p-n Junction Diode under Forward Bias

The figure has two parts. Panel (a) shows the physical circuit: a p-n junction diode with the p-region on the left and the n-region on the right. A battery is connected so that its positive terminal goes to the p-side and its negative terminal to the n-side — this is the forward bias arrangement. The depletion region at the junction is drawn noticeably narrow, which is the key visual cue: forward bias shrinks the depletion layer.

Panel (b) is a graph of electric potential (vertical axis) against position (horizontal axis) across the diode. Three curves are plotted, labelled 1, 2, and 3. Curve 1 corresponds to the equilibrium case (no battery, $V = 0$): the potential jumps by the built-in barrier height $V_0$ across the depletion region. Curve 2 shows what happens when a small forward voltage $V$ is applied — the barrier height drops to $V_0 - V$, but only slightly below $V_0$. Curve 3 is for a larger forward voltage: the barrier is reduced further, again to $V_0 - V$, but now the reduction is substantial. The horizontal axis is marked with "p" on the left and "n" on the right, so the potential is higher on the n-side than on the p-side in equilibrium, and the applied voltage opposes this built-in difference.

The physical idea is straightforward. In forward bias, the external battery pushes electrons from the n-side toward the junction and holes from the p-side toward the junction. Because the applied voltage $V$ opposes the built-in potential $V_0$, the net barrier that carriers face becomes $V_0 - V$. A smaller barrier means more carriers have enough thermal energy to cross the junction. The depletion width shrinks because the electric field that originally held the fixed ions apart is weakened by the external field.

The textbook's own treatment of this figure stays qualitative: a smaller barrier means more carriers have enough thermal energy to cross the junction, so forward current rises steeply as the applied voltage increases and the barrier keeps shrinking. (The exponential diode current-voltage equation and the diffusion-based derivation of the reverse saturation current $I_0$ are not part of the current rationalized NCERT syllabus for this chapter and are not shown here.)

Fig. 14.14 is a single schematic panel (NOT a two-part diagram with a separate concentration-vs-distance graph -- the real printed figure has no such second panel). It shows a cross-section of a p-n junction bar under forward bias, with the p-region on the left and the n-region on the right, and the depletion/junction region drawn as a band containing a mix of small circled + and - symbols in the middle.

Two upward-pointing arrows lead out of that band: one labelled "Injected electrons," the other labelled "Injected holes." These arrows show the two things that happen at the same time once forward bias is applied and the barrier is lowered: electrons cross from the n-side into the p-side (where they become minority carriers), and holes cross from the p-side into the n-side (where they also become minority carriers). A battery symbol below the bar completes the forward-bias circuit, matching the same battery-polarity convention as Fig. 14.13(a).

The physical idea is this: under forward bias, the barrier height is reduced from $V_0$ to $V_0 - V$ (as shown in Fig. 14.13(b)). This allows a significant number of majority carriers to cross the junction. Once across, they become minority carriers on the other side -- electrons are minority carriers in the p-region, holes are minority carriers in the n-region. These injected carriers then diffuse away from the junction into the bulk of the region they've entered, and this diffusion of injected minority carriers is what constitutes the diode's forward current.