A procedure is described to determine the carrier density profile in the channel of a FET by evaluating S-parameters measured over a broad frequency range. Applying this method to GaAs/AlGaAs heterostructures, a frequency dispersion of the gate capacitance has been found, which is attributed to a parasitic conducting channel

Berroth, Manfred

Bosch, Roland

Hurm, Volker

Online Publikationen der Universität Stuttgart

Frequency Dependent CV Measurements 
of GaAs/ AIGaAs Heterostructures 
M. Berroth, IL Bosch and V. Hurm 
Fraunhofer Iusti tut fuer Angewandte Festkoerperphysik 
Eckerstr. 4, 7800 Freiburg, FRG 
Summary 
A procedure is described to determine the carrier density profile in the chan-
nel of a FET by evaluating S-parameters measured over a broad frequency 
range. Applying this method to GaAs/ AlGaAs hctcrostructures, a frequency 
dispersion of the gate cnpacitance has been found, which is attributed to a 
parasitic conducting channel. 
Introduction 
The capacitance measurement of a Schottky contact versus the applied bias 
voltage is a commonly used technique to determine the carrier concentra-
tion profile of a device. Due to the operating frequency of about 1 MHz, 
which is very low compared to applications of field effect transistors far in 
the GHz range, relatively large structures are required, so that the cnrrier 
profile can not be measured at the tro.nsistors used in microwave circuits. 
Further the junction resistance becomes rapidly smaller than the impedo.nce 
of the junction capacitance at low frequencies, if au increasing forward bias is 
applied thus making CV measurements difficult. In addition, some of these 
transisors like MESFETs and MODFETs are showing low frequency effects; 
CV measurement at hig:h frequencies would therefore be of great interest. 
Measuremeuts 
The measurements arc performed using u HP 8510 network analyzer with 
a Cascade prober for on-wafer testing in a 50 ohms environment. Different 
samples !tnve been measured using a standurd m icrowave transistor with 1 
620 
gate length and 250 l-£m width. The drain to source voltage is set to zero and 
the scattering parameter S11 is measured at various gate voltages from be-
yond pinch-off up to far positive gate voltages. The S-parameters are then 
converted to the Z-pnrameters to simplify the deembedding. 
c, 
Fig. 1: Equivnlc11t cin:uit or u Schotllcy coutnct 
Decmbedding Procedure 
As shown in fig. 1, the equivalent circuit of a Schottky contact at high fre-
quencies consists of four element:;: a parasistic ohmic resistance in series 
with an inductance and the intrinsic Schottky contact, described as a junc-
tion resistance and a junction capacitance in parallel. The impedance of the 
Schottky contact is given by the following equation: 
Z R . L R; 11 ::: , + JW , + - - -. ---
1 + JWR;G; 
(1) 
Assuming that the parasitic ohmic resistance and inductance are independent 
of the bias, both can be determined precisely if the gate is strongly positive 
biased, because at these high current levels the junction resistance is neglige-
able and shunts the junction capacitance. The real part of the Z-parameters 
describes the ohmic series resis tance, while the imagiuary part determines 
621 
the inductance at strongly positive gate voltage. Using this procedure to 
determine the parasitic series resistance and inductance, the measured Z-
parameters at all bias points and frequencies can now simply be deembedded 
by subtracting Rs from the real part and wL, from the imaginary part of Z11 • 
The resultant Z-parameters are then converted to Y-parameters, the real 
part of which represents the juntion conductance, and the imaginary part 
the junction capacitance. 
<loped 
300 A Gni\~ 
--<lopc_d __ j 
10 
_
3 I · 10 Clll dope< I 
----
1100 A Gill\~ llll(IOI)Cd Sp,JC('I 2 {)[(; 
-- fl llilCIOI)<:<i 
50 1\I(IJ!~? --------'---
•100 A AIGal\s eloped 
2000 A AIGai\S unclopc<l 
Fig. 2: Cross-sectional view of the heterostructure transistor 
Results 
We hnve measured MESFETs grown by MBE on an AIGaAs buffer layer. 
The CV curves determined were very close to those measut•ed at 1 MHz for 
the whole range from 50 MHz up to 25 GHz. However a modulation doped 
field effect transistor with a vertical structure, as shown in fig. 2, exhibited a 
strong frequency dcpendeuce of the junctiou capacitance, as presented in fig. 
3. The proposed explanation for this behaviour is that there is a parasitic 
conductive channel in the doped AIGaAs layer. The diminishing of this path 
at higher frequencies is thought to be due to the increase<l contact resistance 
622 
to this layer and the reduced mobility of the carriers, establishing a low pass 
response. 
Cote Copociloncc "1.00 
IPFI 
l7$ 
-2.() · lS -LO -o.s 
VollCtJO 
Fit;. 3: Gut~ cupuciluncc cxtruct<~d from S-parnmeter dntu for various frc-
queucics 
Conclusion 
A method to determine the gate capacitance of even submicrometer gate 
transistors is proposed. Due to the broad band $-parameter measurement 
this procedure is of great interest for the characterization of heterostructure 
FETs, e.g. GaAs/ AlGaAs MODFETs. Unlike other methods (1), no fit-
ting procedure is necessary. Especially for the characterization of thin layers 
needed in enhancement FETs can this procedure be used to determine the 
junction capacitance of a Schottky contact up to mod erately positive bias 
voltages. 
References 
1. Y. K. Fukui and M. Muraguch i, "New Method for Measuring Carrier 
Concentration Profile Near a GaAs Surface Through a Network Analyzer," 
IEEE Electron Device Letters, vol. 9, pp. 74-76, Feb. 1988. 


Frequency dependent CV measurements of GaAs/AlGaAs heterostructures

Crossref

http://elib.uni-stuttgart.de/bitstream/11682/8222/1/ber21.pdf

Frequency dependent CV measurements of GaAs/AlGaAs heterostructures

Abstract

Similar works

Full text

Available Versions

Online Publikationen der Universität Stuttgart

Crossref