Field Effect Transistors (FETs)

Why it matters

Field Effect Transistors (FETs) are essential in modern electronics due to their high input impedance and low power consumption, making them ideal for use in amplifiers and switching applications. They are widely used in integrated circuits and various electronic devices, contributing to the miniaturization and efficiency of electronic systems.

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 affects the conductivity of a channel between the source and drain terminals.
• Characteristics: FETs have high DC input impedance. Noise depends on device, bias, frequency and source impedance. A BJT needs base current but its collector current also depends exponentially on V_BE.
• Applications: Used in amplifiers, oscillators, and switching circuits due to their efficiency and reliability.

Formulas

These relations apply to an n-channel JFET in saturation, with V_P = V_GS(off), V_P ≤ V_GS ≤ 0, and sufficient V_DS.

• I_D = I_{DSS} (1 - V_{GS}/V_P)^2

  • I_D: Drain current (A)
  • I_{DSS}: Maximum drain current for zero gate-source voltage (A)
  • V_{GS}: Gate-source voltage (V)
  • V_P: Pinch-off voltage (V)

• g_m0 = 2 * I_{DSS} / |V_P|, and g_m = g_m0 (1 - V_GS/V_P)

  • g_m: Transconductance (S)
  • I_{DSS}: Maximum drain current for zero gate-source voltage (A)
  • V_P: Pinch-off voltage (V)

Worked example

Given: I_{DSS} = 10 mA, V_{GS} = -2 V, V_P = -4 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 the expression: I_D = 10 mA * (1 - 0.5)^2

4. Calculate: I_D = 10 mA * 0.25

5. Result: I_D = 2.5 mA. At this bias, g_m0 = 5 mS and g_m = 2.5 mS.

Common mistakes

• Confusing the operation of FETs with BJTs, especially in terms of control (voltage vs. current).
• Incorrectly applying the pinch-off voltage in calculations.
• Neglecting the effect of temperature on FET characteristics.

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 FETs and BJTs.

Quick check

1. What is the main advantage of FETs over BJTs?
2. Name the two main types of FETs.
3. What parameter does the gate voltage control in a FET?

Answers: 1. High input impedance, 2. JFET and MOSFET, 3. Channel conductivity.