Bias Circuit

junction diodes, the current is larger and is not independent of the value of the reverse applied
voltage. Nonetheless, it is still very small compared to the value of current for normal forward
bias.
As mentioned above, the bipolar junction transistor is made up to two junctions, which share a
common region. A pnp example is shown in Fig. B.2. The individual pn junctions of a transistor
also exhibit diode characteristics, and the junction currents are related to the junction voltages
by equations similar to (B.1). There is an essential difference that can be explained for the
case of one junction being forward biased while the other junction is zero or reverse biased.
Specifically, suppose that the transistor is a pnp (the other possibility being the npn) and the
pn junction on the left (input junction) is forward biased and the pn junction on the right
(output junction), is made to have zero bias by connecting a wire across this junction, as
shown in Fig. B.2.
Figure B.2. Diagrammatic semiconductor pnp transistor. The input
junction on the left has forward bias voltage VD, and the output
junction on the right is shorted for zero bias. The input pn-junction
diode current ID couples with the output pn junction to flow into the
output p region.
Even though the forward-biased pn junction on the left shares the n region with another pn
junction (on the right), it behaves like a pn-junction diode such that the current – voltage
relation for the junction still has the form of (B.1) and is
Equation B.2
The saturation current is now designated IS, since IS ISd. [If the width of the n region is very
large, the holes injected into the n region will recombine with electrons in the n region, and the
associated current will flow entirely out through the contact to the n region. In this limiting
case, the current – voltage relation for the junction on the left reverts to (B.1). The presence
of the p region on the right will play no role.]
In a transistor, the n-region width is made small and the statistical chance of an injected hole
surviving the transit across this region, without recombining with an electron, is very high.
Upon entering the p region on the right, the holes are accommodated by the exit of an equal
number of holes moving out at the contact to this p region. (Actually, they are converted to
electrons at the interface of the contact and semiconductor to be consistent with having only
electron flow in the metal contact and device connecting metal.)
In an ideal transistor, this is the only current mechanism. The various other current
components of the real transistor are discussed below. These include electron injection that
would be expected from the n region into the p region of the forward-biased junction on the