MOSFETs vs. BJTs: Why Voltage and Current Control Devices

MOSFET vs BJT: Technical Deep Dive

Why MOSFETs “Think” in Voltage and BJTs in Current

Understand the physics of transistor semiconductors: MOSFETs are voltage-controlled, while BJTs are controlled by current injection. Easy-to-understand explanations for students and makers.

1. MOSFET: The Field Effect

“Voltage creates control”

The MOSFET operates as a Voltage-Controlled Resistor. The Gate is physically isolated by a layer of Silicon Dioxide ($SiO_2$), meaning there is no physical path for electrons to move from the gate into the silicon.

The Square Law Equation:

$$I_D = \frac{1}{2} \mu_n C_{ox} \frac{W}{L} (V_{GS} – V_{th})^2$$

✔ High Impedance: Ideally $I_G = 0$, requiring nearly zero power to hold the state.
✔ Majority Carrier Device: No charge storage delays, leading to ultra-fast switching.

2. BJT: The Flow Effect

“Current creates control”

The BJT is a Current-Controlled Current Source. It relies on the injection of minority carriers across a PN junction. Control is maintained by the balance of recombination.

The Beta Relationship:

$$I_C = \beta I_B$$

✔ Exponential Sensitivity: A tiny change in $V_{BE}$ leads to a massive change in $I_C$.
✔ Minority Carrier Device: Electrons must physically cross the base, leading to “Storage Time.”

The Regions of Operation

Ohmic / Linear

In a MOSFET, this is where it acts like a resistor ($V_{DS}$ is low). In a BJT, this is called “Saturation,” which is confusing for beginners!

Active / Saturation

MOSFETs in “Saturation” act as current sources ($I_D$ is independent of $V_{DS}$). BJTs in “Active” region do the same. High precision happens here.

Cutoff

Both devices are OFF. However, MOSFETs suffer from “Sub-threshold Leakage,” a critical issue in modern smartphone CPU design.

MOSFET Output Characteristics ($I_D$ vs $V_{DS}$)

BJT Output Characteristics ($I_C$ vs $V_{CE}$)

3. The Switching Operation

MOSFETs don’t need current at DC, but at high frequencies, the gate acts as a low-impedance path due to capacitance.

Miller Effect

The capacitance between Drain and Gate ($C_{gd}$) is amplified by the gain, making the MOSFET appear much “heavier” to the driver than it actually is.

Gate Charge ($Q_G$)

Engineers use $Q_G$ (Coulombs) instead of Farads to determine how much energy is needed to slam the transistor ON.

4. Thermal Behavior

BJT: Positive Feedback

Heat lowers the $V_{BE}$ required, increasing current, creating more heat. Result: Thermal Runaway.

MOSFET: Negative Feedback

Heat increases mobility scattering, increasing resistance ($R_{DS(on)}$). Result: Natural Current Balancing.

Implementation: Control the Gate vs. Base

MOSFET Gate -“G” Requirements

To toggle a MOSFET quickly, the driver must be capable of sourcing/sinking significant Peak Current to charge/discharge the input capacitance ($C_{iss}$).

• Logic Level FETs: Designed for $V_{GS(th)}$ < 2.5V, drivable by MCUs.
• Floating Drivers: Required for High-side N-Channel FETs (Bootstrap circuits).
• Gate Resistor: Necessary to damp oscillations caused by parasitic inductance and gate capacitance.

BJT Base -“B” Requirements

BJTs require a Continuous Current to stay ON. The design must ensure the Base resistor ($R_B$) provides enough current to keep the device in saturation.

• Forced Beta: Usually designed with $\beta_{forced} \approx 10$ to ensure hard saturation regardless of part variation.
• Baker Clamp: A diode circuit used to prevent deep saturation, reducing storage time for faster switching.
• Darlington Pair: Cascaded BJTs used to achieve ultra-high current gain ($ \beta_{total} = \beta_1 \times \beta_2 $).

Real-World Application Case Studies

Microprocessors (CPU/GPU)

Uses billions of FinFETs (advanced MOSFETs). The zero DC gate current is the only reason modern computing is thermally viable.

MOSFET Dominant

High-Fidelity Audio

BJTs are often preferred in output stages due to superior Linearity and lower harmonic distortion compared to FETs.

BJT Preferred

EV Motor Inverters

Uses IGBTs (Insulated Gate Bipolar Transistors) — a hybrid device with a MOSFET gate and BJT output for high voltage handling.

Hybrid (IGBT)

Transfer Characteristics ($I$ vs $V$)

MOSFET ($I_D$ vs $V_{GS}$)

Parabolic: Power of 2 relationship.

BJT ($I_C$ vs $V_{BE}$)

Exponential: Extremely high sensitivity.

Advanced Non-Ideal Effects

Early Effect (BJT)

Wider collector depletion narrows the Base ($V_A$), increasing $I_C$ slightly in active mode.

Channel Length Mod (MOS)

High $V_{DS}$ shortens effective $L$, increasing $I_D$. Equivalent to Early Effect.

Velocity Saturation

In short-channel FETs, electrons hit a “speed limit,” making $I_D$ linear with $V_{GS}$.

Body Effect (MOS)

Substrate potential shifts $V_{th}$. Crucial for multi-stack CMOS designs.

Feature	MOSFET	BJT
Control Input	Voltage ($V_{GS}$)	Current ($I_B$)
Impedance	$\approx \infty$ (Extremely High)	Low to Medium
Switching Speed	Fast (Nanoseconds)	Slower (Storage delays)
Transconductance	Lower	Higher ($g_m = I_C/V_T$)
Linearity	Lower (Parabolic)	Exceptional (Exponential)

The Static Balloon (MOSFET)

Rubbing a balloon on your hair (Voltage) creates a field that pulls hair (Channel) toward it. No matter how much hair moves, you aren’t “using up” the static charge on the balloon unless you touch it.

The Hydraulic Valve (BJT)

A BJT is like a valve that requires a small stream of pilot water (Base Current) to hold a massive floodgate open. If you stop the pilot flow, the floodgate slams shut.