288 CHAPTER 6. BIPOLAR JUNCTION TRANSISTORS
This is the neutral base width. The actual base width will be larger and we need to
calculate the depletion on the base side at the BCJ due to the biasing of the device. Since
the EBJ is strongly forward biased, there is essentially no depletion of the base at this
junction.
The built-in voltage on the BCJ is
Vbi=
kBT
e
ln
(
NabNdc
n^2 i
)
=0.8V
Using theVbivalue we find that the depletion width on the base side of the EBJ for a 5 volt
bias at the base collector junction is
ΔW(V=5V)=1. 59 × 10 −^5 cm
and the base width becomes
Wb=Wbn+1. 59 × 10 −^5 =1. 659 × 10 −^4 cm
(ii) Two disadvantages of a shorter base:
- The output conductance will suffer and the collector current will have a stronger
dependence onVCB.
- The device may suffer punch through at a lower bias.
Two advantages:
- The current gain will be higher.
- The device speed will be faster.
Example 6.4Consider anpnSi-BJT at 300 K with the following parameters:
Nde =10^18 cm−^3
Nab =10^17 cm−^3
Ndc =10^16 cm−^3
Db =30.0cm^2 s−^1
Lb =10. 0 μm
Wb =1. 0 μm
De =10cm^2 s−^1
Le =10. 0 μm
Emitter thickness =1. 0 μm
Device area =4. 0 × 10 −^6 cm^2
Calculate the emitter efficiency and gainβwhen the EBJ is forward biased at 1.0 V and
the BCJ is reverse biased at 5.0 V.
Calculate the output conductance of the device defined by
go=
ΔIC
ΔVCB
