OUTPUT IMPEDANCE FREQUENCY DISPERSION AND LOW FREQUENCY NOISE IN GaAs MESFETs

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OUTPUT IMPEDANCE FREQUENCY DISPERSION AND LOW FREQUENCY NOISE IN GaAs MESFETs

D. Gitlin, C. Viswanathan, A. Abidi

To cite this version:

D. Gitlin, C. Viswanathan, A. Abidi. OUTPUT IMPEDANCE FREQUENCY DISPERSION AND

LOW FREQUENCY NOISE IN GaAs MESFETs. Journal de Physique Colloques, 1988, 49 (C4),

pp.C4-201-C4-204. �10.1051/jphyscol:1988441�. �jpa-00227939�

JOURNAL DE PHYSIQUE

Colloque C4, supplbment au n09, Tome 49, septembre 1988

OUTPUT IMPEDANCE FREQUENCY DISPERSION AND LOW FREQUENCY NOISE IN GaAs MESFETs

D. GITLIN, C.R. VISWANATHAN and A.A. ABIDI

Electrical Engineering Department, University of California, Los Angeles, CA 90024, U . S . A .

Absrruct- L'investigetion trnlte de la relation entre bruit p-r et 18 dispersion de frhuence de I'Imp6dance de sortle GaAs MESFET. On a employe m e nouveIIe technique pour l'l&nti6catlon des nlveaux pro- fonds qui sont rrsponsabIes du bruit et de la dkperslon.

La

The interrelation between p-r noise and output Impedance frequency dlsperslon In GaAs MESFET's has been investigated. A new technique to Identify the deep levels responsible for the dkperdon b used. It k found that same traps are responsible for both noke and dlsperslon. The blas dependence of the disper- sion was aka Investigated and a model Is presented whlch b conslstent with 2-D simulations.

1. Introduction

Low frequency anomalies in GaAs mesfets such as frequency dispersion in output conductance and Uansconduc- tance, low frequency noise and baclcgating, have received considerable attention recently. Our experimental measurements

intended to clarify the origin and interrelation of these phenomena This paper describes the results of experiments

relate the output impedance frequency dispersion and the low frequency noise in

MESFET's.

new technique is used

identify the deep level responsible for output impedance dispersion.

This technique consists

of

of

various

We have also found that this same trap gives rise

g-r mise

2.

Experiment

Depletion MESFET arrays, fabricated with a recessed gate structure on LEC grown substrates, and with

passivation were used in the experiments. Access regions between the gate and swrceldrain electrodes

long.

pinchoff voltage was -1.3

The noise measurement was done by using a rrans-impedance amplifier as described e1sewhae.l The output impedance was obtained by using a lock-in amplifia to measure the differential voltage acms a 50 ohm resistor

series with the drain, as well as the drain voltage. Both the in-phase and quadrature component were meas-

as a function of frequency, temperature and different bias conditions.

3.

Results

The phase of

output

was plotted as a function of frequency under various bias conditions as shown in Fig. 1. The frequency at which the minimum in the phast

the invase of the trap time constant,

and is found

increase with temperature. G-r noise measurements were also carried out at several temperatures. The output noise power multiplied by m u e n c y was plotted as a function of fxequency (Fig 2).2 The

in this plot

of the phase suggesting a common origin between the output impedance dispersion and the low frequency noise as illustrated in Fig 3 for the measurements at 89C. This leads

the conclusion that the dispersion is not surface state gen- erated3 since it is generally accepted that g-r noise is generated by bulk traps.4

An

Arrhenius plot of

yields an activation enagy of 0.75 eV (Fig 5). which is comparable

the energy

the electron trap EL2 (E,,-325 eV). The temperature gradient between the substrate and the actual device

was

taken into account following Hughes et.

Without any temperature correction an activation energy of 0.68 eV would have been obtained. The method employed here

identify the traps causing the

dispersion has

DLTS methd5 Only the traps involved in the ac.

phenomena

excited, whereas the DLTS involves also other traps associated with the surfacc and depletion

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1988441

C4-202 JOURNAL DE PHYSIQUE

region. We have also studied the frequency dispersion in devices with different channel lengths at various bias voltages. The results show that the dispersion increases as the channel length is reduced and the drain bias is increased. The dispersion is larger at Vv - 0, and is always negligibly small in the linear region of operation.

4. Discussion

A question arises whether temperature gradient alone can explain the bias dependence of the output conductance dispersion in analogy with the bias dependence of g-r noise as explained in Hughes et aL2 Our data indicates that this is not the case. By looking at the phase vs frequency at different bias voltages, we see that the minimum of the phase is reduced strongly by increasing Vj, (Fig 4). On the other hand the frequency at which the minimum in phase occurs is seen to shift with temperature without any appreciable change in the minimum value. Thus, our results indicate that the output impedance is controlled by the electric field.

We can summarize the behavior of the device as follows. At VQS close to the threshold voltage, the current saturates due to a true pinchoff of the conducting channel at the drain end. At Vas - 0, however, the current limiting occurs due to velocity saturation, and an electrostatic dipole region forms close to the drain.6

This modifies the flow of carriers by forcing them into the substrate.7 as We have verified this by 2-D PISCES simulations (Fig 6). Carriers surmount the relatively shallow potential barrier (< 0.7V), and penetrate deeply into the SI substrate where traps are encountered. At low frequencies, a fraction of the carriers is captured and emit- ted by traps in step with changes in VDS, leading to a large z * because only untrapped carriers appear at the drain, and the trapped carriers tend to reduce the channel thickness; whereas at high frequencies, the time con- stants of the traps are too long for this to occur, and the current flows as if it were in a trap-free material, decreasing z^ The amount of dispersion depends on the fraction of total carriers that are trapped. Increasing VDS causes a larger voltage drop across the dipole, thereby strengthening it and causing more carriers to be deflected into the substrate; the amount of dispersion increases accordingly.

5. Conclusion

We have used a new method to measure the deep level responsible for zj, frequency dispersion in GaAs MESFET's. This method! uniquely identifies the origin of this phenomena. The results show that the same traps cause dispersion in output conductance and give rise to low frequency noise in these devices.

Our results indicate that the high electric fields that exist in modem MESFET's are responsible for the output impedance frequency dispersion. This result is consistent with the saturation mechanism that has been previ- ously proposed.7 Our experimental data are also consistent with the experimental observations that a p-type implant under the channel eliminates dispersion and reduces noise, by increasing the barrier experienced by car- riers deflected towards the substrate.8 At shorter channel lengths, however, the dipole gets stronger. Unless the operating voltages are also scaled down, such an implant may be unable to restrain carriers, and these low fre- quency parasitic phenomena, along with an undesirable bipolar action in the p-layer9 will,appear.

References

1. J. Chang, "Flicker noise in Silicon MOSFET's," Master's Thesis UCLA, 1988.

2. B. Hughes, N. G. Fernandez and J. M. Gladstone, "GaAs FET's with a Flicker-Noise Corner Below 1 MHz," IEEE Transactions on Electron Devices, vol. ED-34, pp. 733-741, April 1987.

3. P. Ladbrooke, and S. R. Blight, "Low-Field Low-Frequency Dispersion of Transconductance in GaAs MESFET's with Implications for other Rate-Dependent Anomalies," IEEE Transactions on Electron Dev- ices, vol. ED-35, pp. 257-267, March 1988.

4. C. Su, H. Rohdin and C. Stolte, "1/f Noise in GaAs MESFET's," IEDM Tech. Dig., pp. 601-604, 1983.

5. S. Sriram M. B. Das, "Characterization of Electron traps in Ion Implanted GaAs MESFET's on Undoped and Cr-Doped LEG Semi-Insulating Substrates," IEEE Transactions on Electron Devices, voL ED-30, pp.

586-592, June 1983.

6.

M S. Shur, L. F. Easanan, "I-V Characteristics of GaAs Mesfet with Nonunifam Doping Profile," IEEE

on Electron Devices, voL ED-27, pp. 455-461, February 1980.

7.

P. Bonjour, R Castagne, J-F. Pone, J-P.

O. Bat, O. Nuzillat and U Pelda,

Mechan-

with Channel-Substmte Intafadat Barria," IEEE Trunsactions on Electron D ~ S ,

m n , pp. i o i 9 - i m , JW 1980.

8.

P. Canfield, J. Meclinger, L. Forbes, "Buried-Channel GaAs MESPETs with Frequency Independent Out- put Conductance," IEEE Electron Device Lcners, vol. EDL-8, pp. 88-89, March 1987.

9.

B. J. Van Zeghbroeck, W. Patrick, H. Meia and P. Vettiger, "FWadtic Bipolar Effects in Submicrometer GaAs MESFETs," IEEE Electron Device Letters, voLEDL8, pp. 188-190,411y 1987.

Figure#l

Phase of the output impedance at various tempera- tures Vk=3.S volts V , , a

I

!

0.1 1 .o 10 100

KHz

r - - - - - - ^I vs frequency at ratures V*

-3.~-v -

-tipc _ + C

1

1E- 17

0.1 1 .o 10 100

KHz

JOURNAL DE PHYSIQUE Output noise power times frequency and phase of the

Flgure#3 output impdanse vs frequency at 89 C

I

- 6 8 ~ + " " " ' ' " " " " '

b

1 .o 10 100 E l 7

0.1

KHz

Figure#

Phase of the output impedance vs frequency at 89 C V 8 , a

1' ' " . ' " ' . ' ' " " " " ' " " " " ' 1

1KT

eV

-60-b ^.'... "..."" ' " ' ""' ' I

0.1 1 .o 10 100

KHz