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.