Punch-Through Condition in MOSFET
Definition
Punch-through (also called reach-through) is a non-ideal effect where the drain and source depletion regions merge, causing uncontrolled current flow.
When does it occur?
Occurs when:
VDS becomes very large
- Drain depletion region widens
- Extends toward the source
- Touches the source depletion region
Physical Meaning
Normally:
- Current flows through a controlled channel
In punch-through:
- Direct path forms between drain and source
- Current flows without gate control
Conceptual Condition
Drain depletion width ≈ Channel length
Key Effects
- Sudden increase in drain current
- Loss of gate control
- Abnormal device behavior
- Possible device breakdown
Comparison with Avalanche Breakdown
| Feature | Punch-Through | Avalanche Breakdown |
|---|---|---|
| Cause | Depletion regions meet | High electric field (impact ionization) |
| Gate Control | Lost | Partially controlled |
| Occurs At | Lower VDS | Very high VDS |
Summary
Punch-through occurs when the drain depletion region reaches the source, causing uncontrolled current independent of gate voltage.