276 CHAPTER 6. BIPOLAR JUNCTION TRANSISTORS
|E|
dE
dx=e
(Nd,C−np,C)Figure 6.17: (a) Schematic diagram of a typical bipolar transistor structure. The EBJ is forward
biased, and the BCJ is reverse biased such that the collector is fully depleted. (b) Corresponding
charge profile in the device. The space charge in the device results from depletion charge (light
gray) and injected mobile charge (dark gray). (c) Electric field in the collector region.
electric field at the base-edge reaches zero (and equivalently the depletion region widthxpC=
0 ). The current density when this condition occurs is called the Kirk threshold current density
and is given the symbolJKirk. Its value can be readily calculated by solving equation 6.6.6-
equation 6.6.8 subject to the conditions that the electric field at the base edge of the collector is