create new holes.This movement of electrons,from the negative voltage source
hrough the junction and from the positive termina of semiconductor to positive
VoHage source,creates a current fiow.Thus current will fiow in a semiconductor when
the semiconductor is forward biased.
When the polarity of the power source is reversed,the semiconductor is said to
be reverse biased.The holes are moved away from the junction by the negative
Voltage,whereas the electrons are drawn the junction by the positive voltage.Thus
there is little or no combining of electrons and holes at the junction,and no currem
will flow.
In practical terms,there will always be a few electrons and holes near the
junction,allowing a very small current to pass.This small current is known as leakage
current and is usually in the order of a few microamperes (or possibly pico amperes).
When P-type and N-type regions are formed in same crystal,the semiconductor
isknown as a diode or rectifier.The boundary between the two regions is termed a
junction.The P-region terminal is called the anode,whereas the N-region terminal is
called the cathode
Usually,when such semiconductors are used with signals,the semiconductors
are called diodes or signal diodes. When the device is used for conversion of
alternating current(AC)to direct current(DC),the semiconductor is called a rectifier
4.1.2 Basic Two-junction or Bipolar Transistor
Like a diode,a transistor can be used to prevent (or limit)the flow of current in
one direction.The prime use for a transistor,however,is to control the amount of
current in a circuit.This is done by adding a second junction to the basic diode.
junction,discussed in the above section.For this reason,such transistors are called
two-junction transistors or possibly,bipolar transistors.The author prefers the former
term but recognizes that the latter term is also in common use.
There are two possible arrangements for the two-junction in transistors:NPN
where a positive semiconductor material (holes)is placed between two negativ.
semiconductor materials (electrons),and PNP,where the negative material (electrons