p-n Junction Diode: Construction

p-n Junction Diode: Construction

1. What a p-n Junction Diode Is

p-n junction diode = two-layer semiconductor device formed by joining p-type and n-type regions in a single crystal (metallurgical junction), with ohmic contacts on each side. It is the basic building block for rectifiers, Zener, LED etc.

2. Construction (how it's made)

• Substrate: Start with p-type Si wafer (or n-type).
• Doping: Diffuse/ion-implant donor (P) into part of p-substrate to create n-region (or opposite), forming junction at interface. Alternatively epitaxial growth (n on p).
• Junction: abrupt (step) or graded (diffused profile) — abrupt easier for theory.
• Contacts: metal (Al) deposited on p and n ends, annealed to form ohmic contact (linear, no rectification at metal-semiconductor).
• Package: glass/plastic with leads (anode = p, cathode = n, band marks cathode), planar process with SiO₂ passivation for modern IC diodes.

Structure:

Anode (p) | depletion region | Cathode (n)
   Metal contact | p-Si (acceptor) || n-Si (donor) | Metal contact

Depletion region at junction (no free carriers, ionized donors/acceptors) width $W∝√(1/N_A+1/N_D)$ (~0.5 µm for moderate doping).

3. Types of Junction Fabrication

Method	How	Junction Type
Alloyed	Metal dot melted into semiconductor	Old, abrupt
Diffused	Dopant diffused at high T	Graded, modern discrete
Epitaxial / Ion-implanted	Grow or implant dopant	Precise, IC

4. Why Construction Matters

Doping levels ($N_A,N_D$) set built-in potential

(~0.7V Si) and breakdown voltage (higher doping ⇒ lower breakdown), and capacitance ($C∝1/√V$ for abrupt).

5. Exam Points

• Sketch p-n junction with p and n regions, depletion, contacts, anode/cathode — 5-7 marks, very frequent (almost certain construction diagram).
• Describe diffusion/ion-implant to form junction — 3-5 marks, frequent.
• Mention depletion region and $V_{bi}$ — 3-5 marks, frequent.
• Distinguish abrupt vs graded — 3 marks.