For the emitter layer, the resistivity as well as the thickness of the layer will often be unknown, making the resistance of the top layer difficult to calculate from the resistivity and thickness. However, a value known as the "sheet resistivity", which depends on both the resistivity and the thickness, can be readily measured for the top surface n-type layer. For a uniformly doped layer, the sheet resistivity is defined as:
ρ is the resistivity of the layer; and
t is the thickness of the layer.
The sheet resistivity is normally expressed as ohms/square or Ω/□.
For non-uniformly doped n-type layers, ie., if ρ is non-uniform:
The sheet resistivity of an emitter layer is typically measured with a four-point-probe.