Field Effect Transistors (FETs)

What are Field Effect Transistors (FETs)?

A Field Effect Transistor (FET) is a type of transistor that controls the flow of current using an electric field. In contrast to bipolar junction transistors (BJTs), which use current to control the flow of charge carriers, FETs use voltage to control the flow. FETs are generally more power-efficient than BJTs because they have high input impedance, low power consumption, and are typically faster in many applications.

The basic working principle of FETs is that the current flowing from the source to the drain is controlled by the voltage applied to the gate. The gate controls the flow of charge carriers in the channel between the source and drain, hence the name "Field Effect."

The main types of FETs are:

• JFET (Junction Field Effect Transistor)

• MOSFET (Metal-Oxide-Semiconductor Field Effect Transistor)

Differences Between FETs and Traditional Transistors (BJTs)

The key difference between FETs and BJTs (Bipolar Junction Transistors) is in the way they control current flow:

• BJT (Bipolar Junction Transistor): A BJT uses both electrons and holes (charge carriers) for current conduction and operates with current control. A small current applied to the base terminal controls a much larger current between the collector and emitter. BJTs are therefore "current-controlled devices."

• FET (Field Effect Transistor): FETs, on the other hand, operate by controlling the flow of charge carriers using an electric field created by the voltage applied to the gate terminal. This makes FETs voltage-controlled devices, with no direct current input to the gate.

FETs have the advantage of high input impedance, lower power consumption, and better isolation between the gate and the output compared to BJTs.

D-MOSFET vs. E-MOSFET

MOSFETs are the most commonly used type of FET, and they can be categorized into two types based on the type of channel and how the voltage is applied to the gate:

D-MOSFET (Depletion-mode MOSFET)

• Normally on: In a D-MOSFET, the device is normally on (i.e., the current can flow through the channel) when no voltage is applied to the gate.

• Gate Voltage Action: When a reverse voltage is applied to the gate, it creates a depletion region that shrinks the channel, reducing the flow of current and can eventually turn the device off. This means that the gate voltage is used to deplete charge carriers from the channel to control the current.

• Device Type: D-MOSFETs are typically used in circuits where a device should be "on" by default and can be turned off by applying a gate bias.

E-MOSFET (Enhancement-mode MOSFET)

• Normally off: In an E-MOSFET, the device is normally off (i.e., no current flows through the channel) when no voltage is applied to the gate.

• Gate Voltage Action: When a positive gate voltage is applied to an n-channel E-MOSFET, it enhances the conductivity of the channel by attracting electrons to form a conducting path between the source and drain. Similarly, in a p-channel E-MOSFET, a negative gate voltage enhances the conductivity by attracting holes.

• Device Type: E-MOSFETs are commonly used in most digital logic circuits because they have a simple "on" or "off" behavior with a gate voltage.

Main Differences Between D-MOSFET and E-MOSFET

1. Channel State (Normally On/Off)

   • D-MOSFET: Normally on when no voltage is applied to the gate. It is turned off by applying a reverse bias.

   • E-MOSFET: Normally off when no voltage is applied to the gate. It is turned on by applying a voltage to the gate (positive for n-channel, negative for p-channel).

2. Gate Voltage Behavior

   • D-MOSFET: The gate voltage depletes the channel of charge carriers, reducing the current flow and potentially turning it off.

   • E-MOSFET: The gate voltage enhances the channel, allowing current to flow through the channel when the voltage is applied.

3. Application

   • D-MOSFET: Used in applications where a device is typically on by default and needs to be turned off when required.

   • E-MOSFET: Commonly used in digital electronics and switching circuits, where the device operates in an "off" state and is switched "on" when required.

Conclusion

In summary, FETs are voltage-controlled transistors that control the flow of current through a channel using a gate voltage. The main difference between D-MOSFETs and E-MOSFETs is that D-MOSFETs are normally on and can be turned off with the right gate bias, whereas E-MOSFETs are normally off and require a gate bias to turn them on. E-MOSFETs are more commonly used in digital and switching applications because of their behavior as an ideal "switch."