G.W. Wang

Kink effect in submicrometer-gate MBE-grown InAlAs/InGaAs/InAlAs heterojunction MESFETs

The authors report the influence of the kink effect on the DC and microwave performance of i-InAlAs heterojunction doped-channel MESFETs lattice matched to an InP substrate with submicrometer gates. Kink effects were observed at room temperature as well as at 77 K in the DC measurement. The kinks seem to be related to deep-level electron trapping, and are not present at microwave frequencies. Measured results are presented showing that the existence of kinks at low operating frequencies does not seem to degrade the microwave performance of the devices.<<ETX>>

A 0.1- mu m gate Al/sub 0.5/In/sub 0.5/As/Ga/sub 0.5/In/sub 0.5/As MODFET fabricated on GaAs substrates

Al/sub 0.5/In/sub 0.5/As/Ga/sub 0.5/In/sub 0.5/As MODFET structures have been successfully grown on lattice-mismatched GaAs substrates with a 3.8% difference of lattice constants. MODFETs fabricated with a 0.12- mu m T-shaped gate demonstrate DC and microwave characteristics comparable to those of Al/sub 0.5/In/sub 0.5/As/Ga/sub 0.5/In/sub 0.5/As MODFETs on lattice-matched InP substrates. A peak extrinsic DC transconductance of 585 mS/mm and a full-channel current of 370 mA/mm are achieved at room temperature. Parasitic substrate conduction, which may be the result of the threading dislocations under the FET bonding pads and the active FET channel, affects the device performance. The MODFET shows a high current-gain cutoff frequency of 117 GHz and a maximum available gain cutoff frequency of 125 GHz. The effects of substrate conduction on microwave performance are also investigated. >

An analytical and computer-aided model of the AlGaAs/GaAs high electron mobility transistor

An analytical model for the AlGaAs/GaAs high electron mobility transistor (HEMT) or MODFET has been developed. This model uses a Trofimenkoff-type relation [1] for the electron velocity and electrical field and assumes that the electron velocity saturation inside the two-dimensional electron gas channel cause current saturation. It also takes into account the parasitic conduction in the AlGaAs layer by including a MESFET operation. Based on this model, analytical current-voltage equations suitable for computer simulation have been derived. Calculated results for sub-half-micrometer HEMTs show excellent agreement with measured characteristics.

A high-current pseudomorphic AlGaAs/InGaAs double quantum-well MODFET

Double quantum-well modulation-doped field-effect transistors (MODFETs) with planar-doped lattice-strained AlGaAs/InGaAs structure have been fabricated and characterized at DC and microwave frequencies. At 300 K the 0.3- mu m gate devices show a full channel current of 1100 mA/mm with a constant extrinsic transconductance of 350 mS/mm over a broad gate voltage range of 1.6 V. Excellent microwave performance is also achieved with a maximum available gain cutoff frequency f/sub mag/ of 110 GHz and a current gain cutoff frequency f/sub r/ of 52 GHz. A maximum output power of 0.7 W/mm with 30% efficiency is obtained at 18 GHz.<<ETX>>

Observation of high-frequency high-field instability in GaAs/InGaAs/AlGaAs DH-MODFETs at K band

The authors report the observation of high-field instability at room temperature with oscillation frequency as high as 24 GHz in GaAs/InGaAs/AlGaAs double-heterojunction-MODFETs (DH-MODFETs) of 1.2 mu m gate length. Negative drain differential resistance was also observed in these devices under various forward gate biases. The nature of this instability is believed to be caused by the efficient removal of the real-space transferred hot two-dimensional electrons in the AlGaAs layer through the forward-biased Schottky gate. A tuned oscillator, with a fundamental oscillation frequency as high as 19.68 GHz, has also been demonstrated at a gate bias of 1.3 V.<<ETX>>

Comparisons of microwave performance between single-gate and dual-gate MODFETs

Both a 1.2- mu m and a 0.3- mu m gate length, n/sup +/-GaAs/InGa/n/sup +/-AlGaAs double-heterojunction MODFET have been fabricated with single-gate and dual-gate control electrodes. Extrinsic DC transconductance of 500 mS/mm has been achieved from a 0.3- mu m single-gate MODFET. The device also has a current gain cutoff frequency f/sub T/ of 43 GHz and 14-dB maximum stable gain at 26 GHz with the stability factor k as low as 0.6 from the microwave S-parameter measurements. At low-frequency dual-gate MODFETs demonstrate higher gain than the single-gate MODFETs. However, the k of dual-gate MODFETs approaches unity at a faster rate. Power gain roll-off slopes of 3-, 6-, and 12-dB/octave have been observed for the dual-gate MODFETs.<<ETX>>

Reduction of gate resistance in tenth-micron gate MODFETS for microwave applications

Abstract A T-shaped gate technique has been successfully applied to fabrication of MODFETs with gate length on the order of 0.1 μm. The 0.1 μm T-shaped gate is obtained by a PMMA/P(MMA-MAA)/PMMA triple layer electron beam resist system to achieve fine line width, high aspect ratio, and good lift-off profile. Compared to conventional gates, the T-shaped gate with its larger cross section typically shows a reduction of gate resistance by a factor of 10. Effects of gate resistance on the microwave performance of AllnAs/InGaAs MODFETs with 0.12 μm T-shaped gates are also discussed.

IIA-4 The effect of buried p-doped layers on the current saturation mechanism in AlGaAs/InGaAs/GaAs MODFET's

bottom electron supplying layer. By introducing buried buffer layers with a high potential barrier, we are able to dope the bottom electron supply layer much heavier and hence generate more twodimensional electrons before the onset of a parasitic conducting channel. The structures were grown by MBE in the following sequences on semi-insulating substrates: GaAs buffer/high barrier buffer/n+GaAs/Ino,,5Gao~ssAs/n+-Alo~sGao,7As/n+-GaAs. The high barrier buffer layers were made of either a AlGaAs/GaAs superlattice or a buried atomically planar beryllium-doped P+-GaAs layer. A reference layer with an undoped GaAs buffer layer was also grown for comparison. A high extrinsic DC g, of 408 mS/mm and a full channel current of 610 mA/mm at room temperature are obtained for 1.2-pm gate length FET’s with buried buffer layers while FET’s without the buried high barrier buffer layers could not be pinched off. An fr of 21.5 GHz and an fmax of 80 GHz are extrapolated from measured S-parameter data from 0.5 to 26.5 GHz with a 6dB/octave slope. We have obtained a 2DEG sheet charge density as high as 1.7 X 10”/cm2 before the onset of parasitic conduction in the AlGaAs layers, while Hall data indicates larger densities. Preliminary CW power measurements show 0.45 W/mm with 11db linear gain and 36-percent power-added efficiency at 10 GHz and 0.36 W/mm with 7-dB linear gain and 22-percent efficiency at 18 GHz. More detailed measurements are presently being performed at higher frequencies, and the results will be reported. We have demonstrated the enhancement of 2DEG sheet charge densities while reducing parasitic conduction through electron supplying layers in double heterojunction MODFET’s by introducing buried high electron barrier buffer layers. These structures are capable of extending the operating frequency of multiple heterojunction power MODFET’s while increasing the associated power densities through the improvements in both current density and fmax.

MODFET noise model and properties with hot-electron effects

A noise model to study hot-electron effects on noise properties is derived. This model gives H. Fukuis (1979) coefficient an analytical expression in terms of bias condition and high-field parameters. 0.3- mu m gate length pseudomorphic MODFETs with and without a high-energy barrier buffer are fabricated and characterized. The noise data analyzed by the noise model agree with microwave characteristics where carrier deconfinement of two-dimensional electrons degrades both the noise and high-frequency performance. >

Microwave Performance Of Single-gate And Dual-gate Modfets Using Double Hetero'unction Modulation-doped Structures

1.2-/spl mu/m and 0.3-/spl mu/m gate length n+-GaAs/InGaAs/n+-AlGaAs Double Heterojunction Modulation-Doped Field-Effect Transistors (DH-MODFETs)have been fabricated with single and dual controlling gate electrodes. Extrinsic DC transconcluclance of 500 mS/mm has been achieved for a 0.3-/spl mu/m single-gate FET. The device also has an f/sub T/ of 43 GHz and 14 dB Maximum Stable Gain (MSG) at 26 GHz with the stability factor (k) as low as 0.6 from the small signal s-parameter measurements from 0.5 to 26.5 GHz. Dual gate FETs fabricated on the same wafer demonstrate higher gain at low frequencies, but the gain decreases rapidly at high frequencies with the stability factor reaching unity at a rate faster than that single-gate DH-MODFETs. Distinctive power gain roll-off slopes of -3, -6, and -12 dB/octave have been observed forthe dual-gate MODFETs.