What do you understand by emitter efficiency and transport f

What do you understand by \"emitter efficiency\" and \"transport factor in the base\" in a BJT? What is the origin of \"Current Crowding\" and \"Series Resistance\" in BJTs? Why is it necessary for the base region in a BJT to be narrow\'? What is the precise definition of \"narrow\"? The arrows in the figure shows different current density components in an npn BJT. Identify the biasing mode Write the current density components in the boxes.

Solution

a)

The emitter efficiency, is defined as the ratio of the electron current in the emitter, to the sum of the electron and hole current diffusing across the base-emitter junction.

The transport factor, equals the ratio of the current due to electrons injected in the collector, to the current due to electrons injected in the base.

b)

Large area bipolar transistors can have a very non-uniform current distribution due to the resistance of the base layer. Since the base current is applied through the thin base layer, there can be a significant series resistance in large devices.

c)

Base of a BJT is usually made thin and doped less densily to keep the base current small and allow the electrons to move much more freely between emiiter and collector.

The narroness is precisely defined in base transportation factor. Greater the transportation factor, greater the narrowness.

d)

i) It is active mode biasing. Base emitter junction forward biased and base collector junction reverse biased.

ii) In collector area, the top most box represents electron diffusion current density, resulting from injected electrons successfully crossing the base.

The next box down represents the current density due to combination of minority carrier holes from collector and electron from base at collector.

The next box down represents the current density resulting from injected electrons getting trapped in the base.

In the base region, the top most box represents electron diffusion current density, resulting from injected electrons successfully crossing the base.

The next box down represents base current due to hole electron recombination at base.

In the base emiter junction the box represent current density due to injected electrons from emitter to base.

In the collector, the topmost box represents current density due to injected electrons from emitter to base .

The next box down represents current density due to electron hole recombination at base emitter junction.

The next box down is due to successful hole injection from base to emitter.