Characteristic potential method (CPM) for noise calculation has
been developed for multi-terminal semiconductor devices under the
drift-diffusion scheme. Merit of the CPM is that clear cut definitions of the terminal thermal noise currents and the
terminal excess noise currents can be made for unipolar devices
and homogeneous resistors. We prove that the terminal thermal
noise currents and the terminal excess noise currents are uncorrelated for unipolar devices even when they come from the
same local noise sources. We also suggest a way to define thermal
noise and excess noise in bipolar devices using the derived
formulas from the CPM. As applications of the CPM, we show that
the high frequency excess noise observed in homogenous semiconductor resistors is really shot noise whose noise generating mechanism is just the same as that of vacuum diodes. We also show that the dominant high frequency noise in long-channel MOSFETs is thermal noise in the linear region, but the excess noise is getting more significant as the drain bias increases, and is important in the saturation region. The excess noise in the saturation region of
the long-channel MOSFETs is shown to be shot noise. Finally, we try to explain the shot noise-like behaviors observed in forward-biased pn
junction diodes by the conventional corpuscular theory of shot
noise even though the impedance field method confirms that the
shot noise behaviors are caused by the local noise sources in the
neutral regions, not in the depletion regions.
We study the thermal noise characteristics of the scaled MOSFET devices using the hydrodynamic transport model and the Green's function technique. We compare the result of the hydrodynamic model with that of the drift-diffusion model and study the effect of the nonlocal transport on the drain noise current for the NMOSFET with the Lmet (determined by the metallurgical junctions) about 40 nm. It is found that the nonlocal transport effect broadens the effective vector Green's function and increases the responses of the electrostatic potential and the electric field at the entrance of the channel, which can directly influence the drain noise current. We also study the effect of the spatially nonuniform energy relaxation time on the noise characteristics and find that the region with larger energy relaxation time is less sensitive to the velocity fluctuation noise.
The characteristic potential method(CPM), which has been successfully applied to calculate 1/f noise and thermal noise of multi-terminal homogeneous semiconductor resistors, is extended to calculate 1/f noise in inhomogeneous devices such as MOSFETs. The drain 1/f noise current of MOSFETs in the linear region is calculated using the CPM together with the well-known existing 1/f noise sources based on either Hooge's empirical model or McWhorter's model, and the calculated results are compared with the experimental results. It is shown that the difference of the 1/f noise behaviour between n-MOSFETs and p-MOSFETs in the linear region can be attributed to either the difference in their effective field dependence between the local electron mobility and the local hole mobility near the Si-SiO2 interface in the inversion layer or the difference in degree of Nt(oxide trap density)dependence between the effective electron mobility and the effective hole mobility.
A novel CMOS photosensor with a gate-body tied NMOSFET structure realized in the triple is well presented. The photocurrent is amplified by the lateral and vertical BJT action, which results in two different output photocurrents, which can be used for different applications within a pixel. The lateral action results in the drain current with a higher sensitivity at low light intensity. And the vertical action results in the collector current with uniform responsivity over wider range of the light intensity. The proposed photosensor in compatible with CMOS circuits.
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