One statement of Ohm's law gives the relationship between current I, voltage V, and resistance R in a simple circuit to be I=VR I = V R . Resistance has units of ohms (Ω), related to volts and amperes by 1 Ω = 1 V/A.
A result of the surface resistance measurement de- pends on both the material and the geometry of the electrodes used in the measurement. The physical unit of surface resistivity is Ohm (Ω). The legitimate unit of the surface resistance is also Ohm.
Contact Resistance. Using four probes eliminates measurement errors due to the probe resistance, the spreading resistance under each probe, and the contact resistance between each metal probe and the semiconductor material.
Calculating Volume Resistivity from Sheet Resistance and Thickness: The thickness of the layer in centimeters times the sheet resistance value expressed as ohms-per-square is equal to the volume resistivity in ohms-cm.
Sheet resistance, often called sheet resistivity, is a measure of resistance of thin films that are nominally uniform in thickness. It is commonly used to characterize materials made by semiconductor doping, metal deposition, resistive paste printing, and glass coating.
A four point probe is typically used to measure the sheet resistance of a thin layer or substrate in units of ohms per square by forcing current through two outer probes and reading the voltage across the two inner probes.
Bulk resistance is the sum of resistance values of P & N type semiconductor materials. Because of its minute value, bulk resistance can also be expressed as, rB = ( VF - VB ) / IF. When the value of current is very large, the diode offers the resistance above the barrier voltage in forward direction.
To calculate the resistance R of a wire, we need to know three things:
- its length – the longer the wire, the greater its resistance.
- its cross-sectional area A – the greater the area, the less its resistance.
- the resistivity of the material r – the greater the resistivity, the greater its resistance.
The Four-Probe MethodHere, Rs is the sheet resistance, ΔV is the change in voltage measured between the inner probes, and I is the current applied between the outer probes. This technique is also known as the Kelvin technique, a method of eliminating wire and contact resistances from a resistance measurement.
Table of resistivity for common materials
| Table of Electrical Resistivity for Common Materials |
|---|
| Material | Electrical Resistivity at 20°C Ohm metres |
|---|
| Aluminium | 2.8 x 10-8 |
| Antimony | 3.9 x 10-7 |
| Bismuth | 1.3 x 10-6 |
The insulation resistance of a dielectric is represented by its “Volume Resistivity” and “Surface Resistivity”. Values for plastics typically range from 1010 ohm-cm for Cellulose Acetate to about 1019 ohm-cm for a high-performance polystyrene.
4. Which has higher Rs values? Explanation: The Rs values for p-diffusion is 2.5 times greater than that of the n-diffusion.
The key advantage of sheet resistance over other resistance measurements is that it is independent of the size of the square - enabling an easy comparison between different samples. Another advantage is that it can be measured directly using a four-point probe.
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Four point probe is preferred than two-point probe as the contact and spreading resistances in two point probe are large and the true resistivity cannot be actually separated from measured resistivity. To measure very low resistance values, four probe method is used.
Resistance is a measure of the opposition to current flow in an electrical circuit. Resistance is measured in ohms, symbolized by the Greek letter omega (Ω). Ohms are named after Georg Simon Ohm (1784-1854), a German physicist who studied the relationship between voltage, current and resistance.
