A semiconductor material is one which conducts only when excited. Semiconductors A semiconductor material is one which conducts only when excited. It is neither an Insulator, nor a Conductor. A conductor has normally one carrier per atom, while a semiconductor has one carrier per 1012 at room temperature (Silicon). The devices are built by introducing an impurity into otherwise a pure matter, and the process is called “doping”.

Semiconductors Once phosphorus (z=15) is doped as an impurity onto a pure Silicon (z=14), for every atom one electron is left free. Phosphorus in this case is called Donor atom and this is N-Type semiconductor. Once Aluminum (z=13) is doped onto a pure Silicon (z=14), every atom lacks one electron, or one hole is extra. The Al is called “acceptor”, and such is P-Type semiconductor.

p-n Junction Diode Most of the electronic devices are junction devices. The simplest of these is p-n junction diode. A diode is a device that readily allows charge carriers in one direction but not in the other. This property of the diode has been exploited for application in:- Signals, Rectifications, Voltage Protection, Amplification and Switching

Semiconductor Material Extrinsic Materials

Semiconductor Material Bring them together and join them to make one piece of semiconductor which is doped differently either side of the junction. Two separate bits of semiconductor, one n-type, the other p-type

p-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 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 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 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 Diode Usually, we represent this barrier by 'bending' the conduction and valence bands as they cross the depletion zone. Now we can imagine the electrons having to 'get uphill' to move from the n-type side to the p-type side. The holes behave a bit like balloons bobbing up against a ceiling. On this kind of diagram you require energy to 'pull them down' before they can move from the p-type side to the n-type side. The energy required by the free holes and electrons can be supplied by a suitable voltage applied between the two ends of the pn-junction diode. Notice that this voltage must be supplied the correct way around, this pushes the charges over the barrier. However, applying the voltage the 'wrong' way around makes things worse by pulling what free 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 common: They have two leads like a resistor. The current they pass depends upon the voltage between the leads. They do not obey Ohm's law!

Diode Animation

Diode have similar Behavior cathode Anode

Biasing Diode Forward Biasing

REVERSE BIASED JUNCTION Biasing REVERSE BIASED JUNCTION

The Physicist’s Model

Diode: Physicist’s Equation The equation is fairly complicated and quite difficult to use for analysis. This equation is usually wrong! The reason for this is that the actual current/voltage relationship depends upon the detail of how the diode was made - the choice of materials, doping, etc.

Diode: Engineering Models Electronic engineers deal with these problems by simplifying things and using whichever of the following three models of the diode suits them.

Diode: Engineering Models The 'square law' model assumes that the current, when forward biased, is proportional to the square of the applied voltage. The 'corner' model assumes that the current is zero for any voltage below Vd but rises when we try to apply a voltage greater than this The 'one way' model simplifies things even more by assuming that the corner voltage is so small that we can regard it as being zero!

Diodes 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. 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.

Diodes

A DIODE PUZZLE Which lamps are alight?