Page 975 - Basic Electrical Engineering

Figure 14.16 P–n–p transistor

When both junctions of a transistor are reverse biased, the transistor is said

to be cut off and operating in the cut-off region.
The ratio of the collector current to the emitter current of a transistor is

called α which is greater than 0.95. We can express
dc

I = I + I (i)
C
B
E
and

IC = α I neglecting the reverse saturation current (ii)
dc E

Therefore,
I = (1 – α ) I (iii)
B
dc
E
where α is the emitter to collector current gain.
dc
The value of alpha dc (α ) is normally 0.95 to 0.99. As the collector–base
dc
junction is reverse biased, a small reverse saturation current flows across the
junction due to minority charge carriers which are very small in number, and

can be neglected.

From eqs. (i) and (ii)

I = α I = α (I + I )
B
C
dc
C
dc E