From Diodes to Transistors
The diode has two terminals and one main control variable: the voltage across its junction. A transistor adds another terminal and another degree of control. That small step on a schematic represents a major step in what an electronic circuit can do.
What the diode has already taught us
Before meeting a transistor, we already understand several essential ideas:
- Silicon can be deliberately doped P-type or N-type.
- Electrons and holes are mobile charge carriers.
- A PN junction forms a depletion region and internal electric field.
- Forward bias injects carriers across a junction.
- Reverse bias widens the depletion region and largely blocks current.
- Minority carriers can be stored, transported, and recombined.
Two junctions do not simply make two diodes
A bipolar junction transistor (BJT) contains either an NPN or PNP sequence, so it has two PN junctions. It is tempting to imagine two ordinary diodes connected back-to-back. That model predicts the DC continuity checks reasonably well, but it completely misses transistor action.
The reason is that the center region—the base—is made very thin and is deliberately doped so carriers injected from the emitter can cross it and be collected by the collector. In two separate diodes, carriers do not share one thin common base region this way.
NPN idea
In an NPN transistor, the base-emitter junction is normally forward biased. Electrons are injected from the emitter into the thin P-type base. Only a small fraction recombine there. Most diffuse across the base and reach the reverse- biased base-collector junction, whose electric field sweeps them into the collector.
Thus a relatively small base-emitter input condition controls a much larger collector current.
Current gain
A simple beginner model writes:
where β is transistor current gain. This is useful for first calculations, but β is not a fixed precision constant. It changes with collector current, temperature, device type, and operating conditions.
The base-emitter junction still looks diode-like
A silicon BJT base-emitter junction often has roughly the same forward-voltage territory as an ordinary silicon diode. A multimeter's diode-test function can therefore help identify a BJT's junctions and polarity.
From rectification to amplification
| Device | Main idea | What it enables |
|---|---|---|
| Diode | One junction, two terminals, asymmetric conduction. | Rectification, clipping, protection, detection. |
| BJT | Two interacting junctions, three terminals. | Amplification and switching controlled by carrier injection. |
| JFET | Reverse-biased junction controls a conductive channel. | Voltage-controlled current with high input resistance. |
| MOSFET | Electric field through an insulated gate controls a channel. | Very high input resistance and efficient switching. |
The historical parallel
Vacuum electronics followed a similar conceptual progression. The two-electrode thermionic diode rectified. Adding a control grid produced the triode, which amplified. Semiconductor electronics again moved from a two-terminal rectifying device to a three-terminal controlling device, though using completely different physics.
Next: transistors
When moving to the transistor section, keep one question in mind: How does the third terminal control carriers that would otherwise move through the device? For a BJT the answer involves carrier injection through PN junctions. For field-effect transistors the answer involves an electric field controlling a channel.
That question is the bridge from the diode to nearly all active solid-state electronics.