The ratio s of the coherent magnetic energy term and the incoherent mechanical kinetic energy terms of the drift
motion in the hamiltonian of a current carrying system is calculated for the special cases of a HFET or FET. This ratio
defines the resulting quantum 1/f noise from the coherent and conventional quantum 1/f effects.
In this case of FETs and HFETs of much larger width w>>LDS>t, the kinetic energy Ek of average motion with
drift velocity vd per unit length in the direction of the drain-source distance LDS in the channel of thickness t, is still given by Nmvd 2/2, but the magnetic energy Em per unit length in the direction of LDS is roughly proportional with the first
power of w only, instead of w2, and can be approximated by Em = π[ln(w/2LDS)]LDS[nevS/c]2/w. Here S=wt is the cross
section though which current flows this indicates field-decoherence along the large device width w. This yields a
coherence ratio of s ≡ Em/Ek ≈ πnrotLDSln(w/2LDS), which shows that only an effective width w=weff about equal to LDS
should be used in the calculation of s in this special case; larger widths are subject to de-coherence. This favors lower,
mainly conventional, quantum 1/f noise in these devices, in spite of the large values of w. It also explains for the first
time why the huge widths are possible with impunity, i.e., without causing the much larger coherent quantum 1/f noise to
appear. For non-uniform current distribution across t, and for piezoelectric coupling, improved forms are derived for s.
Specifically, the coherence parameter, called s' for the piezo case, is given by s' = (gN'h/m*vs)( vs/u)3F(u/vs)t/12w,
where F(u/vs) = (2/3)(u/vs) for small drift velocity u, much smaller than the sound velocity vs in the semiconductor. Here N'=nwt.