Question

what is dielectric constnt brek down

Answer 1

Essentially when the electric field is strong enough to overcome the insulative propertives of the material. Basically, an insulator becomes a conductor at breakdown.

In a semiconductor or transistor, this is bad since the electrons can pass through the barrier (electron tunneling iirc from university physics, when the silicon layer is too thin) instead of passing through the gates that they're supposed to.