Andrew D. Koehler

Simulation of AlGaN/GaN high-electron-mobility transistor gauge factor based on two-dimensional electron gas density and electron mobility

The gauge factor of AlGaN/GaN high-electron-mobility transistor was determined theoretically, considering the effect of stress on the two-dimensional electron gas (2DEG) sheet carrier density and electron mobility. Differences in the spontaneous and piezoelectric polarization between the AlGaN and GaN layers, with and without external mechanical stress, were investigated to calculate the stress-altered 2DEG density. Strain was incorporated into a sp3d5–sp3 empirical tight-binding model to obtain the change in electron effective masses under biaxial and uniaxial stress. The simulated longitudinal gauge factor (−7.9±5.2) is consistent with experimental results (−2.4±0.5) obtained from measurements eliminating parasitic charge trapping effects through continuous subbandgap optical excitation.

Extraction of AlGaN/GaN HEMT Gauge Factor in the Presence of Traps

Repeatable gauge factors (GFs) of an AlGaN/GaN high-electron mobility transistor (HEMT) device were obtained after eliminating parasitic charge-trapping effects. Many GFs for AlGaN/GaN HEMTs are reported in the literature, and charge traps could be responsible for the four orders of magnitude variation in reported GFs. By employing continuous subbandgap optical excitation, the effect of nonrepeatable charge-trapping transients was effectively minimized, allowing the GF to be accurately measured. The measured GF (-2.8 0.4) is reasonably close to the simulated GF (-7.9 5.2) calculated from stress-induced changes in the 2-DEG sheet carrier density and mobility.

Temperature dependence of enhanced hole mobility in uniaxial strained p-channel metal-oxide-semiconductor field-effect transistors and insight into the physical mechanisms

Uniaxial stress enhanced hole mobility is measured for (100)/⟨110⟩ silicon (Si) p-channel metal-oxide-semiconductor field-effect transistor from 300 to 87 K. For the technologically important longitudinal compressive stress along ⟨110⟩, the percent change in the uniaxial stress enhanced hole mobility is observed to increase at lower temperatures, which is opposite to the trend for biaxially stressed devices. The stress enhanced mobility is compared with six band k⋅p with finite difference formalism, which shows that the larger mobility gain at lower temperatures results from greater uniaxial stress induced hole conductivity mass reduction. The larger mass reduction results from more holes occupying states at the band edge, which have a light hole conductivity mass in the channel direction. For the uniaxial stress range in this work (<500 MPa), negligible strain altered phonon and surface roughness scattering rates are observed.

Effect of proton irradiation on thermal resistance and breakdown voltage of InAlN/GaN high electron mobility transistors

InAlN/GaN high electron mobility transistors were irradiated from the front side with 340 keV protons to a dose of 5 × 1013 cm−2. Raman thermography showed that the irradiated devices had higher channel temperatures than unirradiated control devices, but only by ∼10% under typical biasing conditions. Accordingly, the irradiated devices have higher thermal resistance (400 °C/W) compared to reference devices (350 °C/W), based on the slope of the power versus channel temperature line. However, increases of 42% in off-state drain breakdown voltage (VBR) and of >92% in critical voltage (Vcri) were observed for the proton irradiated HEMT. This is ascribed to the reduction of the peak electric field at the gate edges by ∼50% through the introduction of negative trap charges created from vacancies generated by the proton irradiation.

Strain Effects in AlGaN/GaN HEMTs

Since stress is a major factor in the operation, performance, and reliability in AlGaN/GaN HEMT devices, a thorough understanding of the impact of stress on performance and reliability can lead to improvements in device design. Mechanical wafer bending is a cost-effective method to investigate the effects of stress on semiconductor devices which has been extensively used to isolate and study the effect of stress in strain-engineered Si MOSFETs. In this chapter, a systematic study of the effects of externally applied mechanical stress on the AlGaN/GaN HEMT channel resistance and gate current is presented to provide insights into the physical mechanisms responsible for stress-related performance and reliability issues.

Topside Nanocrystalline Diamond Integration on AlGaN/GaN HEMTs for High Temperature Operation

GaN high electron mobility transistors (HEMTs) performance is limited by increased channel temperature, particularly resulting from self-heating during high power operation. Topside nanocrystalline diamond (NCD) layers have been integrated on AlGaN/GaN (HEMTs) to improve thermal management. HEMTs with NCD heat-spreading layers exhibit a 20% decrease in peak channel temperature compared to reference HEMTs, measured by Raman thermography, as well as improved sheet carrier density, transconductance, sheet resistance, Hall mobility, on-state resistance, and breakdown voltage. A “gate after diamond” approach is implemented to improve the thermal budget of the deposition process while maintaining the integrity of the Schottky gate electrode in a scalable process. Processing improvements for integrating NCD-capping with the HEMT are being pursued, such as eliminating the SiNx passivation interlayer, such that the NCD film is directly on the AlGaN barrier, as well as a sacrificial gate process. Also, boron doped ...