As you know, source books don't talk about the possibility of an intended human-made AI.
Now, if you are a GM and one of your players want to go that way, here are some things to consider :
The birth of an AI
Most of them were born during Crash 2.0, evolving from
programs or reaching self-awareness during the years since the
Crash, but there are also newborn AIs whose backgrounds remain
An AI is a program who achieved self-awareness. Though the Crash 2.0 seems to have play a big role in some programs achieving self-awareness, nobody knows why such thing would happen. The need of a Crash-like scale gives us clue about the amount of "perturbation in the Force" results in an AI. To preserve background consistency, you shouldn't allow a simple Extended Test, even with crazy high treshold. It rather should be an epic quest resulting in the collapse of a huge node, or being at the at the crossing a huge data flows.
Don't forget that, although Technomancers are the more efficient hackers, they code only with resonnance, while AIs are resonnance-free. Sprites are quite like protosapients AIs, but they have nothing in common, origin-wise. If your PC is a technomancer, it should be actually harder for him to wrap his mind around hard code as "they often find the normal way of doing things
to be hopelessly clumsy and backward." (SR4A p.239)
There are three different kinds of AIs
The protosapient is much like an animal, and
behaves as such within the Matrix. The xenosapient’s thought
processes are alien, little understood by metahumans, and usually
still obsessed with its original programming. The metasapient, on
the other hand, is a lucid human-like intelligence. It is aware of its
surroundings and is capable of forming rational arguments. It has a
real personality, real needs, and real desires. A metasapient is quite
different from the metahuman, but it is nonetheless a sentient and
sapient being by the criteria being presented in the Declaration of
Non-Metahuman Sapient Rights.
(Runner's companion p.68)
Allowing to create a protosapient AI would be like allowing the Street Sam to get a Biodrone hound. Make it cost something, be it nuyens, a contact's favor, etc. It could be really fun if you're up to role-play interactions between AI and the hacker (note: if your hacker is a technomancer, he already get similar relations with his sprites, though they usually are more obedient than regular pets).
Creating a xenosapient AI is entirely up to you. It can be a being with limitless power but with absolutely no good reason to use it, or anything fitting in your setting. This can give your PC advantage, trouble, both, or neither, since Xenosapient AIs are specifically unfathomable.
Now the real deal with power-balance are the Metasapient AIs. Runner's Companion allows to create a Metasapient AI PC, so this means they should be considered as Contacts rather than as programs. You may decide the Connection and Loyalty ratings by the action of the PC and his dice rolls. Providing a node to host the AI may be a good way to improve the Loyalty.
Life of an AI
AIs need a node to live, and social interaction, as any other sentient being. Metasentient ones have rights, so your PC shouldn't treat them as slaves (they can, but a well-informed AI will quickly discover that it's being abused). Don't forget that the AI owes nothing to its creator, and may actually resent him for creating it.