In late February 1940, a Bell Labs metallurgist named Russell Shoemaker Ohl was hunched over a slab of unusually pure silicon, trying to figure out why it behaved erratically as a radio detector. A crack ran through the middle of the sample. When he shone a flashlight on it, a voltmeter wired across the crack jumped — half a volt, out of nowhere. He had just stumbled onto the p-n junction, the single most important structure in modern electronics.

Ohl filed for the patent on May 27, 1941; it issued as US 2,402,662 ("Light-Sensitive Electric Device") on June 25, 1946. The companion patent, US 2,443,542, covered the rectifying action of the same junction.

In plain language: Ohl had purified silicon so well that residual impurities accidentally segregated as the melt froze, creating one zone rich in electron donors (n-type) and an adjacent zone rich in electron acceptors (p-type). At the boundary, an internal electric field formed. Photons knocked electrons loose, and that built-in field swept them across the junction — producing a voltage with no battery, no moving parts, no heat. Ohl's patent claimed a silicon device that converts light directly to electricity.

• Efficiency: ~1%. Useless for power, but a thunderclap in the lab.
• Material: silicon, refined to a purity nobody else in 1940 was achieving — the technique itself became foundational to semiconductor manufacturing.
• The demo that mattered: Ohl showed it to Mervin Kelly and a young theorist named Walter Brattain on March 6, 1940. Brattain reportedly went pale. He understood instantly that a controllable junction in a solid could replace the vacuum tube.

That demo set Bell Labs on the path that produced the point-contact transistor in December 1947 (Bardeen and Brattain), the junction transistor in 1948 (Shockley), and the entire semiconductor industry. Every diode, every transistor, every CMOS gate in the device you're reading this on is a refined descendant of the cracked silicon ingot Ohl held up to a desk lamp.

The solar-cell branch matured separately. Ohl's 1% cell was improved at Bell Labs by Daryl Chapin, Calvin Fuller, and Gerald Pearson, who in 1954 unveiled a 6% silicon photovoltaic — the first practical solar cell. The New York Times declared it "the beginning of a new era." Today's commercial monocrystalline silicon cells hit 22–24%; lab champions using passivated emitter and rear contact (PERC) and tandem perovskite-on-silicon stacks now exceed 33%. The Shockley–Queisser thermodynamic limit for a single junction sits around 33.7% — meaning Ohl's discovery had ~30× headroom that the industry has spent 80 years methodically harvesting.

What makes the patent surprising for its era: in 1941, "electronics" still meant glass tubes glowing orange in cathedral radios. Ohl's claim — that a chunk of doped silicon could generate current from sunlight, rectify AC, detect radio, and (the implication Brattain saw) eventually amplify signals — described, in a single physical structure, the entire post-vacuum-tube world. Wartime secrecy delayed disclosure; the patent finally issued the same month ENIAC was unveiled. The vacuum-tube computer was already obsolete on the day of its birth, and the document proving it was sitting in the USPTO.