FETs and MOSFETs

Basics of FETs and MOSFETs

  • Field Effect Transistors (FETs) are another type of transistor, differing from BJTs.
  • They feature a gate, source and drain, instead of a base, emitter and collector as in BJTs.
  • In FETs, the output current is controlled by the electric field in the device, which is set up by the voltage applied on the gate.

FET Characteristics

  • FETs are unipolar devices, meaning they conduct by electron movement or by hole movement, but not both.
  • They have high input impedance, making them suitable for applications where a high degree of isolation is necessary between stages.
  • Unlike BJTs, FETs don’t have a pn junction that can become forward biased and lead to excess current consumption.

Understanding MOSFETs

  • Metal Oxide Semiconductor Field Effect Transistors (MOSFETs) fall under the family of FETs.
  • The term ‘metal oxide semiconductor’ refers to the physical design of the transistor which involves a metal layer, a silicon dioxide layer and a semiconductor layer.
  • The three types of MOSFETs are Depletion MOSFET (D-MOSFET), Enhancement MOSFET (E-MOSFET) and Complimentary MOSFET (CMOS).

MOSFET Characteristics

  • Depletion MOSFETs allow current flow even with zero voltages, while Enhancement MOSFETs need a threshold voltage to allow current flow.
  • CMOS technology combines p-type and n-type MOSFETs to create a low power consumption mechanism which is widely used in modern computer chips.
  • MOSFETs also boast high input impedance, similar to FETs.
  • They offer superior high-frequency response than BJTs due to the absence of a charge storing layer.

Applications of FETs and MOSFETs

  • Both FETs and MOSFETs are ideal for high input impedance applications, such as in oscilloscope probes.
  • Often used in amplification and switching applications due to their power efficiencies.
  • CMOS technology is the foundation of today’s digital integrated circuits in computers, mobile phones and an array other digital devices.

FETs and MOSFETs Limitations

  • FETs and MOSFETs are highly sensitive to static electricity. This sensitivity can lead to device failure if not properly handled.
  • The high input impedance can also become a disadvantage as it can lead to amplification of unwanted noise signals.
  • Temperature stability can be an issue for MOSFETS, risking thermal runaway if not designed and managed properly.

Remember: Always consult the datasheet and consider the application requirements when selecting a transistor type for a circuit design. Knowledge and understanding of the characteristics and limitations of both BJTs and FETs/MOSFETs is essential.