FETs and MOSFETs
Section A-002-004
Section A-002-004
Field-Effect Transistors (FETs), including Metal-Oxide-Semiconductor FETs (MOSFETs), are crucial components in modern electronics. They have unique characteristics and come in various types, each suitable for specific applications.
All FETs have three terminals: Gate, Drain, and Source. These terminals are integral to its operation and determine the flow of current within the device. The exam specifically asks about the terminals on a JFET but this is true for all FETs.
An enhancement-mode FET is a type of FET that does not have a conductive channel when no gate voltage is applied. As a result, no current flows through the device with zero gate voltage. To turn on the transistor and allow current to flow, a gate voltage must be applied to create a conductive channel.
A depletion-mode FET, in contrast, has a conductive channel even when no gate voltage is applied, allowing current to flow with zero gate voltage. This characteristic differentiates it from enhancement-mode FETs.
Many MOSFET devices include built-in gate protective Zener diodes to safeguard the gate insulation from being damaged by static charges or excessive voltages. This is crucial as the gate insulation in MOSFETs is very thin and susceptible to damage.
FET and CMOS devices require special handling precautions due to their high susceptibility to damage from static charges. Even small amounts of static electricity can cause irreversible damage to these sensitive components.
The input impedance of a field-effect transistor (FET) is significantly higher compared to that of a bipolar transistor. FETs, due to their construction, require very little input current, resulting in high input impedance. In contrast, bipolar transistors have lower input impedance.
JFETs come in two basic types: N-channel and P-channel. These types are defined based on the type of charge carriers (electrons or holes) that flow through the channel.
N-Channel Depletion MOSFET: Electron conduction in an n-channel depletion-type MOSFET is associated with the presence of an n-channel depletion layer. This layer allows electrons to flow when no gate voltage is applied.
N-Channel Enhancement MOSFET: In an n-channel enhancement MOSFET, electron conduction is associated with n-channel enhancement. This means that applying a positive gate voltage enhances the channel's ability to conduct electrons.
P-Channel Depletion MOSFET: Hole conduction in a p-channel depletion-type MOSFET occurs due to the presence of a p-channel depletion layer, which allows holes to flow even without a gate voltage.
P-Channel Enhancement MOSFET: In a p-channel enhancement-type MOSFET, hole conduction is associated with p-channel enhancement. Applying a negative gate voltage enhances the channel's conductivity for holes.
Understanding the different types of FETs and MOSFETs, including their operational modes and characteristics, is fundamental in electronics. From the high input impedance of FETs to the specific behaviors of enhancement and depletion-mode devices, these components are integral to modern electronic circuit design. Their sensitivity to static charges underscores the importance of careful handling and the role of protective elements like gate protective Zener diodes in ensuring the longevity and reliability of electronic systems.