- Email: [email protected]
Hot electron transistors grown by MOCVD
Superlattices
and Microstructure.%
545
Vol. 4, No. 415, 1988
HOT ELECTRON
TRANSISTORS
GROWN BY MXVD
H.Kawai, I.Hase. S.Imanaga. K.Kaneko, and N.Watanabe Sony Corporation Research Center Fujitsukacho-174, Hodogayaku. Yokohama. Japan (Received 17 August 1987)
Using AlGaAs/GaAs hot electron transistors (HETs) grown by MOCVD (metalorganic chemical vapor deposition), we have studied the injected hot electron transport during traversal in the doped GaAs base-layer. An anomalous peak and shoulder in the transfer ratio a (=Ic/Ie) vs. emitter-voltage relation were seen in a HET having a lightly doped base, and they were attributed to the onset of the intervalley scattering from F to L and from F to X band minima. respectively. With increasing electron doping density in the base layer, a decreased drastically and structures ascribed to intervalley scattering6 became obscured. In a addition, depended inverse-exponentially on base thickness. It can be concluded that a strong scattering caused by conduction electrons and/or impurities is involved in the transfer process. Current gain B under the common-emitter configuration exceeded 5 and when strained GaInAs was incorporated in the base layer, p exceeded 10, showing the potentially high performance of this new material system.
1.
Introcbction
Future high speed devices will use ballistic electrons. One way to realizethis is the drasticreductionof the lateraldimension of mlike devices. Another way is the shorteningof the vertical dimensionwhile incorporatingheterostructures, such as in permeablebase transistors, verticalFEB..and hot electrontransistorsUlKTs).Among these. particularly bipolar-mode devices are attractive becauseof theirhigh currentdrive capability.DnipolarHETs are thoughtto be fasterthan HHTs becauseof the absenceof the minoritycarrier-storage effectand a shorter base-transit time. In addition,we note that the threshold voltageof a HWI can be precisely controlled. my decreasingthe turn-onvoltage of the hot electron(HE) injector,the supply voltagecan be reducedto below1 volt.leading to very low powerconsumption whilemaintaining high speed. To apply the HHI as practicaldevices,we have to understand the physicsof HE transport in a smalldevice.Pioneering work by Levi-and Haves111 and HeiblumI21 elucidated the ballisticnatureof electronsin GaAs usinga HEI as a HH spectrometer. However.the data
07494036/88/040545+05
802.00/O
obtainedmay reflectthe gualityof BE-grcwn materials and there remain various issues concerningthe scatteringmechanismof HE in the base. We have reported m (metalorganic chemical vapor deposition) to produce monolayer-s&e superlattices[31[41,and appliedthis methodto AlGaAslGaAs HETs[51.In the presentwork, we reportnew HE transport data in MXNDqrown HEps.The effectof dopins density. base- thickness,base depth,-and collectorbarrierheighton the transferratio a (=Ic/Ie) and its spectrumwith respectto the emitter-voltage is the major concernof this paper. Finally, we propose a new material systemto fabricatehigh performanceHEl?s,a strainedGaInAs base with an AlGaAs injector and collectoron a GaAs substrate. 2. %@le
Preparation
The device structures weremzrcwn by abrcspheric pressure MXVD. source materialsused were 'IM;(trimethylgallium). TMA (trimethyl aluminum). TM1 (trimethyl indium).101 arsineand hydrogenas a carrier gas. Disilanawas used as an n-typedopant.To
0 1988
Academic
Press Limited
Superlattices
546
minimize the gas exchange time, a lmninar flew velocity of I m/s above the wafer was used. The grcwth rate was 350 Almin for GaAs. The growth temperature was 760 "C, which is about 150 'C higher than that for normal MBE growth. Even at this high temperature,a monolayer superlattice was successfullygrm[31[41. It would be this high-temperaturegrowth which produces higher quality AlGaAs layers than the other grc&h method like ME3Edoes. The HET basically consists Of five layers. After the gra&h of a 2000A-thick highly conductive GaAs buffer laver on an n GaAs substrate,an n GaAs collectorlayer (5E17/cm3, 3000A), an AlGaAs collector barrier (undoned, 15OOA), an n GaAs base layer, an AlGaAs emitter barrier (undoped. 100 A). an n GaAs emitter the (5E17/an3, 3000A) and lver n GaAs-capping layer (5ElS/an3. 2000A) w:F! Thickness and doping successively grm. density of the base and the collector-barrier height were varied as device parameters. The mesa type structurewas made by dry etching and base-ohmic cantact was made to the thin AlGaAs layer (100 A) by flush thermal alloying with AulAuGelNi.
3. ResultsandDiscussion 3.1 DirectBvi&ntx? of IntervalleyScatteri.q A HDI structure is shown in Figure l(a) with typical device dimensions, together with band structure and bias ncmenclature. The emitter-baseheterostructureis a hot electron injectorand the base-collectorheterostructure is a collector of hot electrons.The collector barrier can functionas a hot electron analyzer if the effective barrier height can be controlled by changing the external collector bias with reqect to the base. The -n-base characteristicsmeasured at 4.2 K are shown in Fig.l(b). The collector current Ic for different emitter currents Ie was plotted as a function of Vcb. Several tenths of the transfer ratio a(=Ic/Ie) were obtained before the beginning of the tunneling breakdownat the base/collectorhetero-barrier. When the collector was negatively biased with respectto the base, negligiblecurrentsflowed over the collector.Since the collectorbarrier is relatively thick, the traversing electrons lose their kinetic energy en route before surmounting the collector &rErierand are forced to go back to the base. We confirmed that the HEZCshawn in Fig.l(a)has a negligible base resistanceat the current levels shawn in Fig.l(b). When Ie was increased up to levels where voltage drop in the base could not be neglected, the Ic characteristicsshifted to the neoative Vcb side (shiftino left in Fig.l(bj).The characteristicswere-similar to those reported by Heiblum[Zl and Reddy[Cl in which ballistic electrons were doserved. A
and Microstructures,
Vol. 4, No. 415, 1988
possible explanation of the Ic shift is that the lptential in the base layer was raised with respct to the external-baselzadby the base resistance and the effective collector/base bias was increased. Figure l(c) plots transfer ratio a for different collector biases as a function of emitter voltage Veb. We estimated an average voltage drop in the base by measuring the voltage drop between the two separated base pads. The voltage drop was less than 3mV at Veb of 0.5 Volts. The injected HE energy in the base thus can be equated with the sum of the emitter voltage and the Fermi energy (-33 meV. 5E17/cm3).The offset voltage in Veb decreases with increasingVcb. This decrease results fran the reduction of the effective barrier height due to electron accumulation and image force lcnvering. A peak around 0.35 V and a shoulder or a steep decrease around 0.5 V appeared in the spectrum of the transfer ratio, and a careful observation of the figure revealed weak irregularitiesin a around 0.28 V. This peak and shoulder have not yet been reported. A Monte Carlo particle simulationof the HDT done by Imanaga et al.[71 showed a peak of a in the a vs. Veb relation.The origin of this peak was ascribed to the onset of the intervalley scattering.The agreement between the present experiment and Imanaga's simulation suggests that the peak and the shoulder in Fig.l(c) relate to the transitionfrom I' to L and fran r to X valleys, respectively. To determine the onset energy of the scatterings.namely to assign the peak and the shoulder -to energies of -the two indirect band-edgesprecisely,the differentialtransfer ratio defined by -dIc/dIe was plotted as a function of Veb and is shy in Fig.2. ‘ItJo peaks (270 mV, 300 a-iW and a clear shoulder t-460 mV) appeared, giving precise onset energies of the individual scatterings. The three energy positions are -303, -333 and -493 meV above the r band-edge, respectively, taking the Fermi energy (-33 meV at 5E17/cm3) into account, [See Fig.l(a)l. The energy positions of the indirect L and X band minima are established"by Aqnes experimental work ;rtiannre 296 nW and 462 meV. above the , respectively.The second peak and the shoulder in Fig.2 thus are about 30 meV higher than the respective band edges. Therefore we can attribute these two energies to transitionsfrom the I' to L band and fran the r to X band minima accompaniedby L0 or LA phonon emission. The lower energy peak (-303 meV) is about 30 meV (Lo *onon energy at L points) belcw the second peak and alnvost r-minimum to L-minima coincides with the separation.We thus attribute this peak to the onset of the elastic intervalley transition. The origin of the elastic transfer may involve ionized impurities.The shoulder around 460 mV
Superlattices
and Microstructures,
547
Vol. 4, No. 415, 1988 X Minima _______~ L ---___
‘.____
L ‘-____
---
~
---___
Minima y------.____
I
(a)
1
Emitter
-0.2
I I
veb
0
E PX
ErL
“cb
A
1500
>
.: ,.:, ,;
.,....... ;..... ‘;:. ..._... .;.
Alo.zGao.BAs
n GcAs
Aio.2 Goa8As
n GaAs
Emitter
n = 5 x 10”cmm3
Collector
n=5x10’7cm-3
Barrier
Base
Barrier
Collector
0.2
0.4
0.6
0.6
v,,,(v)
0.5
0 v eb
1
(V)
Fig.1, (a): An energy band diagrm of a IiEI under bias, together with the device dimensions and nmenclature. (b): Out@ characteristics configuration. No of the HLT in a amaon-bag negatively could sllrmWnt the electron biased-collector barrier as the barrier was as
thick as 1500 A. Cc): Transfer ratio a=(Ic/Ie) as a function of the injection voltage at different collector biases. The spectra show clear evidence of the intervalley scatterimgs of injected hot electrons.
is broad and it canmt be clearly decided whether an elastic r-X transition is involved in this shoulder or not.
voltage, Vbd. was the sams for samples with the same x. Under the base-grounded configuration, a was measured just below this voltage Vbd. a did not strongly depend on Vcb around VW.
3.2
Electrorrelectron
Interaction and
RmtlReemtltfJfBotElectram Figure 3 sham plots of the transfer ratio a of HETs having a base 1000~h-thick as a function of electron doping density in the base. Al concentration x in the collector was break-dawn either 0.2 or 0.3. The collector
A dramatic decrease of a with increasing electron doping density was seen. a peak seen in Fig. l(c) disappeared when the electron density was only doubled (i~i8/cm3). These results indicate that conduction electrons and/or iqurities in the base cause a large relaxation of hot electron energy or manentun. The steep decrease in a with increasing
Superlattxes
548
and Mlcrostructures,
r1 ..
Vol. 4, No. 415. 1988
!OO __
-.
‘..
-..
.
‘i
.
;b
\
\
\\
x = 0.20
i4.2
I
1
0.5
Veb
\
lOI
CARRIER
I
1o18 CONCENTRATION
J
‘\
n = 1 x 1018
I
I
1
I
I
1000
0
Logarithnic
0.1
f
X = 0.25
BASE
titott
\
(V)
Fig.2 Differential transfer ratio dIc/dIe as a function of the injection voltage at different collector bias voltages. The energy position of the two peaks and shoulder can be assigned to -300, -330, and -490 meV, which correspond to, respectively, an elastic r to L. inelastic r r to x band-minima to L, and inelastic transitions.
I
n = 5 x1017
\
” = 2 x1018
0
BASE
‘\.*
I
x = 0.35
K
0
1
??
THICKNESS
plots
J
2000
wb
(A)
of a as a function
of
diffusion moQ1[91, suggests that a strong scattering plays a decisive role to drop the hot electron below the top of the collector barrier. The effective mean free path de&oed from the slope in the figure is, respectively, 2600 A for n_SE17/cm3, 850 A for n=lE18/an3 and 450 A for n=2E18/cm3. again showing a strong electron density deperxdence.
1o19 (cm-3l
Fig.3 Dependence of the transfer ratio on the base electron density. The lzaraneter x stands for Al concentration in the collector barrier. Injection voltage was around 0.35 V.
electron density could not be explained by a Monte Carlo simulation[71 which includes a simple plasmon mode interaction. We could not rely soley on the plasmon mode to interpret such a strong density dependence. The origin of the steep decrease in (I is not yet clear, electron-electron though a single-prticle interaction may be a candidate. shcws the base thickness Figure 4 dependence of a for various electron densities In a series of samples and barrier heights. having the same electron density and the same barrier height, a increased exponentially with decreasing base thickness. The result that the exponential shape appeared rather than an error-function shape, which is deduced from a
axnaon-titter Figure shows the 3(a) transistor characteristics of a IiEZ having a 500-A thick base and a collector barrier with x=0.2. With collector bias helm junction breakdown, we had a current gain of 5.6, which is the highest value reported so far in an AlGaAsKaAs HEX’. Fran a practical viewpoint, however, it is necessary for a HEI to have a higher current gain, a low base resistance and a relatively high breakdown voltage. Although further -rovemerit may be possible, HWPsusing only an AlGaAs a&erial system have been found in 3.2 to have a strict tr&e-off relation between the gain and both the base resistanoe and the breakdown voltage. &a way to solve this trade-off problem is to incorporate GaInAs in the base layer instead of GaAs. while the other constituents ranain unchanged. !l!h.i.sstrained layer system is useful because we can employ mature MXXD technology as well as use anraon GaAs substrates. Figure 5(b) &owe the transistor characteristics of a
Superlattices
and Microstructures,
549
Vol. 4, No. 415, 1988 Ga,..ln,.,As
base,
300A
0
x=0.35 Ib;to,,Astep
2~
500
2 -I
0
v,,
(VI
Fig.S (a): Ccsrnon-emitter transistor characteristics of an AlGaAslGaks HEI havinga base 500 A thick and a SE17/an3dopinglevel. The currentgain 8 obtainedis 5.6.T-4.2K (bl:Characteristics of a HEX'in whichthe base EDm hav’ a strainedGa(O.S)In(O.Z)As-base -i layer 300-thick.'IheGaInAs-baselayer was
thinnerthan criticalthickness(about400 A for Ga(0.8)In(O.2)As[101), hence the lattice mismatchis accDrrmoda ted entirelyby elastic strain, free of misfit dislocations. The Al content in the AlGaAs collector barrier adjacent to the base was graded to 1-r the top of the collectorbarriereffectively with the increaseof the collectorbias. In this device,the energyseparation betweenthe L-bandminima of the base and the top of the collectorbarrier is wider than that of the AlGaAslGaAsHEIs with the same collector barrierheight.A currentgain 8 increases with the collectorbias and exceeds10 as shcwnin Fig. S(b). This devicehas a high break down voltageand a large offset voltage.A large collectorbarrierheightis probablythe cause for these voltages.The devicehas a high Al concentration(0.35) barrierlayer.In addition, the incorporation of an IrGaAsbase increases the collectorbarrierheightmeasuredfran the conduction-band minimunof the base. Residual p-type impuritiesin the thickAlCaAsbarrier might also lift up the collectorbarrier.By optimizingthe structure,for instance by decreasing the Al concentrationin the collectorbarrierand increasing the thickness of the gradinglayeras well as optimizingthe grckvthconditions, the offsetvoltagecan be reducedand a HEl'withhigh performance could be possibleusingthismaterialsystem. 4. w AlGaAslGaAshot electrontransistors were successfully grcwn lq KXVD and the transfer characteristics were investigated.We have observedclearpeaksand a shouldernotonlyin the differential currentgain but also in the normala vs. Veb relations whichwere predicted by MonteCarlocalculation. Thesepeaksand the shoulderwele foundto originatefromthe onset of three kinds of intervalley transitions. A steep decreasein the transferratio (Lwith increasing electrondopingdensityand a clear
L
5
6
7 Vce
6
9
IV)
is replacedby 300 A thick Ga(O.S)In(O.P)As instead of GaAs. The lattice mismatch is accam&ated by elasticstrain.Currentgain 8 exceeded10 at 77 K. inverseexponentialdependenceof D on base thicknesswere observed.These resultssuggest that electron-electronand/or impurity scatteringcause a large relaxationof hot electron energy or manentlm. Under the canaon-emitter configuration, current gain 8 exceeded5, and this is the largestso far reportedin the AlGaAslGaAs system.To improve furtherdevice performance, a GaInAs-strained layer was used for the base, while the other constituents remainunchanged.Currentgain 8 exceeding10 and a very high breakdownvoltage were obtained,indicating the potentialof this materialsystem. mledgaentWe wish to thank K.Tairafor helpfuldiscussions. This work was performed under the managementof the Research and DevelopmentAssociationfor Furture Electron Devices as a part of the Research and DevelopmentProject of Basic Technologyfor FurtureIn&stries sponsored& the Agencyof Industrial ScienceandTechnology, MlTI. References
[llA.F.J.Levi, J.R.Hayes. P.M.Platxman. and W.Wiegmann. Phy6.Rev.Lett.55.2071~1985) M.I.Nathan. D.C.l%unas. and [21M.Heiblun. C.M.Knoedler, Phys.Rev.Lett.S5.2200(1985) N.Watanabe, r31K.Kajiwara.H.Kawai,K.Kaneko.and [email protected].,24.L85(1985) r41 N.Watanabe,and Y.Mori, Surface Sci.174 ,10(1986) I51 I.Hase.H.Kawai.S.Imanaga.K.Kaneko. and N.Watanabe, Electron.Lett.21,757(1985) I61U.K.Reddy.J.Chen.C.K.Peng, and H.Morkoc, Appl.Phys.Lett.48.1799(1986) and N.Wa+znabe, [71S.Imanaga,H.Kawai,K.Kaneko, J.AFp1.Phys.S9,3281(1986) Phys.Rev.Bl4.5331(1976) [SlD.E.Asples, Solid-State Electronics, I91B.K.Ridley, 24,147(1981). I101P.J.Orders and B.F.Usher. Appl.Phys.Lett. 50,980(1987)
and Microstructure.%
545
Vol. 4, No. 415, 1988
HOT ELECTRON
TRANSISTORS
GROWN BY MXVD
H.Kawai, I.Hase. S.Imanaga. K.Kaneko, and N.Watanabe Sony Corporation Research Center Fujitsukacho-174, Hodogayaku. Yokohama. Japan (Received 17 August 1987)
Using AlGaAs/GaAs hot electron transistors (HETs) grown by MOCVD (metalorganic chemical vapor deposition), we have studied the injected hot electron transport during traversal in the doped GaAs base-layer. An anomalous peak and shoulder in the transfer ratio a (=Ic/Ie) vs. emitter-voltage relation were seen in a HET having a lightly doped base, and they were attributed to the onset of the intervalley scattering from F to L and from F to X band minima. respectively. With increasing electron doping density in the base layer, a decreased drastically and structures ascribed to intervalley scattering6 became obscured. In a addition, depended inverse-exponentially on base thickness. It can be concluded that a strong scattering caused by conduction electrons and/or impurities is involved in the transfer process. Current gain B under the common-emitter configuration exceeded 5 and when strained GaInAs was incorporated in the base layer, p exceeded 10, showing the potentially high performance of this new material system.
1.
Introcbction
Future high speed devices will use ballistic electrons. One way to realizethis is the drasticreductionof the lateraldimension of mlike devices. Another way is the shorteningof the vertical dimensionwhile incorporatingheterostructures, such as in permeablebase transistors, verticalFEB..and hot electrontransistorsUlKTs).Among these. particularly bipolar-mode devices are attractive becauseof theirhigh currentdrive capability.DnipolarHETs are thoughtto be fasterthan HHTs becauseof the absenceof the minoritycarrier-storage effectand a shorter base-transit time. In addition,we note that the threshold voltageof a HWI can be precisely controlled. my decreasingthe turn-onvoltage of the hot electron(HE) injector,the supply voltagecan be reducedto below1 volt.leading to very low powerconsumption whilemaintaining high speed. To apply the HHI as practicaldevices,we have to understand the physicsof HE transport in a smalldevice.Pioneering work by Levi-and Haves111 and HeiblumI21 elucidated the ballisticnatureof electronsin GaAs usinga HEI as a HH spectrometer. However.the data
07494036/88/040545+05
802.00/O
obtainedmay reflectthe gualityof BE-grcwn materials and there remain various issues concerningthe scatteringmechanismof HE in the base. We have reported m (metalorganic chemical vapor deposition) to produce monolayer-s&e superlattices[31[41,and appliedthis methodto AlGaAslGaAs HETs[51.In the presentwork, we reportnew HE transport data in MXNDqrown HEps.The effectof dopins density. base- thickness,base depth,-and collectorbarrierheighton the transferratio a (=Ic/Ie) and its spectrumwith respectto the emitter-voltage is the major concernof this paper. Finally, we propose a new material systemto fabricatehigh performanceHEl?s,a strainedGaInAs base with an AlGaAs injector and collectoron a GaAs substrate. 2. %@le
Preparation
The device structures weremzrcwn by abrcspheric pressure MXVD. source materialsused were 'IM;(trimethylgallium). TMA (trimethyl aluminum). TM1 (trimethyl indium).101 arsineand hydrogenas a carrier gas. Disilanawas used as an n-typedopant.To
0 1988
Academic
Press Limited
Superlattices
546
minimize the gas exchange time, a lmninar flew velocity of I m/s above the wafer was used. The grcwth rate was 350 Almin for GaAs. The growth temperature was 760 "C, which is about 150 'C higher than that for normal MBE growth. Even at this high temperature,a monolayer superlattice was successfullygrm[31[41. It would be this high-temperaturegrowth which produces higher quality AlGaAs layers than the other grc&h method like ME3Edoes. The HET basically consists Of five layers. After the gra&h of a 2000A-thick highly conductive GaAs buffer laver on an n GaAs substrate,an n GaAs collectorlayer (5E17/cm3, 3000A), an AlGaAs collector barrier (undoned, 15OOA), an n GaAs base layer, an AlGaAs emitter barrier (undoped. 100 A). an n GaAs emitter the (5E17/an3, 3000A) and lver n GaAs-capping layer (5ElS/an3. 2000A) w:F! Thickness and doping successively grm. density of the base and the collector-barrier height were varied as device parameters. The mesa type structurewas made by dry etching and base-ohmic cantact was made to the thin AlGaAs layer (100 A) by flush thermal alloying with AulAuGelNi.
3. ResultsandDiscussion 3.1 DirectBvi&ntx? of IntervalleyScatteri.q A HDI structure is shown in Figure l(a) with typical device dimensions, together with band structure and bias ncmenclature. The emitter-baseheterostructureis a hot electron injectorand the base-collectorheterostructure is a collector of hot electrons.The collector barrier can functionas a hot electron analyzer if the effective barrier height can be controlled by changing the external collector bias with reqect to the base. The -n-base characteristicsmeasured at 4.2 K are shown in Fig.l(b). The collector current Ic for different emitter currents Ie was plotted as a function of Vcb. Several tenths of the transfer ratio a(=Ic/Ie) were obtained before the beginning of the tunneling breakdownat the base/collectorhetero-barrier. When the collector was negatively biased with respectto the base, negligiblecurrentsflowed over the collector.Since the collectorbarrier is relatively thick, the traversing electrons lose their kinetic energy en route before surmounting the collector &rErierand are forced to go back to the base. We confirmed that the HEZCshawn in Fig.l(a)has a negligible base resistanceat the current levels shawn in Fig.l(b). When Ie was increased up to levels where voltage drop in the base could not be neglected, the Ic characteristicsshifted to the neoative Vcb side (shiftino left in Fig.l(bj).The characteristicswere-similar to those reported by Heiblum[Zl and Reddy[Cl in which ballistic electrons were doserved. A
and Microstructures,
Vol. 4, No. 415, 1988
possible explanation of the Ic shift is that the lptential in the base layer was raised with respct to the external-baselzadby the base resistance and the effective collector/base bias was increased. Figure l(c) plots transfer ratio a for different collector biases as a function of emitter voltage Veb. We estimated an average voltage drop in the base by measuring the voltage drop between the two separated base pads. The voltage drop was less than 3mV at Veb of 0.5 Volts. The injected HE energy in the base thus can be equated with the sum of the emitter voltage and the Fermi energy (-33 meV. 5E17/cm3).The offset voltage in Veb decreases with increasingVcb. This decrease results fran the reduction of the effective barrier height due to electron accumulation and image force lcnvering. A peak around 0.35 V and a shoulder or a steep decrease around 0.5 V appeared in the spectrum of the transfer ratio, and a careful observation of the figure revealed weak irregularitiesin a around 0.28 V. This peak and shoulder have not yet been reported. A Monte Carlo particle simulationof the HDT done by Imanaga et al.[71 showed a peak of a in the a vs. Veb relation.The origin of this peak was ascribed to the onset of the intervalley scattering.The agreement between the present experiment and Imanaga's simulation suggests that the peak and the shoulder in Fig.l(c) relate to the transitionfrom I' to L and fran r to X valleys, respectively. To determine the onset energy of the scatterings.namely to assign the peak and the shoulder -to energies of -the two indirect band-edgesprecisely,the differentialtransfer ratio defined by -dIc/dIe was plotted as a function of Veb and is shy in Fig.2. ‘ItJo peaks (270 mV, 300 a-iW and a clear shoulder t-460 mV) appeared, giving precise onset energies of the individual scatterings. The three energy positions are -303, -333 and -493 meV above the r band-edge, respectively, taking the Fermi energy (-33 meV at 5E17/cm3) into account, [See Fig.l(a)l. The energy positions of the indirect L and X band minima are established"by Aqnes experimental work ;rtiannre 296 nW and 462 meV. above the , respectively.The second peak and the shoulder in Fig.2 thus are about 30 meV higher than the respective band edges. Therefore we can attribute these two energies to transitionsfrom the I' to L band and fran the r to X band minima accompaniedby L0 or LA phonon emission. The lower energy peak (-303 meV) is about 30 meV (Lo *onon energy at L points) belcw the second peak and alnvost r-minimum to L-minima coincides with the separation.We thus attribute this peak to the onset of the elastic intervalley transition. The origin of the elastic transfer may involve ionized impurities.The shoulder around 460 mV
Superlattices
and Microstructures,
547
Vol. 4, No. 415, 1988 X Minima _______~ L ---___
‘.____
L ‘-____
---
~
---___
Minima y------.____
I
(a)
1
Emitter
-0.2
I I
veb
0
E PX
ErL
“cb
A
1500
>
.: ,.:, ,;
.,....... ;..... ‘;:. ..._... .;.
Alo.zGao.BAs
n GcAs
Aio.2 Goa8As
n GaAs
Emitter
n = 5 x 10”cmm3
Collector
n=5x10’7cm-3
Barrier
Base
Barrier
Collector
0.2
0.4
0.6
0.6
v,,,(v)
0.5
0 v eb
1
(V)
Fig.1, (a): An energy band diagrm of a IiEI under bias, together with the device dimensions and nmenclature. (b): Out@ characteristics configuration. No of the HLT in a amaon-bag negatively could sllrmWnt the electron biased-collector barrier as the barrier was as
thick as 1500 A. Cc): Transfer ratio a=(Ic/Ie) as a function of the injection voltage at different collector biases. The spectra show clear evidence of the intervalley scatterimgs of injected hot electrons.
is broad and it canmt be clearly decided whether an elastic r-X transition is involved in this shoulder or not.
voltage, Vbd. was the sams for samples with the same x. Under the base-grounded configuration, a was measured just below this voltage Vbd. a did not strongly depend on Vcb around VW.
3.2
Electrorrelectron
Interaction and
RmtlReemtltfJfBotElectram Figure 3 sham plots of the transfer ratio a of HETs having a base 1000~h-thick as a function of electron doping density in the base. Al concentration x in the collector was break-dawn either 0.2 or 0.3. The collector
A dramatic decrease of a with increasing electron doping density was seen. a peak seen in Fig. l(c) disappeared when the electron density was only doubled (i~i8/cm3). These results indicate that conduction electrons and/or iqurities in the base cause a large relaxation of hot electron energy or manentun. The steep decrease in a with increasing
Superlattxes
548
and Mlcrostructures,
r1 ..
Vol. 4, No. 415. 1988
!OO __
-.
‘..
-..
.
‘i
.
;b
\
\
\\
x = 0.20
i4.2
I
1
0.5
Veb
\
lOI
CARRIER
I
1o18 CONCENTRATION
J
‘\
n = 1 x 1018
I
I
1
I
I
1000
0
Logarithnic
0.1
f
X = 0.25
BASE
titott
\
(V)
Fig.2 Differential transfer ratio dIc/dIe as a function of the injection voltage at different collector bias voltages. The energy position of the two peaks and shoulder can be assigned to -300, -330, and -490 meV, which correspond to, respectively, an elastic r to L. inelastic r r to x band-minima to L, and inelastic transitions.
I
n = 5 x1017
\
” = 2 x1018
0
BASE
‘\.*
I
x = 0.35
K
0
1
??
THICKNESS
plots
J
2000
wb
(A)
of a as a function
of
diffusion moQ1[91, suggests that a strong scattering plays a decisive role to drop the hot electron below the top of the collector barrier. The effective mean free path de&oed from the slope in the figure is, respectively, 2600 A for n_SE17/cm3, 850 A for n=lE18/an3 and 450 A for n=2E18/cm3. again showing a strong electron density deperxdence.
1o19 (cm-3l
Fig.3 Dependence of the transfer ratio on the base electron density. The lzaraneter x stands for Al concentration in the collector barrier. Injection voltage was around 0.35 V.
electron density could not be explained by a Monte Carlo simulation[71 which includes a simple plasmon mode interaction. We could not rely soley on the plasmon mode to interpret such a strong density dependence. The origin of the steep decrease in (I is not yet clear, electron-electron though a single-prticle interaction may be a candidate. shcws the base thickness Figure 4 dependence of a for various electron densities In a series of samples and barrier heights. having the same electron density and the same barrier height, a increased exponentially with decreasing base thickness. The result that the exponential shape appeared rather than an error-function shape, which is deduced from a
axnaon-titter Figure shows the 3(a) transistor characteristics of a IiEZ having a 500-A thick base and a collector barrier with x=0.2. With collector bias helm junction breakdown, we had a current gain of 5.6, which is the highest value reported so far in an AlGaAsKaAs HEX’. Fran a practical viewpoint, however, it is necessary for a HEI to have a higher current gain, a low base resistance and a relatively high breakdown voltage. Although further -rovemerit may be possible, HWPsusing only an AlGaAs a&erial system have been found in 3.2 to have a strict tr&e-off relation between the gain and both the base resistanoe and the breakdown voltage. &a way to solve this trade-off problem is to incorporate GaInAs in the base layer instead of GaAs. while the other constituents ranain unchanged. !l!h.i.sstrained layer system is useful because we can employ mature MXXD technology as well as use anraon GaAs substrates. Figure 5(b) &owe the transistor characteristics of a
Superlattices
and Microstructures,
549
Vol. 4, No. 415, 1988 Ga,..ln,.,As
base,
300A
0
x=0.35 Ib;to,,Astep
2~
500
2 -I
0
v,,
(VI
Fig.S (a): Ccsrnon-emitter transistor characteristics of an AlGaAslGaks HEI havinga base 500 A thick and a SE17/an3dopinglevel. The currentgain 8 obtainedis 5.6.T-4.2K (bl:Characteristics of a HEX'in whichthe base EDm hav’ a strainedGa(O.S)In(O.Z)As-base -i layer 300-thick.'IheGaInAs-baselayer was
thinnerthan criticalthickness(about400 A for Ga(0.8)In(O.2)As[101), hence the lattice mismatchis accDrrmoda ted entirelyby elastic strain, free of misfit dislocations. The Al content in the AlGaAs collector barrier adjacent to the base was graded to 1-r the top of the collectorbarriereffectively with the increaseof the collectorbias. In this device,the energyseparation betweenthe L-bandminima of the base and the top of the collectorbarrier is wider than that of the AlGaAslGaAsHEIs with the same collector barrierheight.A currentgain 8 increases with the collectorbias and exceeds10 as shcwnin Fig. S(b). This devicehas a high break down voltageand a large offset voltage.A large collectorbarrierheightis probablythe cause for these voltages.The devicehas a high Al concentration(0.35) barrierlayer.In addition, the incorporation of an IrGaAsbase increases the collectorbarrierheightmeasuredfran the conduction-band minimunof the base. Residual p-type impuritiesin the thickAlCaAsbarrier might also lift up the collectorbarrier.By optimizingthe structure,for instance by decreasing the Al concentrationin the collectorbarrierand increasing the thickness of the gradinglayeras well as optimizingthe grckvthconditions, the offsetvoltagecan be reducedand a HEl'withhigh performance could be possibleusingthismaterialsystem. 4. w AlGaAslGaAshot electrontransistors were successfully grcwn lq KXVD and the transfer characteristics were investigated.We have observedclearpeaksand a shouldernotonlyin the differential currentgain but also in the normala vs. Veb relations whichwere predicted by MonteCarlocalculation. Thesepeaksand the shoulderwele foundto originatefromthe onset of three kinds of intervalley transitions. A steep decreasein the transferratio (Lwith increasing electrondopingdensityand a clear
L
5
6
7 Vce
6
9
IV)
is replacedby 300 A thick Ga(O.S)In(O.P)As instead of GaAs. The lattice mismatch is accam&ated by elasticstrain.Currentgain 8 exceeded10 at 77 K. inverseexponentialdependenceof D on base thicknesswere observed.These resultssuggest that electron-electronand/or impurity scatteringcause a large relaxationof hot electron energy or manentlm. Under the canaon-emitter configuration, current gain 8 exceeded5, and this is the largestso far reportedin the AlGaAslGaAs system.To improve furtherdevice performance, a GaInAs-strained layer was used for the base, while the other constituents remainunchanged.Currentgain 8 exceeding10 and a very high breakdownvoltage were obtained,indicating the potentialof this materialsystem. mledgaentWe wish to thank K.Tairafor helpfuldiscussions. This work was performed under the managementof the Research and DevelopmentAssociationfor Furture Electron Devices as a part of the Research and DevelopmentProject of Basic Technologyfor FurtureIn&stries sponsored& the Agencyof Industrial ScienceandTechnology, MlTI. References
[llA.F.J.Levi, J.R.Hayes. P.M.Platxman. and W.Wiegmann. Phy6.Rev.Lett.55.2071~1985) M.I.Nathan. D.C.l%unas. and [21M.Heiblun. C.M.Knoedler, Phys.Rev.Lett.S5.2200(1985) N.Watanabe, r31K.Kajiwara.H.Kawai,K.Kaneko.and [email protected].,24.L85(1985) r41 N.Watanabe,and Y.Mori, Surface Sci.174 ,10(1986) I51 I.Hase.H.Kawai.S.Imanaga.K.Kaneko. and N.Watanabe, Electron.Lett.21,757(1985) I61U.K.Reddy.J.Chen.C.K.Peng, and H.Morkoc, Appl.Phys.Lett.48.1799(1986) and N.Wa+znabe, [71S.Imanaga,H.Kawai,K.Kaneko, J.AFp1.Phys.S9,3281(1986) Phys.Rev.Bl4.5331(1976) [SlD.E.Asples, Solid-State Electronics, I91B.K.Ridley, 24,147(1981). I101P.J.Orders and B.F.Usher. Appl.Phys.Lett. 50,980(1987)