P – n junction Prof.Dr.Beşire GÖNÜL.  the basic element of all bipolar devices. >photodiode, light sensitive diode, >LED- ligth emitting diode, >laser.

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Presentation transcript:

p – n junction Prof.Dr.Beşire GÖNÜL

 the basic element of all bipolar devices. >photodiode, light sensitive diode, >LED- ligth emitting diode, >laser diode-variable capacitance diode p – n junction

 The p-n junction can be formed by  p-type silicon into close contact  with a piecce of n-type silicon. But forming a p-n junction is not so simply. Because; 1) very few points of contact 2) exposed to the air – always has a thin oxide coating. 3)Bonding arrangement is interrupted at the surface The formation of p-n junction :

To overcome these surface states problems p-n junction can be formed in the bulk of the semiconductor, away from the surface as much as possible. Surface states

 Lots of electrons on the left hand side of the junction want to diffuse to the right and lots of holes on the right hand side of the junction want to move to the left.  The donors and acceptors fixed,don’t move (unless you heat up semiconductors, so they can diffuse) because they are elements (such as arsenic and boron) which are incorporated to lattice.  However, the electrons and holes that come from them are free to move. p – n junction

 Holes diffuse to the left of the metalurgical junction and combine with the electrons on that side. They leave behind negatively charged acceptor centres.  Similarly, electrons diffusing to the right will leave behind positively charged donor centres. This diffusion process can not go on forever. Because, the increasing amount of fixed charge wants to electrostatically attract the carriers that are trying to diffuse away(donor centres want to keep the electrons and acceptor centres want to keep the holes). Equlibrium is reached.  This fixed charges produce an electric field which slows down the diffusion process.  This fixed charge region is known as depletion region or space charge region which is the region the free carriers have left.  It is called as depletion region since it is depleted of free carriers. Idealized p-n junction

 Energy level diagram for the p-n junction in thermal equilibrium ECEC EVEV EfEf p-type n-type ECEC EVEV EfEf Depletion region Hole Diffussion Hole Drift Electron Diffusion Electron Drift Neutral p-region Neutral n-region

Thermal equilibrium; no applied field; no net current flow

 The drift and diffusion currents are flowing all the time. But, in thermal equilibrium, the net current flow is zero since the currents oppose each other.  Under non-equilibrium condition, one of the current flow mechanism is going to dominate over the other, resulting a net current flow.  The electrons that want to diffuse from the n- type layer to the p-layer have potential barier. Proof