0195136047.pdf

7.2 DIODES 341

corresponding to a switch. A diode acts like a switch that closes to allow current flow in the
forward direction, but opens to prevent current flow in thereverse direction. The diode thus acts
like a unilateral circuit element providing an on–off characteristic.
The physical operation of the junction can be described in terms of the charge-flow processes.
Usually there is a greater concentration of holes in thep-region than in then-region; similarly,
the electron concentration in then-region is greater than that in thep-region. The differences in
concentration establish a potential gradient across the junction, resulting in adiffusionof carriers,
as indicated in Figure 7.2.2(a). Holes diffuse from thep-region to then-region, electrons from
then-region to thep-region. The result of the diffusion is to produce immobile ions of opposite
charge on each side of the junction, as shown in Figure 7.2.2(b), and cause adepletion region(or
space-charge region) in which no mobile carriers exist.
The immobile ions (or space charge), being of opposite polarity on each side of the junction,
establish an electric field because of which a potential barrier is formed anddrift currentis
produced. The drift current causes holes to move from then-tothep-region and electrons to
move from thep-tothen- region, as shown in Figure 7.2.2(c). In equilibrium and with no
external circuit, the drift and diffusion components of current are equal and oppositely directed.
The potential barrier established across the depletion region prohibits the flow of carriers across
the junction without the application of energy from an external source.

pn-Junction under Bias

Let an external source be connected between thep- andn-regions, as shown in Figure 7.2.3(a).
Figure 7.2.3(b) shows the circuit representation of thepn-junction or diode, and its external
circuit. The voltage sourceV, called thebias, either decreases or increases the potential barrier,
thereby controlling the flow of carriers across the junction. WithV=0, the barrier is unaffected
and thepn-junction has zero current. Positive values ofV, known asforward biasing,decrease
the potential barrier, thereby increasing the number of electrons and holes diffusing across the
junction. The increased diffusion results in a net current, called the forward current, from the
p-tothen-region. With increasedV, the forward current further increases rapidly because the
barrier is reduced even further. MakingVnegative (reverse biasing), on the other hand, increases
the potential barrier and reduces the number of carriers diffusing across the boundary. The drift
component produced by the electric field from then-tothep-region causes a small current,
called thereverse current(orsaturation current)IS. The magnitude of the saturation current
depends on the doping levels in thep-andn-type materials and on the physical size of the

+

−

vD

p

anode

cathode

Practical

device

+

−

vD

p

iD

iD

vD

Ideal

diode

Metal

(ohmic)

contact

On

Off

0

(a)

p

n

(b) (c)

iD

Figure 7.2.1pn-junction.(a)Circuit symbol forpn-junction diode.(b)Physical structure.(c)Volt–ampere
characteristic of an ideal (or perfect) diode.