Studies on the Influences of i-GaN, n-GaN, p-GaN and InGaN Cap Layers in AlGaN/GaN High-Electron-Mobility Transistors

Abstract

Systematic studies were performed on the influence of different cap layers of i-GaN, n-GaN, p-GaN and InGaN on AlGaN/ GaN high-electron-mobility transistors (HEMTs) grown on sapphire by metal organic chemical vapor deposition. The decrease of maximum extrinsic transconductance (g m ) and maximum drain current density (I Dmax ) values agrees with the product values of two-dimensional electron gas (2DEG) mobility ( H ) and 2DEG sheet concentration (n s ) of AlGaN/GaN HEMT structures. An improved Schottky barrier height with low surface roughness has been observed in AlGaN/GaN HEMT structure with i-GaN and n-GaN cap layers. The HEMTs with i-GaN cap layer exhibited low gate leakage current with high breakdown voltage among the other HEMTs. Though the HEMTs with n-GaN cap layer and without cap layers exhibited good H , g m , I Dmax values, the ac characteristics are not up to the extent of HEMTs with i-GaN cap layer. All the devices except the HEMTs with InGaN cap layers were operational even up to the measurement temperature of 350 C. The HEMTs with i-GaN cap layer exhibited collapse-free I DS -V DS characteristics with small I D hysteresis width variations among the other HEMTs. The observation of small threshold voltage variation, small drain current hysteresis width and small white light illumination effects confirms the existence of small trapping effects in HEMTs with i-GaN cap layers. Only one thermally activated trap level at À0:161 eV was observed on AlGaN/GaN HEMTs with i-GaN cap layer. However, each of the three trap levels has been observed in HEMTs with other cap layers and HEMTs without cap layers. From this, it is concluded that the collapserelated traps are screened/passivated from two-dimensional electron gas by the addition of thin i-GaN cap layer on AlGaN/ GaN HEMTs. The cap layer i-GaN (3 nm) is a promising candidate to get collapse-free AlGaN/GaN HEMTs.

Under-gate defect formation in Ni-gate AlGaN/GaN high electron mobility transistors

“…A $20 nm-thick layer of Al 0.28 Ga 0.72 N with an n-type GaN capping layer were grown on the epitaxial GaN layer via molecular beam epitaxy [27]; the n-type GaN cap was incorporated to reduce current collapse during biasing [28,29]. Further details regarding semiconductor epilayer growth are described elsewhere [30][31][32].…”

Mechanism of mobility increase of the two-dimensional electron gas in AlGaN∕GaN heterostructures under small dose gamma irradiation

“…1 It is believed that this current degradation is a result of the trapping effect of the electrons by surface states. Therefore, different kinds of passivation materials 2-7 ͑mainly concentrated on more effective Si 3 N 4 passivation͒ or cap layers [8][9][10] were investigated to decrease surface trap effect on the formation of two-dimensional electron gas ͑2DEG͒ at the heterostructure interface. In spite of temporary stabilization of surface properties, an additional surface layer causes stress-induced changes in the barrier layer [11][12][13][14] and changes the conditions for the formation of 2DEG as a result of piezoelectric and spontaneous polarization effects.…”

AlGaN/GaN HEMT grown on large size silicon substrates by MOVPE capped with in-situ deposited Si3N4

“…Current dispersion is one of the most serious problems in AlGaN/GaN HEMT and has been observed by various groups [7][8][9] [10][11][12][13] and also in-situ deposited insulating, p-type, n-type or low temperature GaN cap layers [14][15][16]. Promising results have been reported so far but ex-situ depositions are always carried out on a surface which has been exposed to air, while GaN passivation layers may lead to lower 2DEG density (7-8 Â 10 12 cm À2 ) [14][15][16] and consequently higher sheet resistance (4400 O/&).…”