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JUNE 2024 — Physics STD 12 Science — Question

Gujarat BoardEnglish MediumSTD 12 SciencePhysicsJUNE 20242 Marks
Question
Discuss $p-n$ junction diode under forward bias.
✓

Answer

→ When an external voltage V is applied across a semi-conductor diode such that $p$-side is connected to the positive terminal of the battery and the $n$-side to the negative terminal (fig. (a)), it is said to be forward biased.
Image
→ Here, the voltage applied to the diode across the two terminals of the depletion region and the direction of the applied voltage
(V) is opposite to the built-in potential (V).
→ As a result, the depletion layer width decreases and the barrier height is reduced (Fig. (b)). The effective barrier height under forward bias is $\left( V _0- V \right)$.
→ If the applied voltage is small, the barrier potential will be reduced only slightly below the equilibrium value and only a small number of carriers in the material - those that happen to be in the uppermost energy levels - will possess enough energy to cross the junction. So the current will be small.
→ If we increase the applied voltage significantly, the height of the barrier potential reduces, and more number of charge carriers gain enough energy to cross the depletion region, due to which the current also increases.
→ "Due to the applied voltage, electrons from the n -side cross the depletion region and reach p -side (Where they are minority carriers). Similarly, holes from the p -side cross the junction and reach the n -side. (Where they are minority carriers.) This process under forward bias is known as minority carrier injection."
→ At the junction boundary, on each side, the minority carrier concentration increases significantly compared to the locations far from the junction.
→ Due to this concentration gradient, the injected electrons on $p$-side diffuse from the junction edge of $p$-side to the other end of $p$ side. Likewise, the injected holes on $n$-side diffuse from the junction edge of $n$-side to the other end of $n$-side. This is shown in fig. below.
Image
→ This motion of charged carriers on either side gives rise to current. The total diode forward current is sum of hole diffusion current and conventional current due to electron diffusion. The magnitude of this current is usually in mA .

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