what's the minimal input current for the Ib current of NPN transisor?

Can the input base current to be as low as 1pA for the npn transistor? Then what's the typical current gain for such low base current? thanks.

It can be, but even an integrated transistor of small
size is liable to be well down from peak beta by that
point. And at such low currents you will see a lot
more process and temperature variation in the gain
at that base current level. Having worked in several
bipolar technologies I would not be able to state a
"typical" - they have all been different in both peak
hFE, and how wide & flat the hFE vs Ic curve is.

Offhand I'd say that trying to apply a BJT at such
low base current is not going to end reliably well.
You may find some that work but expect many to
not.

You might explore base current cancellation schemes
if you have a complementary-bipolar technology.
This can reduce Ib(external) by one or two orders
of magnitude, allowing you to run the transistor
one or two orders of magnitude "hotter". But this
is only good for pin bias current type concerns and
won't get you to a low overall power dissipation.

1pA will be greatly exceeded by noise current which I believe depends on gold doping of junction. e.g. 2N5088

For a practical limit consider low noise general purpose NPN below


When Ic = 0.01mA hFE typ=240, thus Ib=42nA

For lower currents use a low noise FET.