6.3 Experimental Measurements
The structures used to evaluate the resistivity of the bulk material are the Hall effect resistor and the collector four-probe resistor [68]. The four-probe resistor has four point contacts arranged in a square array. The structure (see Appendix A) is fabricated by diffusing a base over a large area except at the four point contacts which are protected from the base diffusion by oxide islands. Emitters are diffused at these points in order to make low resistance contacts to the. collector material. The purpose of the base diffusion is to eliminate surface currents. The bulk resistivity is determined by forcing a current, I, between two adjacent probes and measuring the voltage, V, between the other two probes. The resistivity of the material is determined from [69]
2SV (6.1)
(2-v72)IC'
where S is the probe spacing and C' is a correction factor dependent on the ratio of probe spacing to the thickness of the chip [70]. This correction factor is given by
4 4n 2w2 -1/2
C' - 1 + - (_I)n 1 + 4nw
- 2-v2 n=l S2
2 -2 nO )n [1 2n2w2]-I/2
2 -,/ n=l S (62
where 'w is the thickness of the chip.
The collector Hall effect resistor is a four-terminal 'esistor
formed in a square chip 100 mils on a side. Contacts are formed on the