Transistors

Transistors are the fundamental active component in electronics. Every logic gate, amplifier stage, and memory cell is built from transistors. They do two things: switch and amplify. Understanding the basic types and how to use them in simple circuits unlocks most of practical electronics design. The two major families you'll encounter are BJTs and MOSFETs — different physics, different strengths.

BJTs — Bipolar Junction Transistors

A BJT has three terminals: Base, Collector, Emitter. A small current into the base controls a larger current between collector and emitter. The ratio of collector current to base current is the current gain β (beta or hFE), typically 50–300.

Polarity	How it turns on	Current direction	Common parts
NPN	Positive voltage at Base (relative to Emitter)	Collector → Emitter	2N2222, 2N3904, BC547, TIP31
PNP	Base pulled low (relative to Emitter)	Emitter → Collector	2N2907, 2N3906, BC557, TIP32

BJT Operating Regions

Region	Condition	Behaviour	Use
Cutoff	Vbe < 0.6V, no base current	Transistor fully off — acts like open circuit	Switch in OFF state
Active (linear)	Vbe ≈ 0.7V, base current controlled	Ic = β × Ib — linear amplification	Amplifier stages
Saturation	Vbe > 0.7V, base driven hard	Vce ≈ 0.2V, transistor fully on	Switch in ON state

NPN as a Switch

To turn the NPN transistor ON, you must provide enough base current to guarantee saturation (fully on). To turn it OFF, you remove the base current.

Calculating the base resistor:
Example: switching 100mA load, β = 100
Minimum Ib = Ic / β = 100mA / 100 = 1mA
Target Ib = 10mA (overdrive for reliable saturation)

Control voltage = 5V, Vbe = 0.7V
R_base = (5V - 0.7V) / 10mA = 430Ω → use 470Ω or 560Ω

MOSFETs — Metal-Oxide-Semiconductor FETs

MOSFETs have Gate, Drain, Source terminals. They're voltage-controlled — a voltage on the Gate controls current between Drain and Source with essentially zero gate current. This makes them more efficient as switches than BJTs.

Type	Turns on when	Use case	Common parts
N-channel enhancement	Vgs > Vth (gate above source)	Low-side switching (load between VCC and drain)	2N7000, IRF540N, IRLZ44N
P-channel enhancement	Vgs < Vth (gate below source)	High-side switching (load between drain and GND)	IRF9540N, SI2307

N-Channel MOSFET as a Switch

For logic-level control from a 3.3V or 5V microcontroller, use a logic-level MOSFET with Vth well below your control voltage. Standard power MOSFETs often need 8–10V on the gate to fully turn on — they won't work reliably directly from a GPIO pin.

BJT vs MOSFET — Comparison

Property	BJT	MOSFET
Control type	Current (base current required)	Voltage (no gate current in DC steady state)
Input impedance	Low (base-emitter junction ~0.7V drop)	Very high (gate is insulated)
On-resistance	Vce_sat ≈ 0.2V (moderate loss)	RDS_on can be milliohms (very low loss)
Switching speed	Slower (charge storage in base)	Faster, especially at high frequencies
Parallel operation	Difficult (thermal runaway risk)	Easy (positive temperature coefficient for RDS_on)
Best for	Linear amplifiers, audio, precision analog	Power switching, motor control, digital logic
Drive complexity	Simple — just enough base current	Gate capacitance matters at high frequency; needs gate driver for fast switching

Common Part Numbers and Specs

Part	Type	Ic / Id max	Vce / Vds max	Notes
2N3904	NPN BJT	200 mA	40 V	Classic small-signal NPN. Everywhere.
2N3906	PNP BJT	200 mA	40 V	Complement to 2N3904
2N2222A	NPN BJT	600 mA	40 V	Slightly higher current than 2N3904
TIP31C	NPN BJT	3 A	100 V	Power transistor, TO-220 package
2N7000	N-ch MOSFET	200 mA	60 V	Small signal, logic-level gate
IRLZ44N	N-ch MOSFET	47 A	55 V	Logic-level, low RDS_on — good for 5V Arduino projects
IRF540N	N-ch MOSFET	33 A	100 V	Power switching, requires 10V+ gate drive

Darlington Pairs

Two BJTs cascaded so the first drives the second — the combined gain is β₁ × β₂, which can reach several thousand. Useful when driving a high-current load from a very small signal (e.g., a microcontroller output driving a large relay):

Combined β ≈ β1 × β2 (e.g., 100 × 100 = 10,000)

The ULN2003 and ULN2803 ICs are arrays of 7–8 Darlington drivers in a single package, each capable of 500 mA. The downside: higher saturation voltage (~1.2–1.5V instead of 0.2V), which means more power loss.

Safe Operating Area and Heat

Both BJTs and MOSFETs have a Safe Operating Area (SOA) — a region of voltage and current they can handle simultaneously without damage. At high current AND high voltage, even a brief spike can destroy the transistor. For power applications:

• Attach a heatsink when the case temperature will rise more than 20–30°C above ambient
• Use thermal paste between the transistor and heatsink
• Add a flyback diode across any inductive load (relay, motor) to suppress voltage spikes when switching off

References

• Horowitz & Hill — The Art of Electronics, 3rd ed. Chapter 2
• Sedra & Smith — Microelectronic Circuits
• Electronics-Tutorials — BJT ↗
• Electronics-Tutorials — MOSFET ↗