2. BJT Static Characteristics
Semiconductor Device Physics
Chapter 11
BJT Static Characteristics

3. Deviations from the Ideal
Chapter 11
BJT Static Characteristics
Deviations from the Ideal
Common base
Deviations, due to model limitations
Common emitter

4. Base-Width Modulation
Chapter 11
BJT Static Characteristics
Base-Width Modulation
Common-Emitter Configuration Active Mode Operation
Recalling two formulas, W P+ N P + 
VEB IE IC
DpB(x)
If reverse bias –VCB increases
W decreases
bdc increases
IC increases
(VCB = 0)
Increasing reverse bias –VCB x W

5. Chapter 11
BJT Static Characteristics
Punch-Through
Punch-Through: E-B and C-B depletion regions in the base touch each other, so that W = 0.
As –VCB increases beyond the punch-through point, the E-B potential hill decreases and therefore increases the carrier injections and IC.
WB = xnEB + xnCB

6. Increasing reverse bias of C-B junction
Chapter 11
BJT Static Characteristics
Breakdown Mechanisms
In the common-emitter configuration, for high output voltage VCE, the output current IC will increase rapidly due to the two mechanisms: punch-through and avalanche.
Punch-through
Avalanche
Increasing reverse bias of C-B junction

7. Avalanche Multiplication
Chapter 11
BJT Static Characteristics
Avalanche Multiplication
Holes [0] are injected into the base [1], then collected by the C-B junction.
Some holes in the C-B depletion region have enough energy do impact ionization [2].
The generated electrons are swept into the base [3], then injected into the emitter [4].
Each injected electron results in the injection of IEp/IEn holes from the emitter into the base [5].
pnp BJT
For each pair created in the C-B depletion region by impact ionization, (IEp/IEn) + 1 > bdc additional holes flow into the collector.
This means that carrier multiplication in C-B depletion region is internally amplified.
M : multiplication factor

8. Geometrical Effects Emitter area is not equal to collector area.
Chapter 11
BJT Static Characteristics
Geometrical Effects
Emitter area is not equal to collector area.
Current does not flow in one direction only.
Series resistance.
Voltage drop occurs not only across the junction.
Current crowding.
Due to lateral flow, current is larger around emitter periphery than the collector periphery.

9. Graded Base Dopants are injected through diffusion.
Chapter 11
BJT Static Characteristics
Graded Base
Dopants are injected through diffusion.
More or less falling exponential distribution with distance into beneath of the semiconductor.
The doping within the base is not constant as assumed in ideal analysis.
A function of position, having maximum at E-B junction and minimum at C-B junction.
Creating a built-in electric field.
The electric field enhances the transport of minority carrier across the quasineutral width of the base.
Increase of IE and IC.
xdiff : exponential decay constant

10. Due to recombination in emitter depletion region
Chapter 11
BJT Static Characteristics
Figures of Merit
Due to recombination in emitter depletion region
Gummel Plot
Due to high level injection in base, base series resistance, and current crowding

11. Polysilicon Emitter BJT
Chapter 11
BJT Static Characteristics
Polysilicon Emitter BJT
bdc is larger for a poly-Si emitter BJT as compared with an all-crystalline emitter BJT.
This is due to reduced dpE(x)/dx at the edge of the emitter depletion region.
Lower mp
Continuity of hole current in emitter
Shallower slope  less JP
 higher g, b

12. Summary on BJT Performance
Chapter 11
BJT Static Characteristics
Summary on BJT Performance
High gain (bdc >> 1)
One-sided emitter junction, so that emitter efficiency g  1
Emitter doped much more heavily than base (NE >> NB).
Narrow base, so base transport factor aT  1.
Quasi-neutral base width << minority-carrier diffusion length (W << LB).
IC determined only by IB (IC  function of VCE or VCB)
One-sided collector junction, so that quasineutral base width W does not change drastically with changes in VCE or VCB.
Base doped more heavily than collector (NB > NC), W = WB – xnEB – xnCB for pnp BJT.

13. BJT Dynamic Response Modeling
Semiconductor Device Physics
Chapter 12
BJT Dynamic Response Modeling

14. Qualitative Transient Response
Chapter 12
BJT Dynamic Response Modeling
Qualitative Transient Response
Saturation
(ON)
Load line
Idealized switching circuit
Cut-off
(OFF)

15. Qualitative Transient Response
Chapter 12
BJT Dynamic Response Modeling
Qualitative Transient Response 1
cutoff 2 3
active
saturation 3 4 2 4 5 5 1
saturation
active

16. Charge Control Relationships
Chapter 12
BJT Dynamic Response Modeling
Charge Control Relationships
A pnp BJT biased in the active mode has excess minority-carrier charge QB stored in the quasineutral base.
While
Small
In steady state,
Interpreted as average lifetime of an excess minority carrier

17. Base Transit Time tt (Active mode)
Chapter 12
BJT Dynamic Response Modeling
Base Transit Time tt
(Active mode)
Interpreted as average time taken by minority carriers to diffuse across the quasineutral base

18. Relationship between tB and tt
Chapter 12
BJT Dynamic Response Modeling
Relationship between tB and tt
The lifetime of a minority carrier before it recombines in the base is much longer than the time it requires to cross the quasineutral base region

19. Example Given an npn BJT with W = 0.1 μm and NB = 1017cm-3. Find tt.
Chapter 12
BJT Dynamic Response Modeling
Example
Given an npn BJT with W = 0.1 μm and NB = 1017cm-3. Find tt.
From Figure 3.5, mn = 801 cm2/(Vs)

20. Turn-On Transient During the turn-on transient:
Chapter 12
BJT Dynamic Response Modeling
Turn-On Transient
During the turn-on transient:
The general solution is:
Initial condition: QB(0)=0, since transistor is in cutoff:
tr : rise time, period of active mode

21. bdc > bdc(saturation)
Chapter 12
BJT Dynamic Response Modeling
Turn-On Transient
IBBtB > ICCtt
bdc IBB > ICC
bdc > ICC/IBB
In saturation mode:
bdc > bdc(saturation)

22. Turn-Off Transient During the turn-off transient:
Chapter 12
BJT Dynamic Response Modeling
Turn-Off Transient
During the turn-off transient:
The general solution is:
Initial condition: QB(0)=IBBtB:
tsd : storage delay time

23. Chapter 12
BJT Dynamic Response Modeling
Turn-Off Transient
The transient speed of a BJT depends on the amount of excess minority-carrier charge stored in the base and also the recombination lifetime tB.
By reducing tB, the carrier removal rate is increased, for example by adding recombination centers (Au atoms) in the base.
Tradeoff: bdc= tB/tt will decrease.

24. Practical Considerations
Chapter 12
BJT Dynamic Response Modeling
Practical Considerations
The foregoing analysis was highly simplified to avoid excessive amount of mathematics.
More realistic iC transient response is shown below.
Added delay time td (due to charging of junction capacitance) and fall time tf.
Collector current (mathematical model)
Collector current (actual form)

25. Chapter 12
BJT Dynamic Response Modeling
Homework 10
1. (E11.36-B)
The base of a silicon npn bipolar transistor is WB = 0.8 μm wide. The doping concentrations are NB = 2×1016 cm–3 and NC = 1015 cm–3. Determine the punch-through voltage.
2. (E11.37-B)
The base impurity doping level is NB = 3×1016 cm–3 and the base width is WB = 0.7 μm. If the required minimum punch-through voltage is determined to be Vpt = 70 V, calculate the highest level of collector doping. 3.
Next slide.
Deadline: Monday, 5 December 2016.

26. Chapter 12
BJT Dynamic Response Modeling
Homework 10
3. (EIE130.AF00F)
Consider a pnp silicon BJT of area A = 10–6 cm2 at 300 K. It operates in active region with VEB = 0.6 V and VCB = –1 V, so that WB = 0.6 μm. Assume that the emitter and collector regions are much longer than the respective minority carrier diffusion length.
Further data are given as follows: NE = 1019 cm–3, NB = 1017 cm–3, NC = cm–3, DE = 2 cm2/s, DB = 20 cm2/s, DC = 12 cm2/s, τE= 10–7 s, τB= τC= 10–6 s.
What is the common-emitter d.c current gain of this transistor?
What are the excess minority-carrier concentrations at the edges of the depletion regions (at A, B, C, and D in the diagram above).
Calculate the collector current.