Hot-electron camel transistorJ.M.Shannon

Indexing terms: Hot electrons, Transistors

Abstract: A transistor is proposed in which hot electrons cross a degenerate semiconductor base region andovercome a potential barrier in the bulk of the semiconductor which forms a collector. Structures in siliconcorresponding to this concept have been fabricated using low-energy ion implantation and have giventransistor action consistent with hot-electron transport.

1 Introduction

Hot-electron transistors, being majority-carrier devices withnegligible charge storage and a low base resistance offer thepossibility of a high-frequency performance superior to thatof a bipolar transistor.1"3 For this reason, considerableeffort has been directed in the past to the realisation andassessment of metal-base hot-electron transistor structureswith a variety of materials chosen to provide potentialbarriers for hot-electron emission and collection. It wouldseem, however, that fundamental problems related to areduction in the collection efficiency of the collector barrierdue to quantum mechanical reflections and phononscattering as well as electron-phonon scattering in theemitter, limit the common emitter current gain to values— 0-4,4>s and thus the application of such a device isseverely limited. Moreover, there are considerable techno-

Base contact

_L Collector contact

V

-—wW ~ 250 A

logical problems when attempting to make a metal-basetransistor because the formation of high-quality barrierson either side of the metal having the desired barrier heightsis incompatible with a single-crystal technology. Theconcept reported here is based on highly doped surfacelayers in a semiconductor and replaces the metal base witha thin degenerate layer with both electron emitter andcollector formed in the same semiconductor material.

2 Hot-electron camel transistor concept

The band diagram of an h.e.c.t. is shown in its basic form inFig. 1. The base region consists of a thin, highly dopeddegenerate semiconductor and is bounded by a potentialbarrier forming the hot-electron collector and an emitterof hot electrons which in this case is a reverse-biased metal-semiconductor Schottky diode where electrons are emittedvia field and thermionic-field emission.6"8 An importantfeature when making the base of semiconducting materialoccurs because band bending in the base raises the electronenergy distribution relative to the collector and, in principle,electrons can be injected with energies many kT above thecollector peak thereby aiding the collection efficiency.

The collector of hot electrons is a majority-carrier diodenamed a 'camel diode'9 which uses a thin highly doped p+

region to form a potential hump in the bulk of the semi-conductor to control transport of majority carriers. The p+

region is sufficiently thin such that it is fully depleted ofholes by the natural band bending within the barrier regionand provided there is a high density of ionised acceptors thebarrier height is insensitive to reverse bias leading to strongcurrent saturation. Camel diodes have been made withideality factors < 2 using low-energy ion implantation.9

Since both the base and collector region are made in thesame semiconductor material improved collection efficiencyis expected compared with a metal base transistor due to areduction in quantum mechanical reflections.10 This shouldarise because the absence of a strong image force decreasesthe abruptness of the barrier, and hot carriers do not haveto pass across a potential barrier into a different material,having an entirely different electronic structure. A furtherpractical advantage of a camel collector arises because thebarrier to hot electrons can be adjusted simply by changingthe doping level in the p+ region.

Fig. 1 Band diagram for a monolithic hot electron transistor

a In thermal equilibriumb Under operating conditions with a bias applied to the base and

collector regions

Paper T419 S, received 9th July 1979Dr. Shannon is with Philips Research Laboratories, Cross Oak Lane,Redhill, Surrey RH1 5HA, England

3 Implanted structures in silicon

Attempts to make structures having an energy band diagramcorresponding to Fig. 1 have concentrated on implantedstructures in silicon. Ion implantation at low energies hasbeen shown to be very suited to the formation of highlydoped surface layers and has been used to control theproperties of Schottky barriers11"14 and camel diodes.9

The high solubility of the common donor and acceptor

142 SOLID-STATE AND ELECTRON DEVICES, SEPTEMBER 1979, Vol. 3, No. 5

0308 6968/79/050142 + 03 $01-50/0

impurities in silicon enables band bending to occur overdistances comparable with the electron mean free pathhelped by the fact that the nonequilibrium nature of theimplantation process often enables high dopant concen-trations to be obtained at temperatures where diffusionand spreading of the implanted impurity profile does notoccur. Resistivities < 2 x 10~4 ft cm can be obtained usingion implantation15 which is within a factor 50 to 100 ofthat in a metal base and a further reduction is likely usingpulse or laser annealing.16

The planar monolithic hot-electron structure investi-gated is shown schematically in Fig. 2. Implants were madethrough a 70 jum diameter oxide window into the silicon atlow energies to form the camel-diode collector junction andthe degenerate base before the sample was annealed in avacuum at a temperature in the region 750-850° C. Insome cases an additional shallow implant was made toreduce the net doping concentration at the surface under-neath the emitter contact and reduce the amount of barrierlowering which occurs when making a Schottky contact toa highly doped layer. Following the annealing stage contactswere made using Al or Ni for the emitter and Al or Ti forthe base. The emitter metal was extended out over theoxide to form a guard-ring structure which helped topassivate the transistor by reducing the concentration ofelectric field lines which inevitably occurred around theperiphery.

On probing these structures, transistor action was indeedfound with characteristics similar to those shown in Fig. 3.In this case, 4-5 keV boron derived from a BF2 implant wasused to form a camel-diode structure with a dose of 4 x1014/cm2 lOkeV arsenic to form the base. The peak of thearsenic distribution was estimated to be ^ 150 A below thesurface with an arsenic concentration «* 5 x 1020 cm"3.The distance from the emitter to the potential maximumat the collector was estimated to be » 250 A. The sheetresistance of the base calculated from the current/voltagecharacteristics between base and emitter contacts indicatedthat — 20% of the arsenic was electrically active giving apeak donor concentration «102 Ocm~3. It follows fromFig. 3 that, assuming the transistor characteristics aredue to hot-electron transport across the base region (seeSection 4), then a common emitter current gain of 1-5implies a net effective collection efficiency of ^ 60%. Thehighest value of collection efficiency obtained from thisbatch of devices was =* 75% giving a 0 « 3. Invariably, nochange in collector current occurred during application ofthe first few current steps suggesting that a potential dropbetween emitter and base was required before a significant

n**

number of the emitted electrons were energetic enough toovercome the collector barrier.

4 Possible parasitic effects

In Section 3 it was assumed that the transistor charac-teristics obtained from the h.e.c.t. structures were due tohot-electron effects. There are, however, a number ofparasitic effects which could conceivably lead to transistoraction, since these structures contain both n and p typeregions. The first of these could arise from conventionalbipolar action either through the bulk of the device or viaits periphery. The diffusion of electrons, across the p*barrier region to the collector, for example, might constitutethe collector current rather than hot-electron injection.Since, however, a camel-diode structure is substantiallydepleted of holes, the conductivity along the p+ regionnecessarily should be very low and unable to supply thebase current but this might not be the case around theperiphery, because the thickness of the p+ regions and itsdoping concentration are less well defined. In order toeliminate these uncertainties, measurements were made(Fig. 3) with the base contact in the centre of the deviceas above in Fig. 2. Since the base contact was positive withrespect to the emitter and in this configuration the emittercontact overlaps the p+ region, the p+ln++ junction wasalways reversed biased, thus preventing electron injectioninto the p+ region and bipolar action.

A further possible parasitic effect arises because there isa potential minimum for minority carriers at the camelcollector (Fig. 1) and if sufficient charge is able to accumu-late in this minimum then the space charge due to theionised acceptors could be affected and the barrier heightmodulated. This complication is unlikely to arise in asimple camel diode because the space-charge density in thep+ region is large (=^ 1013 cm"2) and typical minority carrierleakage currents are insufficient to significantly affect itwithout having an abnormally long lifetime in the potentialminimum.9 In the h.e.c.t., however, there may be anadditional source of holes from the forward-biased basecontact. Assuming the n++ base region to be uniformlydoped, a worst-case condition would be when all the holesthat are able to pass from the contact over the barrier 4>h

(Fig. la but assuming a base contact rather than an emitter)reach the potential minimum, the current being limited onlyby the velocity of the carriers. In this case, neglecting barrier

Fig. 2 Schematic section of an experimental monolithic hotelectron transistor

A central base contact is surrounded by a Schottky emitter whichalso makes a good Schottky contact to the n~ epitaxial layer

IV

Fig. 3 Common emitter characteristics obtained from a siliconh.e.c.t. on a 10 Si cm n/n * epitaxial layer

The implants used to form the degenerate base and camel col-lector were 4 X 1014 cm"2 lOkeVAs and 5 X 1013 cm"2 4-5 keV B,respectively. A 5 X 1013 cm"2 1 keV B implant was also made toreduce barrier lowering at the emitter contact. X = 1 V/division,Y = 1 mA/division, 1 mA/step base current.-

SOLID-STATE AND ELECTRON DEVICES, SEPTEMBER 1979, Vol. 3, No. 5 143

6mA

4mA

200ns

Fig. 4 Response of collector current to a 8 mA base current stepX = 200 ns/division

lowering for the electrons, the ratio of holes to electroncurrent is

7 =

exp — (0h -

expAtT2 exp — (0B - VF) kT\q

where A^e is Richardon's constant for holes and electrons,VF is the applied forward bias and the Fermi level in thebase is assumed to be coincident with the conduction bandedge. Assuming that the barrier height 0B of the ohmiccontact is at the very most 0-7 V the injection ratio of holesto electrons is < 10~8 and modulation of the space chargewithin the collector region would only occur with a veryhigh current density and a long recombination lifetime.

In order to rule out the possibility that hole injectionfrom the base contact was modulating the collector barrierand giving apparent transistor action, an experiment wasperformed in which the response of the collector currentwas measured during a step in base current. The results ofsuch an experiment are shown in Fig. 4. Here it is seen thatthe collector current response to an 8 mA base current stepis < 40 ns and limited by the apparatus. During this periodonly 109 carriers cross the base contact and, in order tomodulate the collector barrier, the injection ratio y wouldneed to be > 10~3, a factor 10s higher than the worst-casecondition given above.

These considerations suggest that parasitic effectsinvolving minority carriers are not responsible for thetransistor action observed in these structures and one is ledto believe the hot-electron effects are indeed involved. Asignificant factor in support of this conclusion was a corre-lation between the gain of the transistor and the height ofthe emitter barrier. The metallisation giving the highestpotential barrier always gave the highest current gainirrespective of whether it was situated in the centre of thedevice or around the periphery, and no significant transistoraction was observed when making an 'ohmic' emittercontact using a metal with a lower barrier such as titanium.

5 Conclusions

A transistor has been proposed which uses hot-electronemission and collection in a single piece of semiconductingmaterial. Structures corresponding to this concept havebeen made using low-energy ion implantation into siliconand transistor action observed. It is argued that thesecharacteristics are due to hot-electron effects. Initial resultshave shown that overall collection efficiencies can be ashigh as 75% leading to 0 « 3 and the possibility of usefulgain at high frequencies.

6 Acknowledgments

It is a pleasure to acknowledge help with the fabrication ofthe silicon devices from A. Gill and B.F. Martin. Muchuseful discussion and help was also provided by J.A.G.Slatter, particularly in relation to the response measurement.

7 References

1 MEAD, C.A.: 'Tunnel emission amplifiers', Proc. IRE, 1960,48,p. 359

2 ATALLA, M.M., and SOSHEA, R.W.: 'Hot-carrier triodes withthin film bases', Solid-State Electron., 1963, 6, pp. 245-50

3 MOLL, J.L.: 'Comparison of hot electron and related amplifiers',IEEE Trans., 1963, ED-10, pp. 299-304

4 SZE, S.M., and GUMMEL, H.K.: 'Appraisal of semiconductor-metal-semiconductor transistor', Solid-State Electron., 1966, 9,pp. 751-769

5 CROWELL, C.R., and SZE, S.M.: 'Hot electron transport andelectron tunnelling in thin-film structures. Physics of thin films4' (Academic Press, 1967)

6 PADOVONI, F.A., and STRATTON, R.: 'Field and thermionic-field emission in Schottky barriers', Solid-State Electron., 1966, ^9, pp. 695-707

7 CROWELL, C.R., and RIDEOUT, V.L.: 'Normalised thermionic-field (t-f) emission in metal-semiconductor Schottky barriers',ibid., 1969,12, pp. 89-105

8 SHANNON, J.M.: 'Thermionic field emission through siliconschottky barriers at room temperature', ibid., 1977, 20,pp. 869-872

9 SHANNON, J.M.: 'A majority carrier camel diode', Appl. Phys.Lett., 1979, 35, p. 63

10 CROWELL, C.R., and SZE, S.M.: 'Quantum mechanicalreflection of electrons at metal-semiconductor barriers: electrontransport in semiconductor-metal-semiconductor structures', J.Appl. Phys., 1966, 37, pp. 2683-2689

11 SHANNON, J.M.: 'Reducing the effective height of a Schottkybarrier using low-energy ion implantation', Appl. Phys. Lett.,1974,24, pp. 369-371

12 SHANNON, J.M.: 'Increasing the effective height of a Schottkybarrier using low-energy ion implantation', 1974, 25, pp. 75-77

13 SHANNON, J.M.: 'Control of Schottky barrier height usinghighly doped surface layers', Solid-State Electron., 1976, 19,pp. 537-543

14 SHANNON, J.M.: 'Recoil-implanted antimony-doped surfacelayers in silicon', IoP Conference Series 28, 1976, chap. 1, p. 39

15 SHANNON, J.M., FORD, R.A., and GARD, G.A.: 'Annealingcharacteristics of highly doped ion implanted phosphorus layersin silicon', Radiat. Eff., 1970, 6, pp. 217-221

16 SHTYRKOV, E.I., KHAIBULLIN, I.B., ZARIPOV, M.M.,GALYATUDINOV, M.F., and BAYAZITOR, R.M.: 'Local laserannealing of implantation doped semiconductor layers', Fiz.Tech. Poluprovidn., 1975, 9, pp. 2000-2002

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