Philips tech. Rev. 36, 205-210,1976, No. 7 205

.The avalanche photodiode

L. J. M. Bollen, J. J. Goedbloed and E. T. J. M. Smeets

Introduetion

If semiconductor photodetectors now available areconsidered in terms of their usefulness in an opticalcommunication system, the most promising seems tobe the avalanche photodiode. In this article we shallconsider a number of device-appraisal criteria and seewhat consequences they have for the manufacture ofavalanche photodiodes.When' a voltage is applied across a semiconductor

diode in the reverse direction, an electric field ofstrength E is generated in a region around the PoNjunction, since the free charge carriers are drawn awayfrom the junction by the applied voltage. The fieldgradient dEfdx can be derived from the local dopingconcentration nd by using Poisson's equation:

dx 8

where e is the elementary charge and 8 the absolutedielectric constant of the semiconductor material. Fora given doping profile the strength of the field and thethickness of the depletion layer can be varied withincertain limits by varying the applied voltage; seefig. 1.

When a window transparent to the radiation to bedetected is made in one ofthe diode contacts, the radia-tion is able to penetrate into the diode material. Aradiation quantum (photon) that has an energy greaterthan the energy gap between valence band and conduc-tion band of the semiconductor material can then beabsorbed, resulting in the generation of an electron-hole pair. In the depletion layer the electric field sep-arates the electron-hole pair (electrons go to the Nregion, holes to the P region), and this has the resultthat a current, the photocurrent, starts to flow in thecircuit in which the diode is incorporated.The situation so far described is what is found in a

'normal' semiconductor photodiode. The avalanchephotodiode makes use of the fact that the free chargecarriers are accelerated in the electric field and therebygain kinetic energy. They may gain so much energy thatthey can ionize lattice atoms with which they collide,giving rise to new electron-hole pairs. In their turn theelectrons and holes of these pairs can be accelerated and

Ir L. J. M. Bollen, Dr J. J. Goedbloed and Ir E. T. J. M. Smeetsare with Philips Research Laboratories, Eindhoven.

cause ionization by collision, with the result that asingle incoming photon can give rise to an avalancheof electrons and holes. In this case the photocurrentin the external circuit is greater than in the ordinaryphotodiode described above. The avalanche photo-diode is thus a photodiode with internal amplification(multiplication), and may therefore be regarded as asolid-state analogue of the photomultiplier tube.

E

t

p

(I)

-xFig. I. The field-strength E in an N+-P-P+ diode as a function ofthe depth x below the diode surface for increasing values VI, V2and V3 of the reverse voltage across the diode.

The main factors that determine the usefulness ofa detector in an optical communication system are thefollowing:- The quantum efficiency, which indicates the pro-portion of incoming photons detected as a current(without multiplication).- The response time r, which determines the maxi-mum modulation frequency of the light for gooddetection.- The multiplication factor M of a detector withinternal amplification.- The noise characteristics of the detector.- Various practical requirements, such as a not un-duly high voltage and the avoidance of unwanted sideeffects.

Some numerical values of M and. for a number ofsemiconductor photodetectors are given in Table J.We shall now consider the consequences of the vari-

ous factors above for the case of an avalanche photo-diode, with silicon as the semiconductor material.

206 L. J. M. BoLLEN et al. Philips tech. Rev. 36, No. 7

is negligible. Finally, if we look at the drift-velocitycharacteristic of the charge carriers as a function of theelectric field-strength we see that at field-strengthsgreater than about 40 kV/cm in silicon there is nofurther increase in the drift velocity, which reaches asaturation value of about 107 cm/soThe requirements imposed on quantum efficiency

and response time, taken together, result in an evenstricter requirement: the diode must have a depletionregion about 25 (Lmlong, the part in which the highfield-strengths occur must be relatively small, and thefield-strength throughout the depletion region must be

Quantum efficiency greater than 40 kV/cm. The time required for crossingThe quantum efficiency is determined primarily by the depletion region is then 0.25 ns, which is short

the absorption coefficient of the detection radiation' -enough for optical communication purposes. Thereand by the length of the light path in the sensitive part must be no tailing effect due to diffusion of charge car-ofthe diode. The radiation intensity decreases exponen- riers from outside the depletion region.tially along this path. The radiation originating fromthe GaAs semiconductor laser has a wavelength of0.9 (Lm. Consequently the energy ofthe photons is onlyslightly greater than the band gap in silicon (1.1 eV)and the absorption is low. For radiation of 0.9 (Lmtheabsorption coefficient in silicon is about 400 cm-I,which means that the intensity has decreased to l/e ofthe initial value over a distance of 25 (Lm. The fractionof the incoming radiation that is absorbed in the deple-tion layer is detected in any case, and there is still somedetection in zones bordering on the depletion layer.Some of the electrons generated in the P region outsidethe depletion layer are able to reach the depletionregion by diffusion and are then detected, and the sameapplies to some of the holes from the N+ region.To achieve a good quantum efficiency it is therefore

desirable to have an active detection region of lengthabout 25 (Lmin the diode.

Table I. Values ofthe internal multiplication M and the responsetime. for a number of semiconductor radiation detectors.

Detector M •Photoconductor 105 10-3 sP-Ndiode 1 10-11P-I-N diode 1 10-10Phototransistor 102 10-8Avalanche photodiode 103 10-9Field-effect transistor 102 10-7

Speed of response

Important factors that determine the speed of re-sponse are:- The finite speed of the multiplication process (thecharge carriers must pass through a field of a certainstrength before they acquire sufficient energy to causeionization).- The time that charge carriers generated outside thedepletion layer require to reach the depletion regionby diffusion.-=-;- The ,time .a charge carrier requires for crossing thedepletion layer.'The first factor can be eliminated in practice by

making -the multiplication zone small: an~ the field-strengths in it, yery high. Next, since -the _diffusionalways takes place rel~tively,sloWly,we must make thedic~dé'in:'sucha y.ay that the contribution.made J)Y

cthe

diffusing c4a.rg;·c~rriers ('tailiu"g1)to the' photocurrentt,-·· .

Noise characteristics

The generation of electron-hole pairs by the absorp-tion of photons, like the subsequent internal ampli-fication by the multiplication of charge carriers, is astatistical process. It is therefore not surprising thatnoise plays an important role in the avalanche photo-diode. Nevertheless, when the signal-to-noise ratio isconsidered, this detector comes out wellwhen the otherrequirements are taken into account. From studies ofthe noise behaviour of the avalanche photodiode itfollows that the CI./P ratio, where Cl. is the ionizationcoefficient of the electrons and P that of the holes, is ofessential importance. In silicon it varies from about 10at a field-strength of about 350 kV/cm to 3 at 500 kV/cm(these are common field-strengths in the multiplicationregion). The fact that Cl. and /3 are quite different meansthat given an otherwise identical distribution of theelectrical field in the diode, a better signal-to-noiseratio is found when the charge carriers arriving in themultiplication region are mainly electrons than whenthe holes are in the majority [11. This has the result thata new requirement has to be added to those mentionedat the end of the previous section: the part of the deple-tion region that does not belong to the multiplicationregion should be situated on only one side of thatregion and be made of P-type material.The noise power in a diode of this kind is propor-

tional to M2(2 + kM), where M is the multiplicationby the avalanche process and k is a figure' of merit forthe diode that depends on the ratio CI.//3 [21, The pro-portionality only applies if k e; 1 and M» 1, condi-'tioris that are usually fulfilled in practice. In the total2M2 +.kM.3, half ofthe term 2M2 is due to multipliedshot noise during detection, and the other half to shotnoise' during unilateral multiplication (multiplicationinvolving charge carders of one kind only). The term

Philips tech. Rev. 36, No. 7 AVALANCHE PHOTODIODE 207

kM3 is due to non-unilateral multiplication. It ispossible to make diodes in which k is about 0.01; adiode of this type can have a gain of 200 before thenoise in the internal amplification exceeds the noise inthe detection process.

Practical requirements

In addition to the rather fundamental requirement-discussed above, there are various practical requiresments that an avalanche photodiode has to meet. Thus,the dark current, which is due to the electron-hole pairsresulting from the thermal generation in the diodematerial, must be low. So must the leakage currentacross the surface of the diode. This is because bothcurrents produce a background and consequently deter-mine the minimum intensity that a light signal musthave if it is to be detected.

Another requirement is that the multiplication overthe whole active surface of the diode must be as con-stant as possible. All parts of the diode then have thesame amplification and there are no places wherepremature local breakdown of the diode might occur.From a highly simplified model the relation betweenthe multiplication factor M and the doping concentra-tion nd is found to be

1 dM dl1d

M M R::! ---;;;.

Thus, with a gain of lOO a relative variation of 10% inthe gain means that the doping concentration must beconstant to within 0.1 % over the surface of a diode.

Another desirable feature for future applications ofthe avalanche photodiode is that the diode voltageshould be kept below 200 V. An N+-P diode with adepletion region 25 um thick cannot satisfy thisrequirement. If, however, the P region consists of arelatively thick, very lightly doped layer (the n layer)with a thin, heavily doped P layer on top of it, asindicated by curve a in fig. 2, the voltage requirementcan then be met as well as the other requirements men-tioned above. For the voltage requirement a corn-plementarystructure, aP+-N-v-N+ diode, could be used,but as we have already noted an N+-P-n-P+ diode is

Fig.3. Pictures obtained with aflying-spot scanning microscopeof the photocurrent distributionover the surface of threeN+-P-n-P+ avalanche photo-diodes. The material used wasdeposited epitaxially on a p+substrate. The N+ upper layerfor the left-hand diode was madeby diffusion, for the centre oneepitaxially and for the right-hand one by ion implantation.

preferable because of the better noise characteristics ofa diode in which most of the multiplication is due tothe electrons.

To make avalanche photodiodes that meet the spec-ifications listed above we started from a substrate ofdislocation-free P+ material. A n layer and then a Player were first epitaxially deposited on the substrate,and the upper part of the P layer was then convertedinto N+ material by ion implantation. In the develop-

E

t

p I~

-xFig. 2. Curve G, the field-strength E in an N+-P-n-P+ diode as afunction of the depth x below the diode surface. Here the totalvoltage across the diode (area under the curve) is lower than inan N+-P-P+ diode (curve b), although in both cases the maximumfield-strength and the thickness of the depletion layer are aboutthe same.

ment of this method of fabrication a fast flying-spotscanning microscope was found to be of great assist-ance [31. Fig. 3 shows a number of photomicrographs,made with this microscope, of mesa diodes fabricatedIII vanous ways.

We studied two forms of avalanche photodiodes, themesa diode and the planar diode. Both types, whicheach have their own advantages and disadvantages,will now be briefly discussed.

[1) P. P. Webb, R. J. McIntyre and J. Conradi, RCA Rev. 35,234, 1974.K. Mouthaan and R. M. Snoeren, Electronics Letters 10,118, 1974.

[2) R. J. Mcl ntyre, IEEE Trans. ED-13, 164, 1966.[3) P. M. Boers and L. J. M. Bollen, Philips tech. Rev. 35, 23,

1975.

208 L. J. M. BOLLEN el al. Philips tech. Rev. 36, No. 7

The mesa diode

Fig. 4 shows the cross-section of a mesa diode. Thisdevice was made by etching slightly cone-shaped discsfrom a silicon wafer with the desired doping profile.The substrate used for making the epi taxi al layers waspreviously etched away down to a thickness of about5 (.Lmand metal contacts were applied to the front andthe back. The mesa is given its conical shape by etchingwith an etchant that attacks the silicon but not thecontacts. The effect of this shape is that when the volt-age is applied to the diode the depletion layer willextend further at the circumference of the diode thanin the middle.

This can be understood as follows. It follows from (1) that inthe central part of the diode the distances over which the deple-tion layer extends on either side of the P-N junction must beinversely proportional to the doping concentrations in the PandN regions. Toward the P-N junction the field-strength on bothsides must increase to the maximum value. If a and b are thedistances over which the depletion layer extends on both sides,then aN! = bN2 (fig. 5). Since along the edge there is no materialat one side of the P-N junction to provide for this charge neutral-ity, because of the conical shape, the absence of material theremust be compensated by a greater extension of the depletion layeron that side.

Fig.4. Cross-section ofan N+-P-n-P+ mesa diode. The dashed lineindicates the boundary ofthe depletion region in the n layer. Thecurvature of this boundary at the edge of the diode (greatly exag-gerated here) is due to the conical shape of the mesa. M metal-lization layers.

p

E

t

a-x

Fig. 5. Field distribution in an N+-P diode. For the field gradientson eitherside ofthejunction the relations: tan Cl: = dE!/dx = Nieleand tan f3 = dE2/dx = Nse]e must apply. This means thataN! = bN2 for the distances a and b over which the depletionlayer extends on both sides of the junction.

Because of the greater length thus available alongthe edge of the diode for accommodating the sametotal voltage, the field-strength along the edge will notbe so high as in the centre of the diode. This reducesthe risk of breakdown along the edge. It is necessary,however, to protect the edges from the effects of waterand ions in the atmosphere. The TC layer in particular,which is very weakly doped, can easily acquire anN-type skin by adsorption of impurities, giving rise toconducting channels. The edge must therefore be pro-tected by a 'passivating' layer.

The passivation of a mesa diode does not alwaysoffer sufficient long-term proteetion from the forma-tion of N-type channels over the surface of the TC region,so that in time the leakage currents may increase. Inthis respect the use of planar techniques offers betterprospects.

Planar diodes

The semiplanar diode

The same material from which the mesa diodes aremade, i.e. epitaxjal TC and P layers on a dislocation-freeP+ substrate, can also be used for etching out a planardiode structure (fig. 6). The N+ layer is again made byion implantation. The critical region ofthe diode, wheresurface breakdowns might occur, is now located at thefront, where proteetion can be applied more easily andmore reproducibly than on the mesa diode.

To prevent the extension of conducting channels,which here again may grow across the surface from theN+ region, a ring of P-type material around the actualdiode is left untouched by the etching process.

At the edge of the thin N+ region the field-strengthcan become so high that premature breakdown mayoccur here (fig. 7). To avoid such high field-strengthsa film of slightly conducting material can be applied.This prevents breakdowns at the edge, but the leakagecurrents increase to an impermissibly high level. Anacceptable solution for this problem is difficult to find.

The planar diode

If we start from material consisting of an epitaxial TC

layer on a P+ substrate, a diode of the cross-sectionillustrated in fig. 8 can be made by purely planar tech-niques. The N+ layer is surrounded here by a relativelythick guard ring with the same type of doping, so thatthe outer edge of the N+ region is rounded and highlocal field-strengths are avoided. In addition the TC

surface is covered with a silicon-dioxide layer to reduceleakage currents. Here again a ring of P material isapplied to stop conducting channels. In this construc-tion both the N+ layer and P layer are made byimplantation in the final stages of the process. This

Philips tech. Rev. 36, No. 7 AVALANCHE PHOTODIODE 209

allows high temperatures to be used in the earlier stagesof the process, in forming the oxide layer and in diffu-sing the N+ protecting layer, without the risk of dama-ging any existingthin N+ andP layers. Both the localiza-tion and the concentration of the doping can be ac-curately and reproducibly controlled by using animplantation process.

The thickness of the P layer is limited by the avail-able implantation energy to 1.5 [Lm. However, toachieve the desired multiplication it is necessary tohave a sufficiently wide region where the field-strengthIS high enough for avalanche amplification. A conse-

N+

Prrp+

Fig. 6. Cross-section of a semiplanar N+-P-n-P+ diode. The sur-face ofthe it region is given a protecting layer ofslightly conduct-ing material to avoid high local field-strengths. On the actualdiode, i.e. on the N+ material, a special coating is deposited tominimize reflection of the radiation to be detected. M metal-ization. C conducting layer. R anti-reflection layer.

Fig.7. Effect of a conducting layer on the behaviour of a semi-planar diode. Above: a diode without conducting layer: break-down occurs along the edge. Below: a diode with conductingglass layer: no breakdown occurs along the edge. The con-duction through the glass increases the dark current of the diode.

Prrp'

Fig. 8. Cross-section through a planar N+-P-n-P+ diode. The edgeof the N+ region is rounded off by an additional N+ diffusion toreduce the field-strength at the edge and thus prevent prematurebreakdown at this location. M metallization layers. Si02 silicon-dioxide layer.

quence of this is that the field gradients are steep andthe maximum field-strength high (fig. 9), and this hasan adverse effect on the noise characteristics of thediode. These drawbacks can be overcome by applyinga doping profile like that oî fig. 10 during the implanta-tion of the P region.

In planar diodes the depletion layer in the n regionalong the circumference of the diode is curved awayfrom the P+ substrate; this differs from the situation inthe mesa diode, for which we saw that the depletionlayer curves toward the substrate. It is therefore pos-sible to choose the parameters of the n layer such that

-xFig. 9. Effect of the limited thickness of N+ and P regions in thefabrication of these regions by ion implantation. If the field-strength in the depletion layer is to be raised above the thresholdvalue Ea for avalanche amplification over a sufficiently largedistance, the field gradients must be relatively high and conse-quently the maximum field-strength is high.

(III •lPIII

-x

E

t

-xFig. JO. The net doping concentration nd and the field-strength Eas functions of the depth x below the surface of a diode with a'low-high-low' structure. In this structure an excessively highfield-strength is avoided. The successive regions are not drawnto scale in the x-direction.

210 AVALANCHE PHOTODIODE Philips tech. Rev. 36, No. 7

Fig. 12. Left: pulse response of an N+-P-n-P+ diode in which the depletion region is so narrowthat the n region is field-free. Right: pulse response of the same diode in which the voltage isso high that the whole n region is depleted.

In the first case the charge carriers generated in the n. layer have to reach the depletion regionby diffusion. This process is relatively slowand a 'tail' appears on the current pulse producedupon detection of a sharp radiation pulse. In the second case the field-strength in the n regionis so high that the charge carriers acquire the saturated drift velocity almost immediately, andthere is therefore virtually no tailing. Charge carriers generated in the p+ layer have such ashort life that they have very little effect on the pulse response. The oscillations in the tail aredue to the instrurnents.

at the operating voltage the depletion layer completelyreaches the p+ substrate (fig. 11). This means that thefield-strength throughout the n region is so high thatthe charge carriers there acquire the saturated driftvelocity, while tailing from the substrate is negligible- which helps to keep the response speed of the diodehigh. Fig. 12 shows how the pulse response of anexperimental planar N+-P-n-P+ diode can be affectedby the tailing effect. In one case the voltage is made solow that the depletion region hardly goes beyond theP region; there is then considerable tailing. In the othercase the voltage is so high that the whole it. region isdepleted; there is then very little tailing and the pulseresponse is greatly improved.

Summary. An avalanche photodiode is a photodiode with internalamplification. Charge carriers generated in the diode by photonsare accelerated in the internal field of the diode so that on col-lision with the lattice atoms they generate more charge carriers.This process repeats itself, giving rise to an avalanche of chargecarriers for each photon absorbed; the diode is a solid-stateanalogue of the photomultiplier tube. In a diode built up froman N+, a P, a n and a p+ layer the charge carriers taking part in

E

t

p p+

-xFig. 11. A relatively slight increase in the voltage across anN+-P-n-P+ diode gives rise to such a high field-strength in thewhole region that the charge carriers generated there acquire thesaturated drift velocity almost immediately.

the multiplication process are predominantly electrons, whichresults in good noise characteristics. Another advantage of thisstructure is that even with the thick depletion layer necessary toachieve sufficient absorption of the radiation to be detected, thediode voltage remains within reasonable limits. Three versionsof the diode are discussed: a mesa diode, a semiplanar diode anda planar diode. The planar diode, made partly by ion implanta-tion in epitaxial material, offers the best prospects.