Passage VI

A semiconductor is a substance whose electrical prop­erties can be fine-tuned by doping (replacing some of the substance's atoms with atoms of another substance). Silicon is one example of a semiconductor. A scientist per­formed 2 experiments to study the electrical resistance, R, of silicon wafers (thin blocks) that had been doped.

Experiment 1

The scientist placed 15 silicon wafers into a tempera­ture-controlled chamber. The length and width of each wafer was 10.95 mm, and the thickness of each wafer was 0.95 mm. One wafer was pure silicon, 7 of the wafers had been doped with boron atoms, and 7 of the wafers had been doped with arsenic atoms. Each of the boron-doped wafers and each of the arsenic-doped wafers had a different doping concentration, N_d (the number of doping atoms per cubic centimeter, atoms/cm³). The scientist then connected each wafer to a separate ohmmeter (a device that measures R). Figure 1 shows this apparatus; for clarity only 1 wafer is shown connected.

The scientist set the temperature, T, of the chamber at 300 kelvins (K). Then, after the wafers had been in the chamber for 30 min, she measured R (in ohms, Ω) for each wafer. Figure 2 shows R for the doped wafers; R for the pure silicon wafer was 3.2 × 10⁷Ω.

Experiment 2

The scientist removed each wafer from the chamber except for the two having N_d = 10¹⁶ atoms/cm³ . She measured R for these 2 wafers at various temperatures, waiting 30 min between setting T and measuring R (see Figure 3).

Figures 2 and 3 adapted from Narain D. Arora, John R. Hauser, and David J. Roulston, "Electron and Hole Mobilities in Silicon as a Function of Concentration and Temperature." @ 1982 by The Institute of Electrical and Electronics Engineers, Inc.