Wafer-fused n-AlGaAs/p-GaAs/n-GaN Heterojunction Bipolar Transistor with uid-GaAs Base-Collector Setback

Abstract

Recently we reported the first AlGaAs-GaAs-GaN heterojunction bipolar transistor (HBT), a device that potentially combines the high-breakdown voltage of an n-GaN collector with the high mobility of an AlGaAs-GaAs emitter-base. Because of the high degree of lattice mismatch between GaAs (lattice constant of 5.65A) and GaN (3.19A), we formed these devices through wafer fusion, also called direct wafer bonding. Measurements on the first generations of wafer fused HBTs revealed good current modulation, with modest output current (0.83 KA/cm2) and a current gain of 1.2. Limitations to the current gain may be related to traps and defects introduced by the fusion process, or may be a consequence of the natural conduction band offset between GaAs and GaN, which is not well known. This paper describes our new HBT structure that included a thin (20nm) uid-GaAs base-collector “setback” layer. The setback layer shifted the fused GaAs-GaN interface slightly into the collector. This new HBT structure also incorporated a reduced base thickness of 100 nm. HBTs with setback layers demonstrate increased output current (1.7 KA/cm2) and increased current gain (1.9).

DC Characteristics of AlGaAs/GaAs/GaN HBTs Formed by Direct Wafer Fusion

“…The HBTs fused at both 600 • C and 650 • C also exhibit wellbehaved family I-V s but with smaller and similar current gains β ∼ 2−3, which are consistent with what has been observed in the previous studies [7], [9]. Besides the lower current gain, the base-collector reverse leakage currents of both devices are also found to be ∼2-3 times higher than that of the HBT fused at 550 • C. High-temperature annealing alone may degrade the base layer, reducing the minority carrier lifetime and thus the current gain.…”

“…AlGaAs/GaAs/GaN HBTs formed by wafer fusion have been proposed and demonstrated recently [7], [8]. Employing a low fusion temperature of 600 • C and inserting an unintentionally doped setback layer, a current gain of ∼2 was obtained [9]. In this letter, we show fused HBTs with current gains of up to ∼9, produced by inserting a p-GaAs setback layer and using a lower fusion temperature of 550 • C.…”

Gain degradation mechanisms in wafer fused AlGaAs∕GaAs∕GaN heterojunction bipolar transistors

“…1,2 On the other hand, combining the unity emitter injection efficiency and large base transport factor of AlGaAs͑emitter͒/GaAs͑base͒ structures and the high breakdown field of GaN ͑collector͒, AlGaAs/ GaAs/ GaN HBTs can be promising candidates for high-speed high-power applications. [3][4][5] Since the large lattice mismatch between GaAs and GaN makes it very difficult to obtain high quality GaAs/ GaN epitaxially ͑dislocations tend to penetrate the entire structure͒, wafer fusion has been exploited, [3][4][5] which has been used widely to integrate materials with different lattice constants, crystal structures, or orientations while retaining defects near the fusion interface. 6 Recently, dc gain as high as ϳ10 has been reported by Lian et al in AlGaAs/ GaAs/ GaN HBTs formed by wafer fusion.…”