The statistical properties of TDDB in 7.6~14.5nm gate oxide films under high stress field (＞11.8MV/cm) have been studied in this paper.It is proved that gate voltage increment at breakdown Δ

V

bd

represents magnitude and position distribution of charge occupied traps

which reflects quality and uniformity of dielectric film.The calculated critical trap density

N

bd

by total charge to breakdown

Q

bd

and Δ

V

bd

is valuab

le for quantitative evaluation of the reliability of thin gate dielectric film.Experimental results show that

N

bd

of same thickness films is independent with field stress but related to film quality.It also shows that

N

bd

decreases with thinner films due to the statistical property of trap generation.

N

bd

is about 10

20

cm

-3

with 10nm oxide films

which means dielectric breakdown occurs when trap density reaches 1% of the molecular density.As

N

bd

is an intrinsic indicator of reliability of thin dielectric films

it is more important in real applications than

Q

bd

.