Shares of SigmaTel ( SGTL) jumped in after-hours trading Tuesday after the circuit maker reported that fourth-quarter earnings more than doubled.Recently, SigmaTel shares were up $4.19, or 12.4%, to $38.09 after falling 55 cents, or 1.6%, to $33.90 in regular trading. The Austin, Texas-based company reported fourth-quarter net earnings rose to $19.5 million, or 52 cents a share, compared with $8.2 million, or 24 cents a share, a year ago. Revenue jumped to $78.6 million from $35 million last year. Analysts were expecting the company to post earnings of 42 cents on sales of $63.88 million, according to Thomson First Call. Looking forward, SigmaTel forecast first-quarter earnings of 39 cents to 44 cents a share on revenue of $76 million to $83 million. That compares favorably to consensus estimates of earnings of 25 cents and revenue of $53.67 million. SigmaTel designs, develops and markets analog-intensive, mixed-signal integrated circuits (ICs) for products in the consumer electronics and computing markets, including portable compressed audio players, such as MP3 players, notebook and desktop personal computers, digital video disc (DVD) players, digital televisions, set-top boxes and Universal Serial Bus (USB) storage devices.