Start with the core idea

A FET (Field-Effect Transistor) is a transistor where voltage at one terminal controls current through a channel. In simple terms, it is an electronically controlled valve.

The two families people usually meet first are JFET and MOSFET. Both are voltage-controlled and both can have very high input impedance, but they do not behave the same way at startup, biasing, or power switching.

If you remember one rule early, remember this:

• Most JFETs are normally ON at zero gate bias.
• Most MOSFETs you use in switching are normally OFF at zero gate bias.

That single difference already explains why MOSFETs dominate digital and power switching, while JFETs still appear in some analog paths.

What a JFET is

A JFET is usually a depletion-mode device. That means with gate-source voltage near zero, current can already flow from drain to source. You control current by reverse-biasing the gate and narrowing the channel.

In practical terms:

• Think of JFET as “open by default, then pinch down current with gate bias.”
• Gate current is very small, so input loading can be low.
• JFET behavior can be smooth for analog signal control in some circuits.

Why people still use JFETs:

• High-impedance input stages in some analog front ends.
• Low-noise signal paths in specific designs.
• Constant-current style circuits using simple bias networks.

What confuses people:

1. They expect JFET to behave like a logic switch (hard OFF at 0V gate), which is usually not true.
2. They mix MOSFET gate-drive assumptions into JFET designs.
3. They ignore device-to-device variation in analog operating points.

So for JFET work, biasing matters more than “just turning on and off.”

A common first-order JFET model (for many analog use cases) is:

Where:

• $I_D$ is drain current
• $I_{DSS}$ is drain current when $V_{GS}=0$
• $V_{GS}$ is gate-source voltage
• $V_P$ is pinch-off voltage

This equation is useful because it shows the core JFET behavior clearly: as gate bias moves toward pinch-off, current falls nonlinearly.

What a MOSFET is

A MOSFET is the FET you will use most often for switching loads. For common enhancement-mode MOSFETs, no channel exists at zero gate-source voltage, so the device stays OFF until gate voltage rises above the required level.

In practical terms:

• Think of MOSFET as “off by default, then turned on by gate voltage.”
• It can switch fast and efficiently when chosen and driven correctly.
• It is heavily used in converters, motor drivers, battery systems, and digital power control.

An important warning: threshold voltage is not the same as fully ON voltage.

Many people read Vgs(th) and assume the MOSFET is fully usable at that voltage. In reality, Vgs(th) only indicates where conduction begins. For low heat and low loss, you usually need enough gate drive to reach low Rds(on) in the datasheet conditions.

For power switching, always check:

• Drain-source voltage rating (with margin)
• Continuous and pulse current ratings
• Rds(on) at your actual gate voltage
• Gate charge (Qg) and switching frequency
• Thermal path and expected temperature rise

For MOSFETs, these two equations are the most useful starting point:

Linear (ohmic) region:

Saturation region (idealized):

Where:

• $I_D$ is drain current
• $V_{TH}$ is threshold voltage
• $V_{DS}$ is drain-source voltage
• $k_n = \mu_n C_{ox}\frac{W}{L}$ (device/process constant for a given transistor geometry)

For switching designs, a practical power estimate you will use often is:

This is why datasheet $R_{DS(on)}$ at your real gate drive voltage matters so much.

JFET vs MOSFET, plus practical use-cases

Now the useful comparison for real design decisions:

• Default state: JFET is commonly normally ON; enhancement MOSFET is commonly normally OFF.
• Control style: JFET is controlled by narrowing an existing channel; MOSFET is controlled by forming/enhancing a channel.
• Typical role: JFET is often analog/signal-focused; MOSFET is often switching/power-focused.
• Ecosystem: MOSFET options, drivers, and references are much wider for modern embedded/power work.

A quick practical decision path:

1. If your main goal is efficient switching of motors, LEDs, heaters, or converter stages, start with MOSFET.
2. If your goal is a specific analog behavior with very high input impedance and smooth bias control, JFET may be a good fit.
3. If you are unsure, prototype both at small scale and compare measurable outcomes: noise, heat, efficiency, and control simplicity.

Real use-cases:

• MOSFET case: low-side switching of a DC load from a microcontroller, with proper gate resistor, pull-down, and flyback path for inductive loads.
• JFET case: analog input stage where low input loading and bias behavior are more important than hard digital switching.

Common mistakes to avoid in both:

• Choosing by headline specs only, without checking operating conditions.
• Ignoring thermal behavior during continuous operation.
• Assuming one part’s behavior represents the whole family.

If this topic is clear, you should be able to look at a requirement and quickly answer: “Is this mostly an analog-bias problem or a power-switching problem?” That answer usually tells you whether to begin with JFET or MOSFET.