Investor Carl Icahn announced Tuesday on Twitter that he had made a large investment in Apple, calling the stock "undervalued." Billionaire Carl Icahn took a "large" position in Apple shares. He believes "Apple has the ability to do a $150 billion buyback now by borrowing funds at 3 percent.

Icahn also said he is requesting that Apple Chief Executive Tim Cook and company officials pursue a bigger buyback of its stock to help raise the value. Earlier this year, under investor pressure, Apple decided to increase the size of a dividend-and-buyback program to $100 billion, including $60 billion to repurchase stock through 2015.

Earlier on Tuesday, Icahn sent a tweet, saying: "We currently have a large position in APPLE. We believe the company to be extremely undervalued. Spoke to Tim Cook today. More to come."

He also tweeted: "Had a nice conversation with Tim Cook today. Discussed my opinion that a larger buyback should be done now. We plan to speak again shortly."

The investor has become increasingly active in the tech sector in recent years.