I am joining high tech company in Silicon Valley very soon. HR didn't ask me to bring any particular documents on first day.
What should I need to show security at entrance on my first day? Just an offer letter will do? Or my name is enough?
The most important thing to bring is yourself, ready to learn and work. Make sure you know who to ask for when you turn up at the front desk. They'll tell you what else they need.
But I've brought multiple interns onboard, so I know as well.
You're going to need to fill out an I-9 form (right to work in the US). This requires some identification. Look at the last page of this pdf for a list. http://www.uscis.gov/sites/default/files/files/form/i-9.pdf Other commentary on your question notwithstanding, a valid passport will do if you're a US national.
If you're getting paid you'll need a blank and voided check from your bank account, so they can set up your direct deposit correctly.
It will be fine if you don't have these documents on your first day, especially if they haven't written you a letter or email saying what to bring. They'll let you know what and when.
Some form of identification (ID / drivers licence / passport) will probably do for most purposes (although even that might not be necessary).
I can't imagine the offer letter would be necessary, as they were the ones who made you the offer, after all, but it certainly can't hurt to take it along.
Without explicitly telling you what you should take along, they shouldn't expect you to take much more than just a form of identification.
I do suggest you also have the name of someone in the company who can confirm your internship handy on the off chance that they forgot to tell security about you, or the security guidelines they have in place makes preconfirmed entry difficult or impossible.
I can't guarantee that this will be sufficient though - if you'd rather be safe than sorry, you'll have to confirm with them (with an e-mail or phone call to whomever you liaised with).
Pen and paper also doesn't hurt - having a pen (that works) handy to fill in or sign anything and popping out a notepad to make notes of important things might help with the first impressions.