Palash Das

Graded barrier AlGaN/AlN/GaN heterostructure for improved 2-dimensional electron gas carrier concentration and mobility

AbstractThis paper presents an approach of compositional grading of the barrier in AlGaN/GaN quantum well heterostructure to achieve high two dimensional electron gas (2DEG) carrier concentration and mobility for RF power amplifier applications. Plasma assisted Molecular Beam Epitaxy (PAMBE) has been used to grow compositionally graded AlGaN/GaN and AlGaN/AlN/GaN heterostructures. In-situ cathodoluminescence (CL) and ex-situ high resolution x-ray diffraction (HRXRD) along with high resolution transmission electron microscopy (HRTEM) techniques were used to study the compositions and thicknesses of grown heterostructures. Ohmic contact formation for all the samples were found to be challenging due to unusual surface behavior and thus addressed with three different metallization schemes. The graded AlGaN/GaN and AlGaN/AlN/GaN heterostructures show 2DEG carrier concentrations of 2.0 × 1013 cm−2 and 2.3 × 1013 cm−2 with carrier mobility of 764 cm2v−1s−1 and 960 cm2v−1s−1, respectively at room temperature. A performance index has been proposed to correlate the obtained results with its suitability for particular RF applications.

Reverse bias leakage current mechanism of AlGaN/InGaN/GaN heterostructure

The reverse bias leakage current mechanism of AlGaN/InGaN/GaN heterostructure is investigated by current-voltage measurement in temperature range from 298 K to 423 K. The Higher electric field across the AlGaN barrier layer of AlGaN/InGaN/GaN double heterostructure due to higher polarization charge is found to be responsible for strong Fowler-Nordheim (FN) tunnelling in the electric field higher than 3.66 MV/cm. For electric field less than 3.56 MV/cm, the reverse bias leakage current is also found to follow the trap assisted Frenkel-Poole (FP) emission in low negative bias region. Analysis of reverse FP emission yielded the barrier height of trap energy level of 0.34 eV with respect to Fermi level.

A Strategic Review of Recent Progress in Metamorphic Quantum Well Based Heterostructure Electronic Devices

Recent expansion in the demand for high performance applications require high performance devices. It can be achieved by utilizing features of the quantum well based heterostructures on metamorphic buffer. Based on this metamorphic technique two electronic devices, named high electron mobility transistors (MHEMTs) & heterojunction bipolar transistors (MHBTs) are the areas of interest now-a-days. This paper reviews the remarkable progress being made in the development of InP based MHEMT & MHBT in the context of material properties, device structures, DC and RF performances for the development of low cost, high performance power amplifier (PA) and low noise amplifier (LNA) with high linearity applications.

Simplified 2DEG carrier concentration model for composite barrier AlGaN/GaN HEMT

The self consistent solution of Schrodinger and Poisson equations is used along with the total charge depletion model and applied with a novel approach of composite AlGaN barrier based HEMT heterostructure. The solution leaded to a completely new analytical model for Fermi energy level vs. 2DEG carrier concentration. This was eventually used to demonstrate a new analytical model for the temperature dependent 2DEG carrier concentration in AlGaN/GaN HEMT.

Fowler–Nordheim Tunnelling Contribution in AlGaN/GaN on Si (111) Schottky Current

ABSTRACT AlGaN/GaN heterojunction with Schottky metal contact can be modelled with two back-to-back diodes. The forward-biased diode between metal and AlGaN barrier acts at the onset of current with positive bias. Fowler– Nordheim tunnelling is mainly responsible for the electron transport at the low positive bias level. Downward energy band bending of AlGaN barrier with further positive voltage reduces the tunnelling probability due to lowering of the barrier height of the first diode, causing a dramatic change in the current.

An Alternative X-ray Diffraction Analysis for Comprehensive Determination of Structural Properties in Compositionally Graded Strained AlGaN Epilayers

In this letter, a standard deviation based optimization technique has been applied on High Resolution X-ray Diffraction symmetric and asymmetric scan results to accurately determine the Aluminum molar fraction and lattice relaxation of Molecular Beam Epitaxy grown compositionally graded Aluminum Gallium Nitride (AlGaN)/Aluminum Nitride/Gallium Nitride (GaN) heterostructures. Mathews–Blakeslee critical thickness model has been applied in an alternative way to determine the partially relaxed AlGaN epilayer thicknesses. The coupling coefficient determination has been presented in a different perspective involving sample tilt method by offset between the asymmetric planes of GaN and AlGaN. Sample tilt is further increased to determine mosaic tilt ranging between 0.01° and 0.1°.Graphical Abstract

Quantitative investigation into the source of current slump in AlGaN/GaN HEMT on both Si (111) and sapphire: Self-heating and trapping

We have experimentally studied trapping and self-heating effect in terms of current slump in AlGaN/GaN HEMT grown and identically processed on Silicon (111) and Sapphire (0001) substrates. Different responses have been observed through DC characterization of different duty cycle (100%, 50%, 5% and 0.5%) of pulses at drain end. Effect of self-heating is more in case of HEMT on Sapphire due to its comparative poor thermal conductivity whereas trapped charges have strong contribution in current drop of HEMT on Si (111) due to larger lattice as well as thermal expansion coefficient mismatched epitaxy between GaN and Si (111). These results have been compared among substrates that lead us to find out optimal source of current slump quantitatively between traps and self-heating.

Analytical Expression of Barrier Layer for Enhancement Mode AlGaN/GaN HEMT

In this paper, we have investigated analytically the required thickness of barrier layer for the enhancement mode of AlGaN/GaN high electron mobility transistor (HEMT). A mathematical expression is derived for barrier layer of AlGaN/GaN high electron mobility transistor (HEMT) so that the device can work in enhancement mode. This critical value of barrier layer is fixed for a particular Al composition, gate barrier height and relaxation factor. The device will work in enhancement mode if the barrier layer thickness is below the critical value. This critical value of barrier layer is a function of polarization charge. It is seen from derived result that critical value of barrier layer increases if the polarized charge is reduced. Threshold voltage is calculated to show the dependence of critical barrier layer thickness and the gate barrier height.

Effect of Vertical and Longitudinal Electric Field on 2DEG of AlGaN/GaN HEMT on Silicon: A Qualitative Reliability Study

Effect of both vertical and longitudinal electric field on AlGaN/GaN HEMT 2DEG channel is studied under long pulses with different duty cycles at two separate VGS (i.e. at +2 and -2 V). The duty cycles of applied pulses are of 100, 50, 5 and 0.5 %. Separate responses are being observed to confirm different non-ideal reliability issues like scatterings, effects of surface states and trapped electrons. It also raises an optimization scenario between available 2DEG concentration in channel and its various scattering and depletion phenomenon.

Reduction of negative differential conductivity effect of AlGaN/GaN HEMTs using gate scaling

Gate characteristic is one of the most important parts for the HEMTs. In this paper the DC and RF performance improvement using gate length scaling has been presented. The results show that reduction of the gate length from 1 µm to 0.15 µm, the current gain cutoff frequency increases from 12 GHz to 56 GHz and power gain cutoff frequency increases from 20.5 GHz to 63 GHz respectively. In this paper we report the first ever simulation based device structure design for reduction of Negative Differential Conductivity (NDC) effect in AlGaN/GaN HEMTs by gate length scaling.