Field Effect Transistors (FET): Topics, Subtopics, Study Flow, and Working Steps

A Field Effect Transistor, or FET, is a semiconductor device in which an electric field controls current through a conducting channel. Unlike a BJT, which needs base current for control, a FET is mainly controlled by voltage applied at the gate terminal.

FETs are important because modern electronics needs devices that consume very little input current, switch fast, occupy small chip area, and can be packed in huge numbers inside integrated circuits.

• Industry relevance: MOSFETs are the backbone of CMOS ICs, microprocessors, memory, power electronics, SMPS, motor drives, RF circuits, and analog switches.
• Analog relevance: FETs are used in common-source amplifiers, source followers, current sources, active loads, differential pairs, and high-input-impedance sensor circuits.
• Exam relevance: GATE and university exams test JFET pinch-off, MOSFET threshold voltage, operating regions, drain-current equations, transconductance, and FET amplifier gain.
• Interview relevance: strong answers explain channel formation and gate-field control instead of only quoting drain-current formulas.

• PN junction and depletion region
• Doping and majority carriers
• Electric field effect on charge carriers
• Voltage, current, and resistance concepts
• Basic amplifier gain and biasing
• Small-signal model and transconductance idea

Imagine a water pipe with a flexible wall. The water flow is drain current. The pipe opening is the channel. The gate voltage presses on the channel electrically and changes how wide the path is. A wider channel allows more current; a narrower channel allows less current.

In a JFET, the channel already exists and gate reverse bias squeezes it. In an enhancement MOSFET, the channel does not exist at zero gate voltage; it forms only when gate voltage crosses threshold.

Simple memory: BJT uses input current for control; FET uses gate voltage and electric field for control.

A FET has three main terminals: gate, drain, and source. Current flows mainly between drain and source through a channel. The gate controls that channel through an electric field.

• JFET: gate-channel junction is reverse biased; increasing reverse bias narrows the channel.
• Depletion MOSFET: channel exists initially and can be depleted or enhanced by gate voltage.
• Enhancement MOSFET: channel is induced only when gate-source voltage exceeds threshold voltage.
• FET amplifier action: a small change in gate voltage causes a larger change in drain current, which creates output voltage across a load.

The key physical concept is channel control. The gate does not need to inject significant DC current. It creates an electric field, and that field controls the carrier density or channel width between drain and source.

1. JFET Drain Current

For a JFET, the channel is widest when gate-source voltage is zero. As reverse gate voltage increases, the depletion region expands and the channel becomes narrower. Shockley's equation models this behavior:

$$ I_D = I_{DSS}\left(1-\frac{V_{GS}}{V_P}\right)^2 $$

• $$ I_D $$ is drain current.
• $$ I_{DSS} $$ is maximum drain current when gate-source voltage is zero.
• $$ V_{GS} $$ is gate-source voltage.
• $$ V_P $$ is pinch-off voltage, the gate voltage that almost closes the channel.

Physical meaning: the squared term tells us that channel current does not reduce linearly. As the channel is squeezed, current falls faster.

2. MOSFET Threshold Condition

In an enhancement MOSFET, drain current does not start properly until a conducting channel is formed. This happens when gate-source voltage crosses threshold voltage.

Channel forms when $$ V_{GS} > V_T $$

Below threshold, the gate field is not strong enough to create a useful inversion channel. Above threshold, carriers gather under the gate oxide and create a controllable path from drain to source.

3. MOSFET Drain Current in Saturation

In saturation region, a long-channel enhancement MOSFET behaves approximately as a voltage-controlled current source:

$$ I_D = \frac{1}{2}k(V_{GS}-V_T)^2 $$

• $$ k $$ depends on device geometry, mobility, oxide capacitance, and channel dimensions.
• $$ V_{GS}-V_T $$ is called overdrive voltage.
• The larger the overdrive voltage, the stronger the channel and the larger the drain current.

4. Transconductance

Transconductance measures how effectively gate voltage controls drain current. It is the small-signal slope of the drain-current curve.

$$ g_m = \frac{\Delta I_D}{\Delta V_{GS}} $$

Plain meaning: if a tiny change in gate voltage produces a large change in drain current, the device has high transconductance and can provide strong amplification.

5. Common-Source Voltage Gain

In a common-source amplifier, gate voltage changes drain current. Drain current variation creates voltage variation across the drain resistor.

$$ A_v \approx -g_m R_D $$

The negative sign means phase inversion. When gate voltage increases, drain current increases, voltage drop across the drain resistor increases, and drain voltage falls.

1. Apply gate-source voltage to create or control the channel.
2. Drain-source voltage pulls carriers through the channel.
3. Gate electric field changes channel width or channel charge density.
4. Drain current changes according to gate-source voltage.
5. In amplifier use, drain-current change is converted into output voltage across a load.
6. In switching use, the FET moves between cutoff and low-resistance ON state.

The structure diagram should show gate, drain, source, oxide or PN junction, and channel. The characteristics graph should show how drain current changes with drain-source voltage for different gate-source voltages.

• CMOS logic gates and microprocessors
• Memory cells and digital ICs
• SMPS and DC-DC converters
• Motor drivers and power inverters
• Low-noise sensor input stages
• Analog switches and multiplexers
• RF amplifiers and mixers
• Source followers and impedance buffers

Beginner Example

An enhancement MOSFET has $$ V_T=2\,V $$. If $$ V_{GS}=1.5\,V $$, is a strong channel formed?

Since gate-source voltage is less than threshold voltage, a strong inversion channel is not formed. The MOSFET remains OFF for basic circuit analysis.

Intermediate Numerical

A MOSFET has $$ k=2\,mA/V^2 $$, $$ V_T=1\,V $$, and $$ V_{GS}=3\,V $$. Find saturation drain current.

Overdrive voltage: $$ V_{OV}=V_{GS}-V_T=3-1=2\,V $$

$$ I_D=\frac{1}{2}kV_{OV}^2=\frac{1}{2}(2)(2)^2=4\,mA $$

Advanced Problem

A common-source amplifier has $$ g_m=4\,mS $$ and $$ R_D=5\,k\Omega $$. Estimate voltage gain.

$$ A_v\approx -g_mR_D=-(4\times10^{-3})(5\times10^3)=-20 $$

The magnitude of gain is 20, and the negative sign means the output is 180 degrees out of phase with the input.

• Thinking MOSFET gate draws large DC current. Ideally, the insulated gate draws almost no DC current.
• Confusing JFET pinch-off voltage with MOSFET threshold voltage.
• Using saturation current equation when the MOSFET is actually in triode region.
• Forgetting that enhancement MOSFET needs gate voltage above threshold to form a strong channel.
• Ignoring the negative sign in common-source voltage gain.
• Assuming all FETs are normally OFF; depletion-mode devices can be normally ON.

• Why is a FET called a voltage-controlled device?
• Why is MOSFET input resistance very high?
• What is threshold voltage in an enhancement MOSFET?
• What is pinch-off in a JFET?
• What is transconductance, and why does it matter in amplifiers?
• Why does common-source amplifier invert phase?
• How is a MOSFET used as a switch?
• What is the difference between triode and saturation regions?

• For enhancement MOSFET, first check whether gate-source voltage is above threshold.
• Use triode condition $$ V_{DS}<V_{GS}-V_T $$ before applying triode-region equations.
• Use saturation condition $$ V_{DS}\ge V_{GS}-V_T $$ before applying saturation equation.
• JFET is normally ON; enhancement MOSFET is normally OFF.
• Common-source amplifier gives voltage gain with phase inversion.
• Common-drain circuit is a source follower used for buffering.
• High input resistance is a major advantage of FETs in sensor and amplifier input stages.

• FET current is controlled by gate electric field.
• Gate current is ideally very small, giving high input resistance.
• JFET channel exists initially and is squeezed by reverse gate bias.
• Enhancement MOSFET channel forms only after threshold voltage.
• Transconductance tells how strongly gate voltage controls drain current.
• Common-source gives voltage gain and phase inversion.
• Key formulas: $$ I_D=I_{DSS}(1-V_{GS}/V_P)^2 $$, $$ I_D=\frac{1}{2}k(V_{GS}-V_T)^2 $$, and $$ A_v\approx -g_mR_D $$.

Conceptual

• Explain FET operation using channel-width control.
• Why is an enhancement MOSFET normally OFF?
• Why is common-drain amplifier called a source follower?

Numerical

• For a MOSFET with $$ V_T=1.5\,V $$ and $$ V_{GS}=4\,V $$, find overdrive voltage.
• If $$ k=1\,mA/V^2 $$ and $$ V_{OV}=3\,V $$, find saturation drain current.
• Find common-source gain when $$ g_m=2.5\,mS $$ and $$ R_D=8\,k\Omega $$.

MCQs

• Which device is normally OFF: JFET, depletion MOSFET, or enhancement MOSFET?
• Which FET amplifier gives phase inversion: common source, common drain, or common gate?
• What does transconductance relate: voltage to current, current to current, or voltage to voltage?