Study of GaN-based double heterostructure high electron mobility transistor (DH-HEMT)

For higher frequency operation, the device such as high electron mobility transistor (HEMT) needs to be scaled down by reducing the gate length. However, in the case of high voltage operation with reduced gate length, the high concentration of two-dimensional electron gas (2DEG) obtained in the HEMT epistructures may spill over from the quantum well to the GaN buffer which may lead to the decrease in peak currents and RF power densities. AlGaN back barrier in AlGaN/GaN double heterostructure (DH-HEMT) helps to confine the 2DEG to increase the frequency bandwidth of HEMTs. This dissertation describes about the growth and characterization of AlGaN/GaN DH-HEMT on Si using metal organic chemical vapour deposition (MOCVD) process. The wafer bow, surface morphology and electrical characterization such as 2DEG properties, DC characteristics and buffer leakage were done on AlGaN/GaN DH-HEMT epistructure and compared with AlGaN/GaN single heterostructure HEMT (SH-HEMT) epistructure. AlN nucleation layer was grown on Si substrate and step graded AlGaN layer was grown subsequently to compensate the lattice mismatch between AlN and GaN and prevent cracking. Crystal quality of GaN buffer layer was fine-tuned by varying growth temperature, V/III ratio and growth pressure. AlGaN barrier layer and GaN cap layer were optimized to give good 2DEG electrical properties. Coalesced and smooth GaN surface with RMS roughness of 0.3 nm was achieved and surface was observed to be slip plane and crack-free with bow < 20 μm. Contactless Hall was done on the AlGaN/GaN SH-HEMT to measure the 2DEG electrical properties. The sheet resistance, mobility and sheet carrier density were measured to be 459 Ω/sq, 1244 cm2/V.s and 1.09 x 1013 cm-2 respectively. Good 2DEG characteristics together with controlled bowing of AlGaN/GaN SH-HEMT epistructure on Si was achieved. AlGaN/GaN DH-HEMT epistructure with AlGaN back barrier was grown and studied. Similar to AlGaN/GaN SH-HEMT epistructure, AlN nucleation layer was grown on Si substrate and step graded AlGaN buffer layers were subsequently grown. A 1000 nm thick AlGaN back barrier with Al composition of 6 % was grown on the step graded AlGaN layers and good channel current was achieved using 200 nm thick GaN channel. Al composition of AlGaN barrier was kept to 31 % with 15 nm AlGaN barrier thickness. Average sheet resistance, mobility and sheet carrier density of 389 Ω/sq, 1135 cm2/V.s and 1.42 × 1013 cm-2 were obtained. The Idmax of around 700 mA/mm was lower in AlGaN/GaN DH-HEMT as compared to Idmax of around 853 mA/mm measured in AlGaN/GaN SH-HEMT. Lower buffer leakage for AlGaN/GaN DH-HEMT was achieved as compared to AlGaN/GaN SH-HEMT and this is a good indication for higher buffer breakdown. Thus, AlGaN/GaN DH-HEMT epistructure exhibited better device performances in terms of lower buffer leakage.Master of Science (Electronics

Source of two-dimensional electron gas in unintentionally doped AlGaN/GaN multichannel high-electron-mobility transistor heterostructures

Unintentionally doped (UID) AlGaN/GaN-based multichannel high electron mobility transistor (MC-HEMT) heterostructures have been demonstrated on the SiC substrate using plasma-assisted molecular beam epitaxy. The MC-HEMT heterostructure with a GaN channel thickness of 100 nm resulted in a cumulative two-dimensional electron gas (2DEG) concentration of 4.3 × 1013 cm-2 across six GaN channels. The sample showed sheet resistances of 170 ω/sq. and 101 ω/sq. at room temperature and 90 K, respectively. The source of 2DEG in the buried GaN channels of the heterostructure was investigated. The C-V measurements conducted on UID MC-HEMTs excluded the possibility of the valence band being the source of 2DEG and the consequent formation of two-dimensional hole gas at the buried GaN-channel/AlGaN-barrier interfaces. A comparison of the experimentally obtained 2DEG concentration with the simulated data suggests the presence of donor-like trap states, situated at 0.6 to 0.8 eV above the valence band at the buried GaN-channel/AlGaN-barrier interfaces, which act as the source of 2DEG in UID MC-HEMT heterostructures.National Research Foundation (NRF)Published versionThis publication is made possible by the Singapore National Research Foundation under the NRF2017-NRF-ANR003 GaNGUN project