Hydrogen degradation in InP HEMTs

Abstract

Hydrogen degradation is a serious reliability concern for III-V FETs. Exposure can occur when hydrogen out-gasses from packaging material and becomes trapped inside hermetically sealed packages. The detailed mechanism by which H2 degrades FETs is not understood and a device-level solution to this problem has not been reported.This work investigates the hydrogen degradation of InP High-Electron-Mobility Transistors (HEMTs) fabricated at MIT. We show that there are three independent degradation mechanisms: two that affect the intrinsic portion of the device and one that affects the extrinsic portion of the device.In the intrinsic region, H reacts with Ti in the Ti/Pt/Au gate metallization to form TiH,. Since TiH has a larger lattice than Ti, this causes compressive stress in the gate. This stress affects the transistor threshold voltage, VT, through the piezoelectric effect, which alters the equilibrium carrier concentration in the channel. This hypothesis is supported with both theoretical calculations and material characterization (Auger spectroscopy and stress measurements) of Ti/Pt bilayers. Along with the piezoelectric effect, there is an additional, negative AV which we hypothesize is due to H+1 penetration into the semiconductor. Both AVT mechanisms are found to be largely recoverable with further annealing in N 2 .In the recessed regions next to the gate, hydrogen modifies the surface stoichiometry of the exposed InAlAs by facilitating As desorption through the formation of AsH 3 . This causes a reduction in the sheet carrier concentration underneath, and an improvement in the breakdown voltage. Changes in the InAlAs surface stoichiometry have been confirmed through XPS. This mechanism is not found to be recoverable.The physical understanding obtained through this work should be instrumental in identifying a device-level solution to this problem. That I have emerged from this process wiser, more confident, and most importantly still smiling, is testimony to the support and encouragement that I have received from many people. It will be impossible to do justice to them all.To my advisor, Prof. Jesd's del Alamo, I owe a debt of gratitude that I can only repay by trying to maintain his same standards of excellence, dedication and integrity in my own career. It has been an honor and privilege to work under his direction.This work has benefitted greatly from interactions with Sanders Lockheed-Martin.

A model for hydrogen-induced piezoelectric effect in InP HEMTs and GaAs PHEMTs

“…Here, the forward-biased gate current increases significantly and the parasitic resistances decreases. Previous observations of the H-induced piezoelectric effect did not show any H-induced changes in the Schottky-barrier height[37].…”

Hydrogen-induced changes in the breakdown voltage of InP HEMTs

“…The reduction of required to realize the increase in could occur through donor passivation [3], through changes in the surface in the extrinsic region [10], [11], or through stress-induced piezoelectric charges in the extrinsic region [1], [2]. Previous work has shown that exposure to H leads to compressive stress in Ti/Pt/Au gates due to the formation of TiH [1], [2] and the creation of a piezoelectric volume charge distribution in the device. Piezoelectric charge is ruled out here as the main cause of the decrease because there is no orientation dependence to .…”

“…Over time, hydrogen causes changes in device characteristics that can ultimately lead to parametric module failures. Compared with more extensive studies of H degradation of GaAs MESFETs and PHEMTs [3]- [6], only limited data on the H sensitivity of InP HEMTs is available [1], [2], [5]. To our knowledge, a device-level solution to this problem has not been reported for either InP or GaAs technologies.…”

“…This recovery anneal is designed to see if H degradation can be annealed out with further thermal processing in pure N . Recovery behavior has been frequently reported following H degradation [1], [4], [5].…”

“…H YDROGEN degradation of III-V FETs is a serious and well-documented reliability concern [1]- [6]. Exposure occurs when hydrogen out-gasses from packaging material and gets trapped inside hermetically sealed packages.…”