A semiconductor diode is made by joining two specially treated pieces of semiconductor. It has a low resistance to current in one direction and a very high resistance in the other, so current effectively passes only one way.
When the diode is connected so that it conducts, it is forward biased and current flows. When it is connected the other way it is reverse biased and blocks the current. This gives it its key job: rectification, turning alternating current, which reverses direction many times a second, into current that flows in a single direction.
A single diode passes only the half of each cycle that is in the forward direction, giving half-wave rectification. Diodes in a circuit are what let a phone charger or radio run from the mains supply, which is alternating current.
In SPM you should describe forward and reverse bias and explain how a diode rectifies alternating current.
Common misconceptions
- A diode lets current flow equally both ways -> It conducts in the forward direction and blocks the reverse direction.
- A diode changes alternating current by storing charge -> It rectifies by only passing current one way; storing charge is what a capacitor does.
- Any way round, a diode works the same -> Connecting it forward or reverse biased gives completely different behaviour.
The physics behind it
A semiconductor diode is formed by joining a piece of p-type semiconductor to a piece of n-type semiconductor. The junction between them has a low resistance to current in one direction and a very high resistance in the other, so current effectively flows only one way.
When the positive terminal of a supply is connected to the p-side, the diode is forward biased and conducts, letting current in amperes (A) flow once the voltage in volts (V) rises above a small threshold, about 0.7 V for a silicon diode. Connected the other way it is reverse biased and blocks the current.
This one-way behaviour makes the diode useful for rectification, changing alternating current, which reverses many times a second, into current that flows in a single direction. A single diode passes only the forward half of each cycle, giving half-wave rectification; a bridge of four diodes redirects both halves the same way, giving full-wave rectification, and a capacitor can then smooth the output.
See it in daily life
The mains supply in your home is alternating current, but almost every electronic device, from a phone to a laptop to a radio, actually runs on direct current. The adapter or charger that sits between them contains diodes that carry out rectification, turning the two-way mains current into the one-way current the circuits need.
Diodes appear in many other places too. A light-emitting diode, or LED, is a special diode that gives out light when it conducts, which is why LED bulbs and indicator lamps only light when connected the correct way round.
Solar panels are built from the same kind of junction working in reverse, generating a voltage when light falls on them.
Because a diode protects a circuit from current in the wrong direction, it is often placed at the input of a device so that connecting a battery backwards does no harm. This quiet, one-way gatekeeping is happening inside nearly every gadget you own.
How this comes up in SPM
In Paper 2 you are commonly asked to describe the behaviour of a diode under forward bias and reverse bias, and to explain how a diode rectifies alternating current. You should be able to state that a diode allows current in one direction only.
Questions may show a rectification circuit and ask you to sketch the output voltage against time for half-wave and full-wave rectification, or to explain the effect of adding a capacitor to smooth the output. You may also be asked to identify which way round a diode must be connected to conduct.
This topic sits within electronics, next to the transistor and logic gates, so be ready to place the diode in a wider circuit and to link rectification back to the alternating supply produced by a generator.
Source: DSKP KSSM Physics Form 4 and 5 (Versi English) (Bahagian Pembangunan Kurikulum (BPK), KPM)