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You need to choose a MOSFET N-channel transistor for your application. What are the aspects to focus on? You can see multiple aspects. Some of them are more important than others. Let me explain my 5 cents.

Package size – a natural and easily recognized difference. No, this is not what I want to speak about.

1️⃣ 𝗠𝗮𝘅𝗶𝗺𝗮𝗹 𝗱𝗿𝗮𝗶𝗻 𝗰𝘂𝗿𝗿𝗲𝗻𝘁 – You should know the maximal current you need to switch or transfer. If designing a power source switch, focus on the maximal current + Rds on + Maximal power dissipation (below).

2️⃣ 𝗩𝗴𝘀 𝘁𝗵𝗿𝗲𝘀𝗵𝗼𝗹𝗱 – What is the control voltage for your FET? An MCU port? Is it power from 1V8, 3V3 or a different voltage level? Remember that the Vgs threshold voltage has a range depending on multiple factors, and your transistor switch can have issues switching some loads.

3️⃣ 𝗩𝗱𝘀 𝗺𝗮𝘅𝗶𝗺𝗮𝗹 𝘃𝗼𝗹𝘁𝗮𝗴𝗲 – What is the maximal voltage the transistor deals with at the Drain or Source electrode? Please remember that some perfectly looking FETs can have the Vdsmax only 20 V, and you may want to switch 24 V power supply. 

4️⃣ 𝗥𝗱𝘀 𝗼𝗻 – What is the transistor channel’s maximum (or even minimal) resistance your application accepts? Do you want to switch high loads? Or are you looking for a small signal transistor? Remember, universal or small signal transistors can have slightly higher minimal channel resistance.

5️⃣ 𝗠𝗮𝘅𝗶𝗺𝗮𝗹 𝗽𝗼𝘄𝗲𝗿 𝗱𝗶𝘀𝘀𝗶𝗽𝗮𝘁𝗶𝗼𝗻 – This parameter is often omitted. A good engineer must verify the maximal power dissipation level will not be exceeded. A simple equation can be Pmax > Rdson x Id^2.

I will repeat the list of 5 points but with real numbers of 2N7002 and IRLML2502 FETs. I will use typical values for continuous conduction mode.

𝗔𝗻 𝗲𝘅𝗮𝗺𝗽𝗹𝗲 𝗰𝗼𝗺𝗽𝗮𝗿𝗶𝘀𝗼𝗻 𝗼𝗳 𝟮𝗡𝟳𝟬𝟬𝟮 𝗮𝗻𝗱 𝗜𝗥𝗟𝗠𝗟𝟮𝟱𝟬𝟮:

1️⃣ Maximal drain current

2N7002 = 115 mA

IRLML2502 = 4.2 A

2️⃣ Vgs threshold

2N7002 = 1-2.5 V

IRLML2502 = 0.65-1.2 V

3️⃣ Vds maximal voltage

2N7002 = 60 V

IRLML2502 = 20 V

4️⃣ Rds on

2N7002 = 1.2-13.5 Ω

IRLML2502 = 0.035-0.080 Ω

5️⃣ Maximal power dissipation

2N7002 = 200 mW

IRLML2502 = 1.25 W

As you can see, those FETs, even when produced in the same package, are completely different players.

MOSFET transistors
MOSFET transistors
MOSFET linear control