Question 12.5: Design the base width required to achieve a base transport f...

The base transport factor applies to both pnp and npn transistors and is given by

\alpha_{T}=\frac{1}{\cosh \left(x_{B} / L_{B}\right)}=0.9967

Then

x_{B} / L_{B}=0.0814

We have

L_{B}=\sqrt{D_{B} \tau_{B 0}}=\sqrt{(10)\left(10^{-7}\right)}=10^{-3} \mathrm{~cm}

so that the base width must then be

x_{B}=0.814 \times 10^{-4} \mathrm{~cm}=0.814 \mu \mathrm{m}

Comment

If the base width is less than approximately 0.8 \mu \mathrm{m}, then the required base transport factor will be achieved. In most cases, the base transport factor will not be the limiting factor in the bipolar transistor current gain.