Transistor

Transistors Overview#

A transistor is a type of semiconductor device that is typically used to amplify a signal or to act as a relay to turn something on and off. The transistors you'll work with come in two main groups: Bipolar Junction Transistors (BJT) and Field Effect Transistors (FET). BJTs are widely used in hobby electronics and Patchr lessons. FETs on the other hand are are more common in industry.

Component Leads#

Base
Collector
Emitter

Bipolar Junction Transistors (BJT)#

A bipolar junction transistor is the most common type of transistor you'll encounter in these lessons, and they come in two main varieties: the PNP and NPN. Each letter in their names stands for P- and N-type silicon and also indicates the order in which they are arranged in the transistor.

Quite physically, a PNP transistor is a sandwich of P-type a sliver of N-type, and another healthy slab of P-type. This isn't hyperbole, the outer P-type of silicon is larger than the small middle N-type. The opposite is true for the NPN transistor. Here the bread in this transistor sandwich is made of N-type silicon.

Types of Silicon Used in BJTs#

N-Type Silicon is manufactured with impurities to have a surplus of electrons.
P-Type Silicon is manufactured with different impurities to have a surplus of holes. For now it's fine to just think of a hole as the absence of an electron in a valence shell of an atom but a place that would readily welcome an electron if one were around.

PNP Overview#

Photograph of a PNP transistor with labelled leads (Collector, Base, Emitter), the Patchr transistor symbol, and a schematic symbol, all on a lime background.

A PNP transistor is a sandwich of P- and N-type silicon with the P acting as the bread of the sandwich.

PNP: Schematic Symbol#

NPN Overview#

Photograph of a NPN transistor with labelled leads (Collector, Base, Emitter), the Patchr transistor symbol, and a schematic symbol, all on a maize background.

An NPN transistor is a sandwich of N- and P-type silicon with the N acting as the bread of the sandwich.

NPN: Patchr Symbol#

NPN: Schematic Symbol#

PNP & NPN Comparison#

PNP	NPN

Current flows emitter to collector.	Current flows from collector to emitter.
PNP is power ON when there is a low signal (GND) is applied at the base then current starts to flow from Collector to an emitter.	NPN is power ON when the positive charge is applied at the base then current starts to flow from Emitter to collector.

The schematic symbol for both the NPN and PNP transistor have an arrow in their emitter leads, however the direct of the arrow is different. On an NPN transistor the arrow faces out and a PNP transistor the arrow points in. What often confuses people is that the direction of the arrows indicates the flow of conventional current not the flow of electrons.

Without going too deep into an explanation on the history of electronic theory it's important to know that historically it was theorized that current flowed from positive to negative. This is in fact the opposite of what takes place. Electrons move from negative to positive.

Today you'll see both conventional current and electron flow discussed, and most electronics books will preface any discussion of current with a note about their approach to conventional current and electron flow. The good new is that so long as you stay consistent with your approach, you'll be OK.

Don't get turned around. The arrow on the schematic symbol of a transistor shows the direction of conventional current not the directions electrons move — negative to positive. This can make understanding a transistor a bit of mental gymnastics.

Historical Importance#

Before the invention of the transistor in December 23, 1947, a large, heavy component called a vacuum tube dominated the electronics world. Due in part to the component's shear size, computers built using vacuum tubes took up entire rooms in laboratories. Fortunately for the world of mobile computing, the transistor provided engineers with a smaller, lighter alternative to the vacuum tube and the miniaturization of computing blossomed.