The Doctor sometimes also left their own key with one of their companions. On one occasion when the Third Doctor thought he was soon to die, he gave his TARDIS key to Jo Grant, so it wouldn't be lost along with him. (AUDIO: The Tyrants of Logic)
In some cases, the Doctor simply needed their companion to retrieve the TARDIS and get them all out of danger, such as when the Seventh Doctor temporarily gave his key to Roz Forrester. (AUDIO: The Jabari Countdown)
The barrier on the Doctor's TARDIS, a Type 40, was a double curtain trimonic. As such, the Chancellery Guard needed the model's cypher indent key to gain entry without the actual key, as opposed to other methods of forced entry. (TV: The Deadly Assassin) In its police box disguise, entry was allowed by inserting a key into the lock, just like a real police box. (TV: Doctor Who, Rose and others) However, the locking mechanism did not respond to police-issued keys. (TV: Blink) According to Susan Foreman, the lock had a defence mechanism in which there were twenty one different holes inside the lock, one being the right place and other twenty being wrong. If a mistake was made, the whole inside of the lock would melt. (TV: "The Survivors") The Brigadier was prevented from opening the TARDIS in the Third Doctor's absence, because of the metabolism detector on the lock. (TV: Spearhead from Space) Nevertheless, there were occasions on which strangers were able to successfully use the Doctor's key, suggesting either a flaw in the metabolism detector or a choice on the part of the TARDIS. (TV: The War Machines, Doctor Who, Blink)
The lock could be manually secured from inside the TARDIS, preventing even authorised individuals from using the key to unlock the doors from the exterior. (TV: The Dalek Invasion of Earth, Utopia) At some point shortly before the Tenth Doctor's regeneration, the TARDIS had a system that allowed him to lock the TARDIS remotely using a fob (as a joke, the TARDIS roof light flashed and a alarm chirp sound was heard, similar to that used on vehicles on Earth). He was also able to open the door remotely. (TV: The End of Time)
There were occasions on which the Doctor or their companions needed to use the key to lock the doors, (TV: "Strangers in Space") while on others (TV: Spearhead from Space, The Christmas Invasion) the act of merely closing the doors locked the TARDIS. It was evident that the TARDIS' lock could be set to secure either automatically or manually.
However, a key was not always needed to open a TARDIS. During the Doctor's first encounter with River Song (from his point of view), she remarked that she had witnessed his future self open the doors of the TARDIS by snapping his fingers. The Tenth Doctor reacted with disbelief, but later successfully opened the doors in this fashion. (TV: Forest of the Dead) After regenerating into his eleventh self, the Doctor again opened the TARDIS by snapping his fingers, demonstrating that this method had become a reliable way for him to do so. (TV: The Eleventh Hour) The Doctor did, however, continue to make regular use of his key to unlock the TARDIS. (TV: The Time of Angels, Flesh and Stone, et. al.) After losing the key in their thirteenth incarnation, the Doctor simply told the TARDIS what had happened and she opened up on her own. (TV: The Ghost Monument)
At some point in his third life, the Doctor began experimenting with new designs for his key, coming up with several alternative designs before settling on a spade-shaped silver key. After his regeneration, the Fourth Doctor experimented with even more radical designs, which included a double-helix apparently made of plastic, before eventually going back to the spade and Yale styles. (PROSE: Interference - Book Two) The Yale key was also used by the Fifth Doctor. (TV: Four to Doomsday)
The Seventh Doctor used yet another design, incorporating the Seal of Rassilon with a fan-shaped motif. (TV: Ghost Light) The Seventh Doctor eventually went back to the spade key, which the Eighth Doctor initially continued to use. (TV: Doctor Who) At some point, the Eighth Doctor returned to using the Yale key. (PROSE: The Crooked World, History 101)
The Ninth, (TV: Aliens of London, Father's Day) Tenth, (TV: 42) and Eleventh Doctors (TV: The Eleventh Hour) used Yale-type keys in a gold/brass colour as opposed to earlier Doctors' silver-coloured Yale keys.
Beyond simply unlocking the TARDIS doors, the Doctor's TARDIS key has often displayed other uses and characteristics.
When the TARDIS fell through a wound in time, the key was hot and glowed, indicating that it was still connected to the TARDIS, leading the Ninth Doctor to use the key in order to call the TARDIS. The Doctor warned that if the key was touched by anything, it would cause the now-re-materialising TARDIS to disappear. After Rose Tyler came into contact with her younger self, it caused a Reaper to emerge which consumed the Doctor. As it tried to fly away, the Reaper touched the key which destroyed it and caused the TARDIS to disappear. (TV: Father's Day)
As a part of the TARDIS, keys had low-level perception filter properties. The Tenth Doctor was able to tune the keys into the psychic network created by the Master to extend these properties to an individual wearing a key. (TV: The Sound of Drums)
When Sarah Jane Smith left the key in the lock without opening the door, it cancelled the pause control. The TARDIS travelled to its set coordinates on Earth, instead of remaining in the copy of Devesham created by the Kraal. (TV: The Android Invasion)
Behind the scenes Edit
- The original spade-type TARDIS key was designed by Jon Pertwee and began use in Season 11. 
- The Rassilon-style key was designed by Mike Tucker. 
- The prop used for the TV movie was a BBC-licenced replica of the classic spade key, sold at the time by now-defunct American memorabilia catalogue 800-TREKKER. 
- The 2005 key was a standard nightlatch key made by UK lock manufacturer ERA, and it was the real key to the working lock on the TARDIS prop. Spares were stored in a secret compartment inside the TARDIS prop.