Semiconductors Semiconductors A semiconductor material is one A semiconductor material is one which conducts only when excited. which conducts only when excited. It is neither an Insulator, nor a It is neither an Insulator, nor a Conductor. Conductor. A conductor has normally one A conductor has normally one carrier per atom, while a carrier per atom, while a semiconductor has one carrier per semiconductor has one carrier per 10 10 12 12 at room temperature (Silicon). at room temperature (Silicon). The devices are built by The devices are built by introducing an impurity into introducing an impurity into otherwise a pure matter, and the otherwise a pure matter, and the process is called “doping”. process is called “doping”.
Transcript
SemiconductorsSemiconductorsA semiconductor material is one which A semiconductor material is one which
conducts only when excited.conducts only when excited.It is neither an Insulator, nor a It is neither an Insulator, nor a Conductor.Conductor.A conductor has normally one carrier per A conductor has normally one carrier per atom, while a semiconductor has one atom, while a semiconductor has one carrier per 10carrier per 1012 12 at room temperature at room temperature (Silicon).(Silicon).The devices are built by introducing an The devices are built by introducing an impurity into otherwise a pure matter, and impurity into otherwise a pure matter, and the process is called “doping”.the process is called “doping”.
SemiconductorsSemiconductors Once phosphorus (z=15) is doped as an Once phosphorus (z=15) is doped as an
impurity onto a pure Silicon (z=14), for impurity onto a pure Silicon (z=14), for every atom one electron is left free. every atom one electron is left free. Phosphorus in this case is called Donor Phosphorus in this case is called Donor atom and this is atom and this is N-Type semiconductor.N-Type semiconductor.
Once Aluminum (z=13) is doped onto a Once Aluminum (z=13) is doped onto a pure Silicon (z=14), every atom lacks pure Silicon (z=14), every atom lacks one electron, or one hole is extra. The Al one electron, or one hole is extra. The Al is called “acceptor”, and such is is called “acceptor”, and such is P-Type P-Type semiconductor.semiconductor.
p-n Junction Diodep-n Junction Diode Most of the electronic devices are Most of the electronic devices are
junction devices.junction devices. The simplest of these is The simplest of these is p-n junction p-n junction
diode.diode. A A diodediode is a device that readily allows is a device that readily allows
charge carriers in one direction but not charge carriers in one direction but not in the other.in the other.
This property of the diode has been This property of the diode has been exploited for application in:-exploited for application in:-Signals, Rectifications, Voltage Protection, Signals, Rectifications, Voltage Protection,
Amplification and SwitchingAmplification and Switching
Semiconductor MaterialSemiconductor Material
Extrinsic MaterialsExtrinsic Materials
Semiconductor MaterialSemiconductor Material
Two separate bits of semiconductor, one n-type, the other p-type
Bring them together and join them to make one piece of semiconductor which is doped differently either side of the junction.
p-n Junction Diodep-n Junction Diode
Free electrons on the n-side and free holes on the p-side can initially wander across the junction. When a free electron meets a free hole it can 'drop into it'. So far as charge movements are concerned this means the hole and electron cancel each other and vanish
p-n Junction Diodep-n Junction Diode
As a result, the free electrons near the junction tend to eat each other, producing a region depleted of any moving charges. This creates what is called the depletion zone.
p-n Junction Diodep-n Junction Diode
Now, any free charge which wanders into the depletion zone finds itself in a region with no other free charges. Locally it sees a lot of positive charges (the donor atoms) on the n-type side and a lot of negative charges (the acceptor atoms) on the p-type side. These exert a force on the free charge, driving it back to its 'own side' of the junction away from the depletion zone.
p-n Junction Diodep-n Junction Diode
The acceptor and donor atoms are 'nailed down' in the solid and cannot move around. However, the negative charge of the acceptor's extra electron and the positive charge of the donor's extra proton (exposed by it's missing electron) tend to keep the depletion zone swept clean of free charges once the zone has formed. A free charge now requires some extra energy to overcome the forces from the donor/acceptor atoms to be able to cross the zone. The junction therefore acts like a barrier, blocking any charge flow (current) across the barrier.
p-n Junction Diodep-n Junction Diode Usually, we represent this barrier by 'bending' the Usually, we represent this barrier by 'bending' the
conduction and valence bands as they cross the conduction and valence bands as they cross the depletion zone. Now we can imagine the electrons depletion zone. Now we can imagine the electrons having to 'get uphill' to move from the n-type side to having to 'get uphill' to move from the n-type side to the p-type side.the p-type side.
The holes behave a bit like balloons bobbing up The holes behave a bit like balloons bobbing up against a ceiling. On this kind of diagram you require against a ceiling. On this kind of diagram you require energy to 'pull them down' before they can move energy to 'pull them down' before they can move from the p-type side to the n-type side. from the p-type side to the n-type side.
The energy required by the free holes and electrons The energy required by the free holes and electrons can be supplied by a suitable voltage applied can be supplied by a suitable voltage applied between the two ends of the pn-junction diode. between the two ends of the pn-junction diode. Notice that this voltage must be supplied the correct Notice that this voltage must be supplied the correct way around, this pushes the charges over the barrier. way around, this pushes the charges over the barrier. However, applying the voltage the 'wrong' way However, applying the voltage the 'wrong' way around makes things worse by pulling what free around makes things worse by pulling what free charges there are away from the junction! charges there are away from the junction!
This is why diodes conduct in one direction but not the other.
All diodes have three things in All diodes have three things in common:common:
1.1. They have two leads like a They have two leads like a resistor. resistor.
2.2. The current they pass depends The current they pass depends upon the voltage between the upon the voltage between the leads. leads.
3.3. They do They do notnot obey obey Ohm's lawOhm's law!!
Diode AnimationDiode Animation
Diode have similar BehaviorDiode have similar Behavior
The equation is fairly complicated The equation is fairly complicated and quite difficult to use for analysis. and quite difficult to use for analysis.
This equation is usually wrong! The This equation is usually wrong! The reason for this is that the actual reason for this is that the actual current/voltage relationship depends current/voltage relationship depends upon the detail of how the diode was upon the detail of how the diode was made - the choice of materials, made - the choice of materials, doping, etc. doping, etc.
Diode: Engineering ModelsDiode: Engineering Models Electronic engineers deal with these Electronic engineers deal with these
problems by simplifying things and using problems by simplifying things and using whichever of the following three models of whichever of the following three models of the diode suits them.the diode suits them.
The 'square law' model assumes that the current, The 'square law' model assumes that the current, when forward biased, is proportional to the square of when forward biased, is proportional to the square of the applied voltage. the applied voltage.
The 'corner' model assumes that the current is zero The 'corner' model assumes that the current is zero for any voltage below for any voltage below VdVd but rises when we try to but rises when we try to apply a voltage greater than this apply a voltage greater than this
The 'one way' model simplifies things even more The 'one way' model simplifies things even more by assuming that the corner voltage is so small that by assuming that the corner voltage is so small that we can regard it as being zero! we can regard it as being zero!
DiodesDiodes
Detailed characteristics of a diode can be found by looking up the type number in a data book. If you know how to measure resistance with a meter then test some diodes. A good one has low resistance in one direction and high in the other.
The pcb is often marked with a + sign for the cathode end. Diodes come in all shapes and sizes. They are often marked with a type number.
DiodesDiodes
A DIODE PUZZLE A DIODE PUZZLE Which lamps are alight? Which lamps are alight?
