Low Turn-on Voltage InGaP / GaAsSb / GaAs DHBT Grown by MOCVD
In this work, an InGaP/GaAsSb/GaAs DHBT with carbon doping up to 4E19/cm GaAsSb base was studied. A current gain of 115 at the collector current density of 25KA/cm2 was achieved for the Rbs of about 530ohm/sq. Over 100mV turn-on voltage and knee voltage reduction and 2/3 offset voltage were obtained for the GaAsSb based HBT compared with the standard InGaP/GaAs SHBT. The DC gain of the device is less temperature dependence (the DC gain variation is less than 3% for the temperature range from 25 degree C to 100 degree C.) due to the larger valence band discontinuity between InGaP emitter and GaAsSb base, compare with the standard InGaP/GaAs SHBT. These results indicate that GaAsSb is a great choice for the base material of a low turn-on voltage HBT.
Pathway for HBT Turn-on Voltage Reduction on a GaAs Platform
The fundamental lower limit on the turn-on voltage of GaAs-based bipolar transistors is reduced with the use of a low energy-gap base material, GaInAsN. A 115 mV reduction in the turn-on voltage over a wide range of base sheet resistance values (250 to 850 Ω/) is established relative to both GaAs BJTs and conventional InGaP/GaAs HBTs with optimized base-emitter interfaces. InGaP/GaInAsN DHBTs structures with high p-type doping levels (~ 3x10 19 cm -3 ) and acceptable DC and RF characteristics (βmax ~ 75, ft ~ 64 GHz) are demonstrated. The reduction in turn-on Vbe enabled by lattice-matched GaInAsN base layers should permit improved management of the voltage budget, and enhance the overall performance, of both wired and wireless GaAsbased RF circuits.
Numerical analysis of surface crack propagation in flexible pavements using XFEM and cohesive zone model
Surface initiated (top-down) cracking in the longitudinal direction of pavements has become a predominant mode of failure in thick asphalt pavements. Most previous researchers studied crack propagation using the traditional Linear Elastic Fracture Mechanics (LEFM) method by calculating the stress intensity factor around the crack tip. In this paper, Extended Finite Element Method (XFEM) was used to study the key factors causing surface initiated crack propagation under surface tension and/or shear. A two-dimensional (2-D) finite element (FE) model for a two-lane flexible pavement was built to predict pavement responses and crack propagation potential under various scenarios. It was found that surface tension is the major driving force for crack propagation when the crack is at some distance away from tire loading. The results show that the distance from the location of crack to tire loading, the depth and direction of the crack, and the modulus ratio between the hot-mix asphalt (HMA) layer and the base layer have significant influences on crack propagation under surface tension. On the other hand, this study shows that shear stress can induce surface initiated crack propagation near the loading area as the shear resistance of material becomes low at the high temperature.