The avalanche noise characteristics of Al0.8Ga0.2 As have been measured in a range of p-i-n and n-i-p diodes with i-region widths ω varying from 1.02 to 0.02 μm. While thick bulk diodes exhibit low excess noise from electron initiated multiplication, owing to the large α/β ratio (1/k), the excess noise of diodes with ω < 0.31 μm were found to be greatly reduced by the effects of dead space. The thinnest diodes exhibit very low excess noise, corresponding to k = 0.08, up to a multiplication value of 90. In contrast to most III-V materials, it was found that both thick and thin Al0.8Ga0.2As multiplication layers can give very low excess noise and that electrons must initiate multiplication to minimize excess noise, even in thin structure

David, J.P.R.

Hill, G.

Hopkinson, M.

Ng, B.K.

Rees, G.J.

Tozer, R.C.

English

White Rose Research Online

This is a repository copy of Excess noise characteristics of Al0.8Ga0.2As avalanche photodiodes .White Rose Research Online URL for this paper:http://eprints.whiterose.ac.uk/904/Article:Ng, B.K., David, J.P.R., Tozer, R.C. et al. (3 more authors) (2002) Excess noise characteristics of Al0.8Ga0.2As avalanche photodiodes. IEEE Photonics Technology Letters, 14 (4). pp. 522-524. ISSN 1041-1135 https://doi.org/10.1109/68.992598eprints@whiterose.ac.ukhttps://eprints.whiterose.ac.uk/Reuse Unless indicated otherwise, fulltext items are protected by copyright with all rights reserved. The copyright exception in section 29 of the Copyright, Designs and Patents Act 1988 allows the making of a single copy solely for the purpose of non-commercial research or private study within the limits of fair dealing. The publisher or other rights-holder may allow further reproduction and re-use of this version - refer to the White Rose Research Online record for this item. Where records identify the publisher as the copyright holder, users can verify any specific terms of use on the publisher’s website. Takedown If you consider content in White Rose Research Online to be in breach of UK law, please notify us by emailing eprints@whiterose.ac.uk including the URL of the record and the reason for the withdrawal request. 522 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 14, NO. 4, APRIL 2002Excess Noise Characteristics of Al0:8Ga0:2AsAvalanche PhotodiodesB. K. Ng, J. P. R. David, Senior Member, IEEE, R. C. Tozer, Senior Member, IEEE, M. Hopkinson, G. Hill, andG. J. ReesAbstract—The avalanche noise characteristics of Al0 8Ga0 2Ashave been measured in a range of p-i-n and n-i-p diodes with i-re-gion widths varying from 1.02 to 0.02 m. While thick bulkdiodes exhibit low excess noise from electron initiated multiplica-tion, owing to the large ratio (1 ), the excess noise of diodeswith 0 31 m were found to be greatly reduced by the effectsof dead space. The thinnest diodes exhibit very low excess noise,corresponding to = 0 08, up to a multiplication value of 90. Incontrast to most III–V materials, it was found that both thick andthin Al 0 8Ga0 2As multiplication layers can give very low excessnoise and that electrons must initiate multiplication to minimizeexcess noise, even in thin structures.Index Terms—Al0 8Ga0 2As, APD, avalanche excess noise,avalanche multiplication, impact ionization, indirect band gap.I. INTRODUCTIONT HE ADVANTAGE of avalanche photodiodes (APDs) liesin their internal current gain mechanism which can im-prove the signal-to-noise ratio of photodetection systems [1].However, this advantage also carries the penalty of excess noisearising from the stochastic nature of the avalanche process. Theconventional local noise model [2] relates the avalanche noiseof a material to its multiplication factor , and the ratio of itshole to electron ionization coefficients . This model in-dicates that low excess noise can only be achieved in materialswith very disparate ionization coefficients and that the carriertype with the larger ionization coefficient should be used to ini-tiate multiplication. This is, however, only true for thick bulkstructures and recent work [3]–[7] has shown that thin devicescan also exhibit low excess noise, regardless of the initiatingcarrier type, owing to nonlocal effects.APDs for use at the telecommunication wavelengths of 1.3and 1.55 m are InP-based, where InP and InGaAs are usedas the multiplication and absorption media, respectively. GaAs-based APDs operating at these wavelengths are also now pos-sible by using GaInAsN [8], which can absorb telecommunica-tions wavelength light and is lattice-matched to GaAs. However,GaAs-based APDs operating at such long wavelengths will alsonecessarily require a separate avalanche region comprising thewider band gap GaAs or AlGa As to reduce tunneling cur-rents. We have recently shown that bulk AlGa As [9] hasManuscript received October 10, 2001; revised December 13, 2001. Thiswork was supported by EPSRC (U.K.). B. K. Ng was supported by a schol-arship from Nanyang Technological University, Singapore.The authors are with the Department of Electronic and ElectricalEngineering, University of Sheffield, S1 3JD Sheffield, U.K. (e-mail:elp99bkn@sheffield.ac.uk; g.rees@sheffield.ac.uk).Publisher Item Identifier S 1041-1135(02)01661-0.a large ratio and exhibits low excess noise. Measurementscarried out by Zhenget al. [10] also showed that the excess noisefrom electron initiated multiplication in Al Ga As diodeswith i-region thicknesses of 0.65 and 0.15m is lower thanthat from devices of similar thickness, but with aluminum com-positions . These results suggest that AlGa Asmay constitute a suitable avalanche material. However, APD de-signs using Al Ga As as the avalanche medium will requirea better knowledge of the excess noise behavior of this material.In this letter, we report measurements of the excess avalanchenoise of a series of Al Ga As homojunction p-i-n/n-i-pdiodes with i-region thicknesses ranging from 1.02 to 0.02m.Excess noise characteristics under both electron and hole injec-tion conditions from the same diode have also been measuredusing a heterojunction p-i-n structure. The excess avalanchenoise behavior of Al Ga As as a function of the carrierinjection type and avalanche width are presented and discussed.II. NOISE MEASUREMENTThe diodes in this work have been reported in [11].The –region thicknesses of the homojunction p-i-n andn-i-p diodes are m andm, respectively, while the heterojunctionp-i-n structure has m. The avalanche multiplicationand excess noise were measured using the method describedpreviously by Li et al. [4]. Photogenerated carriers wereinjected into the high field region of the diodes by focusingmodulated light from a laser to a small spot onto the opticalaccess windows. The noise measurement system utilizes twolock-in amplifiers to measure independently the photocurrentand the avalanche noise power of a diode. This phase-sensitivedetection technique allows us to distinguish unambiguously thephotocurrent and multiplication noise power from the compo-nents not arising from the modulated injection. The avalancheexcess noise of the diodes was measured at a center frequencyof 10 MHz and a noise effective bandwidth of 4.2 MHz.Electrons (holes) are injected into the high field region of thehomojunction p-i-n (n-i-p) diodes by illuminating the top p(n ) cladding with 442-nm wavelength light, which is stronglyabsorbed [12] in the 1-m-thick top cladding. In the case of theheterojunction p-i-n structure, both pure electron and hole initi-ated multiplication are measured on the same diode by illumi-nating it from the top with 442- and 633-nm light, respectively.Measurements were also performed using 542-nm wavelengthlight [12] to investigate the multiplication characteristics undermixed carrier injection conditions.1041-1135/02$17.00 © 2002 IEEENG et al.: EXCESS NOISE CHARACTERISTICS OF Al Ga As AVALANCHE PHOTODIODES 523Fig. 1. Excess noise factor(F ) versus multiplication factor(M) measuredin Al Ga As p-i-n diodes with measuredw of 1.02m (; ), 0.82m( ; ), 0.31m ( ; ), 0.10m (4; ), 0.03m ( ; ) and 0.02m(hexagons). Open and closed symbols are results obtained from pure electronand mixed carrier injection with 442- and 542-nm wavelength light respectively.Dashed lines are predictions from McIntyre’s local model fork = 0 to 1 in stepsof 0.1 andk = 2.The excess noise factorwas determined from the measurednoise power using , where is the equivalentphotocurrent of a reference Si p-i-n diode that gives the samenoise power as the diode measured andis the unmultipliedprimary photocurrent [4]. The value of used was varied be-tween 5 nA–1 A to ensure that the circuit was not saturatedduring measurement. Several diodes across each layer were alsomeasured and the results were found to be consistent and repro-ducible.III. RESULTSFigs. 1 and 2 show the measuredversus of the p-i-nand n-i-p diodes under different injection conditions. The ex-cess noise predictions from McIntyre’s local model [2] are alsoplotted in these figures for comparison. These local noise pre-dictions are given by(1)where for electron initiated multiplication andfor hole initiated multiplication. Hereafter, we use this(which may not correspond to the actual ratio) as an in-dicator to represent approximately the measured excess noisebehavior of a diode with in order to enable a direct compar-ison between the excess noise of different diodes.In Fig. 1, the excess noise for electron initiated multiplicationin the p-i-n diodes increases from towhen is decreased from 1.02 to 0.31m. On the other hand,decreasing further from 0.31 to 0.02 m causes the to de-crease from 0.38 to a very low value of 0.08. The excess noisein the thinner structures has been measured up to a multiplica-Fig. 2. Excess noise factor(F ) versus multiplication factor(M) measuredin Al Ga As n-i-p diodes with nominalw of 1.01m (; ) and 0.09m(4; ). Open and closed symbols of the homojunction structures are resultsobtained from pure hole and mixed carrier injection with 442- and 542-nmwavelength light respectively. Excess noise from pure hole injection in theheterojunction structure withw = 0:82 m ( ) was obtained with 633-nmlight. Dashed lines are predictions from McIntyre’s local model for1=k = 0to 1 in steps of 0.1,1=k = 2 and1=k = 3.tion of 90, as depicted in Fig. 1. In addition, the excess noise inthe thinnest diode is, to the best of our knowledge, the lowestreported to date for III–V materials. The high multiplicationvalues also show that the dark currents in the thin structures arevery low, even when they are biased near . Mixed carrierinjection using 542-nm wavelength light on the p-i-n diodes re-sulted in a larger excess noise for all thicknesses. In the thinnestdiode with m, the excess noise increased by50%,with increasing from 0.08 to 0.13, when 542-nm light isused.For hole initiated multiplication, the excess noise is reducedfrom to when decreases from 1.01to 0.82 m, as shown in Fig. 2. The excess noise was furtherreduced for the - m n-i-p diode, and is found to liesignificantly below the curve. Similar to the thinnerp-i-n diodes, the excess noise in the thin n-i-p diode was alsomeasured up to a multiplication of 90. When 542-nm light isused on the n-i-p structures, the excess noise decreases tofor the m diodes and for them diodes.IV. D ISCUSSIONMultiplication measurements indicate that the ratio ofAl Ga As converges to 1 as the electric field increases [9],[11]. The excess noise characteristics of diodes withm are in qualitative agreement with the local noise model andreflect the converging ratio in Al Ga As. In contrast,the excess noise in thinner diodes ( m) reduces withdecreasing , regardless of the initiating carrier type. Further-more, the measured excess noise corresponding to ob-t ined from hole injection (the carrier with the lower ionization524 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 14, NO. 4, APRIL 2002Fig. 3. k versus measuredw under pure electron( ) and pure hole(4)injection conditions for the range of diodes investigated. Thek valuesunder mixed carrier injection for the homojunction p-i-n diodes( ), thehomojunction n-i-p diodes( ) and the heterojunction p-i-n diode() laybetween those from pure carrier injection. The data obtained by Zhenget al.(X) [10] are also included for comparison. The dashed lines are drawn to aidvisualization.coefficient in Al Ga As) in the thin n-i-p structure cannotbe explained in terms of the local noise model. This nonlocalbehavior is similar to that observed in other materials [3]–[7],[10] and is attributed to the effect of dead space, the minimumdistance a carrier must travel before it gains sufficient energy toimpact ionize, becoming dominant.A summary of the excess noise behavior of AlGa As isshown in Fig. 3, where the values under different carrier in-jection conditions are plotted against. The values undermixed carrier injection were found to lie between those for purecarrier injection. As shown in Fig. 3, the results obtained byZhenget al. [10] for Al Ga As broadly corroborate our mea-surements. The slight discrepancy may be due to uncertaintiesin or doping values. Fig. 3 suggests that, under electron initi-ated multiplication, an avalanche width of0.31 m gives thehighest excess noise and should therefore be avoided.The change in excess noise for all diodes when the initiatingcarrier type becomes mixed is consistent with and alsowith multiplication measurements [9], [12]. Unlike GaAs, theexcess noise of thin Al Ga As diodes with mchanges significantly when injection becomes mixed. It is in-teresting to note that thin Si diodes [6] also exhibit similar char-acteristics. This indicates that it is still necessary for electrons toinitiate multiplication in thin Al Ga As diodes to minimizeexcess noise.Our results suggest that very thin AlGa As multiplica-tion layers can be used to achieve very high speed APDs [13]without limitation from tunneling current because of the largebandgap. In addition, using a very thin AlGa As multipli-cation layer also has the advantages of a very low avalancheexcess noise and low voltage operation.V. CONCLUSIONThe avalanche excess noise characteristics of AlGa Ashave been determined in a series of p-i-n/n-i-p diodes over awide range of avalanche widths, varying from 1.02m down to0.02 m. The thicker structures ( m) exhibit excessnoise characteristics in qualitative agreement with McIntyre’slocal model. As the avalanche width reduces below 0.31mthe excess noise is found to decrease with decreasing, regard-less of initiating carrier type. This reduction in excess noise isattributed to the dead space effect which causes the avalancheprocess to become more deterministic in thin structures. Sim-ilar to Si, the multiplication process in thin Al Ga As APDsmust be initiated by electrons to minimize the avalanche excessnoise. Our results suggest that very thin AlGa As avalanchewidths can be use to achieve APDs with very high speed andvery low excess noise, without the penalty of a large tunnelingcurrent.REFERENCES[1] G. E. Stillman and C. M. Wolfe, “Avalanche photodiodes,” inSemicon-ductor and Semimetals, R. K. Willardson and A. C. Beer, Eds. NewYork: Academic, 1977, vol. 12, pp. 291–393.[2] R. J. McIntyre, “Multiplication noise in uniform avalanche diodes,”IEEE Trans. Electron. Devices, vol. ED-13, pp. 164–168, 1964.[3] C. Hu, K. A. Anselm, B. G. Streetman, and J. C. Campbell, “Noise char-acteristics of thin multiplication region GaAs avalanche photodiodes,”Appl. Phys. Lett., vol. 69, pp. 3734–3736, Dec. 1996.[4] K. F. Li, D. S. Ong, J. P. R. David, G. J. Rees, R. C. Tozer, P. N. Robson,and R. Grey, “Avalanche multiplication noise characteristics in thinGaAs p -i-n diodes,” IEEE Trans. Electron. Devices, vol. 45, pp.2102–2107, Oct. 1998.[5] K. F. Li, S. A. Plimmer, J. P. R. David, R. C. Tozer, G. J. Rees, P. N.Robson, C. C. Button, and J. C. Clark, “Low avalanche noise character-istics in thin InP p -i-n diodes with electron initiated multiplication,”IEEE Photon. Technol. Lett., vol. 11, pp. 364–366, Mar. 1999.[6] C. H. Tan, J. C. Clark, J. P. R. David, G. J. Rees, S. A. Plimmer, R.C. Tozer, D. C. Herbert, D. J. Robbins, W. Y. Leong, and J. Newey,“Avalanche noise measurement in thin Si p-i-n diodes,”Appl. Phys.Lett., vol. 76, pp. 3926–3928, Jun. 2000.[7] C. H. Tan, J. P. R. David, S. A. Plimmer, G. J. Rees, R. C. Tozer, andR. Grey, “Low multiplication noise thin Al Ga As avalanche pho-todiodes,”IEEE Trans. Electron. Devices, vol. 48, pp. 1310–1317, July2001.[8] M. Kondow, K. Uomi, A. Niwa, T. Kitatani, S. Watahiki, and Y. Yazawa,“GaInNAs: A novel material for long-wavelength-range laser diodeswith excellent high-temperature performance,”Jpn. J. Appl. Phys., vol.35, pp. 1273–1275, 1996.[9] B. K. Ng, J. P. R. David, S. A. Plimmer, M. Hopkinson, R. C. Tozer,and G. J. Rees, “Impact ionization coefficients of AlGa As,” Appl.Phys. Lett., vol. 77, pp. 4374–4376, Dec. 2000.[10] X. G. Zheng, X. Sun, S. Wang, P. Yuan, G. S. Kinsey, A. L. Holmes,Jr., B. G. Streetman, and J. C. Campbell, “Multiplication noise ofAl Ga As avalanche photodiodes with high Al concentration andthin multiplication region,”Appl. Phys. Lett., vol. 78, pp. 3833–3835,June 2001.[11] B. K. Ng, J. P. R. David, S. A. Plimmer, G. J. Rees, R. C. Tozer, M.Hopkinson, and G. Hill, “Avalanche multiplication characteristics ofAl Ga As diodes,” IEEE Trans. Electron. Devices, vol. 48, pp.2198–2204, Oct. 2001.[12] D. E. Aspnes, S. M. Kelso, R. A. Logan, and R. Bhat, “Optical propertiesof Al Ga As,” J. Appl. Phys., vol. 60, pp. 754–767, July 1986.[13] B. F. Levine, “Optimization of 10–20 GHz avalanche photodiodes,”IEEE Photon. Technol. Lett., vol. 8, pp. 1528–1530, Nov. 1996.

Excess noise characteristics of Al0.8Ga0.2As avalanche photodiodes

This is a repository copy of Excess noise characteristics of Al0.8Ga0.2As avalanche 
photodiodes .
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http://eprints.whiterose.ac.uk/904/
Article:
Ng, B.K., David, J.P.R., Tozer, R.C. et al. (3 more authors) (2002) Excess noise 
characteristics of Al0.8Ga0.2As avalanche photodiodes. IEEE Photonics Technology 
Letters, 14 (4). pp. 522-524. ISSN 1041-1135 
https://doi.org/10.1109/68.992598
eprints@whiterose.ac.uk
https://eprints.whiterose.ac.uk/
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522 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 14, NO. 4, APRIL 2002
Excess Noise Characteristics of Al0:8Ga0:2As
Avalanche Photodiodes
B. K. Ng, J. P. R. David, Senior Member, IEEE, R. C. Tozer, Senior Member, IEEE, M. Hopkinson, G. Hill, and
G. J. Rees
Abstract—The avalanche noise characteristics of Al0 8Ga0 2As
have been measured in a range of p-i-n and n-i-p diodes with i-re-
gion widths varying from 1.02 to 0.02 m. While thick bulk
diodes exhibit low excess noise from electron initiated multiplica-
tion, owing to the large ratio (1 ), the excess noise of diodes
with 0 31 m were found to be greatly reduced by the effects
of dead space. The thinnest diodes exhibit very low excess noise,
corresponding to = 0 08, up to a multiplication value of 90. In
contrast to most III–V materials, it was found that both thick and
thin Al 0 8Ga0 2As multiplication layers can give very low excess
noise and that electrons must initiate multiplication to minimize
excess noise, even in thin structures.
Index Terms—Al0 8Ga0 2As, APD, avalanche excess noise,
avalanche multiplication, impact ionization, indirect band gap.
I. INTRODUCTION
T HE ADVANTAGE of avalanche photodiodes (APDs) liesin their internal current gain mechanism which can im-
prove the signal-to-noise ratio of photodetection systems [1].
However, this advantage also carries the penalty of excess noise
arising from the stochastic nature of the avalanche process. The
conventional local noise model [2] relates the avalanche noise
of a material to its multiplication factor , and the ratio of its
hole to electron ionization coefficients . This model in-
dicates that low excess noise can only be achieved in materials
with very disparate ionization coefficients and that the carrier
type with the larger ionization coefficient should be used to ini-
tiate multiplication. This is, however, only true for thick bulk
structures and recent work [3]–[7] has shown that thin devices
can also exhibit low excess noise, regardless of the initiating
carrier type, owing to nonlocal effects.
APDs for use at the telecommunication wavelengths of 1.3
and 1.55 m are InP-based, where InP and InGaAs are used
as the multiplication and absorption media, respectively. GaAs-
based APDs operating at these wavelengths are also now pos-
sible by using GaInAsN [8], which can absorb telecommunica-
tions wavelength light and is lattice-matched to GaAs. However,
GaAs-based APDs operating at such long wavelengths will also
necessarily require a separate avalanche region comprising the
wider band gap GaAs or AlGa As to reduce tunneling cur-
rents. We have recently shown that bulk AlGa As [9] has
Manuscript received October 10, 2001; revised December 13, 2001. This
work was supported by EPSRC (U.K.). B. K. Ng was supported by a schol-
arship from Nanyang Technological University, Singapore.
The authors are with the Department of Electronic and Electrical
Engineering, University of Sheffield, S1 3JD Sheffield, U.K. (e-mail:
elp99bkn@sheffield.ac.uk; g.rees@sheffield.ac.uk).
Publisher Item Identifier S 1041-1135(02)01661-0.
a large ratio and exhibits low excess noise. Measurements
carried out by Zhenget al. [10] also showed that the excess noise
from electron initiated multiplication in Al Ga As diodes
with i-region thicknesses of 0.65 and 0.15m is lower than
that from devices of similar thickness, but with aluminum com-
positions . These results suggest that AlGa As
may constitute a suitable avalanche material. However, APD de-
signs using Al Ga As as the avalanche medium will require
a better knowledge of the excess noise behavior of this material.
In this letter, we report measurements of the excess avalanche
noise of a series of Al Ga As homojunction p-i-n/n-i-p
diodes with i-region thicknesses ranging from 1.02 to 0.02m.
Excess noise characteristics under both electron and hole injec-
tion conditions from the same diode have also been measured
using a heterojunction p-i-n structure. The excess avalanche
noise behavior of Al Ga As as a function of the carrier
injection type and avalanche width are presented and discussed.
II. NOISE MEASUREMENT
The diodes in this work have been reported in [11].
The –region thicknesses of the homojunction p-i-n and
n-i-p diodes are m and
m, respectively, while the heterojunction
p-i-n structure has m. The avalanche multiplication
and excess noise were measured using the method described
previously by Li et al. [4]. Photogenerated carriers were
injected into the high field region of the diodes by focusing
modulated light from a laser to a small spot onto the optical
access windows. The noise measurement system utilizes two
lock-in amplifiers to measure independently the photocurrent
and the avalanche noise power of a diode. This phase-sensitive
detection technique allows us to distinguish unambiguously the
photocurrent and multiplication noise power from the compo-
nents not arising from the modulated injection. The avalanche
excess noise of the diodes was measured at a center frequency
of 10 MHz and a noise effective bandwidth of 4.2 MHz.
Electrons (holes) are injected into the high field region of the
homojunction p-i-n (n-i-p) diodes by illuminating the top p
(n ) cladding with 442-nm wavelength light, which is strongly
absorbed [12] in the 1-m-thick top cladding. In the case of the
heterojunction p-i-n structure, both pure electron and hole initi-
ated multiplication are measured on the same diode by illumi-
nating it from the top with 442- and 633-nm light, respectively.
Measurements were also performed using 542-nm wavelength
light [12] to investigate the multiplication characteristics under
mixed carrier injection conditions.
1041-1135/02$17.00 © 2002 IEEE
NG et al.: EXCESS NOISE CHARACTERISTICS OF Al Ga As AVALANCHE PHOTODIODES 523
Fig. 1. Excess noise factor(F ) versus multiplication factor(M) measured
in Al Ga As p-i-n diodes with measuredw of 1.02m (; ), 0.82m
( ; ), 0.31m ( ; ), 0.10m (4; ), 0.03m ( ; ) and 0.02m
(hexagons). Open and closed symbols are results obtained from pure electron
and mixed carrier injection with 442- and 542-nm wavelength light respectively.
Dashed lines are predictions from McIntyre’s local model fork = 0 to 1 in steps
of 0.1 andk = 2.
The excess noise factorwas determined from the measured
noise power using , where is the equivalent
photocurrent of a reference Si p-i-n diode that gives the same
noise power as the diode measured andis the unmultiplied
primary photocurrent [4]. The value of used was varied be-
tween 5 nA–1 A to ensure that the circuit was not saturated
during measurement. Several diodes across each layer were also
measured and the results were found to be consistent and repro-
ducible.
III. RESULTS
Figs. 1 and 2 show the measuredversus of the p-i-n
and n-i-p diodes under different injection conditions. The ex-
cess noise predictions from McIntyre’s local model [2] are also
plotted in these figures for comparison. These local noise pre-
dictions are given by
(1)
where for electron initiated multiplication and
for hole initiated multiplication. Hereafter, we use this
(which may not correspond to the actual ratio) as an in-
dicator to represent approximately the measured excess noise
behavior of a diode with in order to enable a direct compar-
ison between the excess noise of different diodes.
In Fig. 1, the excess noise for electron initiated multiplication
in the p-i-n diodes increases from to
when is decreased from 1.02 to 0.31m. On the other hand,
decreasing further from 0.31 to 0.02 m causes the to de-
crease from 0.38 to a very low value of 0.08. The excess noise
in the thinner structures has been measured up to a multiplica-
Fig. 2. Excess noise factor(F ) versus multiplication factor(M) measured
in Al Ga As n-i-p diodes with nominalw of 1.01m (; ) and 0.09m
(4; ). Open and closed symbols of the homojunction structures are results
obtained from pure hole and mixed carrier injection with 442- and 542-nm
wavelength light respectively. Excess noise from pure hole injection in the
heterojunction structure withw = 0:82 m ( ) was obtained with 633-nm
light. Dashed lines are predictions from McIntyre’s local model for1=k = 0
to 1 in steps of 0.1,1=k = 2 and1=k = 3.
tion of 90, as depicted in Fig. 1. In addition, the excess noise in
the thinnest diode is, to the best of our knowledge, the lowest
reported to date for III–V materials. The high multiplication
values also show that the dark currents in the thin structures are
very low, even when they are biased near . Mixed carrier
injection using 542-nm wavelength light on the p-i-n diodes re-
sulted in a larger excess noise for all thicknesses. In the thinnest
diode with m, the excess noise increased by50%,
with increasing from 0.08 to 0.13, when 542-nm light is
used.
For hole initiated multiplication, the excess noise is reduced
from to when decreases from 1.01
to 0.82 m, as shown in Fig. 2. The excess noise was further
reduced for the - m n-i-p diode, and is found to lie
significantly below the curve. Similar to the thinner
p-i-n diodes, the excess noise in the thin n-i-p diode was also
measured up to a multiplication of 90. When 542-nm light is
used on the n-i-p structures, the excess noise decreases to
for the m diodes and for the
m diodes.
IV. D ISCUSSION
Multiplication measurements indicate that the ratio of
Al Ga As converges to 1 as the electric field increases [9],
[11]. The excess noise characteristics of diodes with
m are in qualitative agreement with the local noise model and
reflect the converging ratio in Al Ga As. In contrast,
the excess noise in thinner diodes ( m) reduces with
decreasing , regardless of the initiating carrier type. Further-
more, the measured excess noise corresponding to ob-
t ined from hole injection (the carrier with the lower ionization
524 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 14, NO. 4, APRIL 2002
Fig. 3. k versus measuredw under pure electron( ) and pure hole(4)
injection conditions for the range of diodes investigated. Thek values
under mixed carrier injection for the homojunction p-i-n diodes( ), the
homojunction n-i-p diodes( ) and the heterojunction p-i-n diode() lay
between those from pure carrier injection. The data obtained by Zhenget al.
(X) [10] are also included for comparison. The dashed lines are drawn to aid
visualization.
coefficient in Al Ga As) in the thin n-i-p structure cannot
be explained in terms of the local noise model. This nonlocal
behavior is similar to that observed in other materials [3]–[7],
[10] and is attributed to the effect of dead space, the minimum
distance a carrier must travel before it gains sufficient energy to
impact ionize, becoming dominant.
A summary of the excess noise behavior of AlGa As is
shown in Fig. 3, where the values under different carrier in-
jection conditions are plotted against. The values under
mixed carrier injection were found to lie between those for pure
carrier injection. As shown in Fig. 3, the results obtained by
Zhenget al. [10] for Al Ga As broadly corroborate our mea-
surements. The slight discrepancy may be due to uncertainties
in or doping values. Fig. 3 suggests that, under electron initi-
ated multiplication, an avalanche width of0.31 m gives the
highest excess noise and should therefore be avoided.
The change in excess noise for all diodes when the initiating
carrier type becomes mixed is consistent with and also
with multiplication measurements [9], [12]. Unlike GaAs, the
excess noise of thin Al Ga As diodes with m
changes significantly when injection becomes mixed. It is in-
teresting to note that thin Si diodes [6] also exhibit similar char-
acteristics. This indicates that it is still necessary for electrons to
initiate multiplication in thin Al Ga As diodes to minimize
excess noise.
Our results suggest that very thin AlGa As multiplica-
tion layers can be used to achieve very high speed APDs [13]
without limitation from tunneling current because of the large
bandgap. In addition, using a very thin AlGa As multipli-
cation layer also has the advantages of a very low avalanche
excess noise and low voltage operation.
V. CONCLUSION
The avalanche excess noise characteristics of AlGa As
have been determined in a series of p-i-n/n-i-p diodes over a
wide range of avalanche widths, varying from 1.02m down to
0.02 m. The thicker structures ( m) exhibit excess
noise characteristics in qualitative agreement with McIntyre’s
local model. As the avalanche width reduces below 0.31m
the excess noise is found to decrease with decreasing, regard-
less of initiating carrier type. This reduction in excess noise is
attributed to the dead space effect which causes the avalanche
process to become more deterministic in thin structures. Sim-
ilar to Si, the multiplication process in thin Al Ga As APDs
must be initiated by electrons to minimize the avalanche excess
noise. Our results suggest that very thin AlGa As avalanche
widths can be use to achieve APDs with very high speed and
very low excess noise, without the penalty of a large tunneling
current.
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Excess noise characteristics of Al0.8Ga0.2As avalanche photodiodes

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