1. Monolithically Coupled Photo Diode - HBT or a Photo - HBT : A Modeled Comparison
BENNY SHEINMAN, DAN RITTER
MICROELECTRONIC RESEARCH CENTER
ELECTRICAL ENGINEERING DEPARTMENT
TECHNION – ISRAEL INSTITUTE OF TECHNOLOGY

2. Workshop outline Introduction.
Phototransistor electrical configurations.
General bandwidth / efficiency limitations in a photo-detector.
Additional limitation in a top illuminated PIN diode / phototransistor.
Electrical modeling of the top illuminated PIN diode / phototransistor.

3. Phototransistor structure

4. Photo-diode, HBT structure

5. Photo-diode and HBT or PhotoHBT ?
HBT+photodiode
same layers
different layers
Transistor performance
Compromised:
1. Miller effect
2. Collector transit time
Compromised
1. Collector transit time
Optimal
Responsivity
Maximal
Optical window
Small ?
Large ?
Large
Technology
Standard
Difficult

6. Phototransistor configuration
Common Base
Hole current flows to ground
 no current gain

7. Phototransistor configuration
Common Collector
High current gain
Low output resistance limits performance.

8. Phototransistor configuration
Common Emitter
High current gain.
Bandwidth limited by Miller effect:

9. Miller Effect
Integrated PIN HBT
Phototransistor

10. Cascode Configuration
Performance comparable
to that of a PIN + HBT ?

11. Current Density [KA/cm Current Density [KA/cm
Kirk effect 50
100
150
200
250
Current Density [KA/cm 2 ]
Frequency [GHz]
650nm Collector F t
max 50
100
150
200
250
300
Current Density [KA/cm 2 ]
Frequency [GHz]
150nm Collector F
max t

12. Kirk effect (cont.) Associated time constant for a 1*10 emitter
and a 10 diameter optical window:

13. Photodetectors bandwidth limitations
Carrier transit time:
RC of detector capacitance and amplifier input resistance:
M. Agethen et al. IPRM 2002

14. Photodetectors quantum efficiency

15. Only transit time limits performance
Ideal Amplifier
Only transit time limits performance

16. Base-collector junction in a phototransistor
GaInAs active layers:
Base layer highly resistive -

17. A top illuminated PIN as a notch filter

18. Additional RC filter bandwidth limitation in a top illuminated PIN diode / phototransistor.

19. RC network in a PIN detectordr r
Physical structure
Electrical equivalent circuit
Io1 R1 C1
Io2 R2 C2
Io3 R3 C3
IoN RN CN I1 I2 I3 I4

20. Spot size radius =12.5

21. Spot size radius =12.5

22. Spot size radius =12.5

23. Spot size radius =12.5

24. Spot size radius =6

25. Spot size radius =6

26. Top illuminated photo-transistor option 1
Optical window
Emitter
Contact
To base
Base Metal
Base Mesa

27. Top illuminated photo-transistor option 2
Optical window
Emitter
Base Metal
Base Mesa
Contact
To base

28. Spot size radius =12.5 2
12.5
Internal
contact

29. Model of top illuminated detector
Solution of current equations is difficult:
- distributed photocurrent
Photodiode capacitance / area
Yet for a known capacitance value,
a single pole fit gives good results
48
0.2pF

31. High efficiency phototransistors
Cover optical window with conducting transparent ITO (Indium Tin Oxide) layer.
Place internal and external contact to the diode:
Backside illumination.

32. Overcoming the limitations
Incorporating novel structures in
photo-transistors:
Wave guide photodetectors
Distributed phototransistors
Resonant-cavity-enhanced photodetector.
Uni-traveling-carrier photodiode.

33. Conclusions
In the cascode configuration, photo-HBT have comparable performance to PIN detector + HBT processed from the same layers.
PIN detector + HBT processed from different layers will have superior performance.
The highly resistive base layer produces an internal filter in the top illuminated PIN detector.
The influence of the filter should be included in the model of the detector.