CoSi2 Schottky collector HBT

Microelectronics Journal, 24 (1993) 823-830

Switching characteristics of pSi/nSil_ xGex/ CoSi2 Schottky collector HBT O. Nur and M. Willander Department of Physics and Measurement Technology, Linkisping University, S-581 83 Linkisping, Sweden

The rise and fall times for both silicon-based Schottky Collector Heterojunction Bipolar Transistor (SCHBT) and Conventional silicon-based Heterojunction Bipolar Transistor (CHBT) were calculated. The results show that SCHBTs have allowed 20% shorter rise and fall times when compared with CHBTs. The effect of the base resistance in charging and discharging the emitter capacitance is included. We use an approximation for the effect of thc base resistance as an additional RC time constant. Analysis of the collector transition layer shows that the main advantage of SCHBT over CHBT is that the cut-off frequency degradation is drastically decreased at high collector current density.

1. Introduction H

eterojunction Bipolar Transistors (HBTs) have a number o f advantages over conventional bipolar transistors, as discussed in detail by Kroemer [1]. The alignment o f the band edges leads to a large suppression o f the injection o f minority carriers from the base into the emitter, leading to nearly unity injection efficiency and thus an improvement o f the current gain and a possibility o f making high

speed transistors. P N P transistors have a lower gain and longer base transit time than N P N transistors with the same impurity profile, because o f the low minority carrier mobility in the base o f P N P . However, the intrinsic base sheet resistance o f P N P is much less than that of N P N , because o f the high majority carrier mobility in the base o f P N P . Since the base resistance is o f importance for high speed switching applications, there is a potential for P N P circuits if the base transit time and the current gain can be improved, which is the case for an H B T with a thin base. Moreover, by inverting the usual transistor structure, better frequency characteristics can be achieved [2-4]. By putting the collector 'on the top', the area o f the base--collector junction can be reduced, leading to a reduction in the capacitance of that junction. O n the other hand, the emitter-base junction area is increased, so the depletion capacitance o f that junction is low. So, as a net result, a significant improvement in speed is achieved. Collector-up H B T s with a current gain o f 2500

0026-2692/93/$6.00 © 1993, Elsevier Science Publishers Ltd.

823

O. Nur and M. Willander/Switching characteristics of SCHBT

are already being fabricated [5]. Again, by replacing the base collector pn .junction by a Schottky junction, higher speeds can be achieved. The advantages o f a Schottky collector depend on the following [6, 7]:

a Schottky Collector Hctcrojunction Bipolar Transistor (SCHBT) and a Conventional Heterojunction Bipolar Transistor ( C H B T ) is included. The advantage o f the S C H B T with no base push-out at high collector current densities is illustrated.

• zero collector resistance; • zero collector transit time, since there is no collector depiction layer, and • zero storage timc. In addition to these advantagcs, avoiding thc transition from a narrow bandgap to a wide bandgap within thc base-collector junction is of great importance. It has been shown [121 that the valence band offset at the collector-base junction of an Si/Sil xG%/Si H B T , combined with velocity saturation, dcgradcs the transistor characteristics. The presence of the valence band offsct at the base-collectorjunction increases the number o f holes by a factor exp(qAEv/kT), leading to a decrease in collector current and an increase in the base transit time. This problem can be avoided by a Schottky collector transistor.

2. Device structure

A bipolar transistor should have, as a first consideration, an acceptable current gain. The proposed structure was analysed with regard to having fi > 50 and good switching characteristics. The connnon emitter current gain was estimated using thc expression fi

_

l)p L,, N c

D,, I/V1,Nt,

exp((E

,

-

E b)/kT)

Thc above cquation is an approximation to the real situation, since it assumes that no recombination occurs in the emitter base junction.

Wc analyse a structure with a p-type Si as emittcr, an n-type Sil ~Gc~ as base, and a Cogi 2 as collector. Junction interface plays an important role in the device performancc. Devices with good interfaces can show cxcellent characteristics. Advanced Si technologT cnables the growth of CoSi2 on Si substratcs [8]. This can bc followcd by the growth o f a very thin laycr o f ntype Si. This layer will act as a very good surface on which to grow n-type Sil xGe× followed by p-type Si. Alternatively, ion implantation can bc used to form buried C o S i 2 o n Si substratcs with a thin Si layer on top. This can bc followed by growing the other laycrs as described above.

The most important factor affecting the emitter to collector delay time is the base transit time. To minimize the influcncc of thc base transit time a thin base is usually needed. The base thickncss, on the other hand, should be above a mininmm value to avoid punch-through. This minimum value depends on the base doping and the base-collector voltage which the device nccds to support. The base-collector voltage used in the calculations is 5V. A high base doping is also needed to reduce thc basc rcsistancc, and this high doping has anothcr effect by reducing ft. To compensatc for this reduction, a thicker emitter may bc choscn. A thicker emitter incrcascs the emitter resistance, and this has a slight effect on the emitter delay, since, as mcntioned beforc, Ct~. is small.

Thc aim of this work is to analyse thc proposed structure for switching characteristics. A comparison of the rise and Fall times with and without the effect of the base resistance between

The basic structure is a dual contact base H B T , as shown in Fig. 1. The analysed design parameters and some o f the properties arc listed in Table 1.

824

Microelectronics Journal Vol. 24

0.5

lo5 L

B

05

E~O

5

B

' J

I

o 5

!

_1

(~)

Emitter

(b) Si Substrate

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

.J

Fig. 1. Transistor gcumctry. (a) Plan view (dimensions in microns); (b) schematic reprcscntation of layers (not to scale). TABLE 1 Design paramctcrs and some of the calculated model parameters Type

SCHBT

CHBT

Emitter doping (cm 3) Thickness (/m0 Base dopmgo(cm ) Thickness (A) (;ermanium (%) Base resistance

2.0 x 101'~ 1.0 1 x 1()tu

2-0 x 1() I'~ 1-{) 1 x 1()I'~

500

500

10 35.5

10 35.5

Collcctor doping (cm 3) Thickness (ym) r~,~(ps) without Rt, r,.,. (ps) with Rb fl (GHz) without Rb fi (GHz) with Rb fi

__ -3-95 6.1 253 163 2200

1 × 1017 0-8 5-15 7-3 194 137 2200

3. Device model T h e c u t - o f f f f e q u e n c y is d e f i n e d as

1 ,t; -

(la)

~Cc"

In calculating %,. w e have i n c l u d e d the effect o f the base resistance in c h a r g i n g a n d d i s c h a r g i n g the e m i t t e r capacitancc. A n u m e r i c a l o n e d i m e n s i o n a l m o d e l [9], w h i c h was e m p l o y e d to p r e d i c t the D C a n d s w i t c h i n g characteristics o f an H B T , s h o w s that the base resistance has a great i n f l u e n c e o n the s w i t c h i n g characteristics. W e can w r i t c

Tec = "re -}- "t'b -}'- "Cd-~ "Cc-r- Teb

(lb)

825

O. Nur and M. Willander/Switching characteristics of SCHBT

%b accounts for the effect o f the base resistancc. W e have assumed that a rough approximation is achievcd by an additional R C time constant. The inner base resistance is assumed to be the most significant part of the base resistance, and hence rob = Cob Rbl

(2)

For a rectangular geometry, thc inner base resistancc is estimated as RI~, -

1

1

1

(3)

m 4h qNb H/b I*N

wherc 1 and h represent horizontal and vertical dimensions o f thc emitter strip respectively, and m represents the number of strips.

(4)

The emitter rcsistance consists of three parts, and is given by ~, _

kT ql~.

+-

( W e - W&) p~,

A,,

+

pm -

-

A~,c

" = P~

(5)

( 1 ~ { - W~d) pm A~. + A~.~.

(8)

Wed is thc extension of thc dcplction laycr into thc collector. The collector area is taken as the area under the base layer A(. = Ab. The last component of M is the collector dcpletion laycr transit time, given by thc cxprcssion

rd --

Thc other transit time was calculated as r~ = C~b D

The collector charging timc is a product of the collector resistance and the collector depletion layer capacitance. For S C H B T the collector resistancc is assumed to be zcro [6, 7], and this implies zero collector charging time. For C H B T the total collcctor resistance is thc sum of thc collector scrics resistance and thc contact resistance

Wd

(9)

21Js

Thc risc r,. and fall rr times for a common emittcr circuit werc calculatcd respectively using the expressions [1 0] 1

r,.

(l

c~).~

-

lb 0.9/~.(1

In

-:Q

(1())

It, W&. is the cxtension of the emitter base depiction layer into the emittcr.

w h c r c / b represents the turn-on current, and

For a uniformly dopcd base the base transit timc can be estimatcd as

Tb =

(wt,-

r~- -

Wdb) 2

(6)

2Dp

where Wdb is the extension o f the base collector depletion layer into the base. The dielectric constant in the base was assumed to depend linearly on thc Ge concentration Sil ~Gex = ( l - x )

826

[ b ,3(

cSi+x~;Ge

(7)

It.

1

(1 - ~)./i

In

(] ~) ().11, - l b ~ 1--~

(11)

wherc lb represents tile turn-offcurrcnt. The tunnelling o f clcctrons from thc base into thc collector can bc analysed by approximating thc Schottky barrier with a triangular one. The probability of a triangular barrier being pcnc-

Microelectronics Journal, Vol. 24

The energy AE (Fig. la) can be estimated as

trated by an electron having energy AE less than the height o f the barrier is given by [13]

A E = ~b + Vcb -- ~(Nb)

( -2AEs/2 ) P = expk,

E'

3E'Va

Since the diffusion potential is given by

h ( Nb ~,/2

Vd = ¢% - ~(Nt,) - Vcb equation

The m a x i m u m o f the energy distribution o f thc emittcd electrons occurs at an cnergy

E,,,

= V~

(15)

(12)

(15)

(16)

will read

exE = Vd - E,,,

qE' ) 2 cos/1~7~

(14)

(:7)

The result of calculating this probability at room temperature shows that the tunneling is negligible.

above the conduction band.

i

Ec

_

I

.

~b = 0 7 1 e V ~-

p si _

Ef

_E~

_

_

_

~

®®®

I

-

[14]

-El m

-

Vbc

CoSi 2

Ev

n Si i -xGe x AE v

(a)

Depletiq,~ layer

N(x)

.

~',.\\\\\\\\\'-~ Emitter

Base

~

N4,)

I

Collector

c°s'2

x x

(b)

1

x=O

Fig. 2. (a) EncrgT band diagram; (b) one-dimensional represcntation of the transistor physical structure.

827

O. Nur and M. Willander/Switching characteristics of SCHBT

4. No cut-off frequency degradation at high current density

7 6

A serious problem observed in the high current density operation regime of bipolar transistors is the degradation of the cut-off frequency. This is due to the base push-out Kirk effect [15]. In analysing thc S C H B T , the following assumptions are made with regard to the physical structure and impurity distribution:

T

Ic = 10 mA

£ ~ 4 .~ 3

"""~BT

-~ 2 ac

(1) Impurity density distribution is constant throughout the base.

0.024 0.048 0.072 0.096 0.12 Turn on current (mA)

(2) CoSi2 as a collector is considcrcd to bchavc like a vcry highly doped n-type senticonductor w h e n compared with the base. (3) A high field exists in thc dcplction layer duc to reversc biasing. This implies that carriers are drifting with the saturation vclocity.

¢o ¢v (1.)

E (I)

The depiction layer lies entirely in thc basc side (Fig. 2b), and the solution to thc depiction transition layer has thc form

CE

9 8 7 6 5 ":'",~\CHBT 4 3 2 SCHBT ~ . 1

Ic : 10 mA

....

eg

_g=

0 X,

--

( 2 r ' ( - Va' + Vd) )~ (

qNdb

1 +

_]c ) ~ qv~Nat, -

0.024

(18)

As sccn from equation (18), as Jc increases, X1 decreases. The effect of the restriction of the cxtension of the depiction layer into the collcctor will imply that the eftiectivc base width will nevcr exceed the total base width. This will lead to a limited cut-off frequency degradation at high current dcnsities. The variation of the collector currcnt density./c with the current gain cutoff frequency is shown in Fig. 6. 5. Results and discussion

The analysed structure has rise and fall timcs as shown in Figs. 3 and 4. The results of the calculations show that the Schottky Collcctor H B T has improved rise and fall times by 20%

828

0.04{3

0072

0.096

012

Turn on c u r r e n t (mA) Fig. 3. Rise time. (Top) Without the cfli:ct o f Rt,; (bottom) with the cffcct o f Rb, for transistors with parameters o f Table 1.

compared with convcntional H B T with the same parameters and profiles. The effect of the base resistance modelled as dcscribcd carlier is clearly seen to be an incrcase of the emitter to collector delay time by about 54% and 41% for S C H B T and C H B T , respectively. Wc havc shown that the base resistance is the most important parameter affecting switching characteristics. In our case we have a high doping in the base, so the term Z~-b is not dominant. For low doping in the base this additional R C time constant must be dividcd by fl, since the charging and discharging current through C,,t, is It, and not I,,.

Microelectronics Journal, Vol. 24

4 3.5

,

Ic :

I0

mA

3 c

2.5

"ILD

~D

E

2

u_

1

SCHBT

-.

~

0.5 i

0

i

T

¢¢1 5 vC (32 4

i

•

0 . 0 2 4 0.048 0.072 0.096 Turn off c u r r e n t (mA) ]

7

0.12

;CHBT

~

SC~HST ~"" -'~

I

0

0.024 0.048 0.072 0096 Turn off current (mA)

0.12

Fig. 4. Fall time. (Top) Without the effect of Rt,; (bottom) with thc cffcct of Rt,, for transistors with parameters of Table 1. 180

3: LD

,

,

160 140

' ~ H

12 o 100 80 60

"q

,

BT

CHBT

,

i

J

4 104

i

6 104

i

8 104

i

1 105

2

105

(A/cm 2)

Fig. 6. The variation of the current gain cut-off frequency l~('r.~14s collector currcnt density, for N,. = 2.1()lScm ~ Nb -- 1-10tScm ~ and I~Vt~ 500A.

Ic = I0 mA

3

2

,

Jc

T

', "', ",

i

45

40 35 30 25 20

C H B T

1.5

-

70 65 60 55 50

T h e advantagc o f the Schottky collector is clearly seen t o be lost for transistors with thick bases (Fig. 5). As seen f r o m Fig. 6, a stable ft exists at high collector current densities. This will imply that this transistor can bc switched on and o f f at high current densities very quickly w i t h o u t any disturbance. This structure can still be pressed towards bctter switching p e r f o r m a n c e by incrcased vertical scaling and incrcasing the base d o p i n g to reduce the base resistance. T h e calculations wcrc carried out tbr collector 'down'. For collcctor 'up' the difference will be small. T h c structure analysed can be used for digital applications, namely for integrated injection logic (12L), w h e n the pSi/nSil ×Gcx/CoSi2 transistor can be c o m b i n e d with an N P N transistor in the same chip to form an 12L gate, with thc S c h o t t k y collector H B T as a current source and the N P N as a switch.

'

4O 2O

References

0

0

i

i

i

i

0.0:52

0.064

0.096

0.128

Base thickness

11] H. Kroemer, Heterostructure bipolar transistors and

0.16

(urn)

Fig. 5. The variation of current gain cut-offfrequency with the basc thickness, for transistors with parameters of Table 1.

integrated circuits, Proc. IEEE, 70 (1982) 13-25. [2] H. Kawi, T. Kobayashi and K. Kaneko, A collector up A1GaAs/GaAs hetcrojunction bipolar transistor fabricated using three stage MOCVI), IEEE Electron. Device Lett., 9(8) (1988) 419-421.

829

O. Nur and M. Willander/Switching characteristics of SCHBT

[3] A. Sadao and 1. Tadao, Collector up HBT's fabricated by Be+ and O+ ion implantation, IEEE Eh'ctnm. 1)el,ice Lett., 7(1) (1986) 32-34. 141 G. Clifton, Consideration of the relative ficqucncy performance potential of inverted hetcrojunction N-P-N transistors, IEEE Electron. Device Lett., 5(3) (1984) 99-1 ()0. I51 o. Saugiura, A. G. l)entai, C. H. Joyncr, S. Chandrasekhar and J. C. Campbell. High current gain lnGaAs/InP double hetcrojunction bipolar transistor grown by rectal vapour phase cpitaxy, 9(5) (1988) 253-255. [6] (;. A. May, The Schottky barrier collector transistor, Solid State Eh'ctrom'c,', 11 ( 1986) 613-619. [7] S. C Blackstonc and l/.. P. Mcrtcns, Schottky collector 12L,IEITEJ. SolidState Circuits, 12(3) (1977). [8] M. Masanobu, O. Takansbi, N. Nobuo and N. Kiyokazu, Growtb of high quality Si/CoSi2/Si double hctcrostructures by sclf aligned and two step MBE, Material Res. Soc. Syrup. Proc., 160, 1990, 275-281. [91 K. Mamoru and Y. Jiro, Modeling and characterization f'or high spccd GaAIAs/GaAs N-P-N hctcrojunction bipolar transistors, IEEE Tra;;s. Eh'ctron Devices, 31 (4) (1984) 467-476. I101 J. L. Moll, Largc signal transient response of'junction transistors, Pn~c. I. R. E., 1954, 1733 1783. [11 ] P. Ashburn, l)es(~m and Realizatio~ ~71Bipolar 7"ra;;sistots, Wiley, Chichcstcr, 1988. 112] P. E. Cottrel, Velocity saturation in the collector of Si/Sil ~Gc×/Si (HBT's), IEEE Eh'ctnm. l)cvlcc Left., 11(10) (1990)431-4,3,3. 113] E. H. B,hoderick and 1~,. H. Williams, Metal Semiconductor CoHtacls, 2nd edition, Clarendon, Oxford, 1988. 1141 j. 1/.. Jimenez, L. M. Hsiung, P,. 1). Thompson, S. Hashimoto, K. V. l
k T

830

Boltzmann's constant absolute temperature

q

electronic charge effective mass dielectric constant g Planck's constant h 1~'c, ~N'b,,N[c doping c o n c e n t r a t i o n in emitter, base and collector fl D C c o m m o n emitter current gain current gain c u t o f f frcqucncy -[t "Co,"Cb,Z'd, 1c transit times in emitter, base dcplction and collector region "c~.~ emitter to collector delay time Rb total basc resistance inner base resistance Rbl emitter, base and collector We, Wb, Wc thicknesses Cob emitter base j u n c t i o n capacitance Cc collector depiction layer capacitance clnittcr total resistance rc electron mobility in the base ,tln /')~., Pc emitter and collcctor resistivities 1)m (A1-Si) contact resistivity emitter, base and collector areas Ac, Ab, Ac emitter and collector contact areas Ac~.,A,.c hole diffusion length in the base Dp Wd collector depletion layer thickness lc collector current rise and fall times Tr' Ttc o n d u c t i o n and valence bands Ec, Ev Schottky barrier height (I)b Vbc, Vcb biasing, emitter base and collector base j u n c t i o n s saturation w'locity l"s tunnelling probability P diffusion potential, base collector Vd junction E t~ ( N b ) = Ec m*

-)

( ~ -fl' \ fl + 1