Texas Instruments (TI) is to pay $6.5bn (£4bn) for National Semiconductor in one of the microchip industry's biggest deals for years.
The deal boosts TI's leadership in analogue chips used in devices such as mobile phones and tablet computers.
Dallas-based TI is paying a 78% premium to National's closing share price on Monday, sparking a sharp rise in rivals' shares.
The deal takes TI past Toshiba as the world's third biggest chipmaker.
Analysts were surprised at the price being paid.
"Texas Instruments has been a pretty prudent company. There is either some kind of buried patent [owned by National Semiconductor] that is unbelievably attractive to them, or there were other bidders and they felt pressured to get it," said Fort Pitt Capital analyst Kim Caughey Forrest.
Analogue chips convert audio and video to digital signals in a wide range of electronic devices from mobile phones to media players.
Apple iPods and the Motorola Xoom tablet computer both use analogue chips made by Texas Instruments.