Roxann R. Blanchard

On-state breakdown in power HEMTs: measurements and modeling

A new definition of and measurement technique for on-state breakdown in high electron mobility transistors (HEMTs) is presented. The new gate current extraction technique is unambiguous, simple, and non-destructive. Using this technique in conjunction with sidegate and temperature-dependent measurements, we illuminate the different roles that thermionic field emission and impact ionization play in HEMT breakdown. This physical understanding allows the creation of a phenomenological model for breakdown, and demonstrates that depending on device design, either on-state or off-state breakdown can limit maximum power.

A new gate current extraction technique for measurement of on-state breakdown voltage in HEMTs

We present a new simple three-terminal technique for measuring the on-state breakdown voltage in HEMTs. The gate current extraction technique involves grounding the source, and extracting a constant current from the gate. The drain current is then ramped from the off-state to the on-state, and the locus of drain voltage is measured. This locus of drain current versus drain voltage provides a simple, unambiguous definition of the on-state breakdown voltage which is consistent with the accepted definition of off-state breakdown. The technique is relatively safe and repeatable so that temperature dependent measurements of on-state breakdown can be carried out. This helps illuminate the physics of both off-state and on-state breakdown.

Hydrogen-induced piezoelectric effects in InP HEMT's

In this letter, we have investigated hydrogen degradation of InP HEMTs with Ti/Pt/Au gates. We have found that V/sub T/ shifts negative after exposure to hydrogen, and exhibits an L/sub G/ and orientation dependence. We postulate that /spl Delta/V/sub T/ is at least in part due to the piezoelectric effect. Hydrogen exposure leads to the formation of TiH/sub x/, producing compressive stress in the gate. This stress induces a piezoelectric charge distribution in the semiconductor that shifts the threshold voltage. We have independently confirmed TiH/sub x/ formation under our experimental conditions through Auger measurements. Separate radius-of-curvature measurements have also independently confirmed that Ti/Pt films become compressively stressed relative to their initial state after H/sub 2/ exposure.

Hydrogen degradation in InP HEMTs

In this work we have investigated the degradation of InP HEMTs due to hydrogen exposure. We show for the first time that there are two independent degradation mechanisms that affect, respectively, the intrinsic and extrinsic portions of the device. Under the gate, H reacts with Ti and creates a TiH compound with a larger lattice that Ti. This induces stress and the resulting piezoelectric effect shifts the threshold voltage, V/sub T/, of the transistor. This mechanism is found to be largely reversible. In the recessed region next to the gate, hydrogen modifies the surface stoichiometry of the exposed InAlAs. This results in a reduction in the sheet carrier concentration underneath. This mechanism is not reversible.

Titanium hydride formation in Ti/Pt/Au-gated InP HEMTs

Ti/Pt metal layers are an integral part of the gate stack of many GaAs PHEMTs and InP HEMTs. These devices are known to be affected by H/sub 2/ exposure. In this study, Auger Electron Spectroscopy (AES) measurements of Ti/Pt bilayers are correlated with electrical measurements of InP HEMTs fabricated with Ti/Pt/Au gates. The FET measurements show that H/sub 2/ exposure shifts the device threshold voltage through the piezoelectric effect. AES reveals the formation of titanium hydride (TiH/sub x/) in Ti/Pt bilayers after identical H/sub 2/ exposures. These results indicate that the volume expansion associated with TiH/sub x/ formation causes compressive stress in Ti/Pt/Au gates, leading to the piezoelectric effect. After a subsequent recovery anneal in N/sub 2/, the FET measurements show that V/sub T/ recovers. AES measurements confirm that the TiH/sub x/ in hydrogenated Ti/Pt bilayers also decreases after further annealing in N/sub 2/.

Physical mechanisms limiting the manufacturing uniformity of millimeter-wave power InP HEMT's

We have developed a methodology to diagnose the physical mechanisms limiting the manufacturing uniformity of millimeter-wave power InAlAs/InGaAs HEMTs on InP. A statistical analysis was carried out on dc figures of merit obtained from a large number of actual devices on an experimental wafer. correlation studies and principal component analysis of the results indicated that variations in Si delta-doping concentration introduced during molecular-beam epitaxy accounted for more than half of the manufacturing variance. Variations in the gate-source distance that is determined by the electron-beam alignment in the gate formation process were found to be the second leading source of manufacturing variance. The statistical methodology used in this work is suitable for continuous process yield diagnostics and improvement in a manufacturing environment.

Stress-related hydrogen degradation of 0.1-/spl mu/m InP HEMTs and GaAs PHEMTs

Hydrogen degradation of III-V field-effect transistors (FETs) is a serious reliability concern. Previous work has shown that threshold-voltage shifts induced by H/sub 2/ exposure in 1-/spl mu/m-channel InP high-electron mobility transitors (HEMTs) can be attributed to compressive stress in the gate due to the formation of TiH/sub x/ in Ti/Pt/Au gates. The compressive stress affects the device characteristics through the piezoelectric effect. This paper examined the H/sub 2/ sensitivity of 0.1-/spl mu/m strained-channel InP HEMTs and GaAs pseudomorphic HEMTs. After exposure to H/sub 2/, the threshold voltage V/sub T/ of both types of devices shifted positive. This positive shift in V/sub T/ is predicted by a model for hydrogen-induced piezoelectric effect. In situ V/sub T/ measurements reveal distinct time dependences of the V/sub T/ shifts, which are also consistent with stress-related phenomena.