Field Effect Transistor (FET)

Why it matters

Field Effect Transistors (FETs) are essential components in modern electronics, used in amplifiers, switches, and digital circuits. Their ability to control current flow with voltage makes them integral to designing efficient and compact electronic devices.

Key ideas

• Types of FETs: The main types of FETs are Junction FET (JFET) and Metal-Oxide-Semiconductor FET (MOSFET). Each has unique characteristics and applications.
• Operation Principle: FETs control the flow of current by applying a voltage to the gate terminal, which modulates the conductivity of a channel between the source and drain terminals.
• JFET: In a JFET, the gate is reverse-biased, creating a depletion region that controls the current flow. JFETs are normally-on devices.
• MOSFET: MOSFETs have an insulated gate, allowing for high input impedance. They can be either enhancement-mode (normally off) or depletion-mode (normally on).
• Characteristics: Key parameters include transconductance, drain-source saturation current, and threshold voltage.

Formulas

These are n-channel JFET saturation-region relations for V_P ≤ V_GS ≤ 0 and sufficient positive V_DS; V_P means V_GS(off).

• I_D = I_{DSS} (1 - V_{GS}/V_P)^2
  • I_D: Drain current (A)
  • I_{DSS}: Drain current at V_GS = 0 in saturation (A)
  • V_{GS}: Gate-source voltage (V)
  • V_P: Pinch-off voltage (V)
• g_m0 = 2 * I_{DSS} / |V_P|; at a bias point, g_m = g_m0 * (1 - V_GS/V_P)
  • g_m: Bias-point transconductance (S); here g_m0 = 5 mS and g_m = 2.5 mS.

Worked example

Given:

• I_{DSS} = 10 mA
• V_P = -4 V
• V_{GS} = -2 V

1. Calculate the drain current I_D using the formula: I_D = I_{DSS} (1 - V_{GS}/V_P)^2
2. Substitute the given values: I_D = 10 mA * (1 - (-2 V)/(-4 V))^2
3. Simplify: I_D = 10 mA * (1 - 0.5)^2
4. Calculate: I_D = 10 mA * 0.25
5. Result: I_D = 2.5 mA

Common mistakes

• Confusing the operation modes of JFET and MOSFET.
• Incorrectly applying the formula for drain current, especially the sign of V_{GS} and V_P.
• Overlooking the impact of temperature on FET parameters.

For GATE EC

• Focus on understanding the characteristics and operation of JFETs and MOSFETs.
• Practice problems involving the calculation of drain current and transconductance.
• Be prepared for questions on the differences between enhancement and depletion modes.

Quick check

1. What is the main difference between JFET and MOSFET?
2. How does the gate voltage affect the current in a FET?
3. What is the significance of the threshold voltage in a MOSFET?

Answers: 1. JFET has a junction gate; MOSFET has an insulated gate and may be enhancement or depletion mode; 2. It controls the channel conductivity; 3. It determines the voltage at which the MOSFET starts to conduct.