Q.Explain Non-punch through diode.

In a non-punch-through (NPT) power diode the n⁻ drift region is thick enough that, even at rated reverse voltage, the depletion layer does not reach the n⁺ cathode.

[!ANSWER]

In an NPT diode the reverse depletion region stays within the n⁻ drift region (the field falls to zero before the n⁺ layer, giving a triangular field profile), so it needs a thicker drift region and has a higher forward drop.

[!TLDR]

A non-punch-through diode uses a drift region thick enough that the reverse depletion layer never reaches the n⁺ cathode, giving a triangular field profile.

The reverse-blocking ability of a power diode comes from the wide depletion region that forms in the lightly doped n⁻ drift layer. In a non-punch-through (NPT) design the drift region is made deliberately thick. When a reverse voltage is applied, the depletion region spreads from the $P^+ n^-$ junction into the drift region, but even at the maximum rated reverse voltage it stops short of the n⁺ cathode layer.

Because the depletion layer terminates inside the drift region, the electric field has a triangular profile: it is maximum at the junction and decreases linearly to zero within the drift region before reaching the n⁺ layer. The field never "punches through" to the cathode. This gives a robust reverse-voltage capability, but the extra drift-region thickness increases the on-state (forward) voltage drop and hence the conduction loss.

[!ANSWER]

A non-punch-through diode has a drift region thick enough that the reverse depletion region stays inside the n⁻ layer (triangular field falling to zero before the n⁺ cathode); it blocks high reverse voltage but at the cost of a larger forward drop.