Electronic Parts

MOSFETs

The metal-oxide-semiconductor field-effect transistor—MOSFET—is the most important transistor structure in modern digital integrated circuits and is also central to power electronics.

Schematic symbols

These are the common enhancement-mode MOSFET symbols used on the schematics in LearnTronics. The separated gate line is the visual reminder that the gate is electrically insulated from the drain-source channel.

The insulated gate

The gate electrode is separated from the semiconductor by a very thin insulating layer. Gate voltage creates an electric field that changes carrier concentration in the semiconductor beneath it. This controls a channel between source and drain.

Enhancement-mode MOSFET

The most familiar MOSFET is enhancement mode. At zero gate-to-source voltage, the channel is normally off. Sufficient gate voltage creates a conductive inversion layer and allows drain current to flow.

N-channel and P-channel

N-channel MOSFETs use electron conduction in the channel and generally provide lower on-resistance for a given die area than comparable P-channel devices. P-channel MOSFETs are convenient for high-side switching in simpler circuits because their gate drive can be referenced to the positive supply.

A breadboard-sized example: the 2N7000

The 2N7000 is a small N-channel enhancement-mode MOSFET that is convenient for low-current experiments on a solderless breadboard. It is normally off at zero gate-to-source voltage, and its insulated gate makes it useful when a circuit needs to sense a voltage without drawing much steady current from that voltage source.

Device reference: Microchip 2N7000 product page and datasheet.

Threshold voltage

Threshold voltage is defined at a relatively small drain current. It tells when the device is beginning to turn on, not when it is fully enhanced.

Gate charge

Although steady gate current is nearly zero, the gate behaves like a capacitor. Every switching transition requires charge to be moved into or out of that capacitance. Fast switching therefore requires a gate driver that can source and sink substantial short pulses of current.

Miller effect

Gate-to-drain capacitance couples changing drain voltage back into the gate. During switching, the gate voltage may pause at a “Miller plateau” while drain voltage changes. Gate-charge curves are often more useful than a single capacitance number for estimating switching-drive requirements.

Body diode

Most discrete power MOSFET structures contain an intrinsic body diode. It may carry current in bridge and converter circuits, but its forward drop, reverse-recovery behavior, and current rating still need to be checked.

Static sensitivity

The very thin gate insulation makes MOSFET gates vulnerable to electrostatic discharge. Modern devices include process improvements and sometimes internal protection, but proper ESD handling remains important.

CMOS

Complementary MOS uses N-channel and P-channel transistors together. In a basic CMOS logic gate, one network pulls the output high while the complementary network pulls it low. Ideally, little steady current is required when the logic state is not changing.

Why MOS won digital electronics

• Extremely small transistors can be manufactured lithographically.
• Very little DC gate current is required.
• Complementary logic can have low static power.
• Millions and then billions of devices can be fabricated on one wafer.
• Scaling permits faster operation and denser circuits.

MOSFETs in power electronics

Larger MOSFET structures are optimized for low on-resistance, high current, and useful voltage ratings. They switch motors, solenoids, power supplies, LEDs, battery systems, automotive loads, and converters.