Download DIODE THEORY: The PN Junction Diode Characteristics

DIODE THEORY: The PN Junction Diode Characteristics.
A semiconductor junction is formed by joining together two pieces of material, one
doped as a 'p' type, having a deficiency of electrons called (+) holes, the other 'n' type,
having an excess of (-) electrons. At the point where 'p' and 'n' type materials are
joined some electron/hole recombining takes place. This forms a neutral region with
very few free electrons or holes. Electrons and the absence of electrons 'holes' are the
semiconductor current carriers. Because of the shortage of current carriers, in this
region the 'PN' junction acts as a poor insulator (semi-conduction). By applying a
forward bias voltage, this region can be made to contract, thereby developing a
forward resistance of only a few ohms. This allows a high current to flow. Apply a
reverse bias and the junction expands forming an extremely high resistance.
The I, V curve shows a typical forward and reverse bias arrangement for a PN
junction diode.
DIODE THEORY: Simple Diode Circuit.
This simple diode circuit demonstrates the effect of connecting a diode in a forward or
reverse biased condition. Also shown are a number of alternative symbols found in
electronic circuits. On forward biasing the diode, anode (a) positive to the cathode (k)
the lamp will light as the forward resistance is low, typically < 100ohms. In the
reverse biased direction the diode resistance is high (several Mohms) and the lamp is
extinguished.
DIODE THEORY: Forward Biased PN Junction.
On connecting the battery '+V' to the 'p' type material the junction is forward biased.
Negative electrons will cross to the 'p' region attracted by the battery (+) potential and
positive holes will move into the 'n' region. It takes a potential of about 0.6Volts to
overcome the junction barrier of a silicon diode, (germanium types require
approximatly 0.25Volts), thereafter a large increase in current '+I' flows owing to the
low forward resistance of the diode.
Try a range of bias voltages from 10mV to 2Volts
DIODE THEORY: Reverse biased PN Junction.
Reverse biasing the junction expands the PN region, and makes the junction a high
resistance. Connect a battery as shown, positive (+) to the 'n' type material. Increasing
the reverse bias causes the mobile carriers (holes and electrons) to be repelled, thereby
widen the region of recombination (where the negative electrons are moving into the
vacant positive holes).
A reverse biased junction can develop a resistance of many millions of ohms. As there
is a shortage of mobile current carriers.