Modelling of Frequency Dispersion Effects Hetero Field ... · Modelling of Frequency Dispersion Effects in Hetero Field-Effect Transistors Ingmar Kallfass, H. Schumacher, T.-J

Integrated Circuits in Communications

ikall, MOS-AK 07.05.04 slide 1

Modelling of Frequency Dispersion Effectsin

Hetero Field-Effect Transistors

Ingmar Kallfass*, H. Schumacher*, T.-J. Brazil***University of Ulm

**National University of Ireland, Dublin



Devices

Model

Topology/Equations

Dispersion Modelling

Implementation

Overview

Verification

AlGaAs/GaAs pHEMT(UMS PH15)

Frequency Dispersion Modelling

InAlP/InP HEMT (IPAG)

Si/SiGe MODFET(DaimlerChrysler)



Introduction

Frequency Dispersion EffectsStatic Model

Dynamic Model

Non quasi-static

Power Cons.Biasing

Linear (Gain,Matching,Stability,..)Large-Signal (IP3,Eye,..)

Ultra-broadband applications (e.g. 40Gbps)

MMIC

Modelling

Simulation

MMICApplications

Characterisation

Static/DC

S-Parameters

Pulsed-IV




200 nm InGaAs/InPpHEMT

fT= 96 GHz

fmax,U= 131 GHz

gm,max= 1010 mS/mm

Devices

HFET Devices

150 nm AlGaAs/GaAspHEMT

fT= 110 GHz

fmax,U= 190 GHz

gm,max= 900 mS/mm

Research & Technology

100 nm strained-Si/SiGe MODFET

buried n-channel in strained-Si layer

Graded SiGe buffer

fT= 60 GHz

fmax,U= 110 GHz

gm,max= 240 mS/mm



Equivalent Circuit Topology

Novel capacitance model

Accurate gate current model

Modified COBRA Ids model

Frequency dispersion model


+ de-embedding

Capacitance ModellingNonlinear Capacitance Implementation

Universal charge-conservative expressions

Data extraction: multi-bias S-parameters

Solve intrinsic Y-matrix

Non quasi-static resistors modelled as linear elements

Schottky-gate diodesIncludes reverse breakdown

Upper limit to instantaneous Vgs

Clipping

Diode Operation: Level Shifters…



Static Drain Current CharacteristicsModel Parameter Extraction

COBRA model features

Single expression covering all operating regimes

Globally continuous partial derivatives

Few, mostly graphically extractable parameters (+optimisation)

Also used to model dynamic IV

( ) ( )( )

tbds

bdsds

VnVV

effbdsbds

eff

dsds

dsds

bdseffdsnumeffdsgsds

eVgI

PVI

II

IVVVVVI

−

=

⋅+=

++⋅⋅=

1'

1tanh, ζαβλ

( )( ) dstot

tgstgseff

effds

VVV

VVVVV

VVnum

γβ

δ

ξµ

−+=

��

�� +−+−=

++=

12

1

2211

2

121

1



Static Drain Current CharacteristicsModel Parameter Extraction

GaAs: Pronounced self-heating, impact ionization, gain compression

InP: parasitic channel formation/impact ionization (dispersion effect – see later)

SiGe: low current density, low non-idealities

GaAs 2x50µm InP 2x40µm

SiGe 2x50µm



Frequency Dispersion EffectsFrequency Dispersion Model

Thermal / Self-heating

Reduced carrier mobility

Predominant in GaAs (low thermal conduction), large devices

Charge Traps

Modification of free carrier concentration

Present in all MODFET devices, technology maturity

Interface- / Surface states

Modification of effective gate-channel potential

Present in all MODFET devices, technology maturity

Impact ionisation

Avalanche multiplication of carriers

Predominant in InP (small bandgap energy)

Also in GaAs, SiGe at high drain potential



Transient response reveals exponential characteristic with two dispersion time constants

Modelling Approach: MotivationFrequency Dispersion Model

Pulsed-IV affected by traps and thermal dispersion (impact ion. still present)

( ) 21210

ττt

DS

t

DSDSDS eIeIItI−−

++=

Q

Q: Vgs 0V, Vds 2VPulse: Vgs 1V, Vds 4V

GaAs 2x20µm GaAs 2x20µm



Dispersion Modelling: ApproachesFrequency Dispersion Model

Thermal modelsR-C thermal network models instantaneous channel temperature

Analytical basisCombine static&dynamic IV by empirical relationship deduced from measurements

Physical basisParameters (Vth, γ,..) function of frequency, temperature

Equivalent voltagesModify gate- and drain controlling voltages

Common drawbacks

Often no distinction between different dispersion effects

Frequency/Time transition usually not accuately modelled

May require additional iteration process during circuit analysis



Modelling Approach Frequency Dispersion Model

Dispersive effects affect mainly drain current characteristics

Dynamic IV methodsPulsed IV

Dynamic trans- and output conductance

Nonlinear dispersion correction source

Idsx=Ids,dynamic-Ids,static

Gx for DC convergence

Corner frequency set by Gx-Cxconstant

Exponential transient characteristics



Dynamic IV: Pulsed-IVFrequency Dispersion Model

Pulsed-IV data used as dynamic model

True dynamic large-signal characterisation

Self-heating- and trap-related altering of IV eliminated, but

Self-heating and trap state depends on quiescent point (hot, cold…)

Impact ionization still present

ns

ns

GHzf

pulse

ii

iic

100

5.0

1

..

..,

≥≈

≈

ττ



Pulsed-IVFrequency Dispersion Model

Hot quiescent point Vgs=0V, Vds=2V

0.1µs pulses, duty cycle 1ms

Q




Dynamic IV: Integration of gm/gds

Frequency Dispersion Model

Dynamic gm, gds from multi-bias small-signal model extraction in GHz regime

Numerically integrate for Idsac

Compare: Table-based spline models

All dispersion effects eliminated

Error (quantifiable) introduced since integration path dependant:

( )),(),(),(

dsgsdsacdsgsdsac

dsgsmac VVIgradVVg

VVg≠��

�

��

�



Integration of gm/gds

Frequency Dispersion Model

GaAs 2x50µm



Single Dispersion SourceFrequency Dispersion Model

Consider dispersion model elements only…

),(),(),( 21,21,21 VVIVVIVVI dcdsacdsdsx −=

( )xx

xxdsxds

j

xx

xmxm

cjgcj

gggy

ecjg

cjggy

ωω

ωω ωτ

+++=

��

��

�

++=

22

21



Parallel Dispersion SourcesFrequency Dispersion Model

Place several dispersion sources with different time constants in parallel

1,,

1,,

1,,

−

−

−

<<

−=

ixix

ixix

idsacidsacdsxi

CC

III

ττ



Verification: S22 20kHz-8GHzFrequency Dispersion Model

Bias point Vgs=-0.1V, Vds=2.2V (~gm,max)

Small-signal model using 3 dispersion sources with fc=100kHz..10MHz

GaAs 2x50µm



GaAs HEMT: Full Nonlinear Dispersion ModelFrequency Dispersion Model

Dispersion Sources

Idsx1: eliminate thermal/self-heating, data : 2µs pulse

Idsx2: eliminate traps/interface states, data : 0.1µs pulse

Idsx3: eliminate impact ionisation, data : numerical integration > 5GHz gm error < 5%Other weighing

possible

Note: DC + pulsed-IV contain effect of impact ionisation




Verification GaAs: S-parameters 50MHz-50GHzFrequency Dispersion Model

Bias: Vgs=0.8V, Vds=4.5V

Large-signal model (small-signal more accurate, as usual…)

4.0 GHz

GaAs 2x20µm



Verification GaAs: S-parameters 50MHz-50GHzFrequency Dispersion Model

Bias: Vgs=-0.4V, Vds=3.9V (~NFmin)

Large-signal model

GaAs 2x20µm



Verification GaAs: TransientFrequency Dispersion Model

Transient Pulse Response

Quiescent Point: Vgs=0V, Vds=2V

Pulse to Vgs=1V, Vds=4V

Large-signal model in transient simulationGaAs 2x20µm



InP HEMT: Full Nonlinear Dispersion ModelFrequency Dispersion Model

Dispersion Sources

Idsx1: eliminate thermal and trapping effects, data : 0.1µs pulse

Idsx2: not used

Idsx3: eliminate impact ionisation, data : numerical integration > 5GHz

InP 2x40µm



Verification InP: S-parameters 50MHz-50GHzFrequency Dispersion Model

Bias: Vgs=0V, Vds=2.8V (~gm,max)

Large-signal model

400 MHz

InP 2x40µm



Dispersion in the Si/SiGe HFETFrequency Dispersion Model

Dispersion Sources

Idsx1: not used

Idsx2: not used

Idsx3: eliminate thermal + trapping effects, data: numerical integration > 2GHz



Implementation Overview

CAD platform: ADS (v2003A)

Nonlinear n-port (SDD) implementation

Direct (user-compiled) implementationModel topology fully implemented

All types of simulation

Time delay in transient model: solution available, not yet implemented

Linear scaling model

Model Implementation



1-tone power measurementsModel Verification

GaAsVgs=0V, Vds=2V (~gm,max)

Fundamental 5GHz

InPVgs=0V, Vds=1.5V (~gm,max)

Fundamental 5GHz

InP 2x40µmGaAs 2x50µm



Conclusion

Demonstration of nonlinear model incl. accurate frequency dispersion

Applicable to GaAs-, InP and strained-Si/SiGe HEMTs

Empirical: simulation efficiency, global validity, for circuit-design

User-compiled model implementation in ADS, suitable for all types of simulation

Dispersion model reflectsImpact of individual dispersion effects and their time constants/corner frequencies

Transition between dispersive regions in mag/phase

Dispersion model enablesUltra-broadband simulations (accurate PAE…)

Transient response simulations (e.g. pulsed power amps…)

Dispersion Modelling

Documents

Modelling of Frequency Dispersion Effects Hetero Field ... · Modelling of Frequency Dispersion Effects in Hetero Field-Effect Transistors Ingmar Kallfass*, H. Schumacher*, T.-J

Modelling of Frequency Dispersion Effects Hetero Field ... · Modelling of Frequency Dispersion Effects in Hetero Field-Effect Transistors Ingmar Kallfass, H. Schumacher, T.-J