1. Physics of Semiconductor Devices
Institute of Microelectronics and Optoelectronics
Zhang Wei

2. 3.7 BASE RESISTANCE Base resistance Extrinsic resistance RBX
Intrinsic resistance RBI
Extrinsic resistance RBX n ＋ p
RBX
RBI n
EXRINSIC BASE
INTRINSIC BASE

3. nb: the number of base contacts
RBX＝
RSBX( bb /lb) +RCON nB
RCON: contact resistance lb＆Rb : length and width of the extrinsic base region
nb: the number of base contacts
RBI =
C RSBI( be/le) nB 2
le＆be: emitter length and width C: a constant

4. measurement of RSBI p n + I I n n p n + p n

5. Comparison between RBX & RBI
RBX : a constant and very easy to model
RBI : a strong function of current
In particular, RBI decreases markedly at very high collector currents.
caused by current crowding due to the lateral flow of base current underneath the emitter
the lateral flow of base current underneath the emitter VBE varies with distance VBE is highest at the emitter edge closest to the base contact the emitter current ‘crowds’ towards the base contact

6. 3.8 COLLECTOR/BASE CAPACITANCE
Collector capacitance is another parameter which is very important in determining the AC performance of a bipolar transistor. The importance is decided by the Miller effect. + n P
CJCX
CJCI n
EXTRINSIC AREA
INTRINSIC AREA

7. determined
emitter geometry & collector doping concentration
CJCI
The collector capacitance
the necessity of having space to make contact to the base of the transistor
CJCX
limited
Attention:
The emitter geometry is limited by lithography constraints, and the collector doping concentration by the need to suppress high-current effects such as base widening.
Conclusion:
The designer has very little control over the intrinsic component of the collector capacitance.

8. (exp (exp ) [(exp ) ] (exp ) (exp ) (exp (exp (exp
3.9 THE SPICE BIPOLAR TRANSISTOR MODEL
The SPICE notation has been used in the model equations and in the labeling of the model components. The collector and base currents are written as:
qVB’C’ Is
qVB’E’
(exp ) IS
[(exp ) ] 1
Ic= 1 QB
NRKT
NFKT βR
qVB’C’
(exp
qVB’C’ )
(exp )
ISC 1 1
NCKT
NRKT
qVB’E’ IS Is
(exp
qVB’C’ 1)
(exp 1)
IB= +
ISE + βR
NRKT βF
NFKT
qVB’E’
qVB’C’
(exp 1)
(exp 1) +
ISC
NEKT
NCKT

9. The SPICE base resistance model includes current crowding, and is given by:
RB RBM =
RBB’
RBM + QB
The emitter/base depletion capacitance CJEB is modeled using:
VB’E’
MJE = [1 ]
CJEB
CJE
VJE
The emitter diffusion capacitance CDE is modeled by: ə [
TFFIS
qVB’E’ =
(exp ) ]
CDE 1 ə
VB’E’ QB
NFKT
A total base/collector capacitance CBC is given by:
MJC
VB’C’
qVB’C’ [1 ] [
qIS ]
CBC
= CJC + TR
exp
VJC
NRKT KT

10. Chapter 4 POLYSILICON EMITTERS
4.1 INTRODUCTION
Use: form the gate electrode of the MOS transistor
Reason: low deposition temperature and its ability to withstand the high temperatures

11. Fabrication sequence for a polysilicon emitter

12. Fabrication sequence for a polysilicon contacted emitter

13. The advantages of polysilicon emitters:
Firstly: their suitability for producing shallow emitter/base junctions and also their compatibility with self-aligned fabrication techniques
Secondly: they allow the parasitic resistances and capacitances of a bipolar transistor to be minimized, with the result that a considerable improvement in circuit performance is obtained
Furthermore: significantly higher values of common emitter current gains are obtained

14. The advantages of the improved gain of the polysilicon emitter transistor
First: the additional gain can be traded for an increase in the base doping concentration and hence a decrease in the base resistance. This is likely to lead directly to an improvement in the switching speeding of the bipolar transistor.
In addition: reducing the emitter/base junction depth of a conventional bipolar transistor leads to an equivalent reduction in the current gain. This is in contrast to the situation for a polysilicon emitter, where the emitter/base junction depth can be reduced without any degradation of the current gain.

15. 4.2 BASIC PHYSICS OF THE POLYSILICON EMITTER
Problems:
difficult to characterize and difficult to model
the polysilicon/silicon interface and the polysilicon grain boundaries
Conclusion from the comparison of the current gain:
As is shown in the following Figure, it can be seen that the improvement in gain results from a decrease in base current rather than an increase in collector current.

16. Gummel plots for practical polysilicon emitter transistors with and without a deliberately grown interfacial oxide

17. analysis：
•Ipe is significantly reduced because of the mechanism identified as tunneling. Hence the base current is therefore determined by the tunneling properties of the interfacial oxide, in particular by the interfacial layer thickness and the effective barrier height for holes χh。
In devices with a nominally clean interface are improved gains are again obtained, although in this case the improvement is only by a factor of 2 or 3.These results can be explained by noting that in a polysilicon emitter transistor the effective emitter/base junction depth is given by the sum of the polysilicon and single-crystal emitter thickness.
The physics of polysilicon emitters is further complicated by the presence of the grain boundaries in the polysilicon and also by the large pseudo-grain boundary which is formed at the interface between the polysilicon and single-crystal silicon.
The influence of grain boundaries on the electrical characteristics of polysilicon emitter transistors has been experimentally demonstrated and arsenic segregation identified as being important.

18. Summary
•Both tunneling and transport mechanisms contribute to the improved gains of polysilicon emitter transistors.
•Tunneling offers by far the biggest gain improvement, but it can result in an undesirable increase in the emitter resistance of transistor.
•The transport mechanism offers much lower gain improvements, but the benefit can be maximized if careful attention is paid to the mechanisms which limit the extent of the gain improvement.