Question 40 Answer and Explanation _ACT June 2020 Form C02 Science Section

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.

40. Based on the value of R for the pure silicon wafer and the data in Figure 2, does replacing some of the silicon atoms in a silicon wafer with doping atoms make the wafer a better conductor or a better insulator?

F. A better conductor, because the resistance of each doped wafer was less than that of the pure silicon wafer.

G. A better conductor, because the resistance of each doped wafer was greater than than of the pure silicon wafer.

H. A better insulator, because the resistance of each doped wafer was less than that of the pure silicon wafer.

J. A better insulator, because the resistance of each doped wafer was greater than that of the pure silicon wafer.

Answer and Explanation

Your Answer is

Correct Answer is F

Explanation

Look at the last sentence of the second paragraph of Experiment 1, R for the pure silicon wafer was 3.2 × 10^17 Ω, and the resistance R of all boron-doped wafers and arsenic-doped wafers in Figure 2 reached a maximum of 10^ 4 levels, so the resistance is smaller than pure silicon wafers. This shows that doping can reduce the resistance, which means that the chip becomes a better conductor.