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571

current is expressed as a function of the gate voltage Vos and drain voltage Vos, /D = 2K{(le~s - V T ) V D s -- I/2s/2},

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(1)

where Vx is the threshold voltage, #z0 the electron mobility in the channel, C O the gate capacitance, Wo the gate width, and L o is the gate length. Thus the channel conductance,

g=dlD/dVDs= 2K(VGs- V T - VDs),

(2)

is a linear function of the gate voltage, and the slope is proportional to the electron mobility. In the DQW structure, because of the parallel conduction in the two quantum wells, the apparent mobility is exp.,~, se'l as follows [7]: nl~ 1 + n2/,

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ohmic contacts, i.e., the drain and the source contacts, were made by evaporating A u / G e on the n+-GaAs cap layer followed by annealing at 450 o C. The gate contact was made by depositing A u / A I on the 300 /k n+-GaAs cap layer. The electron mobi!ity in the narrow and wide wells are estimated, respectively, to be 3000 cm2/V s and 5300 c m 2 / V s at 4.2 K. The drain current-voltage characteristics were measdrcd at 4.2 and 77 K. The .ransconductance at the saturation region is typically 1.1 m S / m m (4.2 K) and 1.2 m S / m m (77 K ) w h i c h is not so high because of the long gate of the test device ( ~ 400 tzm). Fig. 2 shows the channel conductance g = dlD/dVDs at VDs = 0 and the second derivative of the current ~g/~Vo as a function of the gate voltage at 4.2 K. Similar characteristics were obtained at 77 K. Obviously, the curve of d l ~ / d V D s is divided into four regions (I ~ IV). The reginn (I) is the off-region below the threshold voltage. (The nonzero conductance in this region is due to t h e " [wo-mmenslonal '" " ' mectron . . . . . . . . . . . . gas . . . . . . at u~c ilt;tgl-ointerface between the A l 0 . 3 G a 0 . 7 A s (500 A ) bottom layer ana the undoped GaAs buffer layer). In the regions (II) and (IV), the channel conductance is a linear function of the gate voltage. According to the linear theo,w of the H E M T [5] or M O S F E T [6], in the linear region, the drain

(3)

ns

m l 0 . 3 G a 0 . 7 A s top layer is at 1.4 x 1018 c m -3. Two

where the subscripts 1 and 2 represent the quantity in the wide well and the narrow one, respectively, and n S is the total sheet electron density. Tim variation of the slope of g in fig. 2 suggests the variation of the electron mobility due to the variation of the conductive channel. Above the threshold voltage, we get a linear increase of g to the point A (Vo = 0.85 V). Since the quantized level in the wide well is lower than that in tFe narrow well. the electrons might be accumu-

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Fig. 2. The channel conductance g =dlD/dVDs (solid line) and the second derivative of the current Og! ~ V G (broken line)

as a function of the gate voltage at 4.2 K. The threshold voltage of the DQW is VT = 0.64 V.

572

E. Okuno et aL / GaAs /AIGa,4s DQWFET

lated in the wide well first. Therefore, this region corresponds to the conduction in the wide quantum well. The point A corresponds to the onset of the reduction in the electron mobility, or the starting point of the real space transfer of electron from the wide well to the narrow one. Above the point A, the electron mobility should be low because of the intersubband scattering or the real space transfer as observed in fig. 2. Above the point C, on the other hand, we get another linear increase in g, which might reflect the conduction in the narrow well. If this is the case, we may interpret the regions defined in fig. 2 as, (1) the off-region, (lI) the high-mobility region due to the conduction in the wide quantum well, (II) the intermediate transition region and (IV) the lowmobility region due to the conduction in the narrow well. As is shown by an arrow B in fig. 2, the curve of g exhibits a dip in the transition region (III). The structure is more clear in the plots of i~g/i~VG as a function of VG. Experimentally, this structure is observed at 4.2 K and becomes obscure at 77 K. Since this is in the intermediate region, this might be attributed ~o the crossing or the resonance of the lowest quantized level in the wide well with that of the narrow well. Because of the large scattering probabilit3, at the resonance, the electron mobility should be abruptly reduced and the curve of g should have a minimum, in agreement with the experimental result. In order to confirm the explanation, we measured the photoluminescence (PL) spectra as a function of the gate voltage. Fig. 3 shows the results. The inset shows the typical spectrum, which has two peaks. The main peak at A = 792.1 nm is due to the wide well and the high-energy peak at A = 779.9 nm is due to the narrow well. Fig. 3a shows the PL intensity at A = 792 nm (main peak) as a function of VG. As is shown in the figure, by increasing VG, above the point ,4 as in fig. 2, the PL intens'ty decreases suggesting the decrease of the electron density in the wide well or the occurrence of the real space transfer of electxons from the wide well to the narrow one, in agreement with the previous result. In the cu~-ve in fig. 3a, we get a kink at point B followed by a peak. Around this point, we measured the

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Fig. 3. The PL intensity and the peak energy of the main peak due to the wide well as a function of the gate voltage. The inset shows the PL spectrum at VG = 0 V.

energy of the main peak as a function of VG. The results are shown in fig. 3b, which shows a sudden change at 1/~ = 1.66 V (point B). This change indicates that an anti-crossing of the levels occurs at the resonance [8]. We have tried similar measurements on the high-energy peak due to the narrow well, but the spectra were subjected to broadening at high VG and we could not get clear evidence. Nevertheless, the results as in fig. 3 support the explanation given earlier. By the way, the PL intensity as a function of l/G has a peak above the point B. Near the resonance, because of the anti-crossing of the two levels, the variation of the energy level by the gate voltage is suppressed slightly, followed by an abrupt change as shown in fig. 3b. Therefore, the electron dens'.ty in the wide well will remain constant before the abrupt change, llowever, since the total density of electrons in the channel should be increased by the increase of Vc., the PL intensity increased there. Therefore, at the abrupt change of level energy, we get an abrupt decrease of the PL intensity to give a peak just after the point B. In summary, the D Q W F E T has been studied. By analyzing the channel conductance as a function of the gate voltage, a switching from the high

E.

Okuno et

al. / GaAs /AIGaAs DQWFET

electron mobility region to the low-mobility one was observed for the first time. In order to confirm the change of the conductive channels and the electron mobility, the PL spectra were investigated as a function of the gate voltage, which shows a clear evidence of the crossing of the quantized levels is DQW's. This work is partly supported by the grant-inaid from the Ministry of Education, Science and Culture of Japan. The authors would like to thank H. Kano (Toyota Central R& D Labs. Inc.) for helpful discussions. We also wish to thank H. Itoh, T. Suzuki, K. Hara (Nippondenso Co. Ltd. R& D Dept.) for preparing some of the samples.

573

References [1] S.M. Sze, Semiconductor Devices Physics and Technology (Wiley, New York, 1985) ch. 11. [2] H. Sakaki, Jpn. J. Appl. Phys. 21 (1982) L381. [3] I.C. Kizilyalli, K. Hess and G.J. lafrate, J. Appl. Phys. 61 (1987) 2395. [4] I.C. KitilyaUi and K. Hess, Jpn. J. Appi. Phys. 26 (1987) 1519. [5] R. Dingle, in: Semiconductor and Semimetals, Vol. 24, eds. R.K. Willardson and A.C. Beer (Academic Press, 1987) ch. 4. [6] K. Seeger, Semiconductor Physics, 3rd Ed. (Springer, Berlin, 1985) ch. 5. [7] K. Seeger, Semiconductor Physics, 3rd E d (Springer, Berlin, 1985)oh. 7. [8] L. Vina, R.T. Collins, E.E. Mendez and W.I. Wang, Phys. Rev. Lett. 58 (1987) 832.