Question

Explain, the formation of 'Depletion-layer' at the junction p-n junction-diode. Explain potential-barrier and Avalanche breakdown.

Hint:

To remember the direction of the internal field in the depletion layer, think about the fixed ions left behind: positive ions are on the N-side and negative ions are on the P-side. The electric field always points from positive to negative, so it points from the N-side to the P-side.

Answer 1

1. Formation of Depletion Layer:
When a p-type semiconductor is joined with an n-type semiconductor to form a p-n junction, two processes begin: diffusion and drift. \begin{itemize} \item Diffusion: Due to the high concentration of holes on the p-side and electrons on the n-side, a concentration gradient exists across the junction. This causes holes to diffuse from the p-side to the n-side, and electrons to diffuse from the n-side to the p-side. \item Recombination and Ion Formation: When a hole diffuses to the n-side, it recombines with an electron. When an electron diffuses to the p-side, it recombines with a hole. This process is not instantaneous. The electron leaving the n-side leaves behind a positively charged (ionized) donor atom. Similarly, the hole leaving the p-side leaves behind a negatively charged (ionized) acceptor atom. \item Depletion Region: This creates a region near the junction which is depleted of mobile charge carriers (electrons and holes) but contains a layer of fixed positive ions on the n-side and fixed negative ions on the p-side. This region is known as the depletion layer or space-charge region. \end{itemize} 2. Potential Barrier:
The accumulation of fixed positive and negative charges in the depletion layer sets up an electric field directed from the positive n-side to the negative p-side. This electric field creates a potential difference across the junction. This potential difference is called the potential barrier or barrier voltage (\(V_B\)). The potential barrier opposes the further diffusion of majority charge carriers. An equilibrium is reached when the electric force due to the potential barrier on the majority carriers becomes equal and opposite to the force due to the concentration gradient. For silicon, \(V_B \approx 0.7\) V, and for germanium, \(V_B \approx 0.3\) V at room temperature.
3. Avalanche Breakdown:
Avalanche breakdown is a phenomenon that occurs in a p-n junction diode under a high reverse bias voltage. \begin{itemize} \item Reverse Bias: In reverse bias, the applied voltage widens the depletion layer and increases the strength of the electric field across it. The current is very small and is due to minority carriers. \item Carrier Acceleration: These minority carriers are accelerated to very high velocities and gain significant kinetic energy as they are swept across the strong electric field in the depletion region. \item Impact Ionization: If the reverse voltage is high enough (the breakdown voltage), these energetic minority carriers can collide with the atoms of the semiconductor crystal lattice with enough force to knock valence electrons out of their covalent bonds. This process is called impact ionization, and it creates new electron-hole pairs. \item Avalanche Effect: The newly created charge carriers are also accelerated by the strong electric field and cause further impact ionizations, creating even more carriers. This process repeats, leading to a rapid, cumulative multiplication of charge carriers, much like an avalanche. \item Breakdown: This rapid multiplication results in a sharp increase in the reverse current, and the diode is said to be in the breakdown region. This process can be destructive to the diode if the current is not limited by an external circuit resistance. \end{itemize}