Inverted High Electron Mobility Transistor Research Articles

Design and simulation of a novel GaN based resonant tunneling high electron mobility transistor on a silicon substrate

For the first time, we have introduced a novel GaN based resonant tunneling high electron mobility transistor (RTHEMT) on a silicon substrate. A monolithically integrated GaN based inverted high electron mobility transistor (HEMT) and a resonant tunneling diode (RTD) are designed and simulated using the ATLAS simulator and MATLAB in this study. The 10% Al composition in the barrier layer of the GaN based RTD structure provides a peak-to-valley current ratio of 2.66 which controls the GaN based HEMT performance. Thus the results indicate an improvement in the current–voltage characteristics of the RTHEMT by controlling the gate voltage in this structure. The introduction of silicon as a substrate is a unique step taken by us for this type of RTHEMT structure.

N‐polar n+ GaN cap development for low ohmic contact resistance to inverted HEMTs

AbstractThis study investigates the development and application of Si‐doped N‐polar n+ GaN capping layers used for the reduction of ohmic contact and access resistances in inverted HEMTs. By applying an optimized n+ GaN capping layer to a standard undoped inverted HEMT, we find that non‐alloyed Ti/Al/Ni/Au contacts demonstrate a very low ohmic contact resistance of 80 Ω‐µm. To eliminate the n+ GaN shunting pathway in HEMT devices, we have used e‐beam lithography and dry etching to define windows that establish an effective source‐drain spacing. 0.3 µm gate‐length inverted HEMTs with n+ GaN caps and 1 µm effective source‐drain spacing were found to have a high maximum current density of 1.94 A/mm and an extrinsic transconductance of 335 mS/mm. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

A Two-Dimensional Electron Gas as a Sensitive Detector for Time-Resolved Tunneling Measurements on Self-Assembled Quantum Dots

A two-dimensional electron gas (2DEG) situated nearby a single layer of self-assembled quantum dots (QDs) in an inverted high electron mobility transistor (HEMT) structure is used as a detector for time-resolved tunneling measurements. We demonstrate a strong influence of charged QDs on the conductance of the 2DEG which allows us to probe the tunneling dynamics between the 2DEG and the QDs time resolved. Measurements of hysteresis curves with different sweep times and real-time conductance measurements in combination with an boxcar-like evaluation method enables us to unambiguously identify the transients as tunneling events between the s- and p-electron QD states and the 2DEG and rule out defect-related transients.

GaAs(111)A/B surface orientation effects on electron density in normal and inverted pseudomorphic HEMTs

We present calculations of the two-dimensional electron density in a Si δ-doped AlGaAs/InGaAs/GaAs pseudomorphic high electron mobility transistor (P-HEMT), at low temperature, for three different growth directions (001) and (111)A/B. The calculations are made using a self-consistent resolution of Schrodinger and Poisson equations. The presence of a strong built-in piezoelectric field in (111)A/B growth directions causes changes of the confining potential shape and the carrier distribution in the InGaAs channel. We discuss the influence of GaAs substrate orientation on the conduction-band structure and thereafter on the two-dimensional electron gas (2DEG) concentration in the channel. Our results show that the calculated 2DEG concentration in the normal P-HEMT structure grown on a (111)A GaAs substrate is significantly higher than those grown on (001) and (111)B GaAs substrates. We also note an increase of the average separation between the ionized donors and the carriers. On the other hand, the GaAs (111)B substrate orientation appears as inadequate for the type of structure (normal P-HEMT) on account of the charge-transfer reduction in the channel compared with the (001) orientation. In contrast, we demonstrate that the calculated 2DEG Si δ-doped GaAs/InGaAs/AlGaAs pseudomorphic inverted high electron mobility transistor (PI-HEMT) grown on aGaAs (111)B substrate is appreciably higher than that grown on (001) and afterwards an enhancement of the spatial separation between confined electrons in the channel and ionized dopants occurs. These effects might result in considerably improved devices of great interest regarding high electron mobility.

Quantum point contacts formed in GaAs/GaAlAs heterostructures by shallow etching and overgrowth

Four types of quantum point contacts (QPC) have been fabricated and their conductance vs gate voltage investigated at various temperatures and biases. Conventional high electron mobility transistor (HEMT) structures and inverted HEMT (I-HEMT) structures were used as starting material. The QPC constriction was formed by a shallow wet-etch and devices with constriction widths between 50 and 400 nm were investigated. On both types of heterostructures, two types of devices were fabricated. In the first type of device, an Al gate electrode was deposited directly on top of the etched constriction. The second type of device was made with a buried constriction by MBE regrowth of AlGaAs on top of the shallow etched sample and finally gate metalisation. For all four types of devices, conductance quantization was observed up to temperatures between 20 and 30 K. Separations between the first and second 1D subbands were found to be up to 20 meV for Al gated devices and up to 10 meV for the regrown devices.

Fabrication of 0.2 μm gate pseudomorphic inverted HEMT by phase-shifting technology

A phase-shifter edge line (PEL) mask method is applied to the gate fabrication process of InGaAs AlGaAs pseudomorphic inverted high electron mobility transistors (HEMTs). In phase-shifting technologies, the PEL mask method suits forming a fine line pattern owing to its high resolution. Mushroom shaped 0.2 μm gate pseudomorphic inverted HEMTs fabricated by using the PEL mask method show high d.c. and r.f. performances. As one of the applications of the 0.2 μm gate pseudomorphic inverted HEMTs, a decision circuit IC for 10 Gb/s optical communication systems is successfully fabricated.

Growth conditions and device performance of InGaAs/AlGaAs pseudomorphic inverted high electron mobility transistor

Growth conditions of InGaAs/AlGaAs pseudomorphic inverted high electron mobility transistor (PI-HEMT) structures have been studied. In addition, a new device structure with an InGaAs channel below the critical thickness has been examined. We also investigated misfit dislocations and electron transport properties of PI-HEMTs with various InGaAs channel layer widths above the critical layer thicknesses.

Improving the characteristics of an [formula omitted] inverted HEMT by inserting an InAs layer into the InGaAs channel

A novel InAlAs InGaAs inverted HEMT with a thin InAs layer inserted into the InGaAs channel (InAs-inserted-channel i-HEMT) is proposed and its electron transport properties and device performances are investigated. By optimizing the thickness of the InAs layer and its distance from the underlying InAlAs spacer layer, a maximum mobility of 13,600 cm 2/Vs at 300 K and 63,500 cm 2/Vs at 77 K were attained. The inverted HEMT structure resulted in an extremely high voltage gain, over 60 in a 0.7 μm gate-length device, and a gate-to-drain breakdown voltage as high as 9 V. This voltage gain resulted in a maximum oscillation frequency for a 0.7 μm gate-length device of 81 GHz, 14% higher than that of an InAs-inserted-channel normal HEMT.

A noise model for high electron mobility transistors

A model to explain the noise properties for AlGaAs/GaAs HEMT's, AlGaAs/InGaAs/GaAs pseudomorphic HEMT's (P-HEMT's) and GaAs/AlGaAs inverted HEMT's (I-HEMT's) is presented. The model Is based on a self-consistent solution of Schrodinger and Poisson's equations. The influence of the drain-source current, frequency and device parameters on the minimum noise figure F/sub min/ and minimum noise temperature T/sub min/, for different HEMT structures are presented. The study shows that P-HEMT's have a better noise performance than the normal and inverted HEMT's. The present model predicts that a long gate P-HEMT device will exhibit a better noise performance than a conventional HEMT. There is a range of doped epilayer thickness where minimum noise figure is a minimum for pseudomorphic HEMT's which is not observed in conventional and inverted HEMT's. The calculated noise properties are compared with experimental data and the results show excellent agreement for all devices. >

Optical emission from the one-dimensional electron gas in narrow modulation-doped GaAs/(InGa)As/(AlGa)As quantum wires fabricated by lateral top barrier modulation

We report on the fabrication and photoluminescence characterisation of n-type doped quantum wires, which are based on a modulation-doped GaAs/(InGa)As/(AlGa)As quantum well structure, as used in inverted high electron mobility transistors. Lateral patterning was performed by electron beam lithography followed by a selective wet etch process to remove the n-type doped GaAs top barrier between the wire regions. The removal of the top barrier was verified by micro-Raman spectroscopy. Spatially indirect emission from the one-dimensional (ID) electron gas formed in the quantum wires is observed in low-temperature photoluminescence, even for the narrowest geometrical wire width of 23 nm. The emission shows a blue-shift for wire widths below 100 nm, which amounts to up to 60 meV for the narrowest wires. PACS: 78.66.Fd, 73.20.Dx, 78.55.Cr

Pseudomorphic inverted HEMT suitable to low supplied voltage application

An enhancement-mode pseudomorphic inverted HEMT with short gate length shows superior saturation properties at low drain voltage. Such saturation properties are suitable for low-supplied-voltage applications. High-frequency properties of FETs were also studied, using two types of frequency-dependent measurement systems which represent active load and common-source circuits. It was confirmed that low knee voltage in the static I-V curve is preserved above 100 kHz. The estimated output power for the device is 50% higher than that of conventional pseudomorphic HEMT at supplied voltage of 1 V. >

Influence of Si segregation on the two-dimensional electron gas mobility of inverted HEMT structures

SIMS depth profiles of Si and Al as well as Hall mobility data are reported for inverted high electron mobility transistor (I-HEMT) structures grown by MBE at different substrate temperatures, T s , between 350 and 750°C. To obtain both high electron mobility μ(cm 2 (Vs) −1) and high electron density of the two-dimensional electron gas, the doped AlGaAs barrier was separated from the undoped GaAs layer by a 5 nm undoped AlGaAs spacer. By high resolution SIMS depth profiling this spacer was indeed found to be free of Si (detection limit 1×10 17 atoms cm −3) for structures grown at T s ⪕ 650°C . For T s = 700° C, however, the space contains more than 10 17 atoms cm −3. For T s = 750° C the heterojunction appears broadened and the spacer is doped with Si up to 1×10 18 atoms cm −3. The low temperature Hall data are in accordance with these observations. The highest mobility ( μ = 61,000 at T = 18 K) was recorded for T s = 500° C, and was found to decrease slowly to μ = 14,000 at T s = 700° C, then dropping abruptly to μ = 2300 for T s = 750° C. It is concluded that charge carrier scattering by ionized Si, which has migrated into the nominally undoped spacer, degrades the electrical performance of I-HEMTs grown at T s ⪖ 500° C. A control experiment demonstrated that the mechanism of Si migration is not diffusion, but segregation. At T s = 750° C a deterioration of the heterojunction and/ or a migration induced doping of the active channel is indicated.

InP-based inverted high electron mobility transistors

The fabrication and characterisation of an inverted high-electron-mobility transistor (HEMT) in the AlInAs/GaInAs-on-InP material system are reported. Inverted HEMTs, which have the donor layer beneath the channel, have the potential for higher transconductance, current gain cutoff frequency, and power gain cutoff frequency than conventional HEMTs because the gate can be placed closer to the two-dimensional electron gas (2DEG).

Enhancement-mode pseudomorphic inverted HEMT for low noise amplifier

Characteristics of the pseudomorphic inverted HEMT (P-I-HEMT) are compared with those of the pseudomorphic HEMT. Both devices were fabricated in enhancement mode by the same process. P-I-HEMT shows a higher maximum transconductance of 590 mS/mm, and higher K-value of 600 mS/Vmm at a threshold voltage of O V, and better pinch-off characteristics than its counterpart. Noise characteristics of P-I-HEMT are reported. Lower noise figure (1.0 dB at 18 GHz) was obtained in the P-I-HEMT. It is concluded that the P-I-HEMT shows far better noise characteristics than the other at low drain voltage and current.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Transport measurements on a high mobility, ultralow carrier concentration inverted GaAs/AlGaAs heterostructure

We have measured the low temperature magnetoresistance and Hall Voltage on an inverted high electron mobility transistor (HEMT) structure with ultra low carrier concentration in the dark . We have observed Shubnikov-de Haas oscillations and quantized Hall voltages at very low magnetic fields.

TEM characterization of MBE-grown interfaces for inverted-HEMT structures

With the advent of the high electron mobility transistor (HEMT), device mobilities in excess of 130,000 cm2/V-sec (77K) for an MBE-grown GaAs/AlxGa1-xAs (100) (0.25&gt;x&gt;0.30) heterojunction have become commonplace. However, the corresponding growth of high quality AlGaAs/GaAs inverted HEMT structures (Figure 1) has been difficult due to the high degree of disorder at the inverted heterointerface (maximum 77K mobility 80,000 cm2/V-sec). The AlGaAs growth front is subject to: 1) slower migration kinetics of Al with respect to Ga, 2) high reactivity of Al to the ambient impurities, and 3) outdiffusion of the silicon dopant along the growth front, which further compromises the uniformity and planarity of the growth. Past attempts to grow inverted HEMTs have been limited by the competing parameters of high substrate temperatures (T) prefered for achieving high migration of Al for smoother interfaces, and lower Ts to minimize silicon outdiffusion toward the critical interface. In this presentation we introduce TEM characterization of a novel MBE technique which optimizes interface growth conditions, achieving mobilities &gt;120,000 cm2/V-sec, for the inverted HEMT structure.

A 0.5- mu m-gate GaAs/AlGaAs inverted HEMT IC-multiplier and D/A converter

A gate recess process for a 0.5- mu m I-HEMT (inverted high electron mobility transistor) has been developed. A drain conductance for the 0.5- mu m I-HEMT as small as 2 mS/mm was achieved, indicating a small short-channel effect. The threshold voltage uniformities were studied in microscopic and macroscopic areas in a 2-in wafer. The uniformities are very high, i.e. the standard deviations of microscopic and macroscopic areas are 10 and 30 mV, respectively, at a threshold voltage of 0.1 V. An 8*4 parallel multiplier was fabricated, and a multiplication time of 1.67 ns was obtained at room temperature. An 8-b digital/analog converter (DAC) was fabricated and operated at a clock rate of 1.2 GHz. The DC linearity of the DAC is better than 0.18 LSB. These results confirm that an I-HEMT is very well suited for high-speed integrated circuits. >