The Native Oxide Shield

Silicon, Eleven-Nines Purity, and Why Germanium Lost the Semiconductor Age

Constructed at Bell Telephone Laboratories in December 1947 by John Bardeen, Walter Brattain, and William Shockley, the first working transistors were fabricated not from silicon, but from germanium. For the first decade of solid-state electronics, germanium dominated production because its crystal metallurgy was forgiving. Germanium melts at a modest $937^\circ\text{C}$, where William Pfann's zone-refining method could easily sweep molten bands along a crystal ingot in a graphite boat to concentrate impurities at one end. Yet germanium carried a fatal electronic flaw. Whenever an early transistor radio or military avionics board sat in the sun or heated past $70^\circ\text{C}$, its germanium circuits went dead.

The thermal failure of germanium is rooted in the width of its electronic bandgap—the forbidden energy interval separating bound valence electrons from mobile conduction electrons. At room temperature, germanium possesses a narrow bandgap of only 0.66 electron-volts. Because the concentration of thermally excited intrinsic charge carriers grows exponentially with temperature, warming a germanium crystal above $70^\circ\text{C}$ excites a flood of electrons across the $0.66\text{ eV}$ gap. Those thermal carriers drown out the engineered p-n dopant junctions in uncontrolled leakage current. Located one row higher in Group 14 of the periodic table, silicon holds its outer $3s^2 3p^2$ valence electrons tighter to the nucleus. Its shorter, stiffer covalent bonds widen the electronic bandgap to 1.12 electron-volts, allowing a silicon transistor to operate cleanly past $150^\circ\text{C}$.

Turning silicon into a viable semiconductor required overcoming extreme chemical reactivity. Silicon melts at $1,414^\circ\text{C}$, and molten silicon attacks the fused-quartz ($\text{SiO}_2$) crucibles used to hold it, dissolving oxygen and trace boron into the melt. To function as a controllable semiconductor, crude metallurgical silicon had to be refined to eleven-nines purity ($99.999999999\text{ percent}$)—permitting no more than one foreign impurity atom among ten billion silicon atoms. In the mid-1950s, engineers at Siemens in Erlangen perfected a chemical vapor distillation bypass. They reacted crude silicon with anhydrous hydrogen chloride gas at $300^\circ\text{C}$ to form volatile liquid trichlorosilane ($\text{SiHCl}_3$, boiling at $31.8^\circ\text{C}$), fractionally distilled the liquid in tall columns, and reduced it with pure hydrogen onto electrically heated silicon rods at $1,100^\circ\text{C}$. In May 1954, chemist Gordon Teal at Texas Instruments pulled doped single crystals from a Czochralski melt to demonstrate the first commercial silicon transistor.

Three decisive physical properties allowed silicon to eradicate germanium from microelectronics: - Bandgap Thermal Margin: Silicon's $1.12\text{ eV}$ bandgap reduces intrinsic room-temperature carrier concentration by four orders of magnitude compared to germanium's $0.66\text{ eV}$ gap, preventing thermal runaway. - Crustal Abundance: Making up 27.7 percent of Earth's crust by mass as silicate rock and quartz sand, silicon is nearly two hundred thousand times more abundant than germanium (1.5 parts per million). - The Native Oxide Passivation Layer: Exposed to oxygen at $1,000^\circ\text{C}$, silicon grows a chemically inert, electrically insulating glass skin of silicon dioxide ($\text{SiO}_2$), whereas germanium dioxide ($\text{GeO}_2$) is hexagonal, hygroscopic, and soluble in warm water.

That third chemical asymmetry created the modern integrated circuit. Early silicon mesa transistors left their microscopic p-n junctions exposed at the etched edges of the crystal, where stray dust, moisture, and surface ions caused erratic short circuits. In December 1957 at Fairchild Semiconductor in Palo Alto, physicist Jean Hoerni realized that instead of etching away the protective $\text{SiO}_2$ glass layer after diffusing dopants into the wafer, he could leave the native oxide permanently bonded over the silicon surface. Because thermal $\text{SiO}_2$ forms a continuous amorphous network with fewer than one dangling bond per thousand surface atoms and a dielectric breakdown strength exceeding ten megavolts per centimeter, it passivated the underlying junctions completely. Six months later, Fairchild co-founder Robert Noyce evaporated thin aluminum metal strips directly atop Hoerni's flat $\text{SiO}_2$ insulator to wire multiple transistors together on a single monolithic silicon die.