Ohmic Contact

Metal-Semiconductor Contacts — the junctions formed where metal interconnects meet semiconductor regions, classified as either ohmic (low resistance) or Schottky (rectifying) based on their electrical behavior.

Ohmic Contact

• Linear I-V characteristic (current proportional to voltage in both directions)
• Goal: Minimum possible resistance between metal and semiconductor
• Achieved by: Very heavy doping at the semiconductor surface (>10²⁰ cm⁻³), making the depletion region so thin that carriers tunnel through
• Contact resistance must be minimized — it adds to total transistor resistance and reduces drive current
• Materials: Ti/TiN barrier + W plug (traditional), Co or Ru (advanced nodes)

Schottky Contact

• Rectifying: Current flows easily in one direction, blocked in reverse (like a diode)
• Forms when metal contacts lightly doped semiconductor
• Schottky barrier height depends on metal work function and semiconductor

Schottky Diode Applications

• Fast switching (no minority carrier storage — faster than pn diodes)
• Low forward voltage drop (~0.3V vs ~0.7V for pn junction)
• Used in: RF detectors, power supply clamping, ESD protection

Contact Scaling Challenge

• As transistors shrink, contact area decreases → contact resistance increases
• At 3nm node, contact resistance can be 30-40% of total device resistance
• This drives research into new silicide/germanide materials

Contacts are a hidden bottleneck — the world's fastest transistor is useless if you can't get current in and out efficiently.