1. Heterojunction Bipolar Transistor
Recent advances in
Heterojunction Bipolar Transistor
Reshma Krishnan Madathil
Centre for Materials Science and Nanotechnology (SMN)
Department of Chemistry
University of Oslo
FYS 9310 and 4310
Materials Science of Semiconductors

2. Bipolar Junction Transistors
The transistor is a three-layer semiconductor device consisting of either two n- and one p- type layers of material or two p- and one n- type layers of material.
The former is called an npn transistor, while the latter is called a pnp transistor
So, there are two types of BJT-
i) npn transistor ii) pnp transistor

3. Modes of operation of BJT
Active mode:
E-B junction is forward biased, B-C junction is reverse-biased
Saturation mode:
both junctions are forward biased
corresponds to small biasing V & large output I – transistor is in a conducting state & acts as a closed (or on) switch
Cutoff mode:
both junctions are reverse biased
corresponds to the open (or off) switch
Inverted mode:
inverted active mode
E-B junction is reverse biased, C-B junction is forward biased
Junction polarities and minority carrier distributions of a BJT transistor under four modes of operation.

4. Advantages of BJT • High voltage handling capability,
• High cutoff frequencies of both ft and fmax
This is because current flow is vertical, through layers whose thickness is precisely controlled to submicron dimensions by epitaxy or ion-implantation processes.
• High current drive capability
because the entire emitter area conducts current (as opposed to the thin channel geometry obtained in FET technologies).
• High transconductance
due to the exponential relationship between input voltage and output current.
• High voltage handling capability,
because of the ease of establishing a thick collector region to absorb the output voltage without breakdown.
• Very uniform threshold voltage for turn-on of the output current.
• Very low 1 / f noise,
a result of the intrinsic device being relatively well shielded from semiconductor surface and bulk traps.

5. Requirements for a bipolar device
High gain
High emitter efficiency
High speed
Demands and problems for a BJT
Demands
Problems
Heavy emitter doping
Band gap shrinkage causing base injection
Low base doping
Narrow base width
High base resistance
Solution: Heterojunction Bipolar Transistor
Emitter can be heavily doped using a Semiconductor with a band gap larger than the base
Base can be heavily doped and be made narrow without increasing base resistance
Collector can be chosen from a material to increase breakdown voltage.

6. Heterojunction Bipolar Transistor (HBT)
The basic idea to use a heterostructure in bipolar transistors is almost as old as the bipolar transistor itself.
In 1948, W. Shockley described the advantage of a bipolar transistor consisting of a wide bandgap emitter and a narrow bandgap base.
It took more than 30 years to materialize Shockley's idea in practical devices
The heterojunction bipolar transistor (HBT) is an improvement of the BJT that can handle signals of very high frequencies up to several hundred GHz.

7. Difference between the BJT and HBT Advantages of HBT over BJT
Use of different semiconductor materials
for the emitter-base junction and the base-collector junction, creating a heterojunction.
Limited injection of holes from the base into the emitter region,
- since the potential barrier in the valence band is higher than in the conduction band.
Allows a high doping density to be used in the base
- reducing the base resistance while maintaining gain
Advantages of HBT over BJT
HBTs extend the advantages BJT of their Si predecessors to considerably higher frequencies
The use of wide band gap emitters allows HBTs to attain -
much higher base doping (up to 1020/cm3) and
lower emitter doping (down to 1017/cm3) than that of Si homojunction bipolar transistors
Reduced device parasitic and smaller electron transit times.
Because of unique emitter/base doping profile and quasi-electric fields possibly implemented by varying material composition

8. Heterojunction Bipolar Transistor (HBT)
Modern BJT Structure
Features:
Narrow base
n+ poly-Si emitter
Self-aligned p+ poly-Si base contacts
Lightly-doped collector
Heavily-doped epitaxial subcollector
Shallow trenches and deep trenches filled
with SiO2 for electrical isolation

9. What made the difference?
The holes and electrons see a different barrier
The holes are not allowed to go back into emitter
Drastic reduction in Hole Injection – high emitter doping needn’t be done Reduction in band narrowing effect too.
So What’s the use ?
High Frequency Device
Lower emitter Doping⇒ Smaller junction Capacitance⇒ Higher Speed
Electron Mobility for npn GaAs is 5 times that of Si⇒ Shorter Base transit Time Cutoff Frequencies of the order of 40 GHz

10. n-AlGaAs / p-GaAs / n+GaAs HBT First HBT in the history of BJT
n+ collector
GaAs
AlGaAs B C
n-AlGaAs / p-GaAs / n+GaAs HBT
First HBT in the history of BJT
Possible because of the lattice match between GaAs and AlGaAs
Solid-State Electronics 38:9( 1995)

11. BJT Performance Parameters
Common emitter current gain, β :
For PNP For NPN
Heterojunction Bipolar Transistor (HBT)
To improve β , we can increase niB by using a base material that has a smaller band gap energy
Note that this allows a large β to be achieved with large NB (even >NE), which is advantageous for
increasing Early voltage (VA)
reducing base resistance

12. The Heterojunction Bipolar Transistor
Emitter – wide bandgap (AlGaAs)
Base – lower bandgap (GaAs)
Large bandgap difference (between E-B)  common-emitter current gain can be extremely large
Homojunction: no bandgap difference – doping concentration in the E & B must be very high
EV increases the valence-band barrier height  reduce injection of holes from B to E
can use heavily doped base, maintain a high E efficiency & current gain
Schematic cross section of an n-p-n heterojunction bipolar transistor (HBT) structure.
(b) Energy band diagram of a HBT operated under active mode.
Semicond. Sci. Technol. 18 No 12 (December 2003)

13. AlGaAs /GaAs /GaAs HBTs fabricated at BELL Labs showed the following:
very low values of  =30
Higher values of  were observed in Devices with larger areas.
The  increased from 30 t0 about when the surface of the base region was passivated by chemical treatment to saturate the dangling bonds with sulfur . But the  values were unstable .
Several approaches have been used to stabilize the . The most successful one has been chemical treatment with (NH4)2Sx and protect with PECVD silicon nitride

14. Silicon Germanium HBT (SiGe HBT)
Band gap of Si1-xGex depends upon x.
Strained layer Si1-xGex without dislocations can be realized with thin layers of base n Si n- Si n+ Si
Benefits of SiGe HBT over Si BJT
Collector Currents IC is larger for a given VBE
IC increase improves 
IC increase decreases the emitter charging time This improves the switching speed.
Thin solid films 294 ( 1997)

15. Si/SiGe material system
high-speed capability –
because the base is heavily doped (bandgap difference)
high current gain at low Ic -
Small trap density at Si surface minimizes the surface recombination current
Lower cutoff frequency –
because lower mobilities in Si
Problem: E efficiency & Ic suffer (caused by EV)
To improve: graded-layer & graded-base heterojunction
Device structure of an n-p-n Si/SiGe/Si HBT
(b) Collector and base current versus VEB for a HBT and bipolar junction transistor (BJT).
IEEE Transactions on Electron Devices 52:3(2005) · 

16. InP-based material systems Advantages: very low surface recombination
Advanced HBTs
InP-based material systems
Advantages:
very low surface recombination
Higher electron mobility in InGaAs than in GaAs – superior high-freq performance (in fig., cutoff freq: 254GHz)
InP collector region has higher velocity at high fields than GaAs collector
InP collector breakdown voltage is higher than GaAs
Current gain as a function of operating frequency for an InP-based HBT.
Cutoff freq., fT= 254GHz
K.Kiziloglu et al. IPRM, 294 (2010)

17. GaInAs/InP Buried Metal HBT - REDUCTION OF BASE-COLLECTOR CAPACITANCE
Buried Tungsten wires of the same area as the emitter metal was used to reduce CBCext
SBCT of BM-HBT was estimated to be 22% that of conventional HBT, CBC 30% of conventional HBT
fT = 86 GHz, fMAX > 135 GHz of device with an emitter area of 0.5 x 2.5 μm2
fT = 82 GHz, fMAX > 200 GHz of device with an emitter area of 0.3 x 1.5 μm2
Schematic view of fabricated BM-HBT
Japanese Journal of Applied Physics, Volume 39, Part 2, Number 6A

18. Layer structure for the buried metal - HBT

19. Conclusion
HBT’s have tremendous potential for high power applications.
Polarization doping gives a promising solution to the p-type conductivity problem.
To date, GaAs HBTs with AlGaAs and InGaP emitters are commercially available and used mostly for power amplification in wireless communication systems.
Two commonly used HBTs are silicon–germanium and aluminum gallium arsenide, though a wide variety of semiconductors may be used for the HBT structure.
Growth technique as well as device design must be carefully chosen.