All about NPN and PNP transistors
A transistor is a semiconductor, meaning that sometimes it conducts electricity, and sometimes it doesn’t. Its internal
resistance varies, depending on the power that you apply to its base.
NPN and PNP transistors are bipolar semiconductors. They contain two slightly different variants of silicon, and conduct
using both polarities of carriers—holes and electrons.
The NPN type is a sandwich with P-type silicon in the middle, and the PNP type is a sandwich with N-type silicon in the
middle. All you need to remember is:
• All bipolar transistors have three connections: Collector, Base, and Emitter, abbreviated as C, B, and E on the
manufacturer’s data sheet, which will identify the pins for you.
• NPN transistors are activated by positive voltage on the base relative to the emitter.
• PNP transistors are activated by negative voltage on the base relative to the emitter.
In their passive state, both types block the flow of electricity between the collector and emitter, just like an SPST relay in
which the contacts are normally open. (Actually a transistor allows a tiny bit of current known as “leakage.”)
You can think of a bipolar transistor as if it contains a little button inside, as shown in the figures below. When the
button is pressed, it allows a large current to flow. To press the button, you inject a much smaller current into the base
by applying a small voltage to the base. In an NPN transistor, the control voltage is positive. In a PNP transistor, the
control voltage is negative.
NPN transistor basics
• To start the flow of current from collector to emitter, apply a relatively positive voltage to the base.
• In the schematic symbol, the arrow points from base to emitter and shows the direction of positive current.
• The base must be at least 0.6 volts “more positive” than the emitter, to start the flow.
• The collector must be “more positive” than the emitter.
PNP transistor basics
• To start the flow of current from emitter to collector, apply a relatively negative voltage to the base. • In the
schematic symbol, the arrow points from emitter to base and shows the direction of positive current.
• The base must be at least 0.6 volts “more negative” than the emitter, to start the flow.
• The emitter must be “more positive” than the collector.
You can think of a bipolar transistor as if it contains a
button that can connect the collector and the emitter. In
an NPN transistor, a small positive potential presses the
button.
In a PNP transistor, a small negative potential has the
same effect. The arrows point in the direction of
“positive current flow.”
All-transistor basics
• Never apply a power supply directly across a transistor. You can burn it out with too much current.
• Protect a transistor with a resistor, in the same way you would protect an LED.
• Avoid reversing the connection of a transistor between positive and negative voltages.
• Sometimes an NPN transistor is more convenient in a circuit; sometimes a PNP happens to fit more easily. They
both function as switches and amplifiers, the only difference being that you apply a relatively positive voltage to
the base of an NPN transistor, and a relatively negative voltage to the base of a PNP transistor.
• PNP transistors are used relatively seldom, mainly because they were more difficult to manufacture in the early
days of semiconductors. People got into the habit of designing circuits around NPN transistors.
• Remember that bipolar transistors amplify current, not voltage. A small fluctuation of current through the base
enables a large change in current between emitter and collector.
• Schematics sometimes show transistors with circles around them, and sometimes don’t.
• Schematics may show the emitter at the top and the collector at the bottom, or vice versa. The base may be on
the left, or on the right, depending on what was most convenient for the person drawing the schematic. Be
careful to look carefully at the arrow in the transistor to see which way up it is, and whether it is NPN or PNP.
You can damage a transistor by connecting it incorrectly.
• Transistors come in various different sizes and configurations. In many of them, there is no way to tell which
wires connect to the emitter, the collector, or the base, and some transistors have no part numbers on them.
Before you throw away the packaging that came with a transistor, check to see whether it identifies the
terminals.
• If you forget which wire is which, some multimeters have a function that will identify emitter, collector, and
base for you. Check your multimeter instruction booklet for more details.
The symbol for an NPN transistor always has an
arrow pointing from its base to its emitter. Some
people include a circle around the transistor;
others don’t bother. The style of the arrow may
vary. But the meaning is always the same.
The symbol for a PNP transistor always has an
arrow pointing from its emitter to its base. Some
people include a circle around the transistor;
others don’t bother. The style of the arrow may
vary. But the meaning is always the same.