Sonic screwdriver

The sonic screwdriver is a tool and defensive weapon used by the Doctor from his second incarnation onward. It is also part of toolbox for the Doctor's TARDIS.

Variants

The Doctor's sonic screwdriver

Mark I

A small, simple device very like a penlight. It was first introduced by the Doctor in his second incarnation. (DW: Fury from the Deep)

Uses:

Opening up hatches and panels (DW: Fury from the Deep, The War Games) and control panels. (DW: The War Games)
As a conventional screwdriver (without touching the screws). (DW: The War Games).
For cutting through a section of a wall. (DW: The Dominators)

Mark II

A larger and more elaborately detailed version, the Doctor introduced in his third incarnation. During his fifth incarnation it was destroyed by the Terileptil leader. (DW: The Visitation). By his seventh incarnation the Doctor had a new one. (DW: Doctor Who: The TV Movie)

Uses:

As a means to open the shed containing garage containing the Doctor's TARDIS and Bessie.(DW: Inferno)
Detector to discover booby traps in the Master's TARDIS. (DW: Colony in Space)
Detecting and setting off land mines from a distance. (DW: The Sea Devils)
To open an electronic door. (DW: The Mutants,
Undoing wrist clamps. (DW: The Mutants)
Detection of anti-matter. (DW: The Three Doctors)
To ignite methane and so start fires. (DW: Carnival of Monsters)

Mark III

In his ninth incarnation the Doctor had a new model with a glowing blue laser-like beam at one end.

Uses;

Detecting and stopping telepathic signals. (DW: Rose)
Interfacing with a computer. (DW: The End of the World)
As a medical scanner and diagnostic tool. (DW: The Empty Child)
Battery charging. (DW: Father's Day)
Rusting barbed wire so to an extent that it crumbles. (DW: The Doctor Dances)
Re-connecting barbed wire. (DW: The Doctor Dances)

Mark IV

The tenth incarnation of the Doctor had a model similar to the previous one, though somewhat larger. It was burnt out after modifying an X-Ray scanner to give off more radiation to defeat a Slab, but was replaced soon after with an identical, though probably the same model.

To transfer its powers to the viewer's remote control. (DW: Attack of the Graske)
As a lighter. (DW: The Girl in the Fireplace)
To somehow make the Cybermen turn away from the Doctor's directions (possibly disrupting their systems). (DW: The Age of Steel)
To threaten the Wire and forcing it to release himself. (DW: The Idiot's Lantern)
To partially reconstitute Ursula Blake. (DW: Love & Monsters)
Blowing doors open. (DW: Doomsday)
Get money from cash machine (DW: The Runaway Bride)
Destroying the Robot Santas with sound. (DW: The Runaway Bride)
Increasing the radiation in a device such as an X-Ray scanner. (DW: Smith and Jones)
Breaking into several motorway cars. (DW: Gridlock)
Trying to strip off pieces of Dalekanium during a Solar flare. (DW: Evolution of the Daleks)
Bypassing and turning off security systems. (DW: The Lazarus Experiment)
Halting the TARDIS during dematerialisation. (DW: Utopia)

Mark designations are used here purely for convenience. The Doctor and others only referred to the tool as "the sonic screwdriver".

Other sonic screwdrivers

Liz Shaw had her own sonic screwdriver which she used to open the Doctor's TARDIS for him. (DW: Inferno)

This appeared like the Doctor's Mark II.

Romana constructed her own sonic screwdriver. The device impressed the Doctor, and he attempted, unsuccessfully, to swap sonic screwdrivers with her. (DW: The Horns of Nimon)

This resembled a smaller, slimmer version of the Doctor's Mark II.

Jack Harkness acquired a sonic blaster in his time travelling days, a more offensive multi-function device (DW: The Empty Child)

Sarah Jane Smith had a tool she called sonic lipstick. (SJA: Invasion of the Bane)

The Master had a similar tool called a laser screwdriver (DW: The Sound of Drums)

Using stolen and incomplete UNIT design plans, Toshiko Sato created a sonic modulator. (TW: Fragments)

Miss Foster had a sonic pen in Partners in Crime, with a similar design to The Doctor's sonic screwdriver

Technology and Functions

The sonic screwdriver is apparently the product of Gallifreyan technology since other Time Lords (i.e. Romana) use the device and understand it enough to construct their own versions of it. The name of the device itself suggests that it functions using soundwaves, although the actual workings of the device have never been explicitly explained.

The different versions of the Doctor's sonic screwdrivers have exhibited different capabilities and uses, such as the interception of signals ranging from transmat beams to conscious thought; medical diagnostics and repair of organic parts; cutting, but also re-attaching together materials such as barbed wire; operating Earth machinery such as computers and even cash machines; creating a spark to light a candle. Although it is primarily a tool, it can also be used as a defensive weapon, such as when the Tenth Doctor put it in a sound board to destroy the Robot Santas. (DW: The Runaway Bride) However, according to the Tenth Doctor, the device cannot be used to wound, maim or kill living things. (DW: Doomsday)

Sonic screwdrivers and similar technology also cannot unlock a deadlock seal. (DW: School Reunion)

Behind the Scenes

In Star Trek, Gary Seven had a sonic-screwdriver-like tool called a servo. In the CG series ReBoot, Bob used a keytool called Glitch.