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

2. 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

3. 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.

4. 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.

5. 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

6. 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 ≈ Ohm-cm)
4. Good wide-band impedance matching
 due to the resistive nature of the input and output impedances.

7. 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

8. 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

9. 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’

10. 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’

11. 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’

12. 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’

13. 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 * 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’

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

15. 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’

16. 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’

17. Thank you for your attention!