BJT: Current Model

Now we go back to the current model of BJT where we have stated that electrons injected from the emitter will diffuse across the Base. One aim in making BJT is always to reduce the base width to reduce this unwanted base current. Good designs tries to make electron transfer from emitter to collector to be maximum and I b (due to the hole back injection) to be minimum.

Electron-Hole diffusion

Electron diffusing across the base and collected in the collector gives rise to the collector current I c given by

where

is the emitter area, is the minority diffusion coefficient, is the Base thickness, is the intrinsic concentration in base and N base is the base doping level.

The base current due to hole back injection is

now the ratio of desired to undesired current component is

The ratio is roughly of the order of 1 and

can be controlled by doping.

To make , the base doping should be lower than the emitter doping.

This works for most transistors, however for high speed operation the base resistance R B and the junction capacitance C BE are required to be low. R B decreases with increase in N base and C BE decreases with decreasing N emit . Therefore is difficult to achieve. So high speed(f T ) and high ( ratio) cannot be both achieved simultaneously and an optimization is needed. A recourse to this is to keep N emit low and N base moderate, but increase the electron injection efficiency by using a hot electron injection and field assisted base transport as done in HBT.