Power semiconductor devices: SCR, MOSFET, IGBT

SCR structure, latching and holding current, turn-on and commutation; power MOSFET and IGBT operation, conduction, switching and gate-drive losses, and device selection, with loss-comparison and SCR gate-pulse-width examples.

Drafted with Aria, reviewed by the AiCanCode.org team. Spotted an error? Use Give Feedback at the bottom of the page.

Why it matters

Every motor drive, servo amplifier, battery charger, SMPS and induction heater in a mechatronic system switches power through a semiconductor device. Picking an SCR, MOSFET or IGBT, driving its gate correctly and estimating its losses decides whether the converter is efficient, how big its heat sink must be, and whether it survives faults.

Key ideas

Power devices as switches. A power device is operated either fully on (low voltage drop, conduction loss) or fully off (blocks the supply voltage, almost no current). Loss occurs while conducting and during each transition, when voltage and current are both large at the same time. Devices differ in how they are turned on and off, how fast they switch, and their voltage and current ratings.

SCR (thyristor). A four-layer P-N-P-N device with anode, cathode and gate, and three junctions J1, J2, J3.

  • With the anode positive and no gate signal it is in forward blocking (J2 reverse-biased). With the anode negative it is in reverse blocking.
  • A short positive gate current pulse turns it on; internally it behaves as two coupled transistors with regenerative feedback, so it latches and the gate then loses control.
  • Latching current I_L: the minimum anode current that must flow by the end of the gate pulse for the SCR to stay on. Holding current I_H: the minimum anode current to remain on once conducting. Typically I_L ≈ 2–3 × I_H.
  • Other (unwanted or special) turn-on mechanisms: forward voltage above the breakover voltage V_BO, high dv/dt across the device, high temperature, and light (LASCR).
  • It cannot be switched off from the gate. It turns off (commutation) only when anode current falls below I_H and a reverse voltage is held for longer than the device turn-off time t_q. Natural (line) commutation happens in AC circuits every half-cycle; DC circuits need forced commutation.
  • Protection: a series inductor limits di/dt at turn-on; an RC snubber across the device limits dv/dt.
  • Ratings extend to several kV and several kA, so SCRs dominate HVDC, large controlled rectifiers and soft-starters, at line frequency.

Power MOSFET. A voltage-controlled, majority-carrier device (gate, drain, source). A gate-source voltage above the threshold V_th forms a conducting channel.

  • In a switch it is driven well above V_th (typically 10–12 V, logic-level types 4.5–5 V) so it operates in the ohmic region and behaves as a resistance R_DS(on).
  • The gate is insulated by oxide, so steady gate current is essentially zero; but the gate capacitance must be charged and discharged each cycle, so the driver supplies current pulses.
  • No minority-carrier storage, so switching times are tens of nanoseconds: the device of choice above about 100 kHz and at low voltages (below roughly 200–600 V).
  • R_DS(on) rises with temperature (positive coefficient), so paralleled MOSFETs share current. R_DS(on) rises steeply with voltage rating, which is why high-voltage silicon MOSFETs have high conduction loss.
  • An intrinsic body diode conducts in reverse; it is used as a freewheeling diode in bridges.
  • Gate oxide is fragile: V_GS beyond about ±20 V destroys it.

IGBT. Insulated-gate bipolar transistor: MOSFET-type gate input with a bipolar output stage (gate, collector, emitter).

  • Voltage-driven like a MOSFET, but conductivity modulation in the drift region gives a low on-state voltage V_CE(sat) (about 1.5–3 V) almost independent of voltage rating.
  • At turn-off, stored minority carriers produce a tail current, so switching losses are higher than a MOSFET's; practical switching frequencies are about 2–30 kHz.
  • Ratings from 600 V to 6.5 kV and up to a few kA in modules: the standard device for motor drives (VFDs), servo drives, UPS, traction and induction heating.

Choosing a device. Line-frequency, very high power, no forced turn-off needed → SCR. Low voltage or high frequency → MOSFET (or SiC/GaN wide-bandgap devices for higher voltage at high frequency). Medium-to-high voltage and power at moderate frequency → IGBT. This links directly to the next topics: SCRs in controlled rectifiers, MOSFETs and IGBTs in choppers and PWM inverters.

Formulas

P_cond,MOSFET = I_rms² · R_DS(on)

  • I_rms: RMS current through the device (A); R_DS(on): on-state resistance at operating temperature (Ω). For a DC current I flowing for duty ratio D: P = I² · R_DS(on) · D.

P_cond,IGBT = V_CE(sat) · I_avg (W)

  • V_CE(sat): on-state voltage (V); I_avg: average device current (A). A refinement uses V_CE0 · I_avg + r_CE · I_rms².

P_sw ≈ ½ · V_d · I_o · (t_on + t_off) · f_s (W)

  • V_d: off-state (DC link) voltage (V); I_o: load current switched (A); t_on, t_off: voltage–current crossover times (s); f_s: switching frequency (Hz). Linear-transition approximation for hard switching of an inductive load with a freewheeling diode.

P_gate = Q_g · V_GS · f_s (W)

  • Q_g: total gate charge (C); gate-drive power for a MOSFET or IGBT.

i_A(t) = (V / R) · (1 − e^(−R·t / L)) (SCR with an RL load, after firing)

  • Minimum gate-pulse width: t_min = −(L / R) · ln(1 − I_L·R / V); for a purely inductive load t_min = L · I_L / V.

t_on = t_d + t_r (SCR turn-on time); for reliable commutation, circuit turn-off time t_c > t_q.

Worked examples

Example 1 (standard: MOSFET versus IGBT losses). A chopper switches I_o = 50 A from a V_d = 400 V DC link at f_s = 20 kHz with duty ratio D = 0.5. Compare (a) a MOSFET with R_DS(on) = 0.04 Ω and total crossover time t_on + t_off = 100 ns, (b) an IGBT with V_CE(sat) = 1.8 V and t_on + t_off = 600 ns.

  1. MOSFET conduction: P = I² · R_DS(on) · D = 50² × 0.04 × 0.5 = 50 W.
  2. MOSFET switching: P_sw = ½ × 400 × 50 × 100 × 10⁻⁹ × 20 000 = 20 W.
  3. MOSFET total = 70 W.
  4. IGBT conduction: P = V_CE(sat) · I · D = 1.8 × 50 × 0.5 = 45 W.
  5. IGBT switching: P_sw = ½ × 400 × 50 × 600 × 10⁻⁹ × 20 000 = 120 W.
  6. IGBT total = 165 W.

MOSFET ≈ 70 W, IGBT ≈ 165 W at 20 kHz. The IGBT conducts better, but its slower switching dominates at this frequency; at a few kHz, or at higher voltage where R_DS(on) is much larger, the IGBT would win.

Example 2 (GATE level: SCR gate-pulse width). An SCR with latching current I_L = 50 mA feeds an RL load, R = 20 Ω and L = 0.5 H, from a 100 V DC supply. Find the minimum gate-pulse width. Repeat with R = 0 (pure L).

  1. Anode current after firing: i = (V/R)·(1 − e^(−R·t/L)), with V/R = 5 A and L/R = 0.025 s.
  2. Set i = I_L: 0.05 = 5·(1 − e^(−t/0.025)), so e^(−t/0.025) = 0.99.
  3. t = −0.025 × ln(0.99) = 0.025 × 0.010050 = 2.513 × 10⁻⁴ s.
  4. Pure inductance: i = V·t/L, so t = L·I_L/V = 0.5 × 0.05 / 100 = 2.50 × 10⁻⁴ s.

Minimum pulse width ≈ 251 μs (RL load) and 250 μs (pure L). A shorter pulse leaves the anode current below I_L, and the SCR turns off when the gate pulse ends.

Common mistakes

  • Confusing latching current (to turn on) with holding current (to stay on); I_L > I_H.
  • Thinking a gate signal can turn an SCR off; it needs anode current below I_H plus reverse bias for longer than t_q.
  • Calling an IGBT or MOSFET current-controlled; both are voltage-controlled, though the gate driver must supply charging current pulses.
  • Using I_avg instead of I_rms for MOSFET conduction loss, or I² R for an IGBT (use V_CE(sat) · I).
  • Ignoring switching loss, which grows linearly with frequency and often dominates.
  • Saying "saturation region" for a fully-on MOSFET switch; a MOSFET switch is in the ohmic (linear) region, while "saturation" in a BJT/IGBT means fully on. The words mean opposite things for the two families.

For GATE ME

Expect conceptual questions on device structure, latching and holding current, turn-on methods and commutation, the controlling quantity (current or voltage) for each device, and device selection for a given frequency and power. Numericals cover minimum gate-pulse width with R, L or RL loads, conduction and switching loss estimation, and turn-on time. Practise the RL exponential and the loss formulas with careful units (ns, kHz, mA).

Quick check

  1. Which of SCR, MOSFET and IGBT cannot be turned off from its gate?
  2. An SCR has I_H = 20 mA. Is its latching current larger or smaller than this?
  3. Find the conduction loss of a MOSFET with R_DS(on) = 0.05 Ω carrying a steady 20 A.
  4. Why do paralleled MOSFETs share current reasonably well?
  5. Which device would you choose for a 1 MHz, 48 V DC-DC converter?

Answers: 1. SCR. 2. Larger. 3. 20 W. 4. R_DS(on) increases with temperature, so a hotter device carries less current. 5. MOSFET.

Interview questions

All Electrical Circuits and Electronics interview questions

Try answering each one aloud before you open it.

  1. 1.What is a Silicon Controlled Rectifier (SCR) and how does it function?Concept

    An SCR is a four-layer P-N-P-N thyristor with anode, cathode and gate. With the anode positive it blocks until a gate current pulse is applied; regenerative feedback between its two internal transistors then latches it on, provided the anode current has reached the latching current by the end of the pulse. After that the gate has no control, and the SCR conducts until the anode current falls below the holding current and a reverse voltage is held for longer than its turn-off time t_q. It is used for controlled rectification and AC power control at line frequency and very high power.

  2. 2.Explain the working principle of a MOSFET.Concept

    A MOSFET is a voltage-controlled, majority-carrier device with gate, drain and source; the gate is insulated from the channel by a thin oxide. A gate-source voltage above the threshold V_th attracts carriers under the oxide and forms an inversion channel that lets current flow from drain to source. As a power switch it is driven well above V_th so it sits in the ohmic region and behaves like a small resistance R_DS(on). Because no steady gate current flows and there is no minority-carrier storage, it switches very fast, although the gate capacitance must be charged and discharged every cycle.

  3. 3.What is an Insulated Gate Bipolar Transistor (IGBT) and where is it commonly used?Concept

    An Insulated Gate Bipolar Transistor (IGBT) is a semiconductor device that combines the high input impedance of a MOSFET with the low on-state power loss of a bipolar transistor. It is commonly used in applications requiring high efficiency and fast switching, such as inverters, motor drives, and power supplies.

  4. 4.Why is a MOSFET preferred over a BJT in high-frequency applications?Application

    A MOSFET conducts only by majority carriers, so there is no stored minority charge to remove at turn-off and its switching times are tens of nanoseconds, while a saturated BJT has a storage time that slows turn-off. Its gate needs only charge to switch, not a continuous base current, so the drive circuit is simpler and drive power is low even at high frequency. Its positive temperature coefficient of R_DS(on) also makes paralleling easy. As a result switching losses stay manageable at hundreds of kHz, where a BJT would be far too lossy.

  5. 5.What happens if the gate voltage of a MOSFET is increased beyond its threshold voltage?Application

    Once V_GS exceeds the threshold V_th, an inversion channel forms and drain current can flow. If V_DS is small (V_DS < V_GS − V_th) the MOSFET is in the ohmic region and acts like a resistance, which falls as V_GS rises; this is where a power switch is operated, with V_GS around 10 V. If V_DS is larger it enters the saturation (active) region, where the drain current is set by V_GS rather than by the load, as used in amplifiers. Raising V_GS above its rated maximum, typically about ±20 V, punctures the gate oxide and destroys the device.

  6. 6.How does an SCR differ from a TRIAC?Concept

    An SCR is a unidirectional device, meaning it can conduct current in only one direction, from anode to cathode. In contrast, a TRIAC is a bidirectional device that can conduct current in both directions, making it suitable for AC applications. TRIACs are often used in light dimmers and motor speed controls.

  7. 7.In what scenario would you choose an IGBT over a MOSFET?Application

    Choose an IGBT for medium-to-high voltages (roughly 600 V to 6.5 kV) and high currents at moderate switching frequencies, about 2–30 kHz, as in VFDs, servo drives, UPS and traction inverters. Its on-state voltage stays about 1.5–3 V regardless of voltage rating, whereas a silicon MOSFET's R_DS(on) rises steeply with voltage rating, so the IGBT has much lower conduction loss at high voltage. At high frequency or low voltage the MOSFET wins, because the IGBT's tail current makes its switching losses larger.

  8. 8.Calculate the power loss in a MOSFET with an on-state resistance of 0.1 Ω and a current of 10 A flowing through it.Numerical

    The power loss in a MOSFET can be calculated using the formula P = I²R, where P is the power loss, I is the current, and R is the on-state resistance. Substituting the given values: P = (10 A)² × 0.1 Ω = 100 A² × 0.1 Ω = 10 W. Therefore, the power loss is 10 watts.

  9. 9.What is the role of the gate in an SCR?Concept

    The gate in an SCR is used to trigger the device into conduction. By applying a small gate current, the SCR is turned on, allowing a larger current to flow from the anode to the cathode. Once the SCR is conducting, the gate loses control, and the device remains on until the current falls below the holding current.

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