Laser screwdriver

A device used by the Master, a laser screwdriver was a device unlike the Doctor's sonic counterpart. It was not only larger but bulkier and generally used as a lethal, killing laser weapon instead of a multi-functioning tool.

Function
Incorporating various technologies, including a miniaturised system based on the age-regression technology developed by Professor Richard Lazarus, the laser screwdriver was powered by an Artron crystal taken from the Doctor's TARDIS. (Doctor Who DVD Files)

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)

As a fatal weapon, the laser screwdriver shoots highly lethal laser energy, in which the force produced was enough to send a human flying several feet. (TV: The Sound of Drums / Last of the Time Lords) However, it's effectiveness dwindled when used against The Doctor, who was protected by a psychic field generated by the Archangel network.

Apart from the killing function, the laser screwdriver is capable of artificially ageing a target. The Master used the function to age The Doctor using a sample of his genetic code taken from his severed hand, turning him into an aged creature. (TV: The Sound of Drums / Last of the Time Lords)

History
The Master unveiled the device aboard Valiant, when he used to stop Jack Harkness from intervening the Toclafane invasion and age The Doctor. He compared it to the Doctor's sonic screwdriver, stating "Laser screwdriver. Who'd have sonic?".

After having destroyed the contents of the bag using the device, the Time Lord later also used the screwdriver to shoot at (and kill) Thomas Milligan, who saves Martha Jones' life by taking the shot for the Doctor's companion. (TV: The Sound of Drums / Last of the Time Lords)

The device 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)

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.

The non-narrative source Doctor Who: The Visual Dictionary gave further information about the laser screwdriver. The laser screwdriver incorporated laser technology, which shot out lethal laser beams at an enemy from its tip.