How Semiconductors Work
What really happens inside a diode, an LED and a transistor.
The question
Why does a diode let current through one way but not the other, and why does a red LED need less voltage than a blue one?
What you’ll discover: what semiconductors are, how doping and the PN junction work, and how bipolar transistors and MOSFETs control current.
In simple terms
BeginnerNot quite a conductor, not quite an insulator
Copper conducts easily; plastic hardly conducts at all. Silicon is in between: a semiconductor. On its own, pure silicon conducts poorly, but adding tiny, carefully controlled amounts of other elements, called doping, changes that completely.
- N-type silicon has a little phosphorus added, which brings extra electrons that are free to move (N for negative).
- P-type silicon has a little boron added, which leaves gaps called holes where electrons are missing. Holes act like moving positive charges (P for positive).
The PN junction
Where P-type and N-type silicon meet, some electrons cross over and fill nearby holes. That leaves a thin depletion region with no free charges, which acts like a barrier.
- Forward bias (+ to the P side): above about 0.6 to 0.7 V for silicon, the barrier is pushed aside and current flows. That’s the diode’s forward voltage from Lesson 9.
- Reverse bias (+ to the N side): the barrier gets wider and almost no current flows.
An LED is a PN junction made from other semiconductor materials. When electrons cross the junction and fill holes, they give off energy as light. The material sets both the colour and the forward voltage: higher-energy light, like blue, needs a higher voltage (about 3 V) than red (about 2 V).
Go deeper
IntermediateBipolar transistors
A bipolar junction transistor (BJT) is a sandwich of three layers: NPN or PNP. In an NPN transistor like the 2N3904 from Lesson 10, a small current into the base lets a much larger current flow from collector to emitter. The ratio is the current gain, hFE or β, often around 100 or more for small transistors, but it varies a lot from part to part, so good designs don’t depend on its exact value.
The base-emitter junction is itself a PN junction, so it needs about 0.6 to 0.7 V before any base current flows. That’s why the base resistor is calculated as R = (Vin − 0.7) ÷ the base current you want.
AdvancedMOSFETs
A MOSFET (metal-oxide-semiconductor field-effect transistor) is controlled by voltage instead of current. Its gate is insulated by a microscopically thin oxide layer, so almost no current flows into it; the gate voltage creates a conducting channel between drain and source.
MOSFETs switch large currents efficiently and are the building block of nearly all modern chips: a microcontroller contains millions of them. For switching from a 3.3 V microcontroller pin, choose a logic-level MOSFET whose datasheet shows it fully on at a gate voltage of 2.5 V or so. The gate is so well insulated that static electricity can damage it, so handle MOSFETs by their body and keep them in their packaging until needed.
Quick check
What does doping do to silicon?
Adding small amounts of other elements creates N-type (extra electrons) or P-type (holes) silicon, changing how it conducts.
About what forward voltage does a silicon PN junction need?
About 0.6 to 0.7 V.
What is the main difference between a BJT and a MOSFET?
A BJT is controlled by base current; a MOSFET is controlled by gate voltage.
Words to know
- Semiconductor
- A material that conducts between a conductor and an insulator, controllable by doping.
- Doping
- Adding tiny amounts of other elements to a semiconductor.
- Depletion region
- The barrier layer at a PN junction.
- MOSFET
- A transistor controlled by voltage on an insulated gate.
Electronics School is part of the Discovery Zone. Every activity here runs on small batteries or a USB cable: never wall outlets. The one plug-in tool is the optional soldering iron, set up by an adult.