JFET & MOSFET Fundamentals

Voltage-controlled transistors. Learn the field-effect concept, JFET structure, pinch-off, I_DSS, V_GS(off) and the Shockley equation; MOSFET structure (gate oxide, channel, body, body diode), enhancement vs depletion, NMOS vs PMOS, threshold voltage, regions (cutoff, triode/linear, saturation, subthreshold), the square law and channel-length modulation, key parameters (V_TH, R_DS(on), g_m, g_mb, r_o, C_gs, C_gd, C_ds, Q_g, Q_gd, Q_oss) and switching loss — and why 'saturation' means opposite things for BJTs and MOSFETs — with Python computing JFET/MOSFET currents, g_m, R_DS(on) and switching losses.

FETs control current with an electric field from the gate: essentially no DC gate current flows, giving very high input impedance. MOSFETs dominate digital ICs, power switching and much of modern analog design; JFETs remain valued for low-noise, high-impedance inputs.

Learning the regions and parameters of FETs lets you use them as precision amplifiers, analog switches and efficient power switches.

An analogy

A MOSFET is like a garden hose with a pinch valve operated by magnetism: pressing the gate never lets water out of the gate itself, but its field squeezes or opens the channel so more or less water flows from drain to source.

JFETs

A JFET is a doped channel with reverse-biased PN gates on its sides. Making V_GS more negative (N-channel) widens the depletion regions and narrows the channel until it pinches off at V_GS(off) = V_P. It is normally ON (depletion-mode) and conducts I_DSS at V_GS = 0. JFETs have very low noise and gate leakage in the pA range — used in low-noise preamps, electrometers and as current regulators.

MOSFET types and regions

Types and regions:

'Saturation' — beware the terminology

For a BJT, saturation means both junctions forward-biased: V_CE ≈ 0.2 V, the switch is fully ON. For a MOSFET, saturation means the channel is pinched off at the drain and the current saturates — the device behaves as a current source (used for amplification). A MOSFET used as a closed switch operates in the triode (linear/ohmic) region. Datasheets for power MOSFETs call the triode region the 'linear region' — yet another name.

Parameters and non-idealities

V_TH (spreads ±0.5–1 V, falls with temperature), R_DS(on) (rises ~1.5–2× from 25 to 125 °C, which helps current sharing), capacitances C_iss = C_gs + C_gd, C_oss, C_rss = C_gd (Miller) and gate-charge figures Q_g, Q_gs, Q_gd (Miller plateau) that set drive requirements and switching times; Q_oss and Q_rr add losses. Body effect raises V_TH when the source is above the body (g_mb). Short-channel devices depart from the square law (velocity saturation), and weak inversion gives the highest g_m/I_D. Gate oxide is fragile — ESD and V_GS(max) (typically ±20 V) must be respected.

Formulas

Worked example: JFET current

Given: I_DSS 8 mA, V_P −3 V, V_GS −1 V

Find: I_D.

Answer: 3.56 mA

Worked example: MOSFET g_m

Given: I_D 1 mA, V_OV 0.2 V

Find: g_m.

Answer: 10 mS

Worked example: Gate drive power

Given: Q_g 50 nC, 12 V, 200 kHz

Find: P_g.

Answer: 0.12 W

N-channel enhancement MOSFET

  Gate (metal/poly)
  ──────────────────
  ░░░░ SiO₂ oxide ░░░░          Regions (NMOS):
  n+ source ═channel═ n+ drain   cutoff:     V_GS < V_TH
        p-type body              triode:     V_DS < V_GS − V_TH  (resistor)
  body diode: body→drain         saturation: V_DS ≥ V_GS − V_TH (current source)

A positive gate voltage above V_TH attracts electrons under the oxide, forming an N channel between source and drain. In triode the channel acts like a voltage-controlled resistor; in saturation it pinches off near the drain and the current depends mainly on V_GS.

FET currents, g_m, R_DS(on) and switching loss

# JFET Shockley equation and MOSFET square law, regions, gm and a switching loss estimate
IDSS, VP = 10e-3, -4.0
for VGS in (0.0, -1.0, -2.0):
    ID = IDSS * (1 - VGS / VP) ** 2
    gm = 2 * IDSS / abs(VP) * (1 - VGS / VP)
    print(f"JFET VGS = {VGS:4.1f} V: ID = {ID*1e3:.2f} mA, gm = {gm*1e3:.2f} mS")
k, VTH, lam = 2e-3, 1.0, 0.02                  # k = mu*Cox*W/L (A/V^2)
VGS, VDS = 2.0, 5.0
VOV = VGS - VTH
ID = 0.5 * k * VOV**2 * (1 + lam * VDS)
print(f"MOSFET saturation (VDS {VDS} > VOV {VOV}): ID = {ID*1e3:.3f} mA, gm = {2*ID/VOV*1e3:.2f} mS, ro = {1/(lam*ID)/1e3:.0f} k")
VDS_tri = 0.2
ID_tri = k * (VOV * VDS_tri - VDS_tri**2 / 2)
print(f"triode at VDS = 0.2 V: ID = {ID_tri*1e3:.3f} mA, RDS(on) ~ {1/(k*VOV):.0f} ohm")
# Power MOSFET switching loss: 48 V, 10 A, 100 kHz, tr + tf = 40 ns, RDS(on) = 10 mOhm, D = 0.5
V, I, f, tsw, Ron, D = 48, 10, 100e3, 40e-9, 0.010, 0.5
print(f"conduction loss {I**2*Ron*D:.2f} W, switching loss {0.5*V*I*tsw*f:.2f} W, gate charge 30 nC at 10 V -> {30e-9*10*f*1e3:.0f} mW driver")
# JFET VGS =  0.0 V: ID = 10.00 mA, gm = 5.00 mS
# JFET VGS = -1.0 V: ID = 5.62 mA, gm = 3.75 mS
# JFET VGS = -2.0 V: ID = 2.50 mA, gm = 2.50 mS
# MOSFET saturation (VDS 5.0 > VOV 1.0): ID = 1.100 mA, gm = 2.20 mS, ro = 45 k
# triode at VDS = 0.2 V: ID = 0.360 mA, RDS(on) ~ 500 ohm
# conduction loss 0.50 W, switching loss 0.96 W, gate charge 30 nC at 10 V -> 30 mW driver

The same MOSFET acts as a current source (saturation, 1.1 mA, g_m 2.2 mS) or as a 500 Ω resistor (triode) depending on V_DS. Compared with the BJT (77 mA/V at 2 mA), the MOSFET's g_m per mA is much lower — the price of voltage control. In the power example, switching loss (0.96 W) exceeds conduction loss (0.5 W): faster edges or soft switching matter at high frequency.

Common mistakes

Applications

Key takeaways

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