Bias Circuit

B.1 Fundamentals of Bipolar Junction Transistor Action
A bipolar junction transistor is made up of a sandwich of two semiconductor pn junctions.
Transistors are either npn or pnp. In the active-mode transistor state, one junction, the input
junction, is forward biased, and the opposite junction, the output junction, is reverse biased.
The behavior of the individual pn junction diode will first be considered to clarify forward and
reverse bias.
A semiconductor pn-junction diode in diagrammatic form is shown in Fig. B.1. The current and
voltage of an ideal pn-junction diode are related by
Equation B.1
Figure B.1. Diagrammatic pn-junction diode. Applied voltage VD is
shown for forward bias for which current freely flows. Opposite
polarity is reverse bias, where the diode is essentially cut off and ID =
–IS.
where VD is the voltage applied between the p and n regions (positive at the p terminal), ID is
the responding current, and ISd is the saturation current. The thermal voltage, VT, is defined as
VT = kT/q, where T is the temperature, k is the Boltzmann constant and q is the electron
charge. At 27°C, VT 26 mV. VD is positive for forward bias and negative for reverse bias.
As illustrated by the arrows in the junction of Fig. B.1, for forward bias, the diode current
consists of injection of holes from the p region (positive free carriers) into the n region and
injection of electrons from the n region (negative free carriers) into the p region. The
exponential factor in (B.1) is associated with the statistics of the free carriers, holes and
electrons, and the effect on the lowering of the barrier to free-carrier flow of the application of
the positive applied voltage.
The magnitude of ISd is dependent on semiconductor electronic properties such as doping
levels and free-carrier lifetime and mobility. The magnitude of current flow for a given VD is
dictated by a combination of the barrier-lowering factor (in the exponential) and the rate of
free-carrier recombination; that is, carriers are annihilated by recombining with the opposite
type of carrier on the opposite side of the junction from which they are injected.
A typical value for saturation current, ISd, is 10 –11 mA, such that, for example, for VD = 0.6 V,
ID 0.1 mA and increases by an order of magnitude for each additional increment of δVD 60
mV (at room temperature). The equation would suggest that for negative VD and |VD| >> VT,
ID = –ISd and therefore is very small compared to the current under forward bias. In real pn-