Confinement Of Two-dimensional Electron Gas Research Articles

Growth and characterization of AlGaN/GaN/AlGaN double-heterojunction high-electron-mobility transistors on 100-mm Si(111) using ammonia-molecular beam epitaxy

To improve the confinement of two-dimensional electron gas (2DEG) in AlGaN/GaN high electron mobility transistor (HEMT) heterostructures, AlGaN/GaN/AlGaN double heterojunction HEMT (DH-HEMT) heterostructures were grown using ammonia-MBE on 100-mm Si substrate. Prior to the growth, single heterojunction HEMT (SH-HEMT) and DH-HEMT heterostructures were simulated using Poisson-Schrödinger equations. From simulations, an AlGaN buffer with “Al” mole fraction of 10% in the DH-HEMT was identified to result in both higher 2DEG concentration (∼1013 cm−2) and improved 2DEG confinement in the channel. Hence, this composition was considered for the growth of the buffer in the DH-HEMT heterostructure. Hall measurements showed a room temperature 2DEG mobility of 1510 cm2/V.s and a sheet carrier concentration (ns) of 0.97 × 1013 cm−2 for the DH-HEMT structure, while they are 1310 cm2/V.s and 1.09 × 1013 cm−2, respectively, for the SH-HEMT. Capacitance-voltage measurements confirmed the improvement in the confinement of 2DEG in the DH-HEMT heterostructure, which helped in the enhancement of its room temperature mobility. DH-HEMT showed 3 times higher buffer break-down voltage compared to SH-HEMT, while both devices showed almost similar drain current density. Small signal RF measurements on the DH-HEMT showed a unity current-gain cut-off frequency (fT) and maximum oscillation frequency (fmax) of 22 and 25 GHz, respectively. Thus, overall, DH-HEMT heterostructure was found to be advantageous due to its higher buffer break-down voltages compared to SH-HEMT heterostructure.

Tunable density of two-dimensional electron gas in GaN-based heterostructures: The effects of buffer acceptor and channel width

This is a theoretical study of GaN-based heterostructures with unintentionally doped (UID) GaN channel layer and high-resistivity (HR) GaN buffer layer doped by deep acceptors. Self-consistent Schrödinger-Poisson (SP) numerical simulation shows that, by increasing the acceptor concentration in the HR buffer or narrowing the width of UID channel, the quantum confinement of two-dimensional electron gas (2DEG) is enhanced, while the sheet density of 2DEG is reduced. The tuning effect of 2DEG density is attributed to the depletion effect of negative space charges composed of ionized acceptors located in the region between the UID channel and the Fermi-level pinned region in the HR buffer. For the heterostructure without the UID channel, the 2DEG can be depleted as the acceptor concentration is beyond a critical value. However, by inserting a UID channel layer, the depletion effect of buffer acceptor on 2DEG density is reduced. To gain a further insight into the physics, a simple analytical model is developed, which reproduces well the results of SP simulation. By comparing our theoretical results with the experimental ones, a good agreement is reached, thus the validity of our model is verified.

The effect of back-barrier layer on the carrier distribution in the AlGaN/GaN double-heterostructure

The one-dimensional self-consistent simulation of the band diagram and carrier distribution of the AlGaN/GaN double hetero-structure is firstly carried out to research the effect of the thickness and Al content of the AlGaN back-barrier layer on the carrier distribution. Then the AlGaN/GaN double hetero-structure materials with different back-barrier layers were grown by low-pressure MOCVD method on c-plane sapphire substrate. The mercury probe CV measurement was carried out to verify the results of theoretical simulation. The results of theoretical simulation and experiment both indicate that with the increase of Al content and thickness of the AlGaN back-barrier layer, the two-dimensional electron Gas density becomes low in the main channel and high in the parasitic channel gradually.The increase of Al content and thickness of the AlGaN back-barrier layer effectively enhances the two-dimensional electron Gas confinement but simultaneity produces higher-density parasitic channel. So a compromise has to be made between the improvement of the two-dimensional electron Gas confinement and the restraint of the carrier density in parasitic channel in designing the double heterostructure.

Improving the mobility of an In0.52Al0.48As/In0.53Ga0.47As inverted modulation-doped structure by inserting a strained InAs quantum well

The mobility of two-dimensional electrons in an In0.52Al0.48As/In0.53Ga0.47As inverted modulation-doped structure improved by inserting an InAs quantum well into the InGaAs channel. This letter addresses the main cause of this mobility improvement. By optimizing the thickness of the InAs quantum well, its distance from the underlying InAlAs spacer layer, and the InAlAs spacer-layer thickness, maximum mobilities of 16 500 cm2/V s at 300 K and 155 000 cm2/V s at 10 K are attained. The improvement in mobility is attributed to a decrease in scattering caused by ionized impurities, interface-roughness, and trap impurities. This decrease is a result of the superior confinement of two-dimensional electron gas in the InAs quantum well.

Electrostatics of edge channels.

We propose a quantitative electrostatic theory of the gate-induced confinement of two-dimensional electron gas (2DEG) in the quantum Hall regime. The self-consistent electrostatic potential in the region occupied by 2DEG changes in a steplike manner due to the formation of alternating strips of compressible and incompressible electron liquids. We obtain the dependence of positions and widths of these strips on the filling factor. Incompressible strips are shown to be much more narrow than the compressible ones. The relationship between the widths of the adjacent compressible and incompressible strips is found to be universal: It does not depend on the strip number, magnetic field, or gate voltage. Our theory enables us to explain results obtained in experimental studies of edge-state equilibration.

Characterization of Ultrahigh-Speed Pseudomorphic InGaAs/AlGaAs Inverted High Electron Mobility Transistors

A detailed characterization of ultrahigh-speed pseudomorphic InGaAs/AlGaAs inverted high electron mobility transistors (pseudomorphic I-HEMT's) is reported. Charge control analysis indicates that the pseudomorphic I-HEMT accomplishes improved transconductance (gm) and reduced short channel effects due to higher concentration and excellent confinement of two-dimensional electron gas (2DEG) in the InGaAs channel. The 0.2 µm gate pseudomorphic I-HEMT's reported here exhibit superior DC and RF performances, i.e., maximum transconductance (gmmax) of 565 mS/mm, cutoff frequency (fT) of 110 GHz and propagation delay times (τpd's) of 6.6 ps/gate at R.T. and 4.9 ps/gate at 120 K. Delay time analysis reveals the reduction of both the electron transit time (τtransit) and the channel charging time (τchannel) in the pseudomorphic I-HEMT, which are significant factors in the ultrahigh-speed performance. Higher electron saturation velocity (υs:2.1×107 cm/s) and improved gm are considered to contribute to the reduction in τtransit and τchannel.