Laser screwdriver

The laser screwdriver was a device used by the Master.

History
The laser screwdriver was larger and bulkier than the Tenth Doctor's sonic screwdriver and generally used as a weapon. It incorporated various technologies, including a miniaturised system based on the age-regression technology developed by Professor Richard Lazarus.

The laser screwdriver had isomorphic controls so only the Master could operate it. This was revealed when the Doctor attempted to shoot him with it. (TV: Last of the Time Lords)

When a large trigger on the grip was pressed, the emitter would extend with a fierce click, and the triple-section tip would slide into place. Another button-push would release a highly lethal beam of bright yellow laser energy with a piercing whine, the force of which was enough to send a human flying several feet. A single hit to the chest proved lethal to Captain Jack Harkness and Thomas Milligan, stopping them both mid-run. A large dial integrated into the bottom of the handgrip, textured similarly to the "cracked porcelain" look of the Doctor's sonic screwdriver and rings on the TARDIS console would activate another series of control settings.

The Master used this to activate the laser screwdriver's ageing technology, copied and improved upon from Professor Lazarus's previous attempts. Using a sample of the Doctor's genetic code taken from his severed hand, the Master was able to forcibly advance the Doctor's age by approximately 100 years, leaving his foe nearly crippled and helpless with age and later still further by around 900 years to turn him into an aged creature. (TV: The Sound of Drums / Last of the Time Lords)

The laser screwdriver was later seen in the Doctor's TARDIS, presumably taken by the Tenth Doctor. Amy Pond attached it to the console to power the ship's tractor beam and pull the Eleventh Doctor out of a riptide in spacetime. (GAME: TARDIS) It remained in the console after that. (GAME: The Gunpowder Plot)

The laser screwdriver was powered by an Artron crystal taken from the Doctor's TARDIS. (Doctor Who DVD Files)

The Visual Dictionary
The non-narrative source Doctor Who: The Visual Dictionary gave further information about the laser screwdriver.

The laser screwdriver's incorporated laser technology, which shot out lethal laser beams at an enemy from its tip.

Behind the scenes

 * Conceptual art released on the BBC website as well as conversations with the cast and crew reveal that the laser screwdriver was intended to be larger and more intimidating than the Doctor's sonic screwdriver, meant to be "shown off". This, as well as its lethal and dangerous settings, fit in line with the Master's portrayal as an antithesis of the Doctor. It was also meant to look like something the Master constructed on Earth, out of local materials.
 * A toy replica of the device was released, similar to the Doctor's sonic screwdriver. It does not feature a pen or any removable parts, save for the battery compartment. A single press of the trigger extends the spring-loaded tip and activates the laser sound effect, along with three bright yellow LED's. Holding the trigger for about one second activates the ageing sound effect. The triple-ended tip does not extend further, as it does on the show. While slightly smaller than the on-screen prop, it does feature the crack-textured setting adjustment dial, with an integrated ratchet joint for further sound effects.
 * A later revision to the design left out the ratchet in favor of a more accurate appearance.
 * Fans of the original series noted that the laser screwdriver was a departure from the Master's other favoured weapon, the Tissue Compression Eliminator. When questioned about this, the current series' cast and crew commented that the Master may have simply preferred to use something different for the element of surprise.
 * Russell T Davies has stated that he had considered having the Master say to the Doctor, "My screwdriver's bigger than yours", but thought better of it. Instead, he said, "Laser screwdriver. Who'd have sonic?"