AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Resistors: The current through a diode flows in one direction only. The diode has a very high resistance in the reverse direction.
A diode is a non-ohmic component that acts as a one-way valve for electric current.
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
A diode is a non-ohmic component that acts as a one-way valve for electric current. In forward bias, conventional current flows easily from the anode to the cathode, so the diode conducts and its resistance is low. In reverse bias, the diode blocks current because its resistance is very high, so the current is negligible. This behaviour is shown by a current–potential difference graph: the forward curve rises steeply after about 0.6 V for a silicon diode, while the reverse curve stays close to zero. A diode is therefore useful for rectification, converting alternating current into direct current, and for protecting circuits from incorrect polarity.
What earns the marks
- A diode conducts only when connected in forward bias, with conventional current flowing from the anode to the cathode.
- In forward bias the diode has a low resistance once the potential difference exceeds about 0.6 V for a silicon diode.
- In reverse bias the diode has a very high resistance, so the current through it is negligible.
- The current–potential difference graph for a diode is not a straight line through the origin, so a diode is a non-ohmic conductor.
- A diode can be used to convert alternating current into direct current in a rectifier circuit.
Common mistakes and how to put them right
Thinking that a diode allows current in both directions but just reduces it.
Put it right: In reverse bias the resistance is very high and the current is effectively zero.
Believing that a diode obeys Ohm's law because it has a resistance.
Put it right: Its resistance changes with potential difference and direction, so it is non-ohmic.
Confusing the anode and cathode when describing forward bias.
Put it right: Conventional current enters the anode and leaves the cathode in forward bias.
Exam tips
- Sketch the current–potential difference graph with the forward branch in the first quadrant and the reverse branch close to the negative potential difference axis.
- State the direction of conventional current, not electron flow, when explaining forward bias.
- Link the high reverse resistance to a practical use such as rectification or reverse-polarity protection.
Try the lesson
The start of the Resistors: The current through a diode flows in one direction only. The diode has a very high resistance in the reverse direction. lesson, made from checked teaching notes for this specification point.
Have a go first
A diode is connected in a circuit. In which direction does current pass through the diode?
Key words
Say what each word means before you turn the card.
Forward bias
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Reverse bias
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Non-ohmic
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Rectification
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See every definition
- Forward bias
- The connection in which conventional current flows easily from the anode to the cathode, so the diode conducts and its resistance is low.
- Reverse bias
- The connection in which the diode blocks current because its resistance is very high, so the current is negligible.
- Non-ohmic
- Describes a component whose current–potential difference graph is not a straight line through the origin, as with a diode.
- Rectification
- The use of a diode to convert alternating current into direct current.
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