.%/id-.%Ur E/echonic8 Vol. 24, No. 8. pp. 717-718. I981
Printed in Great Britain.
0038-1l0l/8l/aW717-ot#n.OOM
Pergmon Press Ltd.
l/f NOISE IN GaAs MESFETS
C. H. SUH, A. VAN DER ZIEL and R. P. JINDAL
Electrical Engineering Department, University of Minnesota, Minneapolis, MN 55455,U.S.A.
(Received 27 September 1980;in reoised form 3 January 1981)
Abstract-An attempt was made to discriminate between number fluctuation and mobility fluctuation I/f noise in
GaAs MESFETs. It was found, that both models could explain the data, even though the mobility fluctuation I/f
model seems more likely.
We report here on some I/f noise measurements on
NEC GaAs MESFETs, made in an attempt to discriminate between the number fluctuation model[l] and
the mobility fluctuation model[2] of l/f noise.
In a previous paper[3] we showed that these devices
had a spectrum of the form
(AU) [l - (%T) tan-‘(oT,)l
(1)
where A and 70 were constants that depend somewhat
on the gate bias condition. We interpreted these results
in terms of the following distribution in time constants T
dT/r
-for
To<T<T
‘(T) dr = ln(T1/TO)
= o otherwise.
versus In f at both drain voltages; here IO = S,,(f)/2q) is
the equivalent saturated diode current of the noise and q
is the electron charge. Since To does not depend on V,+,
the dependence of I., upon Vd must come from the
dependence of the parameter A upon Vd.
Figure 2 shows In I,, versus In Vd at V, = -1.50 V and
f - 1 kHz. At low drain bias the value of I., varies as V:,
as expected theoretically (see below). At higher values of
V,J the value of I., passes through a maximum at Vd =
0.65 V, a minimum at 1.5OV and levels off to a constant
value above 2.0 V.
According to Park et al. [8], the drain noise spectrum
for the number fluctuation model is
(2)
Such a distribution is usually invoked to explain the
occurrence of a l/f spectrum[4-71; it gives a spectrum of
the form (1) for o > l/71 and a l/f spectrum for l/71 <
0 < l/T,.
We measured the drain noise spectrum S,(f) at V, =
- 1.50V at very low drain bias ( Vd = 0.10 V) and at
saturation (V, = l.SOV) and found that the constant TV
depended hardly on V,,, whereas the parameter A did.
This is shown in Fig. 1, which gives a plot of InI,,
I%(:
x)l.ff
dx
(3)
whereas for the mobility fluctuation model
S,,(f)
=qh
=LJcdxMEo, xl dx.
fL I0
(4)
Here w is the device width, L the device length, q the
lo4
Go As
mset
NEC24483
lot. 99-3C47)
IO3
4
IO’
$
IO
Fig. 1. fCq= Sr,,f_f)/Zqas
vd=o.lv;
V,=-1.5V,
a
function of frequency for V, = OV,
V,,=t,.tV and V,=-1.5V,
vd=
1.5v.
Pia. 2. I., as a function of Vdat V, = -1.5 V and at V, = -0.5 V
atf=lkHz.
717
C. H. SrIH et al.
718
electron charge, f the frequency, E,(x) the d.c. field
strength at x, U&X) the drift velocity at x and Id the
drain current at the drain voltage Vd. Furthermore
[&(E~,x)]~R is the effective trap density at the Fermi
level at x, E a tunneling parameter and a(E,,x) the
Hooge parameter at x at the field strength Eo(x).
The characteristic 4( V,,) increased monotonically with
increasing V,, reaching its maximum at saturation, as
expected. Since for low drain bias Vd the drain current Id
and the drift velocity Ud(x) at x are both proportional to
V,, we see that Ies at low Vd should be proportional to
V:, in agreement with our data.
Now in GaAs the drift velocity Ud(x), considered as a
function of EJx) first varies linearly with increasing E,,
passes through a maximum at a critical field strength,
decreases with further increase of E,, passes through a
minimum and finally increases again at very large E,;
this phenomenon is caused by inter valley transfer of
carriers. In elemental semiconductor devices such as
silicon MOSFETs, [,&(E,&R decreases with increasing
V, - V, (Fig. 1) and hence [&(E,, X&R increases with
increasing Vd at a given x and with increasing x at a
given Vd. Putting it all together for GaAs MESFETs, we
see that eq. (3) can qualitatively explain the curve shown
in Fig. 2. That is, the number fluctuation model can
explain the data.
In elemental semiconductors like Si, a(E,, x)
decreases with increasing E,. But in GaAs one would
expect first a decrease, then an increase, followed by a
possible second decrease due to inter valley transfer
effects. That is, the mobility fluctuation model can also
qualitatively explain our data.
Hence our measurements do not permit to discriminate
between the two models. To make such a discrimination
possible, much more should be known about
[NT(E,,x)].s and a(E,,x) than we can surmise at
present. The mobility fluctuation model seems to be
more likely, however.
Perhaps this information could have been obtained by
carrying out many more measurements on the GaAs
MESFETs under study, but this is not certain. Time did
not permit us to carry out such a program since one of us
(C. H. Suh) had to return to Korea. We therefore decided
to publish the available data.
The above qualitative discussion can be made more
quantitative by writing for the mobility CL.
cL=PcLt+U-Pb2
(5)
Here p1 and ~2 are the mobilities in the upper and lower
valley, respectively, and /3(E,) is the relative occupancy
of the lower valley. Hence
from which the dependence of ud upon E, follows.
In the same way, for fluctuations Sp, and Spz in IL,
and w2, respectively,
ijp = p&L1 + (1 -Pm
from which follows
But according to Bosman et al. [9]
S,,W/d= 40, xYIPfN[l +(PIoEIus)~I~
=
/.LE”= /$.L&, + (1 - P)/&>
(6)
(9)
where cr(0,x) is the low-field value of Hooge’s parameter
(Y,plo and pzo the low-field values of the mobilities :L~
and ~2, respectively, and pLs the velocity of sound.
Hence
a(Eo, x) = 40, xl
( +P(PIIl.#
(1-
P)(l.L*Il#
1 (/~,oE/ud~+ I+ bzoE/us)’
>’
(10)
This explains the qualitative discussion of a(E,, x).
Since in eqn (6) the carriers can obtain a limiting
velocity of a few times 10’ cm/set., and us is only of the
order of 106cm/sec., it is most likely that the mobility
fluctuation effect will predominate over the number
fluctuation effect. The mobility fluctuation model may
thus give the more likely explanation of Fig. 2.
Acknowledgemenb-This work was performedunder an Army
Research Office grant. Prof. C. H. Suh was on leave of absence
from Hong-Ik University at Seoul, Korea, and was supported by
a Korean government scholarship.
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;:
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ud
(7)
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