Direct Energy, 2018a

124 6.5 Pn Junctions
an excess of negative charges because some aluminum atoms replace silicon
atoms. Aluminum atoms have one less proton than Si atoms. They also
have one less electron,but the hole is a charge carrier which also diuses
away from the junction.
An electric eld forms across the junction due to the net charge distribution
near the junction. Electric eld intensity is the force per unit
charge,and it has the units m V . There is also necessarily a voltage drop
across a pn junction in equilibrium,and this voltage is called the contact
potential V 0 in the units of volts. While the contact potential is a voltage,
it cannot be measured by placing a voltmeter across a pn junction because
additional junctions would be formed at each lead of the voltmeter with
additional voltages introduced [9,p. 141].
Figure 6.11 illustrates the energy level diagram of a pn junction. The
horizontal axis represents position,and the vertical axis represents energy.
It is related to the gures in Section 6.3. However,Fig. 6.11 is zoomed in
vertically,and it is plotted versus position near the junction. It also shows
the relationship between the energy level diagram and the circuit symbol
for a diode,and the depletion layer is labeled. The vertical distance qV 0 ,
also labeled in Fig. 6.11,represents the amount of energy required to move
an electron across the junction [9,p. 141].
Figure 6.12 shows the energy level diagram for a forward biased pn
junction. In a forward biased pn junction,current ows from the p-type to
n-type side of the junction. More specically,holes ow from the p-type
to n-type region,and some of these holes neutralize excess charges in the
depletion layer. The depletion layer becomes narrower. The electric eld
preventing the ow of charges gets smaller,and the voltage drop across the
junction gets smaller. The energy q (V 0 − V x ) is labeled in Fig. 6.12 for a
forward biased pn junction where the voltage V x is the voltage supplied.
This energy represents the energy needed to get charges to ow across the
junction,and it is smaller than the corresponding energy in the case of the
unbiased junction. Charges ow more easily in the case of a forward biased
pn junction,and the diode acts as a wire.
Figure 6.13 shows the energy level diagram for a reversed biased pn
junction. For a reverse biased pn junction,the voltage across the junction
V 0 + V x is larger than for an unbiased junction,and the energy needed for
charges to ow q (V 0 + V x ) is larger than for an unbiased junction. Reversed
biased pn junctions act as open circuits,and charges do not ow due to
this amount of energy required.
A light emitting diode (LED) is a device that converts electricity to
optical electromagnetic energy,and it is made from a semiconductor pn
junction. In use,a forward bias is put across the LED as shown in Fig.