BJT vs MOSFET: Which Transistor Should You Use?

Detailed Explanation

The choice between a BJT and a MOSFET is one of the most common component-selection decisions in hardware design. Understanding the fundamental difference — current-controlled vs voltage-controlled — and the practical implications for drive circuitry, switching speed, efficiency, and thermal behaviour allows you to make the right choice for each application.

Fundamental Difference: Control Mechanism

BJT (Bipolar Junction Transistor):

• Current-controlled. The collector current (Ic) is proportional to the base current (Ib): Ic = hFE × Ib.
• The base-emitter junction is a diode; driving the base requires sourcing or sinking current continuously while the device is on.
• Input impedance is low: the base draws current from the drive signal.

MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor):

• Voltage-controlled. The drain current is determined by the gate-to-source voltage (VGS).
• The gate is insulated from the channel by a thin oxide layer; virtually no DC gate current flows.
• Input impedance is very high (GΩ at DC); the gate appears as a capacitor.

This single difference has cascading consequences for drive circuitry, efficiency, and speed.

Drive Circuitry Comparison

BJT drive: For a BJT switch carrying 500 mA through a load, with hFE = 100, the required base current is at minimum 5 mA. For a 10× saturation margin: 50 mA. The drive circuit must continuously sink or source 50 mA while the switch is on. If the drive is a GPIO limited to 8 mA, a BJT preamplifier or a higher-current driver stage is required.

Base resistor: R = (V_drive − 0.7V) / Ib_required
Example: (3.3V − 0.7V) / 50 mA = 52 Ω → use 47–56 Ω

MOSFET drive: For a MOSFET switch, the only current required is the charge and discharge of the gate capacitance during switching transitions. In steady state (fully on or fully off), no gate current flows. A GPIO can drive most small logic-level MOSFETs directly.

Gate series resistor: 10–100 Ω (limits peak current during transition; not required for DC)
Pull-down: 10 kΩ to GND (ensures defined-off state during power-up)

Implication: The MOSFET is far easier to drive from microcontroller GPIO pins. For a BJT at high currents, a dedicated driver stage is often needed, adding components and complexity.

Switching Speed and Frequency

BJT switching speed is limited by minority carrier storage in the base region during saturation. When the base drive is removed to turn off, excess carriers must recombine — a process called storage time (ts), typically 0.1–5 µs for small-signal BJTs. This limits practical BJT switching frequency to roughly 100 kHz.

MOSFET switching speed is limited by gate capacitance charging time, which depends on the gate driver's peak current capability. With adequate drive, MOSFETs switch in nanoseconds. Power MOSFETs in well-designed buck converters switch at 200 kHz–2 MHz.

Implication: For DC-DC converters, motor controllers, and any switching application above ~50 kHz, the MOSFET is the only viable choice.

Conduction Losses

BJT in saturation: Vce ≈ 0.1–1V (VCEsat). Conduction loss = Ic × VCEsat.

• At 10A, VCEsat = 0.5V: P_conduction = 5W.
• VCEsat is relatively constant regardless of current.

MOSFET in linear (on) region: Acts as a resistor: Vds = Id × RDSon. Conduction loss = Id² × RDSon.

• At 10A, RDSon = 10 mΩ: P_conduction = 1W (much lower than BJT at the same current).
• RDSon decreases as die size increases; modern power MOSFETs have RDSon of 1–10 mΩ.
• At very low currents, VCEsat is small and the BJT may have lower loss than a MOSFET with moderate RDSon.

Implication: At high currents (> a few amperes), the MOSFET is more efficient. At very low currents (< 100 mA), losses are small for both and the difference is negligible.

Head-to-Head Comparison Table

When to Use a BJT

1. Linear amplification: Op-amp output stages, audio amplifiers, and precision analog circuits use BJTs because the Ic/Ib relationship is more predictable and the device operates smoothly in the active (linear) region. The MOSFET's square-law characteristic is less suitable for small-signal linear design.

2. Current-mirror and bias circuits: Matched BJT pairs are the standard building block for current mirrors (where precise Ic/Ib ratios matter) in analog IC design and discrete precision circuits.

3. Very low current switching: For switching currents below ~10 mA (LED drive, logic-level signal switching), a small NPN BJT like the 2N3904 works well and costs less than a cent. The gate charge concern of MOSFETs at very low current levels is irrelevant.

4. Existing designs: When modifying or maintaining legacy hardware, matching the existing component type avoids re-analysis of bias networks, base resistors, and saturation conditions.

When to Use a MOSFET

1. Any switching application driven by a microcontroller GPIO: Logic-level MOSFETs with VGS(th) ≤ 2V switch fully on from 3.3V or 5V without requiring a current-limited base drive. This is the most common discrete transistor application in embedded systems.

2. DC-DC converters and motor drives: The MOSFET's faster switching speed and lower RDSon at high currents are decisive. All modern switching power supplies use MOSFETs (or IGBTs for very high voltage).

3. Driving relays, solenoids, and motors: An N-channel MOSFET on the low side (source to GND, drain to load) with a pull-down on the gate is a simple, reliable solution for driving inductive loads. Pair with a freewheeling diode across the load to clamp inductive voltage spikes.

4. Battery disconnect and power path control: P-channel MOSFETs or ideal diode controllers (which use a MOSFET as a near-zero drop diode) are standard in battery-powered products.

For power electronics design — component selection, gate drive design, thermal management, and PCB layout for switching circuits — Zeus Design's engineering team handles end-to-end product electronics — contact Zeus Design.

Design Considerations

• For logic-level MOSFET selection, verify RDSon at the actual VGS you'll use. Most MOSFETs specify RDSon at VGS = 10V. A "logic-level" MOSFET specifies RDSon at 4.5V (for 5V drive) or 2.5V (for 3.3V drive). Verify the curve in the datasheet at your actual supply voltage, not just the headline parameter at 10V.
• BJT parallel circuits suffer from current hogging. BJTs have a negative temperature coefficient on VCEsat: a hotter transistor has lower VCEsat, draws more current, and heats further. Paralleling BJTs without emitter resistors to balance current sharing causes one device to carry all the current. MOSFETs have a positive RDSon temperature coefficient, so current balances naturally between parallel MOSFETs.
• Consider thermal runaway with BJTs at elevated temperatures. At high ambient temperatures, BJT leakage current (ICBO) increases with temperature. In some circuit configurations, this increase in leakage causes more base drive, which increases Ic, which increases temperature — a runaway condition. Include adequate thermal margin and thermal shutdown protection for BJT-based power stages.
• Add a gate-to-source pull-down for MOSFETs. A 10 kΩ–100 kΩ resistor from gate to source ensures the gate discharges to GND during power-up, power-down, and when the drive signal is floating. Without this, the MOSFET may partially turn on during power sequencing, causing unexpected current paths.

Common Mistakes

• Choosing a standard power MOSFET (VGS(th) = 2–4V) for 3.3V GPIO drive. At 3.3V gate drive, a MOSFET with VGS(th) = 3V may barely be in the ohmic region, with RDSon many times higher than the specified value. Always choose a logic-level MOSFET for 3.3V drive systems. The forum discussion N-channel MOSFET not switching fully from 3.3V GPIO shows this failure mode in a real circuit.
• Forgetting that BJTs require continuous base current. A GPIO that switches a BJT on at startup but enters a high-impedance state (e.g. during sleep or reset) stops supplying base current, turning off the BJT and disconnecting the load — possibly unexpectedly. Design base drive circuits to hold a defined state during all MCU operating modes, including reset and low-power modes.
• Using a MOSFET body diode as the freewheeling diode in a high-frequency converter. The MOSFET body diode has slow reverse recovery and high forward voltage (0.6–1.2V). In a synchronous buck converter, the body diode conducts only during dead time, which limits the loss. In a non-synchronous converter, add an external Schottky diode instead of relying on the body diode for freewheeling.
• Not accounting for MOSFET gate charge at high switching frequencies. At 1 MHz switching with Qg = 10 nC, the average gate drive current is 10 mA — well within GPIO capability. But the peak gate current during the nanosecond-scale switching transition can be hundreds of milliamperes. A GPIO without current capability will slow the switching edge, increasing switching losses. Use a dedicated gate driver IC for frequencies above ~100 kHz or for large-Qg MOSFETs.