What Are MOSFETs and Why Have They Replaced BJTs in Many Applications?

A MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) is a voltage-controlled transistor that switches or amplifies electrical signals by using an electric field to control current flow through a semiconductor channel. Unlike bipolar junction transistors (BJTs) which require continuous base current to operate, MOSFETs are controlled by voltage alone and draw essentially zero gate current, making them faster, more efficient, and easier to drive — which is why they dominate modern power electronics, digital logic, and switching applications.

Introduction: The Transistor That Changed Everything (Again)

The transistor was already the most transformative invention of the 20th century when it appeared in 1947. By replacing vacuum tubes with tiny, solid-state devices, it enabled the miniaturization of electronics that defined the next seven decades. But as electronics advanced, the original bipolar junction transistor — for all its virtues — showed limitations that became increasingly significant.

BJTs require continuous base current to maintain collector current flow. In switching applications, this means the drive circuit must continuously supply milliamps of base current during the entire time the transistor is on. In digital logic with millions of switching transistors, this current adds up to enormous power consumption. In motor drivers and power supplies, the base drive circuitry adds complexity. In high-frequency applications, the charge stored in the base region limits switching speed.

The MOSFET — Metal-Oxide-Semiconductor Field-Effect Transistor — solves all of these problems simultaneously. A MOSFET is controlled by voltage, not current. Its gate is electrically isolated from the channel by a thin layer of silicon dioxide — essentially perfect insulation. No current flows into the gate under DC conditions. Charging the gate to the appropriate voltage creates an electric field that controls current flow; once charged, the gate holds its voltage indefinitely without consuming power. This fundamental difference — voltage control instead of current control — makes MOSFETs:

• More efficient: No gate drive power wasted in steady state
• Faster switching: No minority carrier storage effects to clear before turning off
• Easier to drive: Logic-level gate voltage directly drives power MOSFETs
• More scalable: MOSFET cells shrink more readily than BJT structures, enabling denser integration

Today, MOSFETs are the dominant transistor type in almost every electronics domain. Every digital logic gate in every integrated circuit uses MOSFETs. Every switching power supply uses MOSFETs. Every motor controller, LED driver, and DC-DC converter uses MOSFETs. The CPU in your computer contains tens of billions of MOSFETs. Understanding MOSFETs — how they work, what their key parameters mean, and how to use them correctly — is essential knowledge for modern electronics.

This article builds a complete understanding of MOSFETs from the physical structure through practical circuit design: the internal physics, the different types (N-channel and P-channel, enhancement and depletion), the critical parameters every designer must know, a thorough comparison with BJTs, and practical guidance for selecting and using MOSFETs in real switching, motor control, and power conversion circuits.

Physical Structure: What Makes a MOSFET Work

Understanding the MOSFET’s physical structure illuminates why it behaves the way it does — why it requires no gate current, why it has threshold behavior, and why its on-resistance matters.

The Four Terminals

A MOSFET has four terminals:

Gate (G): The control terminal. Applying voltage here creates the electric field that controls current flow. The gate is separated from the semiconductor by a thin insulating layer of silicon dioxide (SiO₂) — this insulation is why no DC current flows into the gate.

Drain (D): One end of the controlled current path. Conventional current enters the drain (for N-channel) and exits the source, or vice versa (for P-channel).

Source (S): The other end of the controlled current path. The source is the reference terminal — gate voltage is always specified relative to the source (V_GS).

Body (B): The semiconductor substrate. In discrete MOSFETs, the body is usually connected internally to the source. In integrated circuits, the body connects to the most negative supply (for N-channel) or most positive supply (for P-channel). The body connection creates the body diode — an important parasitic element discussed later.

N-Channel Enhancement MOSFET Structure

The most common MOSFET type is the N-channel enhancement mode device. Its structure, from top to bottom:

Metal gate contact: The gate connection, historically aluminum (now polysilicon in modern devices — the “metal” in MOSFET is somewhat historical).

Silicon dioxide (SiO₂) insulating layer: Extremely thin — typically 2–10nm in modern devices, up to 100nm in older designs. This layer is the insulator that isolates the gate electrically from the channel. Its thinness allows the electric field from the gate voltage to effectively penetrate into the underlying semiconductor.

P-type silicon body: The bulk semiconductor substrate, doped with acceptor atoms to create P-type material (positive holes as majority carriers).

N+ source and drain regions: Heavily doped N-type regions embedded in the P-type body, forming the source and drain connections. Without gate voltage, the P-type body between source and drain creates two back-to-back PN junctions — one from source to body, one from body to drain. No current can flow from drain to source through this path.

The channel region: The thin layer of P-type semiconductor directly beneath the gate oxide. This is where the magic happens.

How the Channel Forms: The Physics of MOSFET Operation

With gate-to-source voltage V_GS = 0:

The P-type body between the N+ source and N+ drain regions forms two back-to-back PN junctions. No current flows from drain to source regardless of drain voltage. The MOSFET is OFF.

With small positive V_GS (below threshold):

The positive gate voltage attracts electrons (negative charges) toward the gate and repels holes (positive charges) away from the surface region beneath the gate. The region beneath the gate oxide becomes depleted of holes. But there are not yet enough attracted electrons to form a conducting channel. Current is still negligible — the device is still effectively OFF.

With V_GS above the threshold voltage (V_GS > V_th):

Enough electrons have been attracted to the surface region beneath the gate oxide to form a continuous N-type channel connecting the N+ source to the N+ drain. This channel is called the inversion layer — the P-type material has been inverted to behave as N-type in this thin layer.

Now the path from drain to source is: N+ drain → N-channel → N+ source — a continuous N-type semiconductor path with no PN junctions blocking current flow. Apply drain voltage, and current flows. The MOSFET is ON.

The threshold voltage V_th (also written V_GS(th)) is the minimum gate-to-source voltage required to form this conducting channel. For logic-level power MOSFETs, V_th is typically 1–3V. For standard power MOSFETs, 2–4V. This threshold behavior is what gives MOSFETs their switch-like characteristic.

The Body Diode: An Inherent Parasitic Element

The interface between the P-type body and the N+ drain region forms a PN junction — a diode. This body diode conducts in the direction from source to drain (for N-channel devices) when the drain voltage falls below the source voltage by about 0.7V.

In many applications, the body diode is an unwanted parasitic. In others — particularly in H-bridge motor drivers and synchronous rectifiers — the body diode is deliberately exploited. Understanding its existence and behavior prevents design surprises:

• In a MOSFET used as a high-side switch (drain connected to power, source to load), the body diode is reverse biased during normal operation — no issue.
• In a MOSFET used as a low-side switch or in a bridge circuit, the body diode may conduct during switching transitions and during freewheeling — this must be accounted for in thermal design.
• The body diode has a relatively slow reverse recovery time compared to Schottky diodes. In fast-switching circuits, the body diode’s reverse recovery can cause shoot-through currents when the opposite transistor turns on before the body diode has recovered.

MOSFET Types: A Complete Classification

MOSFETs come in four fundamental types based on channel type and mode of operation.

Enhancement Mode vs. Depletion Mode

Enhancement mode (E-MOSFET): The channel does not exist at V_GS = 0 — you must apply gate voltage to create it. This is the most common type for switching applications. Enhancement mode MOSFETs are normally OFF (no channel without gate voltage), which is the safe default state for most applications.

Depletion mode (D-MOSFET): The channel exists at V_GS = 0 — the device conducts with zero gate voltage. To turn it OFF, you must apply gate voltage of the opposite polarity to deplete the existing channel. Depletion mode MOSFETs are less common in discrete devices but appear in certain specialized applications and in JFET-based analog circuits.

For almost all practical switching and power applications, enhancement mode MOSFETs are used.

N-Channel vs. P-Channel

N-channel MOSFET: Current flows from drain to source (electrons flow from source to drain). Turned on by positive V_GS (gate more positive than source). N-channel devices generally have lower on-resistance and higher speed than equivalent P-channel devices for the same die size, because electron mobility in silicon is about 2.5× higher than hole mobility.

P-channel MOSFET: Current flows from source to drain (holes flow from source to drain). Turned on by negative V_GS (gate more negative than source). P-channel MOSFETs are used as high-side switches where the source connects to the positive supply — applying a negative gate voltage (relative to source) turns the device on without needing to drive the gate above the supply voltage.

The Four MOSFET Types Summarized

MOSFET Operating Regions

The MOSFET operates in three distinct regions depending on the gate and drain voltages. Understanding these regions is essential for correct circuit design.

Region 1: Cutoff (OFF State)

Condition: V_GS < V_th

No channel exists. Drain current I_D ≈ 0 (only leakage current flows — typically nanoamps to microamps). The MOSFET blocks voltage between drain and source up to its rated breakdown voltage (BV_DSS).

In switching applications, the MOSFET spends most of its time in either cutoff (switch open) or the ohmic region (switch closed). The transition between these regions (through the saturation region) should be as fast as possible to minimize switching losses.

Region 2: Ohmic (Linear or Triode) Region

Condition: V_GS > V_th AND V_DS < (V_GS − V_th)

A conducting channel exists and extends fully from source to drain. The MOSFET behaves like a voltage-controlled resistor. The drain current increases approximately linearly with V_DS:

I_D ≈ k × ((V_GS − V_th) × V_DS − V_DS² / 2)

I_D ≈ k × ((V_GS − V_th) × V_DS − V_DS² / 2)

For small V_DS (fully enhanced channel):

I_D ≈ k × (V_GS − V_th) × V_DS
R_DS(on) = V_DS / I_D ≈ 1 / (k × (V_GS − V_th))

I_D ≈ k × (V_GS − V_th) × V_DS
R_DS(on) = V_DS / I_D ≈ 1 / (k × (V_GS − V_th))

Where k is a device-dependent parameter related to carrier mobility and geometry.

In switching applications, the MOSFET should operate deep in the ohmic region when ON — meaning V_GS should be well above V_th (typically 10V for standard power MOSFETs, 4.5V for logic-level types). A higher V_GS in the ohmic region gives a lower R_DS(on), reducing conduction losses.

Region 3: Saturation (Active) Region

Condition: V_GS > V_th AND V_DS > (V_GS − V_th)

The channel is “pinched off” at the drain end — the electric field from the drain voltage has narrowed the channel to near zero at the drain side. Current becomes approximately independent of V_DS:

I_D ≈ (k/2) × (V_GS − V_th)²

I_D ≈ (k/2) × (V_GS − V_th)²

In this region, I_D is controlled by V_GS only — the MOSFET acts as a voltage-controlled current source. This is the region used for linear amplification (where the MOSFET must remain in saturation throughout the signal swing) and where the MOSFET spends time during switching transitions.

Switching loss occurs in the saturation region. During turn-on, the MOSFET passes through saturation (where both significant current and voltage appear simultaneously), dissipating energy P = V_DS × I_D × t_switch. Fast switching minimizes the time spent in this high-dissipation state.

Key MOSFET Parameters: What the Datasheet Tells You

Selecting the right MOSFET requires understanding the key datasheet parameters and what they mean for circuit performance.

Threshold Voltage V_th (or V_GS(th))

The minimum gate-to-source voltage that creates a conducting channel. Specified at a low drain current (typically 250µA or 1mA — just barely turning on).

Standard power MOSFETs: V_th = 2–4V. These require a 10V gate drive for fully enhanced (low R_DS(on)) operation.

Logic-level MOSFETs: V_th = 1–2V. These achieve specified R_DS(on) with 4.5V or 5V gate drive — directly compatible with 5V logic output signals and 3.3V logic (with some derating).

Sub-threshold swing: Below V_th, a small amount of drain current still flows — it doesn’t cut off perfectly at V_th. For precise switching with minimal leakage, drive the gate to 0V or negative (V_GS = 0 or slightly negative) for the OFF state.

V_th variation: Threshold voltage varies with temperature (typically −5mV/°C for N-channel devices) and between units (±0.5V typical variation). A MOSFET specified with V_th_min = 1V might have V_th as low as 0.7V in some specimens — important for designs where you need the device to be reliably OFF at a specific V_GS level.

On-State Resistance R_DS(on)

The resistance from drain to source when the MOSFET is fully turned on (in the ohmic region at specified V_GS). This is the primary determinant of conduction loss.

Conduction loss:

P_conduction = I_D² × R_DS(on)

P_conduction = I_D² × R_DS(on)

For a MOSFET passing 5A with R_DS(on) = 0.1Ω:

P_conduction = 25 × 0.1 = 2.5W

P_conduction = 25 × 0.1 = 2.5W

R_DS(on) is specified at a particular gate voltage (typically 10V for standard, 4.5V for logic-level devices) and junction temperature (typically 25°C). At elevated temperature, R_DS(on) increases significantly:

R_DS(on)(T) ≈ R_DS(on)(25°C) × (T/300)^2.3

R_DS(on)(T) ≈ R_DS(on)(25°C) × (T/300)^2.3

At 100°C junction temperature, R_DS(on) is approximately 2.5× its 25°C value. A MOSFET specified at 10mΩ at 25°C has about 25mΩ effective resistance at 100°C — this must be accounted for in thermal design.

Trade-off with breakdown voltage: There is a fundamental physics trade-off between R_DS(on) and breakdown voltage. Higher breakdown voltage requires a thicker, more lightly doped drift region — which increases R_DS(on). The relationship approximately follows:

R_DS(on) ∝ BV_DSS^2.5

R_DS(on) ∝ BV_DSS^2.5

A 600V MOSFET has inherently much higher R_DS(on) than a 30V MOSFET for the same die size. This is why low-voltage MOSFETs (for 12V automotive and computer power supplies) can achieve R_DS(on) below 1mΩ, while 600V devices for off-line power supplies might have R_DS(on) of 100–500mΩ.

Drain-to-Source Breakdown Voltage BV_DSS

The maximum voltage the MOSFET can withstand between drain and source without avalanche breakdown. The device must never be subjected to V_DS exceeding this value.

Design margin: Always select a MOSFET with BV_DSS at least 20–50% above the maximum voltage it will ever see, including transient spikes. For a 12V system with inductive loads (which can generate voltage spikes), use a MOSFET rated for at least 30V, preferably 40–60V.

Avalanche energy (E_AS): Some MOSFETs are specified with an avalanche energy rating — the energy the device can absorb while in avalanche breakdown without damage. Avalanche-rated MOSFETs are used in applications where inductive transients might momentarily exceed BV_DSS.

Gate Charge Q_g

The total charge required to switch the MOSFET gate from 0V to the specified gate voltage. Gate charge, not capacitance alone, determines switching speed and gate drive power requirements.

P_gate_drive = Q_g × V_GS × f_switching

P_gate_drive = Q_g × V_GS × f_switching

For a MOSFET with Q_g = 50nC, driven to V_GS = 10V at f = 100kHz:

P_gate_drive = 50×10⁻⁹ × 10 × 100,000 = 50mW

P_gate_drive = 50×10⁻⁹ × 10 × 100,000 = 50mW

This 50mW is dissipated in the gate driver (not the MOSFET itself, since the gate is a near-perfect capacitor — energy is dissipated in the drive resistance during charging and discharging).

Q_g components: Gate charge has three distinct phases visible on a V_GS vs. time curve during switching:

• Q_gs: Charge to bring V_GS from 0 to V_th (no drain current yet)
• Q_gd (Miller charge): Charge during the flat “Miller plateau” where V_GS is constant while V_DS swings — the most critical phase for switching speed
• Q_g_remaining: Charge to bring V_GS from plateau to full gate voltage

The Miller plateau (Miller capacitance × V_DS swing) often dominates total gate charge. Minimizing gate drive resistance speeds through the Miller plateau, reducing switching time and switching losses.

Maximum Drain Current I_D and I_D_pulse

The continuous drain current the device can handle at a specified case temperature (typically 25°C or 100°C). The pulsed drain current (I_D_pulse) rating is much higher — MOSFETs can handle brief current surges well above I_D continuous.

Important caveat: I_D is limited by the package’s thermal resistance, not the silicon itself. A TO-220 package rated for 30A might be limited to 15A in practice without a heatsink, because the junction temperature would exceed the maximum (typically 150°C or 175°C) at higher currents. Always verify thermal calculations rather than relying solely on the I_D rating.

Safe Operating Area (SOA)

The Safe Operating Area graph in the datasheet shows the combinations of V_DS and I_D that the device can handle without damage. The SOA is bounded by:

• Maximum I_D (horizontal line at top)
• Maximum power dissipation P_D = V_DS × I_D (diagonal line)
• Maximum V_DS (vertical line at right)
• Thermal limit at longer pulse widths (additional curves for 1ms, 10ms, DC)

For switching applications (brief on-time), the SOA is generous. For linear applications (motor speed control by varying V_GS, class AB audio amplifiers), the device spends time in the saturation region with both significant V_DS and I_D simultaneously — the SOA must be carefully checked.

MOSFET vs. BJT: A Comprehensive Comparison

Understanding why MOSFETs have displaced BJTs in many applications requires a head-to-head comparison across the parameters that matter most.

When BJTs Still Win

Despite MOSFET’s advantages in switching, BJTs retain advantages in some applications:

Low-voltage linear amplification: BJTs have a more predictable and well-characterized transfer characteristic for linear amplification at low voltages. The transconductance (g_m) of a BJT (g_m = I_C / 26mV at room temperature) is very high, enabling high gain with simple circuits. BJT audio amplifier designs benefit from decades of accumulated knowledge.

Very low collector-emitter saturation voltage: A saturated small-signal BJT might have V_CE(sat) = 0.1–0.2V, lower than the voltage drop of a MOSFET with equivalent R_DS(on) at low currents. For very low supply voltages (1.5V batteries), this matters.

Low-cost switching of small currents: A 2N2222 NPN transistor costs 2–5 cents and handles 600mA — fine for switching LEDs, relays, and small motors. The drive circuit simply connects a resistor from a logic signal to the base. Simple and cheap.

RF and microwave transistors: Specialized BJT variants (HBT — heterojunction bipolar transistors) using compound semiconductors (GaAs, InP) dominate RF and microwave amplification where specific noise and gain characteristics are needed.

When MOSFETs Win

Any switching application above ~1A: MOSFET’s lower on-resistance at higher currents, faster switching, and simpler gate drive make it the clear choice.

High-frequency switching (above ~50kHz): BJTs suffer from stored base charge that must be removed before turn-off. At high frequencies, this charge storage limits switching speed and increases losses. MOSFETs have no minority carrier storage — they switch much faster.

Power supplies and motor drivers: Every modern switching power supply (laptop charger, phone charger, DC-DC converter) uses MOSFETs. Every modern brushed and brushless motor controller uses MOSFETs.

Logic integrated circuits: All CMOS digital logic uses complementary N-channel and P-channel MOSFETs. The ability to scale to nanometer dimensions, combined with near-zero static power consumption in CMOS, made MOSFETs the only viable choice for integrated circuit logic.

Parallel operation: MOSFETs can be easily paralleled to increase current capacity. Because R_DS(on) increases with temperature (positive temperature coefficient), if one MOSFET in a parallel group starts conducting more current and heats up, its R_DS(on) rises, reducing its share of the current — a self-balancing mechanism. BJTs have the opposite behavior (negative temperature coefficient), leading to current hogging and thermal runaway.

Practical MOSFET Gate Drive

Getting the gate drive right is the most common challenge when using MOSFETs for the first time.

Minimum Gate Voltage for Full Enhancement

The datasheet specifies R_DS(on) at a particular gate voltage — typically 10V for standard power MOSFETs, 4.5V for logic-level devices. Driving the gate to a lower voltage than specified gives a higher R_DS(on), increasing conduction losses and potentially damaging the device through excessive heat.

Standard power MOSFET (e.g., IRFZ44N, V_th = 2–4V):

• At V_GS = 4V: Device barely turns on, R_DS(on) >> specified
• At V_GS = 6V: Partially enhanced, R_DS(on) 2–5× higher than spec
• At V_GS = 10V: Fully specified R_DS(on) = 17.5mΩ
• At V_GS = 12V: Slightly lower R_DS(on), negligible improvement

Logic-level MOSFET (e.g., IRLZ44N, V_th = 1–2V):

• At V_GS = 3.3V: Significantly enhanced, R_DS(on) roughly meets spec
• At V_GS = 4.5V: Fully specified R_DS(on) = 22mΩ
• At V_GS = 5V: At or slightly below specified R_DS(on)

Key lesson: A microcontroller’s 3.3V or 5V output can drive a logic-level MOSFET directly to near-specified R_DS(on). Attempting to drive a standard power MOSFET from a 3.3V microcontroller results in a partially on device that dissipates far more heat than the datasheet R_DS(on) would suggest.

Gate Drive Resistor

A resistor in series between the gate driver and the MOSFET gate (R_g, typically 10Ω to 100Ω) controls the rate at which gate charge is delivered and thus switching speed. Slower switching reduces EMI (electromagnetic interference from fast-switching transients) but increases switching losses. Faster switching reduces losses but increases EMI and demands more from the gate driver.

t_switch ≈ R_g × Q_g / V_drive

t_switch ≈ R_g × Q_g / V_drive

For Q_g = 30nC, R_g = 47Ω, V_drive = 12V:

t_switch ≈ 47 × 30×10⁻⁹ / 12 ≈ 118ns

t_switch ≈ 47 × 30×10⁻⁹ / 12 ≈ 118ns

Switching losses:

P_switching = 0.5 × V_DS × I_D × t_switch × f

P_switching = 0.5 × V_DS × I_D × t_switch × f

For V_DS = 12V, I_D = 5A, t_switch = 118ns, f = 100kHz:

P_switching = 0.5 × 12 × 5 × 118×10⁻⁹ × 100,000 = 0.5 × 12 × 5 × 0.0118 = 354mW

P_switching = 0.5 × 12 × 5 × 118×10⁻⁹ × 100,000 = 0.5 × 12 × 5 × 0.0118 = 354mW

This switching loss adds to conduction loss:

P_conduction = I²_D × R_DS(on) × duty_cycle = 25 × 0.01 × 0.5 = 125mW

P_conduction = I²_D × R_DS(on) × duty_cycle = 25 × 0.01 × 0.5 = 125mW

Total dissipation ≈ 480mW — manageable with a small heatsink.

Gate Drive ICs

For high-current applications or fast switching, a dedicated gate driver IC provides adequate peak current to charge the gate capacitance rapidly. The microcontroller or PWM controller output cannot source the 0.5–2A peak current needed for fast gate charging.

Popular gate driver ICs:

• TC4420/TC4429: 6A peak, single output (inverting or non-inverting), SOT-23 or 8-pin DIP
• IR2104: Half-bridge driver with built-in deadtime, 0.6A, for N-channel high and low side
• IR2110: Half-bridge driver, 2A peak, bootstrap high-side supply

Bootstrap High-Side Drive

Driving the gate of a high-side N-channel MOSFET (source connected to the switching node, not ground) requires the gate voltage to be above the source voltage — which means above the supply rail when the switch is on. A bootstrap circuit solves this:

A capacitor (C_boot, typically 100–220nF) is charged to VCC during the low-side switch’s on-time (when the switching node is at ground). When the high-side switch needs to turn on, this bootstrap capacitor provides a floating supply above the switching node, delivering the needed gate voltage above the supply rail.

Bootstrap circuits work well in applications where the high-side switch turns on and off regularly (PWM applications). For applications where the high-side switch must remain on indefinitely, a charge pump or isolated gate driver is needed.

Common MOSFET Families for Hobbyist and Professional Use

Small-Signal / Logic MOSFETs (Low Voltage, Small Current)

2N7000 (N-channel, 60V, 200mA, TO-92): The classic small-signal N-channel MOSFET. R_DS(on) = 5Ω at V_GS = 5V. Suitable for logic-level switching of LEDs, small relays, and sensors from microcontroller outputs.

BS170 (N-channel, 60V, 500mA, TO-92): Low-threshold MOSFET (V_th = 0.8–2.4V) for small loads. Common in RF circuits and small switching applications.

AO3400 (N-channel, 30V, 5.7A, SOT-23): Impressive performance in a tiny package. R_DS(on) = 40mΩ at 4.5V gate. Good for battery-powered applications.

Power MOSFETs (High Current, Moderate Voltage)

IRLZ44N (N-channel, 55V, 47A, TO-220): Logic-level gate (fully enhanced at 5V), R_DS(on) = 22mΩ at 5V. Workhorse for 12V motor control and power switching from microcontroller PWM outputs.

IRFZ44N (N-channel, 55V, 49A, TO-220): Standard (non-logic-level) version, R_DS(on) = 17.5mΩ at 10V. Requires 10V gate drive — use with a dedicated gate driver or 12V logic.

IRF3205 (N-channel, 55V, 110A, TO-220): Extremely low R_DS(on) = 8mΩ at 10V. Overkill for most hobbyist projects but popular for high-current LED drivers and motor controls.

STP36NF06L (N-channel, 60V, 30A, TO-220): Logic-level, R_DS(on) = 35mΩ at 5V. Popular in Arduino and Raspberry Pi motor control projects.

High-Voltage MOSFETs

IRF840 (N-channel, 500V, 8A, TO-220): Classic high-voltage switch for off-line supplies. R_DS(on) = 0.85Ω — higher due to the BV_DSS vs R_DS(on) tradeoff.

STF13NM60N (N-channel, 600V, 11A, TO-220F): SuperMesh technology, R_DS(on) = 0.4Ω, for offline switching power supplies.

P-Channel MOSFETs

IRF9Z34N (P-channel, −55V, −19A, TO-220): P-channel complement for half-bridge circuits. R_DS(on) = 100mΩ at −10V.

FQP27P06 (P-channel, −60V, −27A, TO-220): Popular hobbyist P-channel device for high-side switching without gate driver complexity.

Practical Application: Simple N-Channel MOSFET Switch

The most basic MOSFET application is a low-side N-channel switch — replacing a mechanical switch or relay to control a load under microcontroller direction.

Circuit Design

Load: 12V DC motor drawing 2A maximum. Control: Arduino (5V output, 40mA maximum source current). MOSFET selection: IRLZ44N (logic-level, 55V, 47A, R_DS(on) = 22mΩ at 5V gate).

Circuit connections:

• Motor positive terminal → +12V supply
• Motor negative terminal → MOSFET Drain
• MOSFET Source → GND
• MOSFET Gate → Arduino PWM pin through R_g = 100Ω gate resistor
• 10kΩ resistor from Gate to GND (pull-down — ensures MOSFET is off when Arduino output is tri-stated or during power-up)
• 1N5819 Schottky diode across motor (anode to drain, cathode to +12V) — freewheeling diode for inductive motor load

Why the freewheeling diode is essential: When the MOSFET turns off, the motor’s inductance tries to maintain current flow — “the inductor resists changes in current.” With no current path available, the inductor generates a large voltage spike (L × dI/dt) that can easily reach 100V or more. This spike appears across the MOSFET drain-to-source as V_DS exceeding BV_DSS, potentially destroying the device instantly.

The freewheeling (flyback) diode provides the current path — when the MOSFET turns off, inductor current flows through the diode (now forward biased, conducting from drain through diode to supply, back through motor to drain). The voltage clamps at one diode drop above the supply.

Conduction loss check:

P_conduction = I²_D × R_DS(on) = 4 × 0.022 = 88mW

P_conduction = I²_D × R_DS(on) = 4 × 0.022 = 88mW

No heatsink needed for 88mW in TO-220 package.

Gate drive current from Arduino:

I_gate = V_GS / R_g = 5V / 100Ω = 50mA peak (during switching only)

I_gate = V_GS / R_g = 5V / 100Ω = 50mA peak (during switching only)

The 40mA Arduino output limit is briefly exceeded during switching. For continuous PWM at high frequency, add a gate driver IC (TC4420). For occasional switching (relay replacement), the Arduino can drive directly through R_g.

Thermal Management for Power MOSFETs

For MOSFETs dissipating more than about 1W, thermal management determines whether the device operates safely.

Thermal Resistance Model

Heat flows from the MOSFET junction (the actual silicon) through a series of thermal resistances to the ambient air:

T_junction = T_ambient + P_D × (θ_JC + θ_CS + θ_SA)

T_junction = T_ambient + P_D × (θ_JC + θ_CS + θ_SA)

Where:

• θ_JC = junction-to-case thermal resistance (from datasheet, e.g., 1.67°C/W for TO-220)
• θ_CS = case-to-heatsink thermal resistance (depends on mounting, typically 0.1–1°C/W with thermal paste)
• θ_SA = heatsink-to-ambient thermal resistance (from heatsink datasheet, e.g., 5°C/W for small heatsink)
• T_ambient = ambient temperature (typically 25–40°C)

Example: IRLZ44N dissipating 5W, TO-220 package, small heatsink (θ_SA = 10°C/W), thermal paste (θ_CS = 0.5°C/W), ambient 40°C:

T_junction = 40 + 5 × (1.67 + 0.5 + 10) = 40 + 5 × 12.17 = 40 + 60.85 = 100.85°C

T_junction = 40 + 5 × (1.67 + 0.5 + 10) = 40 + 5 × 12.17 = 40 + 60.85 = 100.85°C

Below the 175°C maximum junction temperature — acceptable. Without heatsink (θ_SA = 62°C/W for TO-220 free air):

T_junction = 40 + 5 × (1.67 + 62) = 40 + 318 = 358°C

T_junction = 40 + 5 × (1.67 + 62) = 40 + 318 = 358°C

Far above maximum — the MOSFET would fail immediately. A heatsink is essential for 5W dissipation.

Summary

The MOSFET’s fundamental advantage over the BJT — voltage-controlled operation with negligible gate current — flows directly from its physical structure: the gate oxide insulates the gate from the channel, allowing an electric field to control carrier concentration and thus current flow without any DC current path into the gate.

Enhancement-mode N-channel MOSFETs are the workhorse of modern electronics: normally off (safe default state), turned on by positive gate voltage, low on-resistance in the fully-enhanced ohmic region, fast switching without minority carrier storage effects, and naturally self-balancing in parallel configurations.

Critical parameters for MOSFET selection are: threshold voltage V_th (determines minimum gate drive voltage), on-resistance R_DS(on) (determines conduction loss — scales with temperature), breakdown voltage BV_DSS (must exceed maximum circuit voltage with margin), and gate charge Q_g (determines switching speed and gate drive power). Logic-level MOSFETs (V_th = 1–2V, R_DS(on) specified at 4.5V) are directly compatible with microcontroller outputs; standard power MOSFETs require 10V gate drive for full enhancement.

MOSFETs have replaced BJTs in virtually all high-current switching, power conversion, and digital logic applications due to superior switching speed, easier gate drive, positive temperature coefficient (enabling safe parallel operation), and scalability to nanometer dimensions. BJTs retain advantages in low-current analog amplification, some RF applications, and cost-sensitive low-current switching.