Marvell Technology Group warned revenue would drop this quarter as tablet sales nibble at the PC market and as it struggles to sell smartphone chips in China, hurting its shares after hours.

Executives told analysts on a Thursday conference call they foresaw a plunge of more than 20 percent in mobile and wireless revenue this quarter, while storage sales will slip by a mid-single digit percentage from the previous quarter.

That came after the company lagged expectations on both revenue and earnings in the fourth quarter ended January and said progress in its long-awaited move into China's affordable smartphone market was slower than expected.

It's almost turned into such a wait-and-see story that investors are getting a little bit weary. You can see that in the stock price, said Williams Financial Group analyst Cody Acree.

Investors also worry that red-hot sales of Apple Inc's iPad and other tablets will hurt the outlook for PCs and the chips that go in their hard drives. Marvell's largest customer is hard-drive maker Western Digital .

Marvell is also fending off intensifying competition in smartphone processors from the likes of Nvidia and Qualcomm , and it supplies chips for BlackBerry-maker Research In Motion.

Hurt by concerns about soft PC sales and increasing competition, Marvell saw its shares fall around 2 percent so far in 2011 before it reported its quarterly results, far underperforming the Philadelphia Semiconductor Index's <.SOX> 13 percent gain.

It said revenue in the first quarter would be between $800 million and $850 million, the midpoint representing an worse-than-expected 8 percent dip from the fourth quarter.

GROWTH ZONES

Marvell is betting on China to increase its smartphone processor sales, amid concerns that Canadian customer Research In Motion faces limited long-term prospects as it competes against the likes of Apple and Google .

Last week, ASUS said its new series of smartphones for China Mobile would use Marvell's processors and WiFi/Bluetooth chips. But Marvell's processors have yet to be chosen for high-profile phones beyond Research In Motion. And analysts questioned why Marvell is taking longer than expected to stake out territory in China's entry-level smartphone market.

We are shifting our business away from PC-based stuff to non-PC based and those businesses are very successful ... Don't judge this by just one or two quarters, Chief Financial Officer Clyde Hosein answered.

Marvell said net profit, excluding items, in the fourth quarter was $273 million or 40 cents a share, compared with $266 million or 40 cents a share in the year-ago period. Analysts had expected fourth-quarter net profit of 42 cents a share.

Broadening its business, the Santa Clara, California-based company has become a major player in supplying networking and storage chips for video game consoles, although that market is more seasonal and is expected to make the company's revenue more volatile.

Microsoft's Kinect motion-sensing device for the Xbox 360, launched last year, uses processors made by Marvell, a major win for the company. Retailers bought more than 8 million Kinects in 2010, well above its initial forecast of 5 million.

The company said revenue in the quarter ended in January rose 7 percent to $901 million year over year, but fell 6 percent from the prior quarter.

Analysts on average had expected Marvell to report fourth-quarter revenue of $924 million, according to Thomson Reuters I/B/E/S.

Shares of Marvell fell 5.6 percent to $17.18 in extended trading following the earnings report. The stock closed up 1.28 percent on the Nasdaq.

(Reporting by Noel Randewich; Editing by Bernard Orr, Richard Chang and Matthew Lewis)