Double-quantum-well field-effect transistors with a grating gate exhibit a sharply resonant, voltage tuned terahertz photoconductivity. The voltage tuned resonance is determined by the plasma oscillations of the composite structure. The resonant photoconductivity requires a double-quantum well but the mechanism whereby plasma oscillations produce changes in device conductance is not understood. The phenomenon is potentially important for fast, tunable terahertz detectors. © 2002 American Institute of Physics. ͓DOI: 10.1063/1.1497433͔ Double-quantum-well ͑DQW͒ heterostructures are important in the scientific exploration of correlated electron states in two-dimensional electron systems 1 and potentially important for novel field-effect transistors that add functionality by controlling electron transfer between the quantum wells. 2 Interwell transfer can also be promoted by terahertz photon assisted tunneling, opening the possibility of fast, voltage tunable terahertz ͑THz͒ detectors. 3 This motivated our research on THz response of DQW field-effect transistors ͑FETs͒. We report the THz photoconductivity of DQW FETs in which the gate is a periodic metallic grating. Strong photoresponse occurs at the plasma resonance of the composite structure. Other detector proposals make use of plasmon modes in single two-dimensional electron gas ͑2DEG͒ systems. 4 But, the relatively strong resonant response that we report here appears to require the presence of a DQW. We model the resonant response with a transmission line model of the collective modes of the 2DEGs and correlate the observed resonances with standing plasmon resonances under the metallic part of the grating gate. While the work was motivated by the concept of interwell transfer, the actual mechanism that gives rise to this response is not understood.The FETs are fabricated from modulation doped GaAs/ AlGaAs DQW heterostructures grown by molecular beam epitaxy. Both wells are 200 Å wide and are separated by a 70 Å barrier. The nominal electron densities are n upper ϭ1.7 ϫ10 11 cm Ϫ2 and n lower ϭ2.57ϫ10 11 cm Ϫ2 : the 4.2 K mobility is ϳ1.7ϫ10 6 cm 2 /V s. A 2ϫ2 mm mesa is defined and ohmic contacts to both quantum wells form source and drain. A 700-Å-thick TiAu grating gate ͑with no metallization between the grating fingers͒ is evaporated with the lines of the grating perpendicular to the current flow. We explored 4 and 8 m periods; half the period is metal. The grating modulates the electron density when a voltage is applied, selects wave vectors of the excited plasmon and, coincidentally, produces both normal and transverse THz electric fields. See inset in Fig. 1 for a cross section of the sample.We apply a constant source-drain current of 100 A, focus the radiation onto the sample, and study the photoconductive response of the DQW as a function of gate voltage, THz frequency, and temperature. The radiation sources are the free-electron lasers at the University of California, Santa Barbara, which cover a frequency range between 120 GHz and 4.8 THz. The respons...

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This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.

Large deviations are measured in the c 1 t e cyc o ron effective mass m near the r energy gap arising in strongly cou led G A e recently observed partial coup e a s double quantum wells ( W's Bll' The energy gap results f s) subject to in-plane magnetic esu s rom an anticrossing of the two W dis 1 ' h b B T" repressed by more than a factor of 3 near the u er y II. The severely distorted dis ersion p sion near the gap yields an I, gap e ge. The data show excellent agreement ' h a ed e. a ac or o near the upper gap edge, and enhanced b -50 ce y -% near the lower n agreement with recent theoretical calculations. because the Double-quantum-well (DQW) syst sys ems are of interest ecause they constitute a system in which an additional electronic degree of freedom in th h d e growt direction can be controllably introduced int t -d' g tron as. Most 'n o a wo-imensional elecmagnetotransport measurements on DQW's have been for high magnetic fields er end' grow h plane B~, where the electron's kinetic enerina e. ' Recent work has investigated th ff e e ects of pure y in-p ane fields BII, in which case sin le-r 1 tun nelin case sing e-particle the two el e ing ynamics dominate the intera t' 1 ac ions etween e wo electron layers. The primary effect of B is a linear transverse shif 'ft in the canonical momentum Ak of e ectrons in one quantum well (QW) 1 re ative to the other. is produces a strong modulation of the tunnelin conductance 6 (B ) due ce~I I ue to the conservation of energ o e tunneling and in-plane k. ergy Very recently, in our previous work " an" or and in work y urobe et a . , it was demonstrated that h h a w ent eincouphng is sufficiently strong, a hi h B stron 1 m g) a ig II also tunneling causes the shifted dispersion curves of the icross, opening a partial energ W's to ant' gy gap, or minie minigap, the elecgap, of width EG. At the edges of th tron group velocities are strongly distorted, and the density of states D(E form swee in B t c orms van Hove singularities. B p g II, the minigap is made to pass through the chemical potential p, with the singularities in D(s) producing two sharp features in R =G II ' a mi imum and a maximum as shown in Fig. 1(a), correspondin h wer gap edges, respectively. The data exhibited excellent agreement with th 1 eoretical predictions, and cG could be extracted from th d t Th e a a. is system is unusual in that its Fermi surface (FS) has several com-A similarl sha ' ar y s aped FS, although not tunable b B and II' arisin from g a different mechanism, was studied man years ago in vicinal planes of Si. ' Here we report measurements of th 1 e cyc otron mass m, o e ectrons near the edges of the field-ind d DQW uce 'gap. 'he masses are measured by addin a sma11 ing a sma o~~, and examining the temperature ( T) dependence of the resulting Shubnikov -de Haas (SdH) oscillations. In th n the upper-energy branch of the dispersion C) II l-CCI CL" C) C) II CD CC 0.065 ( 0.045 C) II 0.025 CC &I 0.005 Bii (T) FIG. 1. Normalized RII vs BII for 0=0 0' showin the anticrossing features. (b) Plo...

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We present a novel de-embedding method applicable to high frequency network analyzer measurements involving bisection of a THRU structure. Although similar methods have been published prior to this paper, to our knowledge this new method is unique in that one single de-embedding structure is required (a symmetric THRU), no other simplifying assumptions regarding the THRU structure are required, and no lumped element approximations are required.We present on-wafer measurements of single transistors and transmission line structures in several integrated circuit technologies comparing results using this de-embedding technique to other more commonly used de-embedding techniques (Y, Z subtraction) as well as to on-wafer calibration techniques. Most measurements are to 110 GHZ, with select measurements to 220 GHz illustrating the utility of the method well beyond frequencies at which lumped element approximations break down.-Index Terms -S-parameters, vector network analyzer, de-embedding, transmission matrix, transmission line, onwafer measurement.