For emitter junction
of n-p-n integral transistors, as an example, the current density can reach 100 A/[mm.
Unfortunately, the forward voltage drop of the additional emitter junction
adds to the on-voltage so that even in a package such as DPAK the typical on-voltages achievable are >1.
By applying of equation (2) for emitter junction, the collector current of the transistor operating in the forward active mode can be presented as follows
Be] is the potential barrier of the emitter junction.
The values of other transistor model parameters such as the emitter junction saturation current ([i.
The equation of the input offset voltage of the differential amplifier for the case when emitter junctions operate at high-current density derived using equation (3) is as follows
With a constant voltage base bias, a decrease in the turn-on voltage of the emitter junction causes an increase in collector current.
The decrease in the emitter junction turn-on voltage can be accounted for by adding a temperature-dependent base voltage source
C = hole and electron current ratio at emitter junction for [DELTA]E=0
The saturation resistance results primarily from the need for a finite collector voltage to reverse bias the collector-base junction when the base emitter junction
is forward biased.
Linearity may be further improved by negative feedback, and emitter ballast resistors are used to perform an additional duty, for example, the task of linearizing the current voltage relationship of the base to emitter junction
that provides almost all the nonlinearity in an HBT.
Furthermore, the strain due to the lattice mismatch may be used to improve the hole mobility in the base parallel to the emitter junction