D.K. Arch

A new and simple model for GaAs heterojunction FET gate characteristics

The gate current-voltage characteristics for modulation-doped field-effect transistors (MODFETs) and heterostructure insulated-gate field-effect transistors (HIGFETs) are described using a simple model. This model, which is physically realistic, consists of two Schottky diodes in series: one is a metal-semiconductor (AlGaAs) Schottky diode and the other is an equivalent Schottky diode due to the heterojunction between the AlGaAs and GaAs. A novel technique is developed to characterize the parameters used for this model. The model is used to estimate the effective electron temperature in the channel close to the drain for MODFETs. The estimated electron temperature with 1 V drain-to-source voltage is as high as 80 degrees C at room temperature. Very good agreement between the calculated and measured results is obtained. This model and characterization technique are also suitable for other heterojunction FETs such as quantum-well MODFETs, etc. >

Quantum-well p-channel AlGaAs/InGaAs/GaAs heterostructure insulated-gate field-effect transistors with very high transconductance

Quantum-well p-channel pseudomorphic AlGaAs/InGaAs/GaAs heterostructure insulated-gate field-effect transistors with enhanced hole mobility are described. The devices exhibit room-temperature transconductance, transconductance parameter, and maximum drain current as high as 113 mS/mm, 305 mS/V/mm, and 94 mA/mm, respectively, in 0.8- mu m-gate devices. Transconductance, transconductance parameter, and maximum drain current as high as 175 mS/mm, 800 mS/V/mm, and 180 mA/mm, respectively were obtained in 1- mu m p-channel devices at 77 K. From the device data hole field-effect mobilities of 860 cm/sup 2//V-s at 300 K and 2815 cm/sup 2//V-s at 77 K have been deduced. The gate current causes the transconductance to drop (and even to change sign) at large voltage swings. Further improvement of the device characteristics may be obtained by minimizing the gate current. To this end, a type of device structure called the dipole heterostructure insulated-gate field-effect transistor is proposed.<<ETX>>

Quantum-well p-channel AlGaAs/InGaAs/GaAs heterostructure insulated-gate field-effect transistors

The authors report experimental and theoretical results for self-aligned gate quantum-well p-channel pseudomorphic GaAs/InGaAs/AlGaAs heterostructure insulated-gate field-effect transistors (HIGFETs). High transconductances and transconductance parameters, 113 mS/mm and 305 mS/mm/V at room temperature for 0.8- mu m gate length, are reported. The authors discuss the effects of built-in strain on the valence bands, analyze the device data on the gate length dependence of the device parameters, and discuss the subthreshold characteristics and evidence for implant straggle. They also show the importance of the gate current for the device characteristics and propose to reduce the gate current and thus to increase the voltage swing by fabricating p-channel devices with p-doped InGaAs channels and n/sup +/ AlGaAs gates on the insulating AlGaAs layer. >

New negative resistance regime of heterostructure insulated gate transistor (HIGFET) operation

We present experimental evidence for negative differential resistance in n-channel heterostructure insulated gate transistors (HIGFETs) at high gate voltages. The negative resistance is explained by an increase in the gate current related to the electron heating in the two-dimensional electron gas. This mechanism is similar to that causing the negative differential resistance in NERFETs. However, much smaller parasitic capacitance in HIGFETs may allow us to reach higher frequencies of operation.

HgCdTe heterojunction contact photoconductor

A new photoconductor device structure is described utilizing a heterojunction contact which incorporates a higher band‐gap HgCdTe alloy between the metal contact and the normal band‐gap photoconductor. A theoretical treatment of the heterojunction contact photoconductor (HCP) device shows that carrier sweepout can be virtually eliminated; the calculations predict elimination of ‘‘saturation’’ of responsivity and a very large increase in responsivity. HCP devices were fabricated; experimental results verified the theory in several ways. A responsivity was measured at 80 K of about 450 000 V/W at 30 V/cm and over 1 500 000 V/W at 125 V/cm.

Layer intermixing in HgTe‐CdTe superlattices

High‐temperature x‐ray diffraction measurements on HgTe‐CdTe superlattices grown by molecular beam epitaxy have been made to determine the extent of intermixing of the individual HgTe and CdTe layers. In situ interdiffusion measurements were carried out at 110, 162, and 185 °C and estimates of the interdiffusion coefficient were made. We find appreciable intermixing of the HgTe and CdTe layers at temperatures as low as 110 °C. Such results have serious implications for the use of this material in optoelectronic devices.

Optical absorption and x‐ray diffraction in narrow‐band‐gap InAs/GaSb superlattices

We report on the optical transmission properties of narrow‐band‐gap (Eg<0.1 eV) InAs/GaSb superlattices grown by molecular‐beam epitaxy. Energy band gaps of 0.15 and 0.085 eV at 4.8 K are determined for a 102‐A‐period and a 124‐A‐period superlattice, respectively. The absorption edge is extremely soft due to the spatial mismatch of hole and electron wave functions. In addition we show the first reported x‐ray diffraction measurements on this materials system.

High responsivity HgCdTe heterojunction photoconductor

We present an experimental and theoretical study of n‐type Hg1−xCdxTe photoconductors in which a large band‐gap alloy was grown on top of a smaller band‐gap active region and contacts were made to the larger gap material. The larger band‐gap material causes an energy barrier to holes which decreases the rate at which they reach the high recombination region of the metal‐semiconductor interface. As a result, this heterojunction contact greatly reduces the effects of carrier sweepout on device performance and leads to much higher detector responsivities. Experimental results in a symmetric device with a cutoff wavelength of 7.8 μm at 77 K show responsivities in excess of 106 V/W and detectivities close to the background limited value and nonsaturation of responsivity with bias voltage. In an asymmetric device, in which only one heterojunction contact was used, an order of magnitude increase in responsivity was observed when the heterojunction contact was biased to attract minority carriers, compared with th...

The influence of electric field and mobility profile on GaAs MESFET characteristics

Analytical approximations for the drain I-V relationship, including the mobility profile and field distribution in the channel from the drain to the source, and GaAs MESFETs are derived. The model includes the extended depletion from the gate to the drain for nonself-aligned devices. The calculation of the electric field along the channel is in very good agreement with existing analytical models and a two-dimensional numerical simulation. Experimentally, the authors fabricated and tested tilted angle lightly doped drain (LDD) GaAs MESFETs. It was found that the LLD MESFET structure suppresses the peak electric field under the gate near the drain region. A lower output conductance and higher drain-to-source breakdown were observed as expected. In addition to the electric field, the mobility profile is another factor that influences the performance of the devices. The accuracy of describing the low-field transconductance is strongly dependent on the mobility profile. Moreover, the mobility profile modifies the electric field along the channel and also influences the shape of the drain I-V curves. It is found that more accurate I-V curves can be obtained once the mobility profile is taken into account. >

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.