Pinched Resistor in IC Fabrication
Correct Answer: (c) By reducing conduction path
Explanation:
A pinched resistor is a thin-film or diffused resistor that achieves very high resistance values in a small area by utilizing the principle of reducing the conduction path (cross-sectional area) and increasing the length of the resistive material.
Pinched Resistor Principle:
Resistance Formula: R = rho * L / A
Where:
- R = resistance
- rho = resistivity of material
- L = length of conductor
- A = cross-sectional area
To get megaohm resistance in a small area, we need to:
1. Reduce A (cross-sectional area) significantly
2. Increase L (length) within the small area (using serpentine or spiral layout)
3. Use high-resistivity material (n-type diffused region)
How It Works:
A pinched resistor uses a narrow strip of high-resistivity semiconductor material (typically diffused n-type or p-type silicon region). By making the width very narrow and the length very long in a folded/serpentine pattern, we can achieve:
- Very high resistance values (megaohms)
- In very small physical chip area
- Using standard IC fabrication processes
Structure:
Length: Folded or serpentine to maximize L in small area
Width: Very narrow (typically 1-10 micrometers)
Thickness: Determined by diffusion depth
Material: Diffused semiconductor (high resistivity n or p region)
Why Each Option:
(a) By increasing fabrication steps: Incorrect - Extra steps don't directly create higher resistance
(b) By offering bulk resistance in n-region: Partially correct but not the primary method
(c) By reducing conduction path: Correct! Narrow width = small cross-sectional area = high resistance
(d) By limiting thickness of area: Incorrect - Thickness is already limited by diffusion process
Advantages:
1. Achieves megaohm resistance in micrometer-scale area
2. Fabricated using standard diffusion and masking processes
3. Better integration with other IC components
4. More area-efficient than conventional resistors
5. Suitable for high-value resistor applications
Applications:
- Biasing resistors in op-amps and comparators
- Load resistors in analog circuits
- Feedback resistors where high resistance needed
- Active filters and precision circuits
- Sensor signal conditioning
Disadvantages:
- High temperature coefficient
- Lower precision compared to thin-film resistors
- Noise characteristics not ideal for some applications
- Parasitic effects due to narrow geometry
Conclusion:
A pinched resistor achieves megaohm resistance in a small area primarily by reducing the conduction path (cross-sectional area) to very narrow dimensions and utilizing a high-resistivity material. This clever layout technique is fundamental to modern IC design where space is critical.