Question

How the ohmic contact is formed in metal semiconductor diode? (AL-Aluminium)

(a) n^+ diffusion in p-region near AL lead

(b) p^+ diffusion in p-region near AL lead

(c) n^+ diffusion in n-region near AL lead

(d) p^+ diffusion in n-region near AL lead

I have been asked this question in an online quiz.

This key question is from Active and Passive Components of IC topic in section IC Fabrication of Linear Integrated Circuits

Answer 1

The correct answer is:

(c) n+n^+n+ diffusion in $n$-region near AL lead

Explanation:

In a metal-semiconductor diode, an ohmic contact is formed when the metal makes contact with a heavily doped region of the semiconductor. This ensures that the potential barrier at the junction is minimal or negligible, allowing current to flow freely in both directions without rectification.

For aluminum (Al), which typically forms a contact with $n$-type silicon:

• n+n^+n+ diffusion (heavily doped $n$-region) ensures a very small barrier height, thereby enabling an ohmic (non-rectifying) behavior.

Why not the other options?

• (a) n+n^+n+ diffusion in $p$-region near AL lead: This would not form an ohmic contact but rather a rectifying junction.
• (b) p+p^+p+ diffusion in $p$-region near AL lead: This is used for forming ohmic contact in $p$-type semiconductors but not relevant for aluminum in $n$-type regions.
• (d) p+p^+p+ diffusion in $n$-region near AL lead: This creates a $p$-$n$ junction, which is rectifying rather than ohmic.