Effects of Total Dose Irradiation on the Gate-Voltage Dependence of the $\hbox{1}/f$ Noise of nMOS and pMOS Transistors

We have found that the room-temperature 1/
f
-noise gate-voltage and frequency dependences of pMOS transistors are affected significantly by moisture exposure and total dose irradiation. The voltage noise power spectral density
S

_Vd
is proportional to (
V

_g
-
V

_t
)
^-ß
, where
Vt
is the threshold voltage,
V

_g
is the gate voltage, and ß is a measure of the gate-voltage dependence. For the pMOS devices, preirradiation ß ranges from 0.4 to 0.9, and the frequency exponent ¿ = -¿ln
S

_Vd
/¿ln
f
is greater than unity. Postirradiation, gate-voltage, and frequency dependences change significantly, with ß >> 1 and ¿ much closer to unity. For nMOS devices, preirradiation ß ¿ 1.6 and ¿ ¿ 1, with little change after irradiation. We attribute these observed changes in pMOS noise to changes in the trap density and energy distribution
D

_t
(
E

_f
) of these devices. Before irradiation,
D

_t
(
E

_f
) increases toward the valence band edge, but after irradiation, the distribution is typically more uniform. Moreover, for some moisture-exposed devices,
S

_Vd
¿ ~ (
V

_g
-
V

_t
)
^-3
after irradiation, indicating a
D

_t
(
E

_f
) that increases toward midgap. We conclude that irradiation and/or moisture exposure can greatly affect the defect energy distributions for these devices and that the observed nMOS and pMOS noise can be described by a simple trapping model with an energy-dependent trap distribution.