AlGaN/GaN polarization-doped field-effect transistor for microwave power applications

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Ultra violet light emitting diodes (UV LEDs) face critical limitations in both the injection efficiency and light extraction efficiency due to the resistive and absorbing p-type contact layers. In this work, we investigate the design and application of polarization engineered tunnel junctions for ultra-wide bandgap AlGaN (Al mole fraction > 50%) materials towards highly efficient UV LEDs. We demonstrate that polarization-induced 3D charge is beneficial in reducing tunneling barriers especially for high composition AlGaN tunnel junctions. The design of graded tunnel junction structures could lead to low tunneling resistance below 10 -3 Ω cm 2 and low voltage consumption below 1 V (at 1 kA/cm 2 ) for high composition AlGaN tunnel junctions. Experimental demonstration of 292 nm emission was achieved through non-equilibrium hole injection into wide bandgap materials with bandgap energy larger than 4.7 eV, and detailed modeling of tunnel junctions shows that they can be engineered to have low resistance, and can enable efficient emitters in the UV-C wavelength range.a) Authors to whom correspondence should be addressed. Electronic mail: zhang.3789@osu.edu, rajan@ece.osu.edu 2The large range of direct bandgaps in the III-Nitride system makes these semiconductors uniquely suitable for achieving ultra violet emission over a wide wavelength range down to 210 nm. III-Nitride ultra-violet light emitting diodes (UV LEDs) have a large variety of applications including water purification and air disinfection, and they provide distinct advantages over traditional gas-based lamps. We propose an approach to realize highly efficient hole injection based on interband tunneling. In this approach, a transparent n-type AlGaN layer is connected to the p-AlGaN layer using an interband tunnel junction ( Fig. 1(b)). Non-equilibrium tunneling injection of holes overcomes limits imposed by thermal ionization of deep acceptors. 9,10 At the same time, it leads to efficient light extraction since it eliminates absorbing p-type top contact layers. 11Making efficient interband p-n tunnel junctions is challenging for ultra-wide bandgap AlGaN materials due to the large band gap, wide depletion barrier, and doping limitations. In this work, we show that nanoscale heterostructure and polarization engineering of tunnel junctions can enable highly efficient tunneling even for ultra-wide bandgap materials where ordinary dopant-based homojunctions would have very low tunneling current density. In this letter, we discuss the design of AlGaN tunnel junctions over the entire Al composition range. We then experimentally demonstrate highly efficient tunnel junctions for AlGaN with Al composition greater than 50%. was used for tunneling probability calculation, and tunneling current was calculated by considering all possible contributions by carriers with different energies. Bandtail states and bandgap narrowing effect due to heavy impurity doping are not included in the simulations, though they could lead to increase in the tunneling probability. ...

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confidence: 99%

mentioning

confidence: 99%

Design and demonstration of ultra-wide bandgap AlGaN tunnel junctions

Zhang

Krishnamoorthy

Akyol

et al. 2016

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“…Transistors based on graded AlGaN alloys have been predicted to be technologically important for microwave applications. 2,3 In this work, we compare measurements and theoretical calculations of mobility for polarization-induced threedimensional electron gases in graded AlGaN alloys. The electrical and physical properties vary in the growth direction in graded AlGaN alloys because of the continuously changing Al composition.…”

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confidence: 99%

Electron mobility in graded AlGaN alloys

Rajan

DenBaars

Mishra

et al. 2006

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Polarization gradients in graded AlGaN alloys induce bulk electron distributions without the use of impurity doping. Since the alloy composition is not constant in these structures, the electron scattering rates vary across the structure. Capacitance and conductivity measurements on field effect transistors were used to find mobility as a function of depth. The effective electron mobility at different depths calculated from theory closely matched the measured mobility. Local bulk mobility values for different AlGaN compositions were found, and the electron mobility in AlGaN as a function of alloy composition was deduced. These were found to match with theoretical calculations.

“…In this work, we take heterostructure engineering approach in which the Al alloy composition in the AlGaN channel is graded from wider bandgap to narrower bandgap under the ohmic contacts, hence grading up electron affinity and presenting a higher electron affinity at the metalsemiconductor interface (GaN electron affinity, χGaN=4.1 eV). AlGaN layers with compositional grading from GaN to AlGaN have been studied extensively, and shown to induce bulk three-dimensional electron distributions due to positive polarization (spontaneous+piezoelectric) charge [16][17][18][19] . The polarization-induced fixed charge, , = − • , where is the sum of spontaneous and piezoelectric polarization in AlGaN alloy.…”

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confidence: 99%

AlGaN channel field effect transistors with graded heterostructure ohmic contacts

Bajaj

Akyol

Krishnamoorthy

et al. 2016

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Abstract:We report on ultra-wide bandgap (UWBG) Al0.75Ga0.25N channel metal-insulatorsemiconductor field-effect transistors (MISFET) with heterostructure engineered low-resistance ohmic contacts. The low intrinsic electron affinity of AlN (0.6 eV) leads to large Schottky barriers at metal-AlGaN interface, resulting in highly resistive ohmic contacts. In this work, we use reverse compositional graded n ++ AlGaN contact layer to achieve upward electron affinity grading, leading to a low specific contact resistance (ρsp) of 1.9x10 -6 Ω.cm 2 to n-Al0.75Ga0.25N channels (bandgap ~ 5.3 eV) with non-alloyed contacts. We also demonstrate UWBG Al0.75Ga0.25N channel MISFET device operation employing compositional graded n ++ ohmic contact layer and 20 nm atomic layer deposited Al2O3 as the gate-dielectric. Several studies have investigated alloyed ohmic contacts to n-AlGaN channels. While alloys using metals such as Titanium, Vanadium and Zirconium 10-15 were reported with low specific contact resistance values below 10 -5 Ω.cm 2 on n-AlGaN channels with Al alloy compositions up to 66%, they are challenging to reproduce reliably due to extremely high temperature processes and typically show non-uniformity in current-voltage characteristics. Alternate heterostructure engineering approaches are therefore needed to realize low contact resistance to large bandgap AlGaN.A low-resistance ohmic contact is formed by reducing the potential barrier between a metal and semiconductor. The ideal n-type ohmic contact would have a zero or low Schottky barrier height at the metal-semiconductor interface, which can be achieved by matching the semiconductor electron affinity 3 and metal work function. However, the intrinsic low electron affinity of AlN (0.6 eV) leads to larger metal-AlGaN Schottky barriers, resulting in a poor tunneling probability for electrons (probability~� . , where φb is the barrier height and W is the tunneling width), and therefore highly resistive ohmics. In this work, we take heterostructure engineering approach in which the Al alloy composition in the AlGaN channel is graded from wider bandgap to narrower bandgap under the ohmic contacts, hence grading up electron affinity and presenting a higher electron affinity at the metalsemiconductor interface (GaN electron affinity, χGaN=4.1 eV). AlGaN layers with compositional grading from GaN to AlGaN have been studied extensively, and shown to induce bulk three-dimensional electron distributions due to positive polarization (spontaneous+piezoelectric) charge [16][17][18][19] . The polarization-induced fixed charge, , = − • , where is the sum of spontaneous and piezoelectric polarization in AlGaN alloy. In case of layers with reverse Al compositional grading from wider to narrower bandgap AlGaN, a negative polarization charge is formed, causing a positive curvature in the energy band profile, and thereby creating a barrier to electron flow. To ensure that the conduction band stays flat, it is necessary to compensate for the negative polarization charge using donors. This...