MOSFETS (Kevin)

Owned by Kevin Song
Last updated: 2022-01-24
3 min read

Background Information

  • Transistors are semiconductor devices used to amplify or switch electrical signals and power and can be divided into two very broad classes: Bipolar Transistors and Field Effect Transistors (FET)

  • Metal oxide semiconductor field-effect transistors, also known as MOSFETS, are the most popular type of transistors for implementing a simple switch

  • There are two different types of MOSFETS, known as n-channel (abbreviated NMOS), and p-channel(abbreviated PMOS)

NMOS transistor
PMOS transistor

  • MOSFETS are the basic building blocks of modern electronics and are the most frequently manufactured device in history

  • Logic gates use the switching behavior of MOS/TTL transistors to implement logical functions: AND,

    OR, NOT.

  • The MOSFET has four terminals: drain (D), source(S), gate(G), and body or substrate.

 

Depletion and enhancement modes

  • Depletion mode and enhancement mode are two major MOSFET types, corresponding to whether the transistor is in an on state or an off state at a zero gate-source voltage respectively

  • Enhancement-mode MOSFETS are commonly used as switching elements in most integrated circuits and are off at zero gate-source voltage

    • NMOS can be turned on by pulling the gate voltage higher than the source voltage, and PMOS can be turned on by pulling the gate voltage lower than the source voltage

  • Depletion-mode MOSFETS are less common than enhancement-mode MOSFETS and are normally on at zero gate-source voltage

    • These can be used as load “resistors” in logic circuits

  • The mode of a MOSFET can be determined by the sign of the threshold voltage on the datasheet. For an N-type FET, enhancement-mode devices have positive thresholds whereas depletion-mode devices have negative thresholds; the reverse is true for P-type FETs

    • It could also just say what it is on the datasheet! (If it is depletion-mode, it will be explicitly stated somewhere. If it just doesn’t say, it's in enhancement-mode)

Why Use MOSFETS?

  • The main reason we use MOSFETS is because of their cost

  • MOSFETS are relatively cheap to assemble

  • Due to advances in technology, MOSFETS have also become denser; modern chips can contain >1 billion transistors in less than 1/4 square inches

  • MOSFETS have low power loss and high switching speed

  • MOSFETS can handle more power than their counterparts in BJTs

  • MOSFETS are easier to operate than BJTs

Motor Control Applications

  • How are MOSFETS useful for motor control purposes?

    • MOSFETS can, depending on size and design switch a few hundred milliamps to tens of amps, and single-digit voltages to thousands of volts

    • Motor controllers often use electromagnetic switches to control direction/speed, in which case MOSFETS can be utilized

  • Examples: Power MOSFETS are commonly used as switching devices in electronic control units (ECU) for automotive electronics

    • ECU’s are embedded systems that control one or more of the electrical systems or subsystems in a car or other motor vehicle (brakes, engine control, etc)

  • Power MOSFETS can also be used as power converters in modern electric vehicles