MOSFET Transistors

A MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) is a great transistor type for switching loads. MOSFET transistors are especially good at being fully on and fully off.

You can also dim or regulate loads with a MOSFET: use a PWM (Pulse Width Modulation) signal. It switches the MOSFET on and off at high frequency, and the duty cycle (the sum of the on phases) can regulate the load, i.e. dim a light. This is possible because MOSFETs can switch very fast at up to 10 MHz (and higher with specialized types).

Overview

MOSFETs - like most transistors - has three legs: two (called source and drain) carry the load. The third (called gate) is the control pin that you can use to turn the MOSFET on and off.

So a MOSFET works pretty much like a mechanical switch:

• Gate:
  acts like the button or rocker and controls whether the switch is on or off. Since the switch state is controlled by a voltage, this makes it easy to electronically control the switch, i.e. via a GPIO on a microcontroller, or via a suitable sensor output.
• Source and Drain: conduct the load when the MOSFET is on

In reality, a MOSFET can work just as simple as a mechanical switch, however while a true mechanical switch is just essentially a strip of wire that can be opened and closed, a MOSFET is a semiconductor and has some requirements to be happy.

N-Channel and P-Channel

There are two fundamental types of MOSFETs: N-Channel (more common) and P-Channel (less common). These types differ in the way how their gate pin controls them:

N-Channel MOSFET (Low-Side-Switch)

• Load current flows from drain to source.
• The gate voltage must be positive and higher than the voltage at pin source to turn the MOSFET ON.
• Typically used for low-side switches:
  • OFF by default when gate connected to ground (pulled low)
  • Turned ON by pulling gate higher than source (e.g., connecting gate to +V)

P-Channel MOSFET (High-Side Switch)

• Load current flows from source to drain.
• The gate voltage must be negative and lower than source to turn the MOSFET ON.
• Typically used for high-side switches:
  • ON by default if the gate is at GND (assuming source is at +V)
  • Turned OFF by pulling gate to the same voltage as source (+V)

Don’t worry too much about this if you are a beginner. Just keep in mind that a MOSFET cannot simply be placed anywhere in your circuit as if it were a simple mechanical switch.

Determine whether you want a high-side or low-side switch, pick the appropriate MOSFET type (N-Channel or P-Channel), and use one of the sample circuit designs below.

Understanding GATE Pin

MOSFETs are controlled via voltage, but a better term might be electrostatic load.

With a N-Channel MOSFET, briefly touching the gate pin with a wire connected to +V is sufficient to turn the MOSFET ON. It stays on even when you remove the wire.

In contrast to BJT transistors and their base pin, there is no continuous current flow into the gate. The charge you apply to the gate stays there, and the charge you applied to gate stays there (and keeps the MOSFET ON) until you actively remove the charge again.

Practical Takeaways

• Sensitive:
  MOSFETs are very sensitive to electrostatic energy. Always properly ground yourself before touching MOSFET pins.

• Pull-Down Resistor:
  To ensure gate returns to a defined state when you stop applying your control signal, use a pull-down resistor or enable the pull down in the controlling GPIO. This ensures the gate is pulled to a voltage lower than the source, properly turning the MOSFET OFF.

• Protective Resistor:
  Unlike BJT transistors, MOSFETs do not require a current-limiting resistor for the gate because they are voltage-controlled devices. However, you may still want to use a protective resistor between the gate and GPIO to limit inrush currents when driving the gate capacitance, especially in larger MOSFETs.

MOSFET Pinout

With the typical TO-220 packages, when looking at the MOSFET from the top (so you can read its type), with the “legs” downwards, the pin order is (from left to right):

The best way to determine the pin order is looking at the MOSFET datasheet though.

Use Cases

Both MOSFET and BJT transistors can perform similar tasks, but each has distinct strengths and weaknesses due to their design:

• Switching Applications:
  • MOSFETs require almost no control current, making them faster and more efficient as switches.
  • MOSFETs are ideal for high-frequency applications, such as switching power supplies or PWM dimming.
• Amplification Applications:
  • BJTs excel at current amplification and are commonly used in analog circuits.
  • While they can be used as switches, they are typically favored for low-frequency applications.

Since digital electronics primarily rely on on/off signals, MOSFETs are now the most widely used transistor type. However, BJTs remain popular in DIY projects due to historical use, affordability, and ease of handling.

Fake MOSFETs

MOSFETs from untrusted sources may not meet specifications and often have high Rds(on), making them unreliable.

• Fake MOSFETs may initially function at low currents but overheat or fail under load.
• To verify authenticity, use a component tester to measure characteristics and compare them with the datasheet.

When you buy MOSFETs from platforms such as Amazon, eBay, or AliExpress, you almost certainly receive counterfeit and relabeled MOSFETs, not the types you ordered.

These MOSFETs may even work well for your use case. However, they almost never fulfill the specifications and will typically have high Rds(on) even at higher voltages, making them usable only for very small currents, if at all.

If you must buy MOSFETs from unknown sources, stick to common, inexpensive generic types—never pay for higher-cost logic-level MOSFETs as they are typically the same low-quality types, just with a new laser engraving claiming to be a shiny logic level or sophisticated MOSFET.

Key Considerations

When selecting a MOSFET type, these are the four key considerations to focus on:

• Control Voltage: is the MOSFET fully turned on at the gate voltage you can provide? Else, it may act like a resistor, cause a voltage drop, and can get very hot.
• Maximum Current: how much current can the MOSFET handle? Can you use it to switch your load? Do you need a heat sink?
• Package: which package type does the MOSFET use, and is it suitable for your type of hobbyist project? Is it i.e. a TO-220 package with legs that fit through-hole soldering? Or is it i.e. a tiny SO-8 surface-mount package that needs tiny contacts, custom PCBs, and reflow soldering?
• N- and P-Channel: do you need a more common N-Channel MOSFET (off by default, controlled by positive voltage), or a less common P-Channel MOSFET (on by default, controlled by negative voltage)?

A common question is whether a MOSFET requires a protective resistor. This question is often asked by users familiar with BJT transistors:

• BJT Transistors: require a resistor at their control pin (base) because such transistors have a very low internal resistance. Without a protective resistor, they would act similar to a LED, and maximum current would quickly heat up and destroy it.

• MOSFET Transistors: do not require a resistor at their control pin (gate).

  However, the circuit that is driving the gate pin may need one. MOSFETs have a high resistance, so no high (continuous) currents flow.

  Larger MOSFETs can have a significant capacitance, leading to a very high momentary inrush current. It may last for just a few nanoseconds which can suffice to damage a microcontroller GPIO.

  That’s why for MOSFETs, too, protective resistors are recommended (at least for larger MOSFETs), just for a different reason.

Finding Suitable MOSFET

Picking an appropriate MOSFET model for a project is crucial. You can easily pick one that is completely unusable for your particular scenario.

Distributors offer online databases for preselection. Once you find MOSFET types that appear suitable, always carefully manually review the datasheet to see if it is really a fit.

• The good news is: You can save all this time and effort if you plan to order MOSFETs at places like Amazon, eBay, or AliExpress.

• The bad news is: that’s because you almost certainly will receive counterfeit fake MOSFETs anyway. You probably won’t get the MOSFET types you carefully selected. So save the time. Or, if you want your MOSFETs to work as expected, order at real prices at real distributors (like Mouser or DigiKey).

  Understanding MOSFET Data Sheets

Reading MOSFET datasheets is not trivial, especially for beginners: there are many specs that all describe how a MOSFET responds to certain circumstances:

It is then your task to translate and adapt these specs to your own use case, and determine if it is a fit or not.

Preselection

Distributors offer distributor online databases that you can use to preselect MOSFETs.

Most online databases allow you to set various filters, and you can select distinct properties (like package type) as well as ranges (at minimum, at maximum) using icons at the bottom of the selection lists.

Here are some useful filters for preselection:

• MOSFET Type: N-Channel versus P-Channel. For example, to search for a MOSFET that is off by default and uses a positive control voltage, set Transistor Polarity = N-Channel.
• Package Type: set Mounting Style = Through Hole if you want to use a regular soldering iron and standard 2.54 mm pitch prototyping board, filtering out tiny surface mount packages.
• Vgs(th): make sure the MOSFET starts to conduct well below the voltage you are intending to use. When Vgs(th) is close to, or above your voltage, the MOSFET is not suited. For 3.3 V, choose Vgs(th) <= 1.2V.

  Do not confuse Vgs(th) (which is a single voltage threshold) to Vgs (which is the allowable voltage range for gate).

• Power: Set Id (drain current) to the current the MOSFET needs to switch. For example, if you plan to switch 1 A loads, set Id >= 1.2 A.
• Channels: If you are looking for single MOSFETs, set Number of Channels = 1 Channel.

You may want to click Apply Filters after each new filter you set. If you set multiple filters at once, the online database may refuse to accept your choices.

After applying all of the above filters, this left me with a list 46 MOSFET types for manual review.

Manual Review

Before you order a MOSFET, always carefully review its data sheet. Never trust recommendations or online databases. They can point you to potentially interesting MOSFETs, but you must ultimately determine if a particular MOSFET is really a match for your specific project.

With a shortlist of MOSFETs, you now need to look at each MOSFET’s data sheet and evaluate the fine print.

On-State Resistance

A MOSFET must turn on fully when you apply a voltage to its gate pin. If it turns on only partially, it acts like a resistor, causing voltage drop and heat. The MOSFET could get extremely hot or even be damaged.

Why Low Resistance Is Crucial

In real life, no MOSFET has a true zero-ohm resistance when on, but it is important to be close to zero ohms, especially when switching high currents:

Rds(on) at Desired Gate Voltage

The Rds(on) value tells you the resistance a MOSFET has when you apply a particular voltage to its gate pin. Rds(on) is different for different voltages, and in the data sheet, you are looking for a graph, not a single number.

A single Rds(on) value isn’t helpful at all. It merely describes the minimum resistance the MOSFET can have under perfect conditions. Much higher voltages may be required, and at the voltage you are going to use, Rds(on) can be much worse.

The online search returned a SUP85N10-10-E3. Let’s see if it really is usable with a 3.3 V microcontroller. Here is what its datasheet reveals:

• Rds(on) is specified as 0.0085Ω, which is very low; however, at a gate voltage Vgs=10V, it is rated for 10 V.
• At 4.5 V, Rds(on) already raises to 0.01Ω, which is still relatively low.

Unfortunately, most datasheets do not contain an Rds(on) graph. But they often provide a Vgs Gate-to-Source Voltage and label this Transfer Characteristics:

This graph reveals:

• At 3V, the MOSFET is off and not suited.
• At 3.3V, the MOSFET barely starts to conduct but still has a considerable resistance.
• When getting hot, the MOSFET isn’t working at 3.3V anymore.

So, even though the distributor database suggested the SUP85N10-10-E3, it turns out to be a great MOSFET for 5V, but not suited at all for 3.3V.

Vds / Id = Rds(on)
3.3 V / 20 A = 0.165Ω

# double-check with known Rds(on):

Vds / Rds(on) = Id
4.5V / 0.01Ω = 450 A
10V / 0.0085Ω = 1.176 A

V_DS / I_D = Rds(on)

verifying data sheet known Rds(on) values:
-> 0.01Ω at 4.5V
-> 0.0085Ω at 10V
for 4V:
2.0 V / 55 A = 0.03Ω
4.0 V / 55 A = 0.07Ω
for 10V:
2.0 V / 235 A = 0.0085Ω

Logic Level MOSFETs

Finding suited MOSFETs for low voltages like 3.3 V can be hard. Most MOSFETs require much higher gate voltages.

Rather than trying to find a MOSFET that has a low Rds(on) at 3.3V, you can also increase the gate voltage. That’s a good approach if you have MOSFETs at hand that don’t work well with logic levels.

• MOSFET Driver:
  use a dedicated MOSFET driver IC. It accepts a logic level and outputs a higher voltage to drive the MOSFET gate.

  MOSFET drivers operate a MOSFET in its perfect range but can be pricey.

• Level Shifter:
  use a simple level shifter breakout board to raise the gate voltage at least from 3.3V to 5V. At 5V gate voltage, the selection of MOSFETs is much bigger.

  Since level shifters are not designed as MOSFET drivers, they may or may not drive the MOSFET well enough. Level shifters designed for I2C interfaces may work fast enough even for high-frequency use cases but do not provide enough current for high-capacity MOSFETs.

• BJT Transistor as Gate Driver:
  a simple 2N2222 NPN transistor with a 1kΩ base resistor can supply a higher gate voltage (provided your circuit has higher voltages that can be used). It is too slow for high-frequency switching, though, so the use cases are somewhat limited.

N-Channel

All prices are considered to be a rough classification and may vary greatly between sources and over time. Prices are for single MOSFETs. Bulk orders (>50) are significantly lower, typically at 50%.

Disclaimer

• This table is a rough summary, and not all MOSFETs listed have been tested. There may be errors. Use this table as a preselection only, and always verify the datasheets yourself.
• All information is provided at your own risk. No warranties or liabilities.
• If you find discrepancies or would like to suggest a new MOSFET to add to this list, please use the comment form at the bottom of this page. Your help and feedback are greatly appreciated!
• This list is a work in progress and is by no means exhaustive or complete.

Key Points

• 3.3V Compatibility:
  Most MOSFETs require at least 4.5V for low Rds(on). You’ll notice that those marked as “+” or “++” typically work at 3.3V with reasonable performance, but for optimal results, a 5V gate drive is often better.

• 5V Compatibility:
  A larger selection of MOSFETs will work well at 5V, and you can usually expect lower Rds(on) and higher current handling.

Recommendations

• For 3.3V logic level, you should aim for MOSFETs marked as “++” or “+++” and check the Rds(on) values at 3.3V to confirm suitability.
• For 5V logic level, you have a much wider range of options with lower Rds(on), which should work efficiently for switching.

(in progress)

P-Channel

(todo)

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