What is a MOSFET and what does it do?

A MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) is a voltage-controlled switch. Apply sufficient gate voltage and current flows drain-to-source. Remove it and current stops. Gate draws virtually no current in steady state, so MOSFETs are easier to drive than BJTs.

What is the difference between a BJT and a MOSFET?

BJTs are current-controlled: base current drives collector current. MOSFETs are voltage-controlled: gate voltage drives drain current with near-zero gate current. MOSFETs switch faster and run cooler at high frequencies. BJTs are cheaper for simple low-frequency switching.

What is an IGBT used for?

IGBTs combine MOSFET gate control with BJT output characteristics. They handle high-voltage, high-current applications above 600V where MOSFETs become inefficient: motor drives, inverters, welders, and UPS systems.

What is the BC547 transistor used for?

The BC547 is a general-purpose NPN BJT rated 45V, 100mA. Common uses include signal amplification, LED driving, relay switching, and interfacing microcontrollers to higher-voltage loads.

Why use ULN2003A instead of individual transistors?

The ULN2003A contains 7 NPN Darlington pairs with built-in base resistors and flyback diodes. One IC replaces 7 transistors plus external components, simplifying PCB layout when driving relay banks or LED arrays from a microcontroller.

Transistors Explained: BJTs, MOSFETs & IGBTs with Real Part Comparisons

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Transistors Explained: BJTs, MOSFETs & IGBTs with Real Part Comparisons

Admin: IDAR Mohamed
13 Jun 2026
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Every circuit board has transistors in it. The part numbers change, the package shrinks from TO-3 to SOT-23, but the job is the same: switch something on and off, or amplify a weak signal. What changes dramatically between circuit types is which transistor family makes sense.

Comparison of common transistor packages including TO-3 and SOT-23.

Figure: Transistor packages have become dramatically smaller over time, from large TO-3 metal power packages to compact SOT-23 surface-mount devices used in modern electronics.

This guide covers BJTs, MOSFETs, and IGBTs with real part numbers, worked example calculations, and a selection table for common applications.

What Is a Transistor?
Transistor Operating Regions Explained
BJT vs MOSFET vs IGBT
Common BJT transistors
MOSFETs explained
What is an IGBT?
Darlington arrays: ULN2003A and ULN2803A
Selecting the right device
FAQs

What Is a Transistor?

A transistor is a three-terminal semiconductor device that amplifies a signal or acts as a controlled switch. In most modern circuits, it's the second role that matters: switching loads on and off, controlling motor speed, or routing logic signals.

Three major families cover most applications. BJTs (Bipolar Junction Transistors) are current-controlled. MOSFETs (Metal-Oxide-Semiconductor Field-Effect Transistors) are voltage-controlled. IGBTs (Insulated Gate Bipolar Transistors) are a hybrid type built for high-voltage work.

Each targets a different operating point. Choosing the wrong one adds heat, gate drive complexity, or efficiency problems.

Transistor Operating Regions Explained

Bipolar Junction Transistors (BJTs) operate in three primary regions, each dictating the transistor's behavior and application within a circuit: the Cutoff Region, the Active Region, and the Saturation Region. Understanding these regions is crucial for designing circuits that utilize transistors effectively as switches or amplifiers.

Figure: Collector current (IC) versus collector-emitter voltage (VCE) showing transistor cutoff, active, and saturation regions for increasing base currents (IB).

Cutoff Region

In the cutoff region, the transistor acts like an open switch. Both the emitter-base junction and the collector-base junction are reverse-biased. For an NPN transistor, this means the base current ( $I_B$ ) is zero or very close to zero. Consequently, there is no significant collector current ( $I_C$ ), and the transistor effectively blocks the flow of current between the collector and emitter. This state is ideal for turning off a load or preventing signal flow.

Active Region

The active region is where the transistor functions as an amplifier. In this region, the emitter-base junction is forward-biased, and the collector-base junction is reverse-biased. A small change in the base current ( $I_B$ ) leads to a proportionally larger change in the collector current ( $I_C$ ). This amplification is governed by the transistor's current gain, beta (β or $h_{FE}$ ), where $I_C = \beta \times I_B$ . The output characteristics graph for the Common-Emitter configuration clearly illustrates this, showing distinct curves for different $I_B$ values, where $I_C$ increases with $V_{CE}$ up to a certain point and then flattens out, indicating a controlled current source. This linear relationship makes the active region suitable for analog amplification applications.

Saturation Region

When the transistor is in the saturation region, it behaves like a closed switch. Both the emitter-base junction and the collector-base junction are forward-biased. This occurs when a sufficiently large base current ( $I_B$ ) is applied, causing the collector current ( $I_C$ ) to reach its maximum possible value, limited by the external circuit components (e.g., collector resistor). In this state, the voltage across the collector-emitter ( $V_{CE}$ ) is very small, typically around 0.2V for silicon transistors, indicating that the transistor is fully turned on. The saturation region is critical for digital switching applications where the transistor needs to fully conduct current.

BJT vs MOSFET vs IGBT

Parameter	BJT	MOSFET	IGBT
Control type	Current (base)	Voltage (gate)	Voltage (gate)
Drive requirement	Resistor + base current	Gate resistor only	Gate resistor only
Switching speed	Moderate	Very fast	Moderate
Voltage range	Up to ~200V typical	Up to 1000V+	600V to 6500V
On-state loss	$V_{CE}(sat) \times I_C$	$I^2 \times R_{DS}(on)$	$V_{CE}(sat) \times I_C$
Best fit	Amplifiers, low-freq switching	Power switching, motor drives	High-voltage inverters

Below 100V at moderate-to-high frequencies, MOSFETs are the default. Above 400 to 600V, IGBTs often deliver better efficiency per dollar. BJTs remain useful for analog amplifiers and simple low-frequency switching.

Common BJT transistors

BC547 — General-purpose NPN

Rated at 45V $V_{CEO}$ and 100mA $I_C$ in TO-92. Standard for relay driving, LED switching, and interfacing microcontroller GPIOs to 12V loads.

Worked example: driving a 12V relay from a 3.3V GPIO

Relay coil: 12V, 70mA
Required base current: $I_B = I_C / h_{FE} = 70mA / 200 = 0.35mA$
With 10x saturation margin: $I_B(target) = 3.5mA$
$R_B = (V_{GPIO} - V_{BE}) / I_B = (3.3 - 0.7) / 0.0035 = 743Ω, \text{use } 680Ω$

The BC547 handles this with headroom. Above 100mA, move to the 2N2222 or 2N3904.

BC547 NPN transistor TO-92 package showing collector, base, and emitter pin locations.

Figure: BC547 general-purpose NPN transistor in TO-92 package with collector (C), base (B), and emitter (E) pin identification.

2N2222 / 2N2222A — Classic NPN switch

Rated at 40V and 600mA, the 2N2222A is available in TO-18 and TO-92 packages. Compared to earlier variants, it offers a higher guaranteed current gain ( $h_{FE}$ ), typically a minimum of 100 instead of 75 at 150mA, along with tighter manufacturing tolerances.

2N2222A NPN transistor package and pinout showing collector, base, and emitter terminals.

Figure: 2N2222A NPN transistor with collector, base, and emitter terminals labeled for circuit design and troubleshooting.

The actual $h_{FE}$ can vary widely—from around 75 up to 300—depending on collector current and operating conditions. For reliable switching design, always base calculations on the minimum $h_{FE}$ specification, not the typical value, to ensure proper saturation under worst-case conditions.

Example 1: Switching a Small DC Motor with a 2N2222A

A common use for the 2N2222A is driving small DC motors from a microcontroller. Since most GPIO pins can only source a few milliamps, the transistor acts as a switch that allows the motor to draw current directly from the power supply.

Components

NPN transistor: 2N2222A
DC motor: 6V, 150mA
Base resistor: calculated
Flyback diode: 1N4007
Microcontroller GPIO: 5V

2N2222A NPN transistor used as a low-side switch driving a 6V DC motor with a flyback diode and GPIO control signal.

Figure: Typical 2N2222A transistor motor driver circuit showing a microcontroller GPIO controlling a 6V DC motor through a base resistor and flyback protection diode.

Step 1: Motor current

Motor running current:

Ic = 150mA

Small DC motors can draw several times their rated current at startup, so it is good practice to leave additional design margin.

Step 2: Calculate required base current

For switching applications, the transistor should operate in saturation.

Instead of using the typical transistor gain, a forced gain of 10 is commonly used:

Ib = \frac{Ic}{10}

Ib = \frac{150mA}{10}

Ib = 15mA

Step 3: Calculate the base resistor

GPIO voltage:

V_{gpio} = 5V

Base-emitter voltage:

V_{be} ≈ 0.7V

R_b = \frac{V_{gpio} − V_{be}}{I_b}

R_b = \frac{5 − 0.7}{0.015}

R_b = 286Ω

Step 4: Choose a standard resistor value

The nearest standard value is:

R_b = 270Ω

This provides slightly more base current and helps guarantee saturation.

Step 5: Power dissipation in saturation

A saturated 2N2222A typically has:

V_{CE}(sat) ≈ 0.2V

Power dissipated by the transistor:

P = V_{CE} \times I_c

P = 0.2 \times 0.15

P = 0.03W

Only 30mW is converted into heat, so the transistor remains cool.

Flyback diode protection

When the motor is switched off, its winding generates a reverse voltage spike.

A 1N4007 diode connected across the motor terminals safely absorbs this energy and protects the transistor from damage.

Operation

GPIO HIGH → transistor saturates → motor runs
GPIO LOW → transistor turns off → motor stops
Flyback diode suppresses inductive voltage spikes

Insights

The 2N2222A can easily switch small motors, relays, buzzers, and solenoids. By forcing the transistor into saturation, voltage drop and power dissipation remain low, improving efficiency and reliability.

2N3904 / MMBT3904 — Low-noise NPN

Rated 40V, 200mA. The 2N3904 is TO-92 through-hole; MMBT3904 is the SOT-23 SMD equivalent on the same die. The noise figure is slightly better than that of the 2N2222, so it tends to be the default for audio pre-amplifier stages and sensor interfaces where the noise floor matters.

2N3904 low-noise NPN transistor in TO-92 package with pinout diagram.

Figure: 2N3904 transistor pinout showing emitter, base, and collector connections used in signal amplification circuits.

2N3055 — Power NPN

60V, 15A, TO-3. The 2N3055 was widely used as a pass transistor in linear power supplies, where it regulates output voltage by operating in its linear region. It also appeared in audio amplifiers and early motor control circuits before MOSFETs became the standard for efficient switching applications. While modern designs prefer MOSFETs for switching, the 2N3055 is still useful in simple linear regulators where circuit simplicity matters more than efficiency.

2N3055 power transistor in TO-3 metal package showing base, emitter, and collector connections.

Figure: 2N3055 power NPN transistor in a TO-3 package. The case acts as the collector terminal.

MOSFETs explained

A MOSFET controls drain current with gate voltage. No gate current flows in steady state because the gate is capacitive. That's the key practical difference from a BJT: no continuous base current to manage. Gate drive circuits are simpler as a result.

N-channel MOSFET showing gate, drain, and source terminals with insulated gate structure.

Figure: N-channel MOSFET structure and pinout. The insulated gate controls current flow between the drain and source without requiring continuous gate current.

For N-channel MOSFETs, the device turns on when $V_{GS}$ exceeds the threshold voltage, typically 2 to 4V for standard parts and 1 to 2V for logic-level types.

IRFZ44N — Workhorse MOSFET

Specs: 55V, 49A, $R_{DS}(on)$ = 17.5mΩ at $V_{GS}$ = 10V, TO-220.

Conduction loss at 10A continuous:

P_{cond} = I^2_D \times R_{DS}(on) = 10^2 \times 0.0175 = 1.75W

IRFZ44N N-channel MOSFET pinout showing gate, drain, and source terminals.

Figure: IRFZ44N MOSFET in TO-220 package with gate (G), drain (D), and source (S) terminal identification.

A small heatsink covers that. The IRFZ44N is common in PWM motor control, DC-DC switching stages, and low-side switching up to 55V.

Example 2: PWM Speed Control of a DC Motor Using IRFZ44N

MOSFETs are ideal for motor control because they require almost no steady-state gate current and have very low on-resistance.