Transistors

Transistor (BJT)

A small current into the base lets a current about 100 times bigger flow from collector to emitter. It works as a switch or an amplifier, and it was the first kind of transistor.

Think of it as: A small hand on a lever that lifts a heavy gate: a little effort (base current) controls a big flow (collector current).

Pins

B (base): : The control input. Always feed it through a resistor: it behaves like a diode to the emitter (about 0.7 V).
C (collector): : Connects to the load for an NPN low-side switch.
E (emitter): : The pin with the arrow. For NPN it goes to ground; the arrow points out, the way current leaves.

Classic small transistors (2N3904, BC547) are black TO-92 "half-moons"; the SMD versions are SOT-23. The pin order differs between parts, so check the datasheet.

What to look for inside

An NPN transistor switching a 12 V load through 1 kΩ. The base current slowly rises from zero and falls again.
With no base current nothing crosses: the transistor is off.
A small base current (pink holes) lets the emitter fire electrons into the very thin base. Almost all of them shoot straight through into the collector; only about 1 in 100 is lost in the base.
That is the gain: the collector current is about β (≈ 100) times the base current, until the load resistor will not allow any more. Then it is saturated, or fully on.

The numbers that matter

Spec	Typical
h_FE (β) Current gain. It varies a lot between parts, so design for the minimum value.	50 to 300
I_C max The most collector current it can carry.	100 mA to 1 A (small signal)
V_CEO max The highest voltage it can block when off.	40 V
V_CE(sat) The voltage left across it when fully on. Times I_C gives the heat.	0.1 V to 0.3 V
V_BE Base–emitter drop, used to work out the base resistor.	≈ 0.7 V

Where you’ll use it

Switching an LED, buzzer or small relay from a microcontroller pin.
Amplifying small signals (audio, sensors).
Level shifting between 3.3 V and 5 V logic.

Common mistakes

No base resistor. The base–emitter junction is a diode, so it draws huge current and the transistor or the pin burns out.
Too little base current, so it never saturates and gets hot. For a switch, give it about I_C ÷ 10 to be sure.
Mixing up the pin order. E-B-C on a 2N3904 is not the same as on other parts.

Lessons that use it

There is no BJT lesson yet. The MOSFET lesson shows the same low-side switching idea.

Lesson 3.2: MOSFET as a Switch: Gate Voltage & Heat

Go deeper

I_C = \beta\, I_B,\quad I_E = I_C + I_B

Active region: the collector current follows the base current.

R_B = \tfrac{U_{pin} - U_{BE}}{I_B}

Base resistor for a switch: e.g. (3.3 V − 0.7 V) ÷ 1 mA ≈ 2.7 kΩ to switch a 10 mA load, using the I_C ÷ 10 rule.

I_C = I_S\,e^{U_{BE}/U_T}

Underneath, the collector current is exponential in U_BE, just like a diode.