In our last post, we looked at how we can turn insulating semiconductor materials into conductors by doping them. This creates either free electrons (n-type) or holes (p-type), allowing current to flow.
But the real magic happens when we bring n-type and p-type semiconductors together to create our first and most fundamental semiconductor device: the diode.
What is a diode?
A diode is the simplest type of semiconductor device and the building block for almost every electronic component that comes after it. It has one key job: it allows electricity to flow in one direction, but not the other.
This simple behaviour is incredibly useful, and diodes have sorts of applications:
- Protecting circuits from current flowing the wrong way
- Rectifiers, which convert alternating current (AC) from the grid into direct current (DC) used by your phone, laptop, or TV
- LEDs (Light Emitting Diodes), which emit light when current flows through them
- Photodiodes, used in solar panels and sensors, which generate current when light hits them
Figure 1: Current (I)-Voltage (V) curve of a diode. Ref: https://commons.wikimedia.org/wiki/File:Diode-IV-Curve.svg
How Do We Understand Diodes?
Meet the I–V Curve. To characterise how semiconductor devices work we use a graph called an I–V curve, which plots current (I) against voltage (V):
- Current (I) is the number of electrons flowing through the circuit
- Voltage (V) is the pressure pushing the electrons through
Fig. 1 shows the I-V curve for a diode:
- In the forward ‘right’ direction, once a small threshold voltage is reached, the diode starts conducting and current flows easily.
- In the reverse ‘wrong’ direction, very little current flows, just a tiny leakage, because no device is perfect. If you push the reverse voltage too far, the diode can break down and start conducting anyway.
How is a Diode Made?
A diode is formed by joining n-type and p-type semiconductors. Where they meet the free electrons (from the n-type side) and the holes (from the p-type side) recombine. Forming an area with no free charge carriers, known as the depletion zone. It’s an insulating region, and no current can flow through it, unless…
Figure 2: A diode showing the effect of with a) reverse voltage – no current flowing and b) forward voltage – current flows!
If we apply voltage in the reverse direction (the way the diode doesn’t conduct), electrons and holes are pulled away from the junction. This makes the depletion zone grow wider, and still no current flows.
But flip the voltage around, and something exciting happens: electrons and holes are pushed toward the junction, they recombine at the boundary, the electrons fill in the holes and the depletion zone disappears. And current flows!
The small “turn-on voltage” (around 0.7 V for silicon) is the energy needed to kick-start this recombination process.
Even though I know how they work, diodes still feel like a little bit of magic to me. From powering LEDs to enabling solar panels, this tiny one-way switch lies at the heart of modern electronics.
In the next post, we’ll take a step further into the world of transistors, where things get even more powerful.