Field Effect Transistors (FET/JFET/MOSFET) — A/L Physics

What must I remember about Field Effect Transistors (FET/JFET/MOSFET)?

• Field Effect Transistor Concept: Unipolar voltage-controlled three-terminal semiconductor device where conduction current through a conducting channel (Drain to Source) is modulated by an electrostatic electric field applied at the Gate terminal.
• JFET Architecture (Junction FET): n-channel JFET consists of an n-type semiconductor bar with two embedded p-type gate regions forming p-n junctions; Gate-Source junction is ALWAYS operated under REVERSE BIAS (Vgs ≤ 0); Gate input impedance is extremely high (> 10⁸ Ω) with essentially zero gate current (Ig ≈ 0).
• Pinch-Off Voltage (Vp): Value of Drain-Source voltage Vds at which the depletion layers from opposing gates meet across the channel, causing drain current Id to saturate at a constant value (Idss for Vgs = 0).
• MOSFET (Metal-Oxide-Semiconductor FET): Insulated gate separated from channel by an ultra-thin silicon dioxide (SiO2) dielectric layer; features virtually infinite input resistance (> 10¹² Ω); available in Enhancement mode (normally OFF, channel induced by gate voltage) and Depletion mode (normally ON).

Test yourself (1 minute)

1. BJT vs FET Key Differences

   BJT is bipolar (conduction by both electrons and holes) and CURRENT-controlled; FET is unipolar (conduction solely by majority carriers) and VOLTAGE-controlled with far higher input impedance and lower thermal noise.

2. Shockley's Equation for JFET

   In the active/saturation region: Id = Idss * [1 - (Vgs / Vp)]², where Idss is drain saturation current at zero gate bias and Vp is pinch-off voltage.

3. Static Electricity Sensitivity of MOSFETs

   The ultra-thin SiO2 gate oxide insulation can easily be punctured and permanently destroyed by electrostatic discharge (ESD) from human hands; stored in conductive antistatic foam.

Practise Field Effect Transistors (FET/JFET/MOSFET) questions today

Source: NIE G.C.E. (A/L) Physics Resource Book - Unit 9: Electronics

What should I study first?

What comes next?

Next in the syllabus

Where does Field Effect Transistors (FET/JFET/MOSFET) fit in the syllabus?

Unit 9.5 · Year 2 · Term 2