Dynamic measurement and analysis for accurate models

Dynamic measurement and analysis for accurate models

Measurement& ana|ysis As devices such as HEMTs play a rote in of growing importance. One preferred ever increasingly solution is to unlock the ...

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Measurement& ana|ysis

As devices such as HEMTs play a rote in

of growing

importance.

One

preferred

ever increasingly

solution is to unlock the value of simulation

Graham Riley & Stephen Blight,

applications, finding a way to shorten the

power to get closer to first time design

Accent Optical Technologies

path to correct devices and circuit designs is

success goals.

complex practical

RF

Dynamic measurement and analysis for accurate models T h e n e e d for b e t t e r device models is driven b y

modelling technology. Dynamic I-VAnalysis

the ever-increasing n e e d for h i g h fidelity simula-

(DiVA) can h e l p p r o v i d e answers a n d break-

tions of increasingly c o m p l e x systems.The drive

t h r o u g h s in this area.

for h i g h e r complexity, l o w e r cost a n d faster time

A c o m m o n e x p e r i e n c e is that designers may only

to market has r u n into a gap b e t w e e n the n e e d

simulate to a certain degree before p r o t o t y p i n g if

a n d t h e ability to simulate. A large p a r t of that

t h e y believe that f u r t h e r a t t e m p t s at h i g h e r

gap is due to inaccurate device models leading

fidelity results will either fail or p r o d u c e incon-

to a typically h i g h n u m b e r of design iterations

sistent or u n p r e d i c t a b l e returns.This m e a n s that

during R&D.

today's c o m p l e x designs are often finished in a

Resulting designs also have to include "margin"

sub-optimal way b e c a u s e of time constraints and

to c o v e r u n k n o w n s and m a r g i n costs money. In

b e c a u s e p r o t o t y p i n g c a n n o t allow e n o u g h free-

addition, w h a t is really possible w i t h a given

d o m (often due to time a n d cost constraints) to

p r o c e s s may n e v e r b e fully realised b e c a u s e

explore t h e design space for b e t t e r options.

designer c a n n o t explore t h e full design space:

As a c o n s e q u e n c e t h e r e is a gap b e t w e e n w h a t a

only that part allowed for b y c u r r e n t m o d e l s a n d

designer could and s h o u l d design w i t h a given

Foundry Capability

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5o

III-Vs REVIEWTHEADVANCEDSEMICONDUCTORMAGAZINEVOL16 - NO4 - MAY2003

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t e c h n o l o g y a n d w h a t h e is able to actually

n e i t h e r of t h e s e really h e l p s w h e n it c o m e s to

achieve, given c u r r e n t s i m u l a t i o n capability.

closing t h e Gap.

However, it d o e s a p p e a r that, despite significant

• Acceptable Investment:

issues, d e s i g n e r s still u s e EDA p r o d u c t s b e c a u s e t h e y do n e e d to simulate their designs. But t h e gap has b e e n g r o w i n g b e t w e e n fab capability a n d d e s i g n / s i m u l a t i o n capability. Simulators provide value in s u p p o r t i n g design choice, b u t are generally s e e n as unreliable or insufficient.

Simulation Software will always c o s t too m u c h ! Paying m o r e w o n ' t e n s u r e t h e Gap is closed. • A c c u r a t e Results: T h e only area w h e r e t h e r e is real s c o p e to close t h e Gap is f t m d a m e n t a l accuracy. F u n d a m e n t a l

H o w c a n w e h e l p i m p r o v e things? It w o u l d b e

a c c u r a c y is t h e d o m i n a n t variable in establishing

entirely logical for d e s i g n e r s to focus only o n

s i m u l a t i o n value, b e c a u s e s l o w e r s o f t w a r e that

areas t h a t allow t h e m to get m o r e value f r o m

gets y o u t h e r e first t i m e is always going to b e a t

s i m u l a t i o n p r o d u c t s , if t h e y see value for their

faster b u t i n a c c u r a t e tools for a r e t u r n o n invest-

i n v e s t m e n t . V a l u e is closely tied to a c c u r a c y of

m e n t in b u s i n e s s e s , w h e r e t h e c o s t of a d e s i g n

results, as m e a s u r e d b y d e s i g n iterations saved. In

t u r n in lost r e v e n u e is so high. l m p r o v e d accura-

o t h e r w o r d s , value is i n c r e a s e d as t h e Gap is

cy will h e l p close t h e Gap.

closed.

We n o w are in a p o s i t i o n to see w h e r e w e c a n

In t h e case of circuit simulators: value = abilitT

practically invest in o r d e r to close t h e gap. Of

to achieve a c c u r a t e results, in a r e a s o n a b l e t i m e

t h e s e factors, c o d i n g a c c u r a c y is ha t h e h a n d s of

for an a c c e p t a b l e i n v e s t m e n t . So h o w will simula-

t h e p e r s o n w h o i m p l e m e n t s a m o d e l in t h e simu-

tion p r o d u c t s deliver m o r e i n h e r e n t value in t h e

lation tool.This is o f t e n t h e s i m u l a t o r v e n d o r b u t

future? H o w will t h e y close t h e Gap?

it c a n also b e an e n d user.

Taking e a c h of t h e above p o i n t s in turn: • Reasonable Time

If F u n d a m e n t a l Simulation A c c u r a c y = Engine A c c u r a c y * Model A c c u r a c y

I m p r o v i n g 'Time to result' is t h e focus of c o n t i n u -

Given t h a t t h e b e s t s i m u l a t i o n e n g i n e s in t h e i n d u s t r y are m a t u r e / a c c u -

ing i n v e s t m e n t . Simulators are already in t h e 'fast

rate/stable w i t h i n t h e c o n s t r a i n t s o f their f u n d a m e n t a l a p p r o a c h , that

e n o u g h ' r e g i o n for tacMing t h e level of p r o b l e m s

leaves M o d e l A c c u r a c y as t h e p r i m a r y driver of s i m u l a t i o n accuracy.

t h a t d e s i g n e r s are c o m f o r t a b l e s u b m i t t i n g . More

F o c u s i n g o n t h e w o r l d of s e m i c o n d u c t o r device models:

s i m u l a t i o n s d o n ' t always p r o d u c e b e t t e r results;

Model A c c u r a c y = Model Type * Coding A c c u r a c y * Extraction

faster s i m u l a t o r s d o n ' t really close t h e Gap.

Methodology * Measurement Accuracy

Making t h i n g s faster a n d c h e a p e r is a given, b u t

III-Vs REVIEW THE ADVANCED

SEMICONDUCTOR

MAGAZINE

VOL 16 - NO 4 - MAY 2003

5~

Measurement & analysis

Pulse versus Static IV Measurement of 600urn pHEMT

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t

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--vGS

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= = = = = = =

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0V 0.6V -1.8V -1.2V -O.6V 0V 0~6V

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10.0

1Z,5

15.0

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v G S = -0 . 2 V --vG$ ~vDS ---VGS VGS VGS ~ VGS

= O.4V

6V. vGS -O.SV = -1.4V = ~0.8V = -g,?V = 0.4V

This is an area that d e m a n d s quality control and good p r o g r a m m i n g m e t h o d s , but is not truly a primary variable• The t h r e e remaining areas do require consideration in improving simulation accuracy, and as such t h e y all require investment. These are fast-pulsed current-voltage measurem e n t s p e r f o r m e d around any c h o s e n bias p o i n t of the device u n d e r consideration.

Model type - s e l e c t i o n / d e v e l o p m e n t . Extraction m e t h o d o l o g y - h o w to turn m e a s u r e d data into m o d e l parameters. M e a s u r e m e n t accuracy - and suitability. Investing h e r e will h e l p close the Gap. Of t h e s e t h r e e areas, the c o m m o n link is dynamic m e a s u r e m e n t and analysis

A useful way to think o f dynamic I-V m e a s u r e m e n t s is as a m u c h m o r e useful r e p l a c e m e n t o f DC I-V m e a s u r e m e n t . T h i s suggests that DC I-V m e a s u r e m e n t s have significant limitations, w h i c h is i n d e e d true as DC m e a s u r e m e n t s only s h o w h o w a device reacts to slow, l o w f r e q u e n c y c h a n g e s in the terminal voltages. Usually, w h a t w e are m o r e interested in is h o w a device r e s p o n d s to fast, high f r e q u e n c y changes in the terminal voltages.This is exactly w h a t dynamic I-V m e a s u r e m e n t s provide. This high-frequency information is m u c h m o r e useful for guiding p r o c e s s d e v e l o p m e n t and Dynamic I- V measurements use short pulses to avoid changes in deep-level trap occupancies and self-heating• Diva is able to show/true hl~Th-freqency device responses• Such measurements are particularly important for compound material systems which have medium or high levels of deep-level traps. This includes GaAs, InP, SiC and GaN based technologies.

5~

IIl-Vs REVIEW THE ADVANCED SEMICONDUCTOR MAGAZINE VOL 16 - NO 4 " MAY 2003

device design, and for developing circuit m o d e l s used in circuit simulators. Given that Dynamic IV s h o w s t h e s e benefits, investing in this t e c h n i q u e should have the affect of investing in precisely the areas that will help provide m o r e accurate models and therefore help to Close the Gap. For f u r t h e r i n f o r m a t i o n contact: W e b site: w w w . a c c e n t o p t o . c o m / d i v a