| Summary: | 碩士 === 國立交通大學 === 電子研究所 === 84 === We have investigated the existence of electron subbands in
delta-doped InGaPmaterial with Si using Subnikov-de Haas
measurement at 0.6K. The Subbandenergy levels and the
electron wave functions of these subbands are calculatedself-
consistently by solving the Poisson and the Schrodinger
eqautions.The desities of 2DEG are 1.73 x 10^12, 0.87 x 10^12,
and 0.086 x 10^12 cm^-2for the 1st, 2nd, and 3rd subband,
respectively. The calculated subband energylevels are -39.33,
-12.31, -8.00, and -4.09 meV below the Fermi level for
thecorresponding four subbands.
An n-In0.49Ga0.51P/GaAs MODFET with 2 um gate length
demostrates a maximumextrinsic transconductance of 74 mS/mm and
a maximum intrinsic transconduc-tance of 121 mS/mm at 300K.
The device has a source resistance of 53 ohms, amaximum
oscillaton frequency (fmax) of 16 GHz, and a cutoff frequency
(ft) of6.2 GHz.
The second device is a delta-doped In0.49Ga0.51P/In0.1Ga0.9As/
GaAs MODFETwith 1.2 um gate length. It demonstrates a maximum
extrinsic transconductanceof 64 mS/mm at 300K. The lower
extrinsic transconductance is due to highersource resistance
(95 ohms) and the lower mobility. The measured famx and ftare
13.4 GHz and 5.2 GHz, respectively.
We have also employed the software, Medici, to predict the D.C.
characteris-tic of the second device. The measured
characteristics are reasonable agree-ment with the simulated
results. It shows that Medici is a good tool to pre-dict and
optimize device performance. An equivalent circuit has been
obtainedusing the Touchstone program along with the measured S-
parameters. From theequivalent circuit, we conclude that
three are three reasons that affect theperformance of these
MODFETs: (1) the transconductance is limted by large ser-ies
resistance and low mobility, (2) ft is restricted by large
parasitic capa-citances, and (3) fmax is poor due to lower
ratios of transconductance to out-put transconductance and gate-
source capacitance to drain-gate capacitance.
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