Also describe the experimental setup for the four probe method for resistivity measurement with the help of neat diagram.

Electrical parameters:

1. Characterization of starting wafer: P or N
2. Resistivity: For measurement of hole and electron concentration
3. Carrier mobility

Four Probe method:

It is a common method to measure resistivity. In this method basically the current flowing for the unknown applied voltage is measured to calculate resistance or vice versa.

The four collinear metal probes with sharpened tips are placed on semiconductor.

It allows the current through the two outer probes; voltage drop is measured with the inner two probes using a high impedance voltmeter.

Problems with probe contacts are eliminated in voltage measurement since no current flows through these contacts.

If we assume that the semiconductor dimensions are large compared to probe spacings then the potential measured at a distance r from a probe carrying a current I in a material with resistivity ρ is simply

$V=\frac{ ρI}{2πr}$

From geometry of figure in which probe spacing are uniform, this reduces to

$ρ=2πs \frac{V}{I} Ω.cm$

Where

I = current driven through two outer probes

V = potential difference measured between the inner two probes

In most instruments of this type s is taken 1.588 mm. i.e 2πs = 1 cm and calculations become simple.

The equation is valid only if wafer diameter & thickness both are very much greater than s.