So, what if you could use the brains and connectivity of such a hand-held computer to drive a laptop-size screen, keyboard and speakers, thus overcoming these limitations? Well, Motorola Mobility has devised a new phone and accessory that aim to do just that: to make the phone the only computer you need.

I’ve been testing this new phone, the Atrix 4G, an Android device that will cost $200 with a two-year contract and will run on AT&T’s network. It’s slated to be available by March 6. I’ve also been testing its unusual and clever accessory called the laptop dock, which looks like a large netbook, with an 11.6-inch screen, full keyboard, touch pad, and stereo speakers. This dock, the price of which depends on when you buy it, has no processor, no file storage and no connectivity of its own. It’s dormant until you plug the Atrix into a slot behind the screen.

When you dock the phone, the faux laptop comes alive. It duplicates the phone’s screen on its larger display and lets you use its connectivity and apps. It also contains a battery that charges the phone. The image of the phone’s screen, and any of its apps you run, can be actual size or blown up to use the dock’s larger screen.

With Motorola’s Atrix 4G smartphone, the laptop is the accessory. The phone shown docked to the laptop dock.

Full-Screen Firefox

Even more interestingly, the dock gives you access to a full, and full-screen, PC version of the Firefox Web browser. Firefox is tucked away inside the Atrix but is available only when the phone is plugged into the laptop dock or a second, smaller dock that’s meant to connect to a TV or desktop monitor. The smaller dock lacks a built-in keyboard, battery or screen.

The laptop dock costs $500, but AT&T will knock the price down to $300, after rebates, if you buy it at the same time you buy the phone. That brings the combined price of both devices to $500—the same as the separate price for the dock. The smaller dock, called the multimedia dock, costs $190.

In my tests, the Atrix and the laptop dock performed mostly as advertised. The phone had no trouble driving the larger screen or the full Firefox browser.

I was even able to insert a flash drive into one of the dock’s two USB ports and copy songs, photos, videos and documents into the phone’s internal memory using the keyboard and touch pad. I edited and wrote text in an app called Quickoffice on the phone using the laptop dock’s keyboard, and ran various other apps, including the popular game Angry Birds, on the larger screen.

The Firefox browser worked as normal, using either the phone’s cellular or Wi-Fi connections to access the Internet. And both the phone itself and Firefox can run Flash videos, which mostly played fine.

But the combination of the phone and dock wasn’t as fast, smooth or versatile as having a real laptop, even though to use them you’re essentially carrying around a light laptop (the dock weighs 2.4 pounds). Many apps on the phone aren’t as polished or powerful as typical PC apps, and I found them clumsier to use with the keyboard and touch pad, as opposed to the touch screen for which they were designed.

Installation Issue

Also, other than Firefox, you can’t install PC programs. You can use Web apps inside Firefox, such as Google Docs or the stripped-down Web versions of Microsoft’s Office apps. For email, you can either use the program based in the phone or any Web-based program via the Firefox browser, such as Gmail or Yahoo Mail. But you can’t, say, install iTunes, or PC-based games, or the full versions of Outlook or Microsoft Word.

And there is only a primitive file system, limited to the capacity of the phone, which is just 16 gigabytes, with an option to expand to 48 gigabytes.

The dock’s screen required a lot of scrolling when using Firefox, partly because the browser has a lot of menus and toolbars. To address this, Motorola lets you convert Web pages to versions with the Firefox controls stripped out, so you just see the content.

There’s another problem with the laptop dock. When you make or receive a voice call while the phone is docked, you must rely on the phone’s microphone and speakers, hidden behind the screen of the dock. As a result, calls sounded muffled on both ends, even though the phone automatically switches into speakerphone mode. Motorola says it is working on this issue.

Despite the drawbacks, some folks will surely be attracted to this innovative combination.

If you mostly do your computing tasks on a phone or a PC Web browser, storing files in the cloud and using phone or Web-based apps, Motorola has you covered. And the fact that the dock can charge the phone is a big plus.

Motorola’s Atrix 4G

The Phone Side

What about the phone itself?

Well, it’s one of the nicest smartphones I’ve tested. Its processor makes it fast, and it has a 4-inch, high-resolution screen—almost as high as the iPhone 4′s, though not quite as sharp to my eye. It runs an older version of Android, but Motorola is promising an upgrade.

The phone also has good battery life. It lasted a full day while I was testing it and Motorola claims up to nine hours of talk time. Photos and videos I took with the phone were sharp, and it has a front camera for video calls.

The Atrix also has two other notable features. First, it can take advantage of AT&T’s souped-up 3G network, which the carrier calls 4G because it can supposedly achieve 4G data speeds.

In my tests, in the D.C. and New York areas, the speed wasn’t especially impressive, averaging just a bit better than 3G speeds on other AT&T phones I’d tested.

There is also a fingerprint sensor built into the phone, which you can use instead of a pass code to secure the phone. It worked fine for me.

Overall, this is a very nice Android phone that can imitate a limited version of a laptop. That may be enough for some folks, but fall short for others.

Write to Walter S. Mossberg at walt.mossberg@wsj.com