124
4.3.10
Biennial Report 2002/03 - Solid-State Electronics Department
A Combined Large-Signal and Small-Signal Model for
InP Based Single HBT
Scientist:
Silja Ehrich
Introduction
The design of complex high-frequency electronic circuits is very difficult and strongly depends on
the models used for the single device. In case of HBT the large-signal behaviour is often described
by using the Gummel-Poon-model as implementd in simulation software such as PSPICE or ADS,
resp.. For modelling the rf-behaviour small-signal equivalent circuits are used.
Combined Model
The investigated HBT all base on a typical three-mesa design of InP based HBT devices as
described in [1]. Therefore all small-signal equivalent elements are associated to specific device
regions and build up the used small-signal equivalent circuit. The model presented here is based on
the physical relevant “T”-like model and combines the large-signal description and the small-signalmodel. The resulting model is given in fig. 1. In case of large-signal application the base-collector
diode DBC and the base-emitter diode DBE is used.
CIO
Cfb Cjc
Rjc
CBP
DBC
B
LB
RB
RBP
Rcc
Rbb
DBE
CIN
Cje
Rje
RC
LC
C
COUT
intrinsic
RE
LE
extrinsic
Fig. 1
E
Combined large-signal and small-signal equivalent circuit of HBT consisting of an
intrinsic and an extrinsic part
Biennial Report 2002/03 - Solid-State Electronics Department
125
Parameter-Determination
First of all standard dc-characterization like measuring the output-characteristic and the forward and
reverse gummel-plot are done. Also s-parameter are measured in a frequency range from 45 MHz
up to 40 GHz at different bias-conditions in respect to the collector-emitter voltage VCE and the
collector current density SC using the HP8510C networkanalyzer. Additional the rf-noise-parameter
characterization (minimum noise figure Fmin, equivalent noise resistance Rn and optimum generator
reflexion coefficient Γopt) is performed in a frequency range from 2 GHz up to 18 GHz using the
ATN NP5 system. Device under test is a carbon doped InP/InGaAs self-aligned HBT with an
emitter-area of AE = 1x2x10 µm², grown by LP-MOVPE with non gaseous sources. Rfcharacterization are done at bias-condition where the HBT is in the active region as well as in the
saturation region.
The parameter determination starts with finding the extrinsic elements in form of a multi-bias sparameter-optimization using evolutionary optimization strategies. Subsequently the intrinsic
elements have to be extracted keeping the extrinsic elements constant. The bias-dependence of the
intrinsic elements can now be described by mathematical equations.
In the end the diode-parameters of the base-emitter and base-collector diode have to be found,
getting a good fit between measured and simulated data for both gummel-plots.
DC-Simulation
To prove the new combined model dc-simulations are done and compared to those resulting from
simulation using the standard Gummel-Poon model as implemented in the simulation software
ADS. In fig. 2 the comparison between the Gummel-Poon Model, the combined model and the
measured data of the output-characteristic of a single HBT with an emitter-area of AE = 1x2x10 µm²
is shown. The agreement between measured and simulated data in case of the combined model is
excellent. This new model takes into account specific effects of InP devices like basewidth
modulation, more accurately.
15
15
mA
mA
9
IC
9
IC
6
6
3
3
0
0
-3
-3
0.0
a)
Fig. 2
0.2
0.4
0.6
0.8
1.0
Vce
1.2
1.4
1.6
V
2.0
0.0
b)
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
V
2.0
Vce
Measured (symbols) and simulated (lines) output-characteristics of a 1x2x10 µm² HBT
(IB = 0 µA up to 400 µA, steps 80 µA)
a) Standard Gummel-Poon Model
b) Combined large- and small-signal model
126
Biennial Report 2002/03 - Solid-State Electronics Department
S-Parameter Simulation
Besides dc-characterization the rf-behaviour of the investigated device is very important. In fig. 3
the agreement between measured and simulated data at constant bias-conditions (VCE = 1V,
IC = 20 mA) is presented for different kinds of models: the standard Gummel-Poon large-signal
model and the new combined model. The excellent agreement for the combined model can clearly
be seen. Also the difficulties using a large-signal-model for s-parameter simulation are pointed out
in fig. 3a.
-20
-15
-10
-5
0
0.10
0.15
0.20
-20
-15
-10
-5
0
0.05
0.10
0.15
0.20
b)
a)
Fig. 3
0.05
Measured (symbols) and modeled (lines) S-Parameters at VCE = 1 V and IC = 20 mA for
a) Standard Gummel-Poon large-signal model
b) combined small- and large-siganl model
Conclusion
Excellent agreement between measured and simulated data using the new combined modell is
demonstrated for both dc- and s-parameter simulation. The main advantage of this combined model
is, that it can be used for all kinds of simulation for large-signal as well as for small-signal
application without changing the parameters. This combined model can be used for all biasconditions in the saturation region as well as in the active region. This is an advantage compared to
the small-signal-model where the intrinsic parameters have to be extracted for each bias-point.
Acknowledgement
Thanks to Innovative Processing AG for providing their HBT.