Introduction to GaAs HBT and current technologies ECE695 Discussion session Dongmin pak

Contents INTRODUCTION GaAs based HBT Why GaAs HBT introduced? Si BJT Performance limitations Advantages of HBTs over Si BJTs GaAs based HBT GaAlAs/GaAs HBT Device Structure Various Materials for High Speed Applications InGaP/GaAs HBT Type -I/II AlInP/GaAsSb/InP DHBTs GaAsSb-Based DHBTs

Why GaAs HBTs Introduced? d - Si BJT Performance Limitations Current Gain Limitations To obtain a high common emitter gain the emitter region should be doped the base width should be made However, at very high emitter doping levels, the band gap narrowing leads to a sharp increase in hole injection into the emitter and reduce gain. Base Resistance Limitations Emitter Current Crowding Effects Base Access Resistance Base Transit Time Limitations Transit time is proportional to the square of the width of the neutral base.  shrinking the base width is an important design parameter for the improvement of BJT high frequency performance.

Why GaAs HBTs Introduced? d - Si BJT Performance Limitations In a classical BJT, the base width must be reduced to achieve high speed. However, if the base width reduced  the base resistance increased, which slows the device response time. The base resistance can be reduced by increasing the base doping concentration, but then the injection into emitter decreases the current gain. Impossible to optimize the base width and doping concentration for high-speed, high gain and low base resistance.  The use of heterojunctions permits improved transit time, current gain and base resistance simultaneously.

Why GaAs HBTs Introduced? d - Advantages of HBTs over BJTs Injection Efficiency = 𝑒𝑙𝑒𝑐𝑡𝑟𝑜𝑛 𝑐𝑢𝑟𝑟𝑒𝑛𝑡 𝑖𝑛𝑗𝑒𝑐𝑡𝑒𝑑 𝑓𝑟𝑜𝑚 𝑡ℎ𝑒 𝐸 𝑖𝑛𝑡𝑜 𝑡ℎ𝑒 𝐵 ℎ𝑜𝑙𝑒 𝑐𝑢𝑟𝑟𝑒𝑛𝑡 𝑖𝑛𝑗𝑒𝑐𝑡𝑒𝑑 𝑓𝑟𝑜𝑚 𝑡ℎ𝑒 𝐵 𝑖𝑛𝑡𝑜 𝑡ℎ𝑒 𝐸 Optimized in classical BJTs by highly doping the emitter and lightly doping the base. High injection efficiency obtained in an HBT by using a material with a larger 𝐸 𝑔 for the emitter than that used for the base material. The large 𝐸 𝑔 emitter blocks injection of holes from the base.  the doping concentration in the base and emitter can be adjusted over a wide range with little effect on injection efficiency. Y.C. Chou and R. Ferro AlGaAs/GaAs HBT: Graded E-B junction

Why GaAs HBTs Introduced? d - Advantages of GaAs HBTs over BJTs 1. Lower forward transit time along with a much lower base resistance  due to the much higher base doping concentration  Increased cut off frequency 𝑓 𝑡 2. Better intrinsic device linearity  due to a higher beta(gain) early-voltage product. 3. Very low collector-substrate capacitance 𝐶 𝑐𝑠  due to the use of semi-insulating GaAs substrate (resistivity ≈ 10 7 Ohm-cm) 4. Good wide-band impedance matching  due to the resistive nature of the input and output impedances.

GaAlAs/GaAs HBT Device Structure Y.C. Chou and R. Ferro Cross section of GaAlAs/GaAs HBT HBT epitaxial layer structure The material on which an HBT is fabricated is grown on a semi-insulating GaAs substrate Epitaxial layers grown by MBE or MOCVD

Various Materials for High-Speed Applications Various compound semiconductor materials can be used for high-speed applications Lattice match is still very important!! Lecture note from Prof. Peide Ye

InGaP/GaAs HBT Replacement of the AlGaAs emitter by InGaP. Y.C. Chou and R. Ferro Replacement of the AlGaAs emitter by InGaP. Advantages over AlGaAs/GaAs HBT The large 𝐸 𝑣 discontinuity (∆ 𝐸 𝑣 ≈0.35 𝑒𝑉) to conduction band discontinuity (∆ 𝐸 𝑐 ≈0.15 𝑒𝑉) ratio of InGaP/GaAs material system. High etching selectivity between InGaP and GaAs layers Low density of DX center(Deep donor level) Low surface recombination velocity of InGaP layer 0.15eV 0.35eV J. Tsai, S Chiu, W Lour and D Guo, Semiconductors 2009’

InGaP/GaAs HBTs with WSi/Ti Base Electrode and Buried SiO2 To achieve high-speed and low-power operation in HBTs Essential to reduce both the emitter size and the parasitic capacitance simultaneously Achieved by using WSi/Ti as the base electrode and by burying SiO2 in the extrinsic base-collector region under the base electrode The buried SiO2 reduces the parasitic capacitance under the base electrode because the dielectric constant of SiO2 is about 1/3 of that of GaAs. T Oka, TED 1998’

InGaP/GaAs HBTs with WSi/Ti Base Electrode and Buried SiO2 Epi-layers grown by MBE The p-GaAs base layer is highly doped to reduce the contact resistance with the WSi/Ti base electrode relatively thick sub-collector layer was used to bury thick SiO2 in the extrinsic collector The base-contact width is 0.3um, the sidewall thickness is 0.1um. The final thickness of the buried SiO2 is about 0.4um T Oka, TED 1998’

InGaP/GaAs HBTs with WSi/Ti Base Electrode and Buried SiO2 𝑓 𝑡 =138 𝐺𝐻𝑧, 𝑓 𝑚𝑎𝑥 =275 𝐺𝐻𝑧 with emitter size 0.6 * 4.6 𝑢𝑚 2 ( 𝑉 𝐶𝐸 =1.6𝑉, 𝐼 𝑐 =4𝑚𝐴) This is an early work! T Oka, TED 1998’

Hot electron injection effect on the mm-wave performance of type -I/II AlInP/GaAsSb/InP DHBTs A type I/II DHBT with AlInP emitter/GaAsSb composition graded base/InP collector Epi-structures grown by MBE on <100> InP:Fe S.I. substrates. The band alignment of type 1 emitter-base interface provides hot electron injection into the base to enhance the effective base velocity to 2.54 * 10 7 cm/s. an abrupt emitter used to create an energy launcher  the reduction in base transit time  improved device gain and high frequency performance K. Y. D. Cheng APL 2011’

Hot electron injection effect on the mm-wave performance of type -I/II AlInP/GaAsSb/InP DHBTs K. Y. D. Cheng APL 2011’

GaAsSb-Based DHBTs with a Reduced Base Access Distance Type-2 InP/GaAsSb/InP DHBTs The use of a binary InP collector provides high thermal conductivity and offers high breakdown voltage GaAsSb-based DHBTs attractive for submm-wave IC applications Epi-layers grown by MOCVD The Ga 𝐴𝑠 𝑥 𝑆𝑏 1−𝑥 base is compositionally graded from x = 0.41 on the collector side to x = 0.61 on the emitter side  a band gap variation of ∼50 meV GaInP composite emitter improves gain and minimizes emitter-size effects. M Alexandrova, EDL 2014’

GaAsSb-Based DHBTs with a Reduced Base Access Distance Fabricated in a self-aligned triple mesa process Highly directional Ar sputtering used during the GaInAs emitter contact layer etching step a minimized emitter mesa undercut and reduces the base access distance the reduction of the base access distance a narrower base-collector mesa, leading to a reduced base-collector capacitance. The combination of lower base access resistance and base-collector capacitance leads to significant increases in both 𝑓 𝑡 and 𝑓 𝑚𝑎𝑥 𝑓 𝑡 =503 𝐺𝐻𝑧 & 𝑓 𝑚𝑎𝑥 =779 𝐺𝐻𝑧 R Fluckiger, EDL 2014’

Thank you for your attention!