Introduced during the second Dr.s tenure featuring Patrick Troughton. It's first appearance was in the episode Fury From the Deep. In appearance it was little more than a penlight. In fact that was all it was. Yet even at this early stage it had multiple functions. Apart from being able to undo screws, it was able to induce intense heat, allowing metal to be welded or cut. Also it could activate or deactivate door locks. In this incarnation the sonic screwdriver was a limited device, only operating at near contact ranges. Soon it was to grow, both in size and importance, as a metaphor for the non-violent nature of the Doctor.
Other shows have their icons, Star Trek with the Enterprise, the Phaser, and the Tricorder. Lost in Space with the Robot B9 however none of these came to represent a vital choice made by the arch protagonist in the manner of the sonic screw driver. Dr Who gives his enemies a choice before he engineers their downfall.
As the life of the second Doctor moved on the sonic screwdriver appeared several more times, until his regeneration forced upon him by the Timelords at the end of War Game. Initially the third Doctor “went hands free” the sonic screwdriver was nowhere to be seen. Its reintroduction was to be prefaced in an early episode, Inferno, revolving around a conversation between the Doctor and a soldier of U.N.I.T. Where a small device was described as a "Door handle"
Shortly after this the screwdriver was reintroduced in all of its classic glory. Resembling a cross between the futuristic ring and fins of a rocket and an old style microphone, the emitter was supported in a ring by three struts. This was placed atop a handle equipped with a sliding portion that activated the screwdriver and regulated it's power and function. The emitter head could be changed to perform different operations. Along with the new design came new capabilities. It became the Swiss army knife of science fiction props. It could detonate mines at a distance, unlock doors, deactivate surveillance cameras, generate a disabling high frequency signal.
Throughout the Pertwee and Tom Baker years, the sonic screwdriver remained relatively unchanged. There were small revisions, colour changes were most notable. But it's essence remained. But this very ubiquitousness presented a problem. When confronted with a locked door or other impassable obstacle, the writers could simply introduce the “Doctor uses his sonic screwdriver” and magically the impassable was passable. So with the 5th Doctor the sonic screwdriver was destroyed in the episode Visitations.
Again the Doctor was “hands free”. The script writers were unleashed to create imaginative methods of overcoming the barriers placed in the path of the Doctor. Yet in the hearts of the fans, there was a small empty spot. We secretly yearned for the sonic screwdriver. Like jelly babies it had become synonymous with the Doctor. Without it he was incomplete, yes dammit the Doctor without his sonic screwdriver was like Spock without his ears. So with the rebirth of the Doctor in the form of Colin Baker, the sonic screwdriver returned. As was said previously, the Doctor generally avoids violence, Colin Baker was an exception, he was not averse to using weapons to attack his enemies, using a Cyberman blaster to eliminate several Cybermen. The Sonic screwdriver however retained its non-lethality. To all intents and purposes identical to the earlier version it's capabilities were reduced and it became fallible.
So onto the Sylvester McCoy years, the screwdriver continued to fade into the background. It was still there, like a trusty friend, but no longer prominent. In the classic form this was to be it's swan song. The show was cancelled, and apparently finally, the hum of the sonic was to be heard no more.
In the later years of the 1980s, the show made a brief reappearance. In the form of a T.V. Movie with Paul McGann in the role of the Doctor, Sylvester McCoy reprised his role for the regeneration, and with him the sonic screwdriver re-emerged. But this was not to be a great resurgence, and the sonic screwdriver proved to be a trivial trinket, serving little purpose in the story. The design followed the established format. With a seeming lack of appetite for the show again it fell, and this time all seemed lost. The Doctor had disappeared into the time vortex, never to be seen again.
Rose, the first of a new series featuring Christopher Eccleston as the Doctor and Billy Piper as his companion, Rose Tyler. burst upon our screens. With it a new sonic screwdriver. The new design could trace it's heritage back to the original version. The new sonic was smaller than the classic version with proportions more akin to a pen. With the head extendible it could perform a variety of functions. It could cut metal and rejoin it, undo locks, scan for alien tech. Perform medical scans, interact with electronics and redirect teleportation technologies.
Now we are but a few weeks from a new Doctor. Peter Capaldi has taken on the mantle of the Timelord.