate voltage (via the push-button) will still rise above the source voltage when the button is pressed, turning the MOSFET on.
  • The LED and load will still receive power from the same battery, as the MOSFET will allow current to flow from the drain to the source when turned on.

Pros and Cons of Using One Battery:

ProsCons
Simplifies the circuit (fewer components).The single battery will drain faster since it powers both the gate control and the load.
Reduces cost and space requirements.May require careful wiring to avoid voltage drops in high-current applications.

You can safely use a single 9V battery for this circuit by connecting it to both the gate control and the load, as long as the wiring is adjusted to maintain the proper voltage references.

Q12.) Would using one battery affect voltage readings of the MOSFET circuit?

Would using a single 9V battery affect voltage readings throughout the example circuit from question Q11 above, instead of using two batteries, or would the voltage readings across components read the same either way?

Using a single 9V battery instead of two separate 9V batteries in the example MOSFET circuit from question Q11 will not significantly affect the voltage readings across components, as long as the circuit is properly wired. Let’s explain why:

  1. Voltage reference remains the same in the original circuit with two batteries; both batteries share a common ground. This means all voltage measurements are referenced to the same ground point.
    • When you switch to a single 9V battery, the same ground reference is maintained, so the voltage differences across components (e.g., the gate-to-source voltage (VGS) or the drain-to-source voltage (VDS)) will remain the same.
  2. The voltage across each component is determined by the circuit’s design and Ohm’s Law, not by the number of batteries used.
    • The gate voltage (VG) is determined by the push-button switch, the 1kΩ resistor (R1), and the pull-down resistor (R2). These components form a voltage divider when the button is pressed, and the resulting voltage at the gate will be the same whether you use one or two batteries.
    • The drain voltage (VD) is determined by the LED and the MOSFET’s operation. The LED will drop a fixed forward voltage (e.g., 3V for the example circuit’s LED), and the remaining voltage will appear across the MOSFET and the 200Ω resistor (R3). This behavior is independent of whether the power comes from one or two batteries.
  3. When using a single battery, it will supply current to both the gate control circuit and the load circuit. This could cause a slight voltage drop if the battery is heavily loaded or nearing depletion, but in your circuit, the current draw is relatively low (e.g., the LED and MOSFET likely draw less than 50mA). A fresh 9V battery should handle this without significant voltage sag.
  4. Some practical considerations are if the single 9V battery is heavily loaded or partially discharged, its output voltage may drop slightly due to its internal resistance. This could cause minor variations in voltage readings compared to using two separate batteries, but these differences are usually negligible in low-power circuits like the example circuit from question Q11, above.

Conclusion

MOSFETs are indispensable components in the world of modern electronics, offering unparalleled versatility and efficiency in a wide range of applications. Through the 12 questions and answers we explored here, we were able to uncover some of the fundamental principles behind MOSFET operation, their advantages and disadvantages, as well as their practical uses.

Hopefully, this structured approach has provided a deeper understanding for you of how MOSFETs function and that you’ve learned why they are preferred in many designs over other types of transistors. As technology continues to advance, the importance of MOSFETs will only grow, solidifying their role as a key building block in the ever-evolving landscape of electronics. Whether you’re designing cutting-edge systems or simply learning the basics, MOSFETs remain a crucial subject for anyone passionate about electronics.

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