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Clarification: It wasn't Air Force One that visited Switzerland, it was the 747-200 E4 carrying Ashton Carter. Source

What is this thing on top of the plane (In red circle): enter image description here

On other "Air Force One" Google image searches, I can't find this thing. I know they have more than one 'doomsday-plane', but whats the purpose / use of this thing? enter image description here

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The aircraft is the Boeing E-4B Advanced Airborne Command Post 'Nightwatch', which serves as the survivable mobile command post for the US National Command Authority. The aircraft are based on the same Boeing 747s as the VC-25s used by US President.

There are four identical aircraft performing this function in the US. The "bulge" on the top is a radome that houses a steerable SHF SATCOM antenna, as can be seen in the following image.

E4B

Image from cnet.com

The following image shows the aircraft systems in a little more detail.

E4

Image from http://everythingisahoax.tumblr.com

As a side note, Air Force One refers specifically to USAF aircraft carrying the US President. From US DoT ORDER JO 7110.65W

When the President is aboard a military aircraft, state the name of the military service, followed by the word "One".

EXAMPLE−

“Air Force One.”

“Army One.”

“Marine One.”

For civil aircraft, it is Executive One.

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    $\begingroup$ And - a bit OT - if the President is on a naval vessel (eg. a submarine or a carrier), it should get the call-sign "Navy One" for the duration. $\endgroup$
    – Baard Kopperud
    Jan 26, 2016 at 16:35
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    $\begingroup$ So, translating from milspeak, it houses a directional microwave antenna for satellite communication, amirite? $\endgroup$
    – Jeff Y
    Jan 26, 2016 at 20:53
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    $\begingroup$ Marine One happens reasonably often — it seems that generally when the president is on a helicopter it's provided by the Marines. $\endgroup$
    – hobbs
    Jan 26, 2016 at 23:57
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    $\begingroup$ @BaardKopperud George W.Bush flew a naval aircrafl (S-3B Viking) while being President. it was called Navy One. $\endgroup$
    – aeroalias
    Jan 27, 2016 at 0:12
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    $\begingroup$ @BaardKopperud Just to hone my nitpicking skills, George W Bush did not fly the S-3B, he was a passenger. $\endgroup$
    – TomMcW
    Jun 26, 2017 at 2:26
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That is not simply an E4, but a "B" variant. The hump houses a SHF antenna. (Source)

December 1979 a fourth aircraft (75-0125) was added. This aircraft was fitted with the distinctive “Hump” on the back of the upper deck, housing the SHF antenna. It also offered upgraded accommodations, additional shielding, upgraded electronics and new CF-6-50E2 engines. This became known as the E-4B model.

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