Schottky Barrier Lowering

Schottky Barrier Lowering is the reduction in effective metal-semiconductor barrier height caused by the attractive image force acting on a carrier near a conducting surface — it causes Schottky diodes and contacts to exhibit higher leakage and lower barrier height than simple workfunction difference calculations would predict.

What Is Schottky Barrier Lowering?

• Definition: The lowering of the peak potential energy barrier at a metal-semiconductor junction due to the image potential created when a charge carrier induces an equal and opposite mirror charge in the adjacent metal.
• Image Force Mechanism: When an electron approaches a metal surface, it induces a positive image charge in the metal. The attractive Coulomb interaction between the electron and its image reduces the total potential energy near the surface, rounding and lowering the barrier peak.
• Barrier Reduction Formula: The barrier lowering is proportional to the square root of the electric field at the junction — delta(Vb) = sqrt(qE / 4*pi*epsilon_s), where E is the peak electric field and epsilon_s is the semiconductor permittivity.
• Field Dependence: The larger the electric field at the junction (achieved by higher reverse bias or higher doping), the greater the barrier lowering — leading to the characteristic field-dependent ideality factor in Schottky diode I-V curves.

Why Schottky Barrier Lowering Matters

• Reverse Leakage: Schottky diodes exhibit higher than theoretically predicted reverse current because the effectively lowered barrier admits more thermionically emitted carriers than the nominal workfunction difference would allow.
• Ideality Factor: Image force lowering contributes to ideality factors above 1 in Schottky diodes, causing measured I-V curves to deviate from ideal diode behavior and complicating barrier height extraction.
• Barrier Height Measurement: Accurate determination of Schottky barrier height from I-V or C-V measurements must account for barrier lowering — omitting the image force correction leads to systematic underestimation of the true zero-field barrier.
• High-Voltage Device Design: In power Schottky rectifiers, the field-enhanced barrier lowering under high reverse bias increases leakage current and blocking losses, setting a tradeoff between on-state voltage drop and reverse leakage.
• Simulation Accuracy: TCAD models for Schottky contacts must include image force boundary conditions to correctly predict reverse leakage and forward current at voltages where field-enhanced lowering is significant.

How Schottky Barrier Lowering Is Managed

• Field Control: Reducing the electric field at the Schottky junction through guard rings, field plates, or graded doping profiles limits barrier lowering in high-voltage diodes, improving blocking performance.
• Material Selection: Higher-permittivity semiconductors have smaller barrier lowering for a given field because the image potential is screened more strongly — a consideration in III-V Schottky contact design.
• Accurate Characterization: Richardson plot analysis that extracts barrier height as a function of temperature provides a reliable method to separate image-force lowering contributions from the zero-field barrier value.

Schottky Barrier Lowering is the image-charge physics that makes every metal-semiconductor interface leakier than workfunction calculations predict — accounting for this field-dependent barrier reduction is essential for accurate Schottky diode characterization, contact resistance modeling, and reliable high-voltage device design.