MOSFETs

The MOSFET — Metal-Oxide-Semiconductor Field Effect Transistor — is the most manufactured semiconductor device in history. Every digital computer, every smartphone processor, and virtually every modern piece of electronic equipment is built almost entirely from MOSFETs. In ham radio they appear as power transistors in linear amplifiers, switching elements in power supplies, and the building blocks of all the integrated circuits used throughout modern transceivers.

The Insulated Gate: What Makes a MOSFET Different

The key difference between a MOSFET and a JFET is how the gate is constructed. In a JFET, the gate is a reverse-biased PN junction — it draws a small leakage current and its voltage range is limited by the junction's breakdown voltage. In a MOSFET, the gate electrode is completely separated from the channel by a thin layer of silicon dioxide (SiO₂) — a high-quality insulator. This gate oxide makes the gate input impedance extremely high — typically 10¹² to 10¹⁴ Ω — because no DC current can flow through an insulator.

This insulated gate also means the gate voltage can be positive or negative relative to the source without forward biasing any junction, giving the MOSFET a wider range of operating modes than the JFET.

N-channel enhancement MOSFET (left) and P-channel enhancement MOSFET (right). The broken line in the channel symbol indicates that no channel exists at zero gate bias — it must be induced by the gate voltage. The arrow direction distinguishes N-channel from P-channel.

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Enhancement Mode and Depletion Mode

MOSFETs come in two fundamental operating modes:

The enhancement-mode N-channel MOSFET is by far the most common type in power and digital applications. With zero gate voltage, no channel exists between drain and source — the device is off. When a positive gate voltage exceeds the threshold voltage (V_th), an inversion layer of electrons is induced in the P-type substrate beneath the gate oxide, forming a conducting channel that allows current to flow from drain to source.

N-Channel and P-Channel MOSFETs

Like JFETs and BJTs, MOSFETs come in complementary N-channel and P-channel types:

• N-channel enhancement MOSFET: Turned on by positive gate-to-source voltage. Current flows from drain (positive) to source. Most common type — lower on-resistance for same die area than P-channel, so preferred for high-current switching. IRF510, IRFZ44, BS170 are common examples.
• P-channel enhancement MOSFET: Turned on by negative gate-to-source voltage. Current flows from source (positive supply) to drain. Used for high-side switching where the source is at the supply rail. IRF9540 is a common P-channel power MOSFET.

Threshold Voltage and On-Resistance

Two key parameters determine a MOSFET's switching behavior:

Threshold voltage (V_th or V_GS(th)) is the minimum gate-to-source voltage needed to start forming the conducting channel. For a logic-level MOSFET designed to be driven by a 3.3 V or 5 V logic output, V_th is typically 1–2.5 V. Standard power MOSFETs often require 4–6 V for full enhancement and must be driven by a proper gate driver circuit, not directly by a logic output.

On-resistance (R_DS(on)) is the resistance of the fully-on channel between drain and source. It determines the power dissipated in the MOSFET when conducting: P = I_D² × R_DS(on). Modern power MOSFETs achieve R_DS(on) values below 10 mΩ, making them extremely efficient switches with very low conduction losses.

Power MOSFETs

Power MOSFETs are designed to handle high currents and voltages. They achieve this by using a vertical structure with millions of parallel cells — each cell is a tiny MOSFET, and they all operate in parallel to share the current. Important power MOSFET ratings to check on a datasheet:

• V_DS(max): Maximum drain-to-source voltage before breakdown. Must be above the peak voltage in your circuit.
• I_D(max): Maximum continuous drain current. Must not be exceeded — add safety margin.
• R_DS(on): On-resistance. Lower is better; choose for your current level.
• P_D(max): Maximum power dissipation. Determines heatsink requirements.
• Q_g: Gate charge. Determines how much charge the gate driver must supply — important at high switching frequencies.

Gate Drive Considerations

Unlike BJTs (which need continuous base current to stay on), a MOSFET needs only enough voltage on its gate — no DC current flows through the oxide. However, the gate oxide forms a capacitance with the channel, and this capacitance must be charged and discharged each time the MOSFET switches. At high switching frequencies this gate capacitance requires significant peak current from the driver, even though the average gate current is zero. This is why high-speed MOSFET switches (in switching power supplies and Class D amplifiers) use dedicated gate driver ICs that can source and sink several amperes peak to charge and discharge the gate rapidly.

MOSFETs in Ham Radio

• Linear RF amplifier output stages: Power MOSFETs designed for RF (such as the RD16HHF1 or BLF188XR) operate at HF and VHF frequencies and handle hundreds of watts. They are increasingly replacing vacuum tubes in high-power amplifiers.
• Switching power supplies: The SMPS in most modern transceivers uses power MOSFETs switching at 50–500 kHz to regulate the output voltage.
• Antenna switching: MOSFETs in full H-bridge configurations drive relay coils and motorised antenna tuner capacitors.
• PA protection circuits: Fast-switching MOSFETs provide overload protection by shutting down the PA in microseconds when SWR or current limits are exceeded.
• All digital circuits: Every microcontroller, DSP, and FPGA inside modern transceivers is built from CMOS (Complementary Metal-Oxide-Semiconductor) — billions of N and P channel MOSFETs working in pairs.