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A semiconductor diode is formed when small quantities of phosphorus and boron are added to adjacent layers of a crystal of silicon to increase its conduction - Leaving Cert Physics - Question 12 (b) - 2009
Question 12 (b)
A semiconductor diode is formed when small quantities of phosphorus and boron are added to adjacent layers of a crystal of silicon to increase its conduction. Expla... show full transcript
Worked Solution & Example Answer:A semiconductor diode is formed when small quantities of phosphorus and boron are added to adjacent layers of a crystal of silicon to increase its conduction - Leaving Cert Physics - Question 12 (b) - 2009
Step 1
Explain how the presence of phosphorus and boron makes the silicon a better conductor.
Answer
The presence of phosphorus and boron in silicon enhances its conductivity by altering the number of charge carriers.
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When phosphorus (a group V element) is added, it contributes extra electrons, which act as negative charge carriers. This increases the overall electron concentration, thereby enhancing conductivity.
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Similarly, the addition of boron (a group III element) creates 'holes' or positively charged carriers. These holes result from the absence of an electron in the silicon lattice, which facilitates movement and further increases conductivity when paired with the increased electron density.
Step 2
What happens at the boundary of the two adjacent layers?
Answer
At the boundary of the two adjacent layers (p-type and n-type), the following events occur:
- Electrons from the n-type layer migrate towards the p-type layer, where they can recombine with holes. This creates a region depleted of charge carriers at the junction known as the depletion region.
- The formation of this depletion region establishes an electric field, which contributes to the junction voltage.
- As a result, the junction voltage aids in regulating the movement of charge carriers across the boundary.
Step 3
Describe what happens at the boundary when the semiconductor diode is (i) forward biased.
Answer
(i) When the semiconductor diode is forward biased, the depletion region width is reduced, allowing charge carriers to move more freely across the junction. This enhancement of conductivity facilitates current flow through the diode.
(ii) In contrast, when reverse biased, the depletion region widens, leading to an increase in the barrier potential. The increased width prevents current flow, effectively creating a non-conducting state.
Step 4