Somna S. Mahajan

Studies on the Thermal Stability of Ni/n–GaN and Pt/n–GaN Schottky Barrier Diodes

High temperature annealing of Ni and Pt Schottky diodes on n-GaN has been investigated. Ni/n–GaN and Pt/n–GaN Schottky diodes were fabricated using ultra high vacuum and annealed at various temperatures (200 °C−700 °C) using rapid thermal anneal system. It was observed that in case of Ni/GaN diodes, the Schottky barrier height increases for 200 °C and then decreases with increase in annealing temperature. For Pt/GaN diodes, the barrier height found to decrease with increase in annealing temperature. The observed variation of barrier height and ideality factor is discussed on the basis of interfacial chemical reactions between metal and semiconductor.

Gate Leakage Current Suppression in AlGaN/GaN HEMT by RTP Annealing

Ni/Au Schottky contacts to AlGaN/GaN HEMT were formed by E-beam evaporation technique and lift-off process. The contacts were Rapid thermal annealed at 300 °C for duration of 2–10 min. A significant suppression in gate leakage current was observed at 2 and 4 min annealing. At further higher annealing duration deterioration in the reverse gate characteristics was observed.

Silicon nitride films for passivation of pHEMT based MMIC

Passivation of psuedomorphic high electron mobility transistor (pHEMT) based MMICs by silicon nitride films deposited by PECVD is reported here. These films were first optimized for passivation of MESFETs to get desired performance. No degradation in pHEMT/MESFET characteristics like Idss, gm Vp and Cgs were observed while only nominal degradation was found in Schottky diode ideality factor eta and its breakdown voltage VB. The refractive index of the films was 1.92 with 1200 A0 thickness. The films have high dielectric strength > 5E6 V/cm, low tensile stress < 5E9 dynes/cm2 and dielectric constant 6.9 -7.1. The etch rate of the film is ~ 900 A0/min in BHF and fine patterns can be etched with 1-2 minutes etch time in BHF. The film composition was analyzed by FTIR and SIMS studies.

Development of GaAs Hyperabrupt Schottky Varactor Diode using Ion-Implanted Active Layer on SI GaAs

We report here the fabrication of ion implanted GaAs hyperabrupt varactor diode that can be integrated in MMIC process. Varactor diodes with constant sensitivity γ of 0.55–0.65 with Cmax/Cmin varying from 2.5 to 3.5 have been fabricated. Varactors diode geometries with different anode lengths, anode width, single and multiple finger anodes with device area varying from 50 to 6,000 μm2 were fabricated. The breakdown voltage of >10 V have been obtained for all the different value capacitors ranging from 0.16 to 11.2 pF.

Ohmic contacts to AlGaN/GaN HEMTs: A Comparison of Two Different Ti/Al Metaletal Ratios

Ohmic contacts formed with two different metal stacks; by varying the Ti/Al thickness ratio were rapid thermal annealed in the temperature range 740–820 °C for 30 s in N2 ambience. The ohmic contact formed with Ti/Al metal thickness ratio 1/5 showed lower Rc values and smoother surface morphology compared to the contact with Ti/Al metal thickness ratio 1/1.5. The difference in behavior for both the contacts was corroborated with the outcome of different metallurgical reactions as observed by X-ray diffraction (XRD) and Energy dispersive X-ray analysis (EDX).

High temperature alloyed Ag based ohmic contacts to pseudomorphic high electron mobility transistor (p-HEMT)

The quality of ohmic contacts in pseudomorphic high electron mobility transistors (p-HEMTs) formed by sequential e-beam evaporation of Ni/Ge/Au/Ag/Au was investigated as a function of alloying cycles in the range from 390degC to 460degC for duration of 30 sec. The contact resistance (Rc) and the specific transfer resistance (ohm-mm) values of ohmic contacts so formed were measured by Transmission line method (TLM) and compared with the contacts formed without Ag interlayer. XRD technique revealed the presence of Au-Ga phase at alloying temperatures 420degC and 440degC indicative of the formation of good quality and low Rc ohmic contacts. The Ag based contacts showed improved specific transfer resistance values and smooth surface morphology at wider alloying temperatures compared to the contacts formed without using Ag as a barrier layer. This factor is beneficial in the fabrication of lower resistance ohmic contacts in HEMTs, where a deeper penetration of ohmic metal upto the 2DEG channel is desirable.