Field Effect Transistors (FETs)

Why it matters

Field Effect Transistors (FETs) are essential in modern electronics for their ability to amplify and switch electronic signals efficiently. They are widely used in integrated circuits, which are the backbone of all electronic devices, from smartphones to industrial machines.

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 alters the conductivity of a channel between the source and drain terminals.
• Advantages: FETs offer high input impedance, low gate DC current in suitable operation; noise and total power depend on device type, bias and application rather than a universal advantage over BJTs.
• Applications: Used in amplifiers, oscillators, and switching devices in various electronic circuits.

Formulas

The following square-law relations describe an n-channel JFET in its saturation/pinch-off region, with VP ≤ VGS ≤ 0 and adequate VDS. They are not universal MOSFET equations.

• 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| at VGS = 0; at another operating point, 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_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. Here gm0 = 5 mS and gm = 2.5 mS at VGS = −2 V.

For a long-channel enhancement NMOS square-law model in saturation, ID = (1/2)μnCox(W/L)(VGS − Vth)², with VGS > Vth and VDS ≥ VGS − Vth, neglecting channel-length modulation. State the model and region before using it.

Common mistakes

• Confusing the gate-source voltage (V_{GS}) with the drain-source voltage (V_{DS}).
• Incorrectly applying the square law for I_D in JFETs.
• Forgetting to consider the sign of V_P when calculating g_m.

For GATE EE

• Focus on understanding the characteristics and operation of JFETs and MOSFETs.
• Practice problems involving the calculation of I_D, g_m, and analyzing FET amplifier circuits.
• 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 does g_m represent in FETs?

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