JFET Operation and Characteristics

Insulated Gate FET / Metal Oxide Silicon FET (MOSFET)

• Uses an insulated layer between the gate and the channel, typically formed from a layer of oxide of the semiconductor
• Gate is made of a layer of metal set down on the silicon oxide, which in turn is on the silicon channel
• Widely used in many areas of electronics and particularly within integrated circuits
• Key feature is the exceedingly high gate impedance

Dual Gate MOSFET

• Specialized form of MOSFET with two gates in series along the channel
• Enables performance improvements, especially at RF, when compared to single gate devices
• Provides additional isolation between the input and output, and can be used in applications like mixing/multiplication

Metal Silicon FET (MESFET)

• Normally fabricated using Gallium Arsenide and often referred to as a GaAs FET
• Often used for RF applications where they can provide high gain, low noise performance
• Drawback is the very small gate structure, making it sensitive to damage from static, ESD

HEMT and PHEMT

• High Electron Mobility Transistor and Pseudomorphic High Electron Mobility Transistor
• Developed to enable very high frequency operation
• Although expensive, they enable very high frequencies and high levels of performance to be achieved

JFET

• Unipolar device, differing in operation from a bipolar junction transistor
• Output current controlled by the voltage on the gate
• Gate voltage creates an electrical field, or depletion region, within the device
• Considered a voltage-driven device rather than a current-driven device like the bipolar junction transistor
• Output current controlled by an electrical field created by the input voltage

JFET Operation

• At zero gate voltage, channel height is maximum and channel resistance is minimum, resulting in current flow
• With a slightly positive voltage, the channel height opens further, allowing maximum current flow
• If the gate is made negative with respect to the channel, the diodes become reverse biased and the depletion regions increase
• At a large enough gate-to-source voltage (VGS), the channel is effectively “pinched off” because of the depletion regions touching

FET Basics

• Concept of the field effect transistor is based around the concept that charge on a nearby object can attract charges within a semiconductor channel
• Operates using an electric field effect - hence the name
• FET consists of a semiconductor channel with electrodes at either end referred to as the drain and the source
• Control electrode called the gate is placed in very close proximity to the channel so that its electric charge is able to affect the channel
• Gate of the FET controls the flow of carriers (electrons or holes) flowing from the source to drain
• Semiconductor channel where the current flow occurs may be either P-type or N-type

Field Effect Transistor Types

• Junction FET (JFET)
• Major types of FET that can be covered in the tree diagram
• Each type has its own characteristics and uses in electronic circuit design