Vol. 2 No. 3 Sep. 1998

JOURNAL OF SHANGHAI UNIVERSITY

Study of a GaAs MESFET Model with Ultra-Low Power Consumption Wang Wenqi

Wang Rongguang

Chen Baolin

Wang Tong

(School of Communication and Information Engineering)

A model of enhancement-mode GaAs MESFET (EFET) for low power consumption and low noise applications has been obtained by using a small-signal equivalent circuit whose component values are derived from the physical parameters and the bias condition. The dependence of the RF performance and DC power consumption on physical, material and technologicalparameters of EFET is also studied. The optimum range of the physical parameters is given which is useful for the design of active device of ultra low power consumption MMIC. Abstract

Key words EFET, ultra low power consumption

1

Introduction

gain/power ratio of 19.1 dB/mW at 1.25GHz [4]. Nakatusgawa et al. c8] successfully developed a low

Active devices play an important role in monolithic microwave integrated circuits (MMICs). Formerly,

noise and low power consumption monolithic amplifier for the personal handy phone system (PHS). The

the depletion mode GaAs M E S F E T ( D F E T ) is preferred usually for the reasons that the process technology of D F E T has come to maturity, and the DC power consumption of the device are not a key problem. With the rapid development of the mobile communications and the increasing demands of smallsized and lightweight portable communication equipment, as well as low cost and prolonged life of batteries, a novel active device with ultra-low power consumption and high RF performance,especially Emode M E F E T , is the best choice. This is because that, under low bias conditions, E F E T is characterized by low power consumption, high transconductance and low noise. Therefore the study and development of E F E T are brisk E1-83.Encouraging progress in ultra-low power consumption MMIC is being made. For example, Cioff has reported an L-band ultra-low power consumption monolithic amplifier with a gain of 15.3dB, total power consumption of 0.8mW and

device has a 2dB noise figure and 12.2dB power gain

Received Feb. 19, 1998 Project supported by Shanghai Natural Science Foundation (93ZD14001) Wang Wenqi, Professor, Shcool of Communication and Information Engineering, Shanghai University, 149 Yanchang Road, Shanghai 200072

at 1.9GHz, and its power consumption is less than 2roW. However, equivalent circuit models and the selection of the physical parameters of E F E T s have rarely been covered in the literature. Based on the physical, material and technological parameters of E F E T , the component values of the equivalent circuit of the device model are given in this paper. The dependence of the DC power consumption (PJc) and RF performance on the gate length ( L c ) , gate width ( Z c ) , the thickness of the active layer (W) and doping density (Nd) is also discussed. The optimum range for the E F E T physical parameters in designing an ultra-low power consumption MMIC is then obtained.

2

E F E T Model Parameters The physical structure of an M E S F E T is shown in

Fig. 1 , and its corresponding small-signal equivalent circuit in Fig. 2. The notations used are as follows. gin--transconductance of the device at low frequencies, Cgs--depletion layer capacitance in the gate-source region, Cgd -- depletion layer capacitance in the gate-drain re-

Journal of Shanghai University

214 gion, Cd,--substrate capacitance between gate and source, Rd,--substrate resistance between gate and source, R~--channel resistance close to the source edge, I = Y.V~ -- equivalent voltage-controlled current source, R,--source series resistance,

where d and x are the depletion thickness and the amplitude of the depleted region toward the drain, respectively (Fig. 1), and g" --

e~e,ZGV.~ d '

(4)

where V,~ is the saturated drift velocity.

Rd--drain series resistance,

Compared with DFET, the thickness of the active layer in an EFET is smaller, and the dopant density

Rg--gate series resistance,

Ym=gme-i'~--small signal transconductance of MESFET, r--carrier transition time from source to drain in the channel,

r -1

IT o-I

is lower. Therefore the requirement of qNdWZ./~r~ -----~[n:] zeo~,-

can be met. The allowable values of W and Nd of both GaAs EFET and DFET are shown in Fig. 3.

I---33 E

~2 '

.

r.~. ~'- ::--2-'" ""

:-~ 1

,I s.i. s,

EFET 0.06 0.88 0.'10 0.12 0.'14 0.'16 0118 J.20 W( ~ m)

Fig. 3 Selection range of Na and W for EFET and DFET

Fig. 1 MESFET geometry

In Fig. 3, the lower and upper regions are for EFET and DFET respectively. For EFETs with N z = 8

R~

× 1 0 ~6Atoms/cm s, W = 0 . 0 8 t t m , LG=0. 5t~m, Z a =

:Cm,

"~Pg~

lYre

gme ~

Rs

200t~m, L , = L D = lttm, and Lsa=LGD=O. 5ttm. The component values in the equivalent circuit are listed in Table 1.

3 S ~L~ Fig. 2 Small signal equivalent circuit of the MESFET

The relation between these parameters and the MESFET physical parameters has been derived in the literature, The main results areCgl°3:

=

1,__ZcLc [V--~°¢~---q~ l -~ , 2,/2 L ~-~,J

The intrinsic part of an EFET device can be characterized by the I-Y-] parameters: Yll --

1

C,,

Relationship between the Physical Parameters and the DC Power Consumption and RF Performance

(1)

where Vs is the voltage of the Schottky potential barrier,

2eoE~ZG Cgd -- 1 + 2 x / L c '

(2)

r --= ~

(3)

Y~z = - - jtOCgd,

gme-~ Y n -- 1 + jRiC~,oJ

)

(5) (6)

jo.ICgd ,

(7)

-- Rd, - 2- + jo~( Cd, + Ced) , Yzz --

(8)

where , D = 1 + tozC~,2R~.

1 + 2-x/LG '

(Cg,

oJzRiCg,2 D + jco - ~ + Cgd ,

Vol. 2 No. 3 Sep. 1998 Table 1

Wang

W. :

Study of a GaAs MESFET Model with...

215

Component values of the EFET's equivalent circuit under various biases (Vg,=0.5V)

Va,(V)

g=(ms)

r(ps)

Cg,(pf)

C#(pf)

Cd,(pf)

R,(O)

R,(O)

Rs(g'2)

Rd, (g'2)

Ra(O)

0.7

40. 24

0. 482

0. 084

0. 0277

0. 0164

53.71

8.18

3.60

156.17

4.16

1.0

40.24

0.886

0.084

0.0197

0.0164

53.71

8.18

3.60

204.97

2.91

2.0

40.24

2.13

0.084

0.0136

0.0164

53.71

8.18

3.60

367.65

/

T h e S parameters of an E F E T can be calculated

TaMe 2

Dependence of G,, ,NF~and Pd~ on W ( N a = 8 X 10a6Atoms/m3, La=0. 5btm,Zc=200/xm)

from the [ Y ] parameters in the following steps. (1) Change intrinsic E F E T l-Y] into [ Z ] .

W(~m) 0.06 0.07 0.08 0.09 0.10 0.12 0.14 0.16

(2) Include the series elements R g , R , , R a and L,, and G,(dB)

then change [ Y ] back to I-Z]. (3) Include the parallel capacitance Cpg and Cm, and change I-Y] into l-Z] again. (4)

Include the series inductance Lg and La, and

NF~i, (dB)

/

20.63 27.28 28.92 29.78 30.73 31.28 31.65

0.292 0.278 0.266 0.256 0.247 0.235 0.225 0.217

Pdc(mW)

/

0.40 2.55 4. 71 6.86 11.16 15.45 19.76

change [ Z ] into I S ] . When this procedure is applied to M M I C , the para-

Table 3

sitic parameters may be omitted. The maximum G~ can be obtained as

follows:

G . = ( 1 - ISx,l ~ ) ( 1 - 1S2~1~)"

(9)

The minimum noise figure of the device can be calcu-

Dependence of G,,,NFmi.and Pac on LG(Nd= 8 × 1016Atoms/m3, ZG= 200~tm,W = 0. 08btm)

Lc (t~m)

0.3

0. 5

0. 8

1.0

1.2

1.4

Gm(dB)

31.83

27.28

22.32

19.77

17.60

15.72

NF=,(dB) 0.177

0.266

0.394

0.477

0.558

0.638

P~(mW)

2.55

2.55

2.55

2.55

2.55

2.55

lated from the following equation: NFm~. = 1 +

2rcKs • f

. C , , [ ( R , + R , ) / g r . ] ½. (10)

W h e n the substrate current in the device is much smaller than the channel current I~h, the DC power consumption Pd~ becomes Pd~ = Vd~ • q • N d • V,~,(W -- d ) Z a .

(11)

T h e dependence of the active layer thickness W , gate

Table 4

Zc(btm)

100

200

300

400

450

500

Gm(dB)

28.76

27.28

25.92

24. 66

24.07

23.50

NF~an(dB) 0.232

0.266

0.311

0.365

0.394

0.423

Pac(mW)

0. 255

3.83

5.11

5.75

6. 39

TaMe 5

length LG, gate width Zc and dopant density N a on the DC power consumption Pa~ and R F performance is s h o w n in Table 2 t h r o u g h Table 5 (at Va, = 0. 7V, and f = 2GHz).

Dependence of Gm,NFmi.and Pdc on Z 6 ( N d = 8 X 1016Atoms/cm3, La = 0. 5/~m,W= 0. 08/~m)

0. 28

Dependence of G=,NF,~.and Pd, on Nd (Lc = O. 5tim, W=0. 08btm, Zc=200/~m)

Na 6X1016 8X1016 1X1017 1.8×1017 2×10" (Atoms/cm 3) Gm(dB)

18. 72

27.28

28. 71

30. 16

30.29

It is observed from Table 2 and Fig. 4 that when W is too small, the power consumption Pa~ decreases

NFmi,(dB)

0. 273

0. 266

0. 261

0. 257

0. 258

but the R F performance becomes worse. When W is

Pdc(mW)

0. 22

0. 255

5.13

16. 74

19.85

between 0.07/Jm and 0.09btm, we have P a ~ S m W ,

G~

> 2 0 d B and NFm~,<0.08dB. Care must be taken to

D F E T . For this reason, a moderate value of W is ap-

maintain a good R F performance while keepingthe

propriate.

power consumption low. When W increases beyond

Table 3 and Fig. 5 show that the R F performance

the above mentioned range, although the R F perfor-

of the device can be improved as Lc becomes s h o r t e r ,

mance is further improved, the DC power consump-

a range from 0.3btm to 1.0t~m is appropriate. If La is

tion

rises

when W =

rapidly.

For

example,

Pa¢ ~ - 2 0 m W

0.16 /~m . The device thus becomes a

greater than 1.2t~m, the R F performance will deteriorate. F r o m Table 4 and Fig. 6, we see that the R F

Journal of Shanghai University

216

G,,,(

32

"(0%,,_ ~ -..

-~.100

@_x~

......

%

1.0~"'~

I

~:OS/co~ s)

/l.O

: ~7~

W

/ r/0.10 ~k.#@

/)'~(I0~'> ~ ~°~le~s)

t Z ~ if

80 60

40

Fig. 4

~

~t, ~,~/

eto

1.5 ~

7 0.5

~

Pdc(mW)

/00

et°~e/etOe)

d,:, 1.0 ~

W

~

~ (~) ; 20 ~ -;"

L ~~1- =~ ~"H _- ~"~-~ ~' ¢- I'¢,-.'' '-,[".~'/,' ./'' /'/¢"' /.- / - '/~ '/-"

2.0

Relationof Gm,NFm~, and Pdc to Nd and W Vd,=0. 7V Lc=0. 5btm Zc= 300ttm V,, = 0.5V f = 2GHz

performance is not very sensitive to the variation of

,,t .) Fig. 5

,.Ok-

II

2.0

Relation of G,, ,NF~.inand Pdc to Nd and LG Vd,=0. 7V W = 0 . 08ttm ZG= 300/~m

Vg~= 0. 5V

f = 2GHz

may be considered.

Za. When ZG is between 100ttm and 450ttm, G ~ >

Table 5, Figs. 4 and 6 indicate t h a t , w h e n the Ne

24dB and NFmi,<0.4dB. Puc becomes sensitive to the Pdc will decrease. When Za-----300ttm, Puc will be less

value is too small, DC power consumption is low, but with a poor RF performance. Under this condition, Gm will decrease and NF~n will increase. As Nd

than 3.9roW. Thus for Zc, a range from 100t~m to

has been selected around 8 N 1016 Atoms/era 3, we

450ttm is appropriate. If a DC Power consumption is

have Gm>27dB, NF,~,
strictly limited, a range of ZG from 100ttm to 300tLm

Nd is too high, although the RF performance is

variation of Za, as Za is reduced, and subsequently

Vol. 2 No. 3 Sep. 1998

Wang

W. :

Study of a GaAs MESFET Model with...

217

g o o d , Pz~ will increase abruptly. For example, P d ~

el equivalent circuit have been obtained based on the

20roW, when N d : 2 × 1017 A t o m s / c m 3. In this case

physical, material, and technological parameters of

the device becomes a D F E T .

the device. The dependence of the dopant density, active layer thickness, gate l e n g t h , and gate width of the device upon the R F performance and DC power consumption has been investigated.

GIn(aBe0

It is concluded t h a t , when W ranges from 0.07/~m

30

0

-'~

/300

[ Soo

"g'(¢'Oz>

1~ S N ~ ~ O l O 0

%%/%0

to 0. 09/zm , Lc from 0. 3t~m to 1. 0/zm, Zc from 100/~m to 450/~m ,and Nd is around 8 X 1016Atoms/ cm 3 , E F E T devices have a good R F performance with a low DC power consumption. The optimum ranges of L c and ZG obtained here are in general agreement

46-

2.0

with the figures given in [-1],[-3~ and [ 4 ] .

References 1

NFn~n(dB) 2

Nair V., Low current enhencement mode MMICs for portable communication application, I E E E GaAs 1(2 Symp. Tech. Dig., Oct. 1989, 67-70 Phillippe P. et al., A 2GHz enhencement mode GaAs down converter IC for satellite TV tuner, I E E E microwave and millimeter-wave monolithic Circuits Syrup.

3

4

5 Pd~(mW) 6

7

_ % 37'/200

8

9 Fig. 6

Relation of Gm ,NFmln and Pa, to Na and Za V~,~ 0. 7V W = 0. 08/~m LG= 0. 5~tm Vg,= O. 5V f = 2GHz

4 Conclusions In this paper, component values of the E F E T mod-

10 11

Dig., June 1991, 61-64 Imail Y. et al., Design and performence for low current GaAs MMICs for L-band front-end application, 1 E E E Trans. Microwave Theory Tech. , 39(2) :209-215(1991) Cioffi K. R. , Ultra-low DC power consumptions in monolithic L-band components, I E E E Trans. Microwave Theory Tech., 40(12):2467-2472 (1992) Nakajima S. et al., Enheneement-mode GaAs MESFET technology for low consumption power and low noise applieation , I E E E Trans. Microwave Theory Tech., 42(12): 2517-2524 (1994) Heaney E. et al. , Ultra low power low noise amplifiers for wireless communication, I E E E GaAs IC Syrup. Dig., 49-51 (1993) Tokumitsu M. et al. , A very low power consumption front-end MMIC for L-band recevers, Proc. 22nd European Microwave Conf., 242-247 (1992) Nakatsugawa M. et al. , An L-band ultra-low-power-consumption monolithic low-noise amplifier , I E E E Trans. M i crowave Thoery Tech., 43(7):1745-1750 (1995) Pengelly R. S. , Microwave Field-Effect Transistors-Theory, Design and Apllications, John Wiley ~ Sons Ltd (1982) Peter H. Ladbrooke, MMIC Design: GaAs FETs and HEMTs, Arteeh House Ltd(1989) Stephenl Long , Gallum Arsenide Digital Integrated Circuit Design, Mcgraw-Hill publishing company(1990)