M. Shur

Distributive nature of gate current and negative transconductance in heterostructure field-effect transistors

Experimental data showing that the dependence of the gate current on the drain voltage in enhancement-mode heterostructure field-effect transistors changes qualitatively when the gate voltage is varied from below to above threshold are presented. The data lead to the conclusion that for gate voltages higher than the threshold voltage and drain voltages larger than the drain saturation voltage, most of the potential drop occurs in a small region near the drain end of the channel. The gate current is distributed along the channel so that electrons in the channel are diverted toward the gate. A model is proposed that takes into account such a distribution of the gate current along the channel. The distributive nature of the gate current leads to negative transconductance in heterostructure field-effect transistors at high gate voltages. Negative transconductance reaching -125 mS/mm in 1- mu m gate devices is observed, and an equivalent circuit model is proposed that describes the dependence of the drain current on the gate voltage in good agreement with present experimental data. >

Short channel effects in submicron self-aligned gate heterostructure field effect transistors

AlGaAs/GaAs self-aligned gate heterostructure FETs with gate lengths varying from 0.3 to 1.5 mu m were fabricated to study short-channel effects. Peak extrinsic transconductance as high as 360 mS/mm was achieved with this process. Short-channel effects such as increases in the output conductance, increases in the subthreshold current, and shifts in the threshold voltage are reported for temperatures ranging from 30 degrees C to 100 degrees C. The observations follow closely predictions from a simple model which attributes the effects to space-charge-limited electron injection into the GaAs buffer layer beneath the actual two-dimensional electron gas channel.<<ETX>>

Doped channel pseudomorphic GaAs/InGaAs/AlGaAs hetero-structure FETs

We report experimental and theoretical results obtained for pseudomorphic InGaAs Doped Channel Heterostructure FETs (DCHFET) that demonstrate the advantages of this device over other heterostructure FETs. We demonstrate high transconductance beta value, and saturation current and low output conductance and subthreshold current. These improvements are due to a higher density of carriers in the channel, the carrier confinement in the quantum well device structure, and the superior transport properties of InGaAs. Transconductances of 350 mS/mm and beta-values of 440 mS/V-mm were measured for 1-µm enhancement mode DCHFETs. Transconductances as high as 471 mS/mm and drain saturation currents as high as 660 mA/mm were measured for 0.6 µm depletion mode DCHFETs.

Superlattice conduction in superlattice modulation‐doped field‐effect transistors

We fabricated superlattice modulation‐doped field‐effect transistors where the doping is concentrated in GaAs narrow quantum wells separated by undoped AlGaAs barriers. As the doped AlGaAs regions are eliminated from such a structure, the concentration of traps generally associated with doping of AlGaAs is low. We observed the threshold voltage shift of only 140 mV with temperature change from 77 to 300 K (which should be compared to the shift of 200–300 mV in conventional modulation‐doped field‐effect transistors). Peak transconductances of 310 mS/mm at 300 K and 321 mS/mm at 77 K have been obtained. An interesting feature of this device is the complicated dependence of the transconductance on the gate voltage which has two peaks at 77 K and one sharp peak at 300 K. These peaks are caused by the parallel conduction paths in the superlattice at high gate voltages and by the gate leakage current. This parallel conduction in GaAs‐doped quantum wells may be used in order to achieve larger voltage swings in s...

Quantum well p-channel AlGaAs/InGaAs/GaAs devices for complementary heterostructure FET applications

The low extrinsic transconductance of the p-channel self-aligned gate heterostructure insulated gate FETs (HIGFETs), resulting from low hole mobility and high source resistance, has limited the performance of these devices. Results are presented for such devices fabricated on an AlGaAs/InGaAs/GaAs strained quantum-well structure. Transconductance, transconductance parameter, and maximum drain current as high as 113 mS/mm, 305 mS/V/mm, and 94 mA/mm, respectively, were achieved in 0.8- mu m devices at room temperature. At 77 K 181 mS/mm, 800 mS/V/mm, and 180 mA/mm, respectively, were obtained in 1- mu m devices. The highest hole field effect mobilities deduced from the device data are 860 and 2815 cm/sup 2//V at room temperature and 77 K, respectively. These device parameters are believed to be the best reported to date, suggesting that a viable complementary heterostructure FET technology based on this structure can be realized. >

Velocity saturation effect in heterostructure field effect transistors

Heterostructure FETs fabricated on a doped InGaAs channel heterostructure yielded device performance superior to that of devices on a conventional superlattice MODFET structure A comparison is made for 0.4-10- mu m-gate-length FETs fabricated with a self-aligned gate process. Transconductance as high as 534 mS/mm was achieved with 0.4- mu m doped channel devices. This device structure achieved better performance through electron velocity saturation during normal device operation. The MODFET structure, in contrast, is limited by charge transfer into the charge control layer and by gate leakage. Ring oscillators fabricated using 0.3*10 mu m/sup 2/ gates on the doped channel structure yielded a minimum gate delay of 8.3 ps at 2.3 mW/gate.<<ETX>>