Reliability issues in III–V compound semiconductor devices: optical devices and GaAs-based HBTs

A survey of thermal-electric feedback coefficients in GaAs-based heterojunction bipolar transistors

In0.49Ga0.51P ( Eg=1.87 eV)/GaAs and In0.38Ga0.62As0.22P0.78 ( Eg=1.77 eV)/GaAs Npn heterojunction bipolar transistors (HBTs) were fabricated. In devices with 110 µm×110 µm emitter areas, the curtent gains of both devices were the same value of 14 at a collector current density of 700 A/cm2. By contrast, the turn-on voltage of InGaAsP/GaAs HBTs was 0.01 V smaller than that of InGaP/GaAs HBTs, which indicated that the conduction band discontinuity of the InGaP/InGaAsP heterojunction was much smaller than the valence band discontinuity.

GaAs/Ga 0.89In 0.11N 0.02As 0.98/GaAs NpN double heterojunction bipolar transistor with low turn-on voltage

The first successful demonstration of a delta-doped InAlGaP/GaAs heterojunction bipolar transistor (HBT) is reported. A comparison to a baseline InAlGaP/GaAs HBT without a delta-doping layer is made. Both of these devices exhibit near-ideal current gain (beta) versus the collector current ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</sub> ) characteristics (i.e., beta independent of <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</sub> ) at high currents. The delta-InAlGaP/GaAs HBT exhibits a 40% reduction in offset voltage ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">CE,</sub> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">offset</sub> ) and a 250-mV reduction in knee voltage ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k</sub> ) without sacrificing beta compared with the baseline InAlGaP/GaAs HBT. At a higher <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</sub> , the decrease in beta of the InAlGaP/GaAs HBTs with increasing temperature is significantly smaller than the corresponding effect measured in the formerly reported GaAs-based HBTs. The rather temperature-insensitive characteristics of these two InAlGaP/GaAs HBTs originate from their large valence-band discontinuity (Delta <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">E</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V</sub> ) at the emitter-base (E-B) junction. Furthermore, at intermediate base current <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">B</sub> levels (0.4-1.6 mA), <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">CE,</sub> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">offset</sub> falls as <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">B</sub> increases, which is a trend contrary to that of most HBTs in the literature. Finally, the experimental dependence of <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">CE,</sub> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">offset</sub> on temperature, <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">B</sub> , and the effective barrier height at the E-B junction is explained with reference to an extended large-signal model.

Base current degradation mechanisms in carbon-doped AlGaAs/GaAs heterojunction bipolar transistors (HBTs) under forward current stress is investigated in transistors with a thin AlGaAs surface protection ledge. Although the base current in HBTs with ledge is relatively stable compared to that in HBTs without ledge, the base current increase in some of these passivated HBTs is still noticeable. An increase of surface recombination current in the region surrounding the AlGaAs ledge is identified as a primary degradation mechanism. To alleviate the base current instability, several passivated emitter-base junction fabrication processes are recommended.

This paper reviews recent studies on the reliability of GaAs heterojunction bipolar transistors (HBTs), mainly carbon-doped-base AlGaAs/GaAs HBTs, and discusses degradation modes in these devices. The carbon-doped-base AlGaAs/GaAs HBTs generally have more stable current-voltage characteristics than beryllium-doped-base ones. However, even carbon-doped-base HBTs show a decrease in the current gain due to an anomalous increase in the base current that occurs under high-current stress. The estimated ohmic contacts of these HBTs show that their stability is high enough under actual operating conditions. Bias stress testing has shown that two factors dominate the reliability of carbon-doped-base AlGaAs/GaAs HBTs: the material and the passivation. HBTs with an AlGaAs surface passivation layer on the extrinsic base region exhibit a longer life than those without one. The base current ideality factor of short-lived HBTs increases to larger than 2 during bias stress testing, suggesting that the origin of the excess base current cannot be simply explained by a space-charge recombination process. The reported activation energies are distributed around 0.5 eV, which is almost the same as those for AlGaAs/GaAs laser diodes. Cross-sectional TEM analysis has revealed that current-induced defects are formed in the base layer. The indium-incorporated-base HBTs show a median life of longer than 15,000 hours at a junction temperature of 215°C with a collector current density of 50 kA/cm 2, implying that GaAs HBTs have sufficient reliability for very-high-speed communication systems.

Ionizing-radiation-induced current gain degradation in NPN bipolar junction transistors is due to an increase in base current as a result of recombination at the surface of the device. A model is presented which identifies the physical mechanism responsible for current gain degradation. The increase in surface recombination velocity due to interface states results in an increase in base current. Besides, changing the surface potential along the base surface induced by the oxide-trapped charges can also lead to an increased base current. By combining the production mechanisms of oxide-trapped charges and interface states, this model can explain the fact that the current gain degradation is more severe at a low dose rate than at a high dose rate. The radiations were performed in a Co60 source up to a total dose of 70 krad(Si). The low dose rate was 0.1 rad(Si)/s and the high dose rate was 10 rad(Si)/s. The model accords well with the experimental results.

Si/SiGe heterojunction transistors were fabricated with 21.4 nm, heavily doped (5*10/sup 19/ cm/sup -3/) bases. Electrical results from these transistors are presented, and compared with results from comparable silicon homojunction control devices. These results indicate that the collector current is enhanced by a factor of 13, but the potential gain enhancement is compromised by a six-fold increase in base current. The base current is shown to be strongly dependent upon the collector/base voltage, indicating that recombination in the neutral base is the mechanism responsible for the increased base current. The recombination can only be modeled if the lifetime near the collector/base junction is significantly lower than elsewhere in the base. A simple two-region lifetime model predicts a value of 3.0*10/sup -13/ s close to the collector-base junction. >

During elevated-temperature bias stress, InGaP/GaAs HBT's grown by MOCVD show a medium-term degradation in current gain of about 20%, with an activation energy of 0.64 eV. They also show a corresponding decrease in base resistance and an increase in turn-on voltage. InGaP/GaAs HBTs grown by GSMBE, however, do not show this degradation. SIMS measurements show a five times greater than GSMBE-epi hydrogen concentration of about 10/sup 19/ cm/sup -3/ in the base layer of the MOCVD-grown epi. The degradation can be explained by acceptor depassivation due to hydrogen out-diffusion from the epi during stress.

The relative importance of the base bulk recombination current and the base-emitter junction space charge recombination current is examined for AlGaAs/GaAs HBTs with different grading schemes in the base-emitter junction. Experimental results demonstrate that, in abrupt HBTs, the base bulk recombination current is larger, and the base current increases with the base-emitter bias with an ideality factor of ∼1. In contrast, in graded HBTs, the space charge recombination current dominates and the base current ideality factor is ∼2. These experimental results agree well with a published theoretical calculation.

Ionizing radiation effects in silicon-germanium (SiGe) heterojunction bipolar transistors (HBTs) at different dose rates were investigated. Experimental results show that the base current increases with increasing accumulated dose for the high and low dose rates of irradiation, causing a significant drop in current gain. Besides, the lower the dose rate, the higher the radiation damage, which demonstrates a significantly enhanced low-date-rate sensitivity (ELDRS) effect in the SiGe HBTs. The different degradation behaviors for high and low dose rates of irradiation are compared with each other and discussed; furthermore, the underlying physical mechanisms are analyzed and investigated in detail.

High-speed InGaP/InGaAsN/GaAs NpN double heterojunction bipolar transistors with low turn-on voltage

Can you think of living without the World Wide Web, e-mail, DVDs, CD-ROMs, the bar code scanner, mobile phones or high-efficiency solar cells? I cannot. These systems are part of the backbone of our modern civilization. And they have something in common: they are all devices based on the double heterostructure. Zhores Alferov (and independently Herbert Krömer) proposed its concept and its usefulness for semiconductor lasers in 1962, two years after the first demonstration of the first (solid-state) laser. InP-based double heterostructure lasers are today, and have been for more than 20 years, the enabling light sources for optical fiber communication, sending their photonic bits around the globe at enormous and ever-increasing rates. In 2009, 61% of the $500+ billion world market for laser systems was based on semiconductor lasers. Obviously a straightforward success story? Not really at the beginning, rather the kind of survival story of somebody who was producing an enormous number of ideas and trying to make them reality; his production of ideas has not yet stopped after 60 years of professional life. Let's have a look back at how it started.

Current status of heterojunction bipolar and high-electron mobility transistor technologies

This paper reports on the design, growth, fabrication, and processing of an integrated Grating-Outcoupled Surface Emitting (GSE) semiconductor laser and a Double Heterojunction Bipolar Transistor (DHBT) driver. The GSE laser has a 380 mum long gain section, 15 mum long out-coupler, and 200 mum long distributed Bragg reflectors (DBRs). The design of the DHBT includes an InP emitter and collector and an InGaAs carbon doped base layer. Advantages of electronic-photonic integrated circuits (EPICs) include 1) increased speed, 2) lower noise, 3) reduced power requirements since the driver and laser no longer need to be matched to a 50-ohm transmission line, 4) on-wafer testing capability and mapping of known good die, 5) reduced packaging costs, and 6) improved reliability.