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The P-N Diode JunctionDramatic Changes in Electrical Properties Controlled by Small Barrier Potential
Jun 21, 2010 © Harry P. Schlanger
A diode consists of p-type and n-type semiconductor materials joined together, forming a depletion region at the junction. Control of this potential determines current flow.
The addition of a small percentage of foreign atoms
in a regular crystal lattice of silicon, or germanium, produces dramatic changes in their electrical
properties.
The Semiconductor Concept
The concept of semiconductors is depicted by the bonding of Silicon with another element Boron, or Antimony.
"Majority carriers" contributing to current flow are then achieved as:
- Holes in p-type semiconductors that bond with Boron
- Electrons in n-type semiconductors that bond with Antimony
When two different types of semiconductor materials are joined intimately as shown, the movement of holes
and electrons combine near the junction, resulting in a depletion layer at the juncture. A small barrier
potential due to charged ions left behind is then created (see first graphic below title).
The Semiconductor Diode
The circuit symbol used for a diode is a large black arrow, indicating conventional current flow in the
direction from p-type to n-type materials. A bar represents the junction. If a battery is
applied to the ends of the diode, the barrier potential provides resistance to any current flow.
Diode Characteristics
Applying a reverse bias potential to the diode causes the depletion layer to widen, increasing resistance
to flow. Majority carriers have more difficulty jumping across the junction and only a tiny current results. On the other hand, applying forward bias causes the depletion region to narrow and so lowers resistance to flow. In this case, majority carriers jump more easily across the junction and a large current ensues.
The circuit on the left shows Reverse Bias - The barrier region is widened and current is small as
resistance to flow is increased. The circuit on the right shows Forward Bias - The barrier region is
narrowed as resistance to flow is reduced.
To summarise, when two different semiconductor materials brought together in intimate contact, the junction
becomes depleted of majority carriers and a small barrier potential forms. Applying external voltage either
reduces the width of the barrier if the voltage is forward-biased, or increases the width of the barrier
when reverse-biased.
The reader might be interested in a follow-up article,
The P-N Diode as a Switch.
References:
- Bail Doug, et al. Heinemann Physics 12. Units 3 and 4. Port Melbourne, Melbourne Australia. 2nd Edition. 2004.
The copyright of the article The P-N Diode Junction: Dramatic Changes in Electrical Properties Controlled by Small Barrier Potential is owned by Harry P. Schlanger. Permission to republish in print or online must be granted by the author in writing.
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