Indium antimonide transistors

Solid-State

Electronics

Pergamon Press 1961. Vol. 3, pp. 159-166.

INDIUM

ANTIMONIDE

Printed in Great Britain

TRANSISTORS*+

H. L. HENNEKE Central Research Laboratories, (Received

1 March

Texas Instruments

Inc., Dallas 22, Texas

1961; in rewised form 1 June 1961)

Abstract-The design, fabrication, and electrical characteristics of an n-p-n indium antimonide transistor which operates at 77°K are discussed. An analysis of the expected high-frequency performance is presented and a comparison made to a p-n* germanium transistor. Because of its high electron mobility, the indium antimonide transistor should have a base transit time l/26 that of a similar germanium transistor. Although these ultimate capabilities have not been approached, transistors withft as high as 300 MC/S and ,3’s of 500 have been observed. Switching speeds approaching those of the best state-of-the-art germanium transistors have been measured. R&urn&-Le dimensionnement, la fabrication, et les caracteristiques d’un transistor n-p-n a l’antimoniure d’indium operant a 77°K sont discutes dans cet article. Une analyse du rendement a haute frequence attendu est presente et une comparaison faite avec un transistor au germanium. Grace a sa forte mobilite d’electrons, le transistor a l’antimoniure d’indium devrait avoir un partours electronique 26 fois plus rapide que celui d’un transistor similaire au germanium. Malgre que ces capacites extremes n’ont pas tte atteintes, des transistors ayant des ft aussi &levees que 300 MC/S et des Bs de 500 ont et4 observes Des vitesses de commutation approchant les meilleurs transistors au germanium ont Cte mesurees. Zusammenfassung-Struktur, Herstellung und die elektrischen Charakteristiken eines n-p-nIndiumantimonid-Transistors, der bei 77°K arbeitet, werden diskutiert. Der zu erwartende Hochfrequenzbereich wird analysiert und mit dem eines p-rz-+Germanium-Transistors verglichen. Wegen seiner betriichtlichen Elektronenbeweglichkeit sollte die Laufzeit in der Basis bei diesem Transistor nur l/26 von der eines lhnlichen Germanium-Transistors sein. Obwohl man die iiussersten Leistungen noch nicht erreicht hat, liessen sich Frequenzen ft bis zu 300 MHz und /3-Werte his 500 beobachten. Die gemessenen Schaltgeschwindigkeiten ngherten sich denen der besten existierenden Germanium-Transistoren.

1. INTRODUCTION

BECAUSE indium

antimonide

has

the

highest

mobility of all III-IV semiconducting compounds, it is a most promising starting material for either a microwave or a high-speed switching transistor. The primary difficulty in attempting to construct bipolar transistors from compound semiconductors like indium antimonide arises from the low carrier-lifetime values. This short lifetime results electron

* The research ported in part by been performed Naval Ordnance

reported in this paper has been supthe Bureau of Naval Weapons and has under the technical direction of the Laboratory, White Oak.

t A limited account of this work was presented at the Solid State Device Research Conference in Pittsburgh, Pennsylvania, June, 1960. L

159

in short carrier diffusion lengths and requires relatively narrow base regions to obtain sufficient current gain. In fact, the base regions required are comparable in width to those achieved on the most recent high-frequency germanium and silicon transistors. To investigate the high-frequency performance expected for an indium antimonide transistor, a theoretical analysis was made to relate the physical parameters of such a transistor to its highfrequency response. An n-p-n structure was chosen for the model.

C, ct! Dn

2. NOTATION Collector transition capacity. Emitter transition capacity. Diffusion constant for electrons.

H.

160

L. HENNEKE

Diffusion constant for holes. cut-off frequency. Maximum frequency of oscillation. Frequency at which 1hfe1 is unity. High-frequency common-emitter current hfe gain. Emitter current density. Boltzmann’s constant. A constant between 1 and 6, depending on 11 the base-layer impurity distribution. NC Donor impurity concentration in the collector. Donor impurity concentration in the Ne emitter. Acceptor concentration in the base region. pb PC Acceptor concentration in the collector region. Acceptor concentration in the emitter Pe region. Electron charge. 4 Base spreading resistance. 7.8' Base transit time. tb Collector depletion layer transit time. tc Emitter charging time. te Emitter-to-collector transit time for tee carriers. T Temperature in “K. vgc nrn Lattice-scattering-limited maximum drift velocity for carriers in the collector barrier region. Base width. W Collector depletion-layer width. X Large signal /3. PO Electron mobility. Pn Hole mobility. PlJ Storage time constant. 78

Ift

where tee is the emitter-to-collector transit time for carriers and is equal to the sum of the emitter charging time te, the base transit time tb, and the collector depletion layer drift time tc. The complete expression for tee(fn= 1/2rtee) i,(2)

kT tee = -Ce+

t

3. ANALYSIS OF THE CAl’ABILI’I’lES OF THE INDlUM ANTIMONIDE TRANSISTOR

The frequency performance of a junction transistor may be characterized fully by the parameters fa,the frequency at which the common-base current gain has fallen off 3 dB, r;, the base series resistance, and Cc, the collector junction capacity. These are related by the expression(r) (power gain)l/3(bandwidth) =

= ii

(

r;hctec

fmax08c=

),,,

(1)

qJe

W2 -+ n&

X 2~8Clim

(4

To obtain a value for tee, we must reduce each term of this equation to measurable physical quantities. If this is done for a drift-transistor structure with graded base-impurity distribution, a comparison may be made between the theoretical limits and capabilities of an indium antimonide and a germanium transistor. Assume a germanium p-n-p transistor with P, = 4 x 1015 cm-3, Pe = 101s cm-3 and an average Nb = lo17 cm-3. Compare this to an antimonide transistor with indium n-P+ N, = 4 x 1014 cm-3, Ne = 101s cm-3 and an average Pb = 101s cn-3. Further, assume the same physical structure, base width and impurity variation for the two transistors. A reasonable value for the drift mobility of minority carriers in the base of the germanium transistor is 1000 cm3/V-set for T = 3OO”K, so that Dp = p&T/q = 26 cm+ec. For the indium antimonide transistor, a value of 100,000 cm2/V-set is reasonable for pn at T = 78°K; so that D, = pnkT/q = 670 cmz/sec. The lattice-scattering-limited maximum drift velocity is approximately 8 x 10s cm/set for germanium(3) and may be greater than 2 x 107 cm/set for indium antimonide.@) From equation (2) the following comparisons may be made for the two transistors : t,-InSb tb-InSb t,InSb

less than Ge by a factor of 6 less than Ge by a factor of 26 less than Ge by a factor of 5

Since the average resistivity of the base layer is the same for the two transistors, r: will be approximately the same. If the lower operating voltage of the indium antimonide transistor is considered, C, is also about the same for the two cases. At this point it is important to note that if the emitter changing time te and the collector transit time to are negligible so that base transit time tb dominates, the a-cut-off frequency will actually

INDIUM

ANTIMONIDE

be higher than the value 1/2rrta for either the germanium or the indium antimonide transistor. KROEMEZR(~) has shown that other factors must be considered in relating base transit time to a cut-off frequency resulting from this transit time. His result, a more accurate expression for fa of a drift transistor, is

Substituting for the value of tb,

fa

N

2_( hpa(w ),I2

(4)

The conclusion from these calculations is that the indium antimonide transistor may be expected to have an u-cut-off frequency of from 5 to greater than 26 times that of a geometrically similar germanium transistor. The actual value will depend on which transit time dominates. 4. METHOD OF FABRICATION

To utilize properly the high electron mobility of indium antimonide, our effort has been concentrated on developing an n-p-n transistor. Because of the proved usefulness of solid-state vapor diffusion for forming narrow base regions, this technique was used to form the p-type base layer. The emitter region could best be formed by alloying because: (1) A transistor capable of highfrequency operation must have a small emitter area to reduce junction capacity. Techniques employed in silicon technology, which allow impurity-diffusion selective masking to reduce emitter area, are not yet developed for indium antimonide. (2) For the highest emitter injection efficiency, an abrupt emitter-base junction (such as those obtained by alloying) is preferable to the graded junction produced by diffusion. (3) Preliminary diffusion studies of normal group VI donors indicate difficulty in obtaining the high concentrations necessary for good emitter efficiency. The starting material is single crystal n-type indium antimonide grown by the Teal-Little technique at the Central Research Laboratories of Texas Instruments. The impurity concentration is about 4 x 1014 cn-3. The electron mobility is greater than 400,000 cm2/V-set, as determined by

TRANSISTORS

161

Hall measurements at 77°K. This material is cut in slices 0.040 in. thick on the (111) plane and is lapped to a thickness of about O-020 in. with a fine abrasive. The slices are then optically polished. Thorough washings in triple-deionized water follow each step. The p-type base region is formed by vacuum diffusion in a sealed quartz ampule. Although a number of impurity-diffusion sources were investigated in elemental and dilute alloy form, none was completely satisfactory. Zinc,(s) which was investigated most thoroughly, was found to be unsatisfactory for several reasons. (1) The surface concentration from an elemental source is too high, approaching the limit of zinc solubility in indium antimonide, 2 x 102s cm-s. (2) The diffusion coefficient is extremely concentration-dependent. Lowering the surface concentration by reducing zinc vapor pressure (“diluting” the zinc in indium) reduces the diffusion coefficient to an impractically small value. (3) The diffusing zinc does not follow a complementary errorfunction distribution. The diffusion profile contains a break where concentration falls off rapidly with distance. Such a profile makes accurate control of the base width under the alloyed emitter contact difficult. Other acceptor impurities-cadmium, magnesium, manganese and mercury-were also investigated. Magnesium shows promise, but no completely satisfactory diffusant has yet been found. After a p-type diffused layer about 10~ thick is obtained, one side of the slice is lapped down to remove this layer and to expose the original n-type crystal. The slices are then diced into smaller wafers about 0.040 by 0.040 in., and mesas are masked and etched on each wafer. The mesas are about 0.010 in. in diameter. The collector connection is formed by soldering the lapped side of each wafer with pure tin to a TO-18 header. The ohmic base connection is made by alloying a 0.002 in. pellet of 98 per cent indium/2 per cent cadmium alloy onto the diffused layer in a stripheater micro-furnace under helium atmosphere. The emitter contact is formed by similarly alloying a O-002 in. pellet of an indium-gallium-tellurium alloy onto the p-type layer. This contact is not allowed to penetrate the base layer and forms the emitter junction of the n-p-n structure. Gold wires O+lOO7in. in diameter connect the header

162

H.

L.

HENNEKE

to these contacts. The transistors are then given a short clean-up etch to reduce surface leakage currents. Fig. 1 is a sketch of the completed diffused base-alloyed emitter structure. 5. EXPERIMENTAL RESULTS Fig. 2 is a photograph of the curve-tracer presentation of an indium antimonide transistor operating common base at 77°K. The CCof the

base-emitter diode characteristics of this transistor. These diodes have less reverse current and more nearly diffusion-limited forward current than any indium antimonide diodes previously reported in the literature. Figs. 6 and 7 are photographs of this same transistor operating common emitter at both high and low current levels. Beta is about 40 at an emitter current of 7 mA.

i In-Ga-Te

98% In -2%Cd. BASE CONTACT

EMITTER

CONTACT

DIFFUSED PURE

p-TYPE

LAYER

TIN n-TYPE

FIG. 1. Structural

view of indium antimonide mesa transistor.

InSb

n-p-n

t >

;3 \ a E N

I

-0.I

VOLT/DIV

I

+

FIG. 2. Indium antimonide transistor characteristics high current level, common base. Emitter current 2 ma/step, Q = O-975.

at is

unit is about O-975 at a collector current of 10 mA. Fig. 3 shows the characteristics of this transistor, illustrating the ability to operate at very low levels. The a is still about 0.75 at 50 PA collector current. Figs. 4 and 5 illustrate the base-collector and

-0.1

VOLT/DIV

-+

FIG. 3. Indium antimonide transistor characteristics low current level, common base. Emitter current 20 PA/step, TV= O-75.

at is

Although most of the transistors had j3’s between 10 and 40, a few exhibited much greater current gains. Fig. 8 is a photograph of the characteristics of one of these higher gain transistors. The a of this unit is about 0.994 at 5 mA collector current.

INDIUM

ANTIMONIDE

The low breakdown voltage is apparent in all these photographs. At present, the breakdown is thought to be due to avalanche breakdown.

-0.1

VOLT/DlV

163

TRANSISTORS

The common-emitter high-frequency current gain hfe was measured by conventional means. Fig. 9 shows the variation of hfe with frequency

-

-0.1 FIG. 4. Indium antimonide transistor, base-collector diode characteristics, emitter open circuited. Icno = 1pA; BVcno = 3V at 10 PA.

VOLT/DIV

+

FIG. 6. Indium antimonide transistor characteristics at high current level, common emitter. Base current is 50 PA/step, /I = 40.

t

t >

>

a

;5

a’

p‘

ZE

L

3

0

%

I -0o.I

VOLT/DlV

i-,

FIG. 5. Indium antimonide transistor, base-emitter diode characteristics, collector open circuited. IEBo = 2 PA; BVEBO = 1 V at 10 PA.

-0.1

VOLT/DIV

_I,

FIG. 7. Indium antimonide transistor characteristics low current level, common emitter. Base current 10 @/step, g = 2.

at is

164

H.

L.

HENNEKE

for one of the better indium antimonide transistors. At the frequency where hfe is unity, ft is about 160 MC/Sfor this transistor.

Table 1. Indium antimonide transistor aarameters at 77°K a

Parameter

Conditions

B

I,=SmA v, = 0.5 v T’s = 0.3 v

ICBO IBBO BVCBO BVEBO RCS hb

hob

ft

-0.2

VOLT/DIV

--_,

FIG. 8. Indium antimonide transistor characteristics, common base. Emitter current is 1 n&/step, u = 0.994.

Typical parameters and the best parameters observed during this period of research are tabulated in Table 1.

Ic=lmA G=lmA Ic = 10mA IE=smA

I,=SmA Ig=3mA

Typical

20 20 ICA 20 LA 3v

0.5

v

1s n 20 n 500 /&lo 80 MC/S

1,=5mA 25

I

I

I MC

IO MC

IOOMC

FREQUENCY FIG. 9. Common

emitter

500
Since electron and hole lifetimes in indium antimonide are very low, indium antimonide transistors are advantageous in saturating circuitry. Accordingly, these transistors were compared with commercial transistors designed for fastswitching circuits. There are two ways to study storage effects: (1) compare storage times for the same amount of overdrive on the base (common-emitter connection), or (2) compare a time constant associated with collector-current decay in the storage mode. The first is widely used. The second may require

35t

-5

Best observed

current

gain vs. frequency.

1000 MC

INDIUM

ANTIMONIDE

some explanation (see Fig. 10). It is assumed(v) that the collector current rises to a level ICM = &Jb, where ,8a is large-signal /3,and Ia is base current. Actually, the collector current is limited to ICS E Vcec/R~, where RL is the load resistor; therefore, storage time is time required for the collector current to fall from ICMto O-9 1~s. The storage time constant TVgoverns this fall.

165

TRANSISTORS

Using the circuit shown in Fig. 11, it is possible to determine lb, RL, V,, and ts. From the Tektronix 575 curve tracer, /?sis found; thus, r8 is calculated from (8). This figure of merit has the advantage of relative independence from the degree of saturation and will be used for comparing various transistors. Storage time results for a number of transistors are tabulated in Table 2. Table 2. Storage-time meawnments

l-l-l-l-lIn&urn antimonide (77”K)-P’cc = 1 V; RL = 390 R; v,, = 1OV P-P-_ 190 0.05 40.5 25*00 0.50 A 70 5.00 33.8 2.00 1.40 B 50 5.00 29.2 l-65 1.6s C

TIME. t -+

E”

FIG. 10. Collector current wave form.

In Fig. 10, the base current is switched off at tl, and the storage time is t8. For t 2 tl,

ic = ICM exp( - t/r8) Applying end-point definitions,

(5)

(6)

=

pdbeXp(-+a)

(7)

RL

The storage time constant is then

ts T” =

1.50 1.60 ---_

180 90

Gmna~~O”K)-_Vcc = 1 V; vb = 1OV --I-___ 160 35.00 1.75 2N501-1 260 32.00 1.60 2N501-2 170 22.80 1.60 2N501-3 230 7.00 1.75 2N240-1 270 8.75 1.75 2N240-2

5.00

49.5 75.0

RL = 390 hz; 5.00 ::: 5+0 5.00

34.0 55.4 39.0 74.4 81.3

values from storage time

or 0.9 v,,

3.00 1.78

In ~&RL/O*9Vc,]

+zz_~~~LLoscopE FIG. 11. Storage-time test circuit.

(8)

6. CONCLUSIONS

The work reported in this paper has demonstrated the feasibility of fabricating transistors from the semiconductor compound indium antimonide. These transistors exhibit low input impedances and operate at very low power levels. In conventional switching circuits, these transistors have switched in times comparable to the best state-of-the-art germanium transistors. The reasons for the relatively low frequency cut-off compared to the theoretical capabilities are not clearly understood. As the limitations on performance of this new transistor are understood, the research will be reported.

AcknowZedgements-The author is grateful to his colleagues in the Device Research Department of Texas

166

H.

L.

HENNEKE

Instruments for many helpful discussions and suggestions. I should also like to acknowledge the early work on indium antimonide devices of R. L. PETRI~ (presently at Texas Instruments Incorporated, Dallas, Texas) of the Naval Ordnance Laboratory. For developing the circuitry and carrying out the switching measurements, I am indebted to R. W. LADE, presently at Marquette University. Special thanks are due Mrs. B. MIMI and Mrs. F. EUBANKSfor assistance in fabricating the transistors.

REFERENCES 1. R. L. PRITCHARD,Frequency Resfionse of GroundedBase and Grounded-Emitter Transistors AIEE Winter Meeting, New York, January, 1954. 2. J. M. EARLY, Proc. I.R.E. 46, 1924 (1958). 3. E. J. RYDER, Phys. Rm. 90,766 (1953). 4. M. GLICKSMAN and M. C. STEELE, Whys. Rew. 110, 1204 (1958). 5. H. KROEMER, Arch. elektr. Ubertr. 8, 363 (1954). 6. D. L. fiNDALL and M. E. JONES. To be published. 7. D. E. DEUITCH, Transistors I. RCA Laboratories (1956).