Frequency- and Temperature-Dependent Gate Reliability of Schottky-Type ${p}$ -GaN Gate HEMTs

In this paper, we carried out a systematic investigation on gate degradation and the physical mechanism of the Schottky-type p-GaN gate HEMTs under positive gate voltage stress. The frequencyand temperature-dependent measurements have been conducted. It is found that the time-dependent gate degradation exhibits weak relevance with frequencies ranging from 10 to 100 kHz under dynamic gate stress and is similar to that in static gate stress. Both the gate breakdown voltage (BV) and mean-time-to-failure (MTTF) show positive temperature dependence. Moreover, the current-voltage (I-V) characteristics and threshold voltage (V_TH) instability of p-GaN devices before/after gate degradation are compared and analyzed. The degraded Schottky junction exhibits an ohmic-like gate behavior. It is revealed that under a large gate bias stress, high-energy electrons accelerated in the depletion region of the p-GaN layer would promote the formation of defect levels near the metal/ p-GaN interface, leading to the initial p-GaN layer degradation. The subsequent high gate leakage density could cause the final degradation of the AlGaN barrier.

I. Introduction

Normally-off GaN-based devices have gained increasing attention in high-frequency power switching applications for the fail-safe operation and simplified gate drive topology [1]–[3]. Among several normally-OFF technologies [4]–[8], the p-GaN gate solution has played a leading role in the commercial market with a good balance among performance, reliability, and cost. Two types of gate contact between the gate metal and -GaN layer are adopted in commercial products, i.e., Ohmic-type [9] and Schottky-type [10]. In particular, the -GaN gate HEMTs with an ohmic-type gate contact have presented impressive stability and reliability [11], [12]. A Schottky gate contact to -GaN layer is the other approach for the merits of reduced gate leakage current and a larger gate swing [10]. However, the -GaN layer sandwiched between a Schottky junction and a heterojunction is electrically floating, and charges (electrons and holes) storage/emission within the floating -GaN layer would induce threshold voltage () instability under both gate stress [13], [14] and drain stress [15], [16]. Moreover, the reverse-biased Schottky junction under a positive (forward) gate bias would withstand a high electric field that causes reliability concerns. There have been extensive studies on the gate degradation of the -GaN gate HEMTs under dc static gate stress. Wu et al. [17] has reported the positive temperature-dependent gate breakdown and related it to avalanche multiplication in the space charge region near the Schottky metal/-GaN interface. Rossetto et al. [18] and Stockman et al. [19] have shown that the -GaN gate presented a time-dependent dielectric breakdown (TDDB) like degradation, following Weibull or lognormal distribution, respectively. The time-dependent gate degradation was ascribed to the formation of a percolation path in the depletion region of the -GaN layer [18]–[24].