P-N JUNCTION DIODE

ABSTRACT

          

This paper presentation will show us, what a P-N junction diode is, its operations, methods of forming P-N junction, P-N junction breakdown, and its applications.

INTRODUCTION

     The semiconductor diode is a two-terminal device containing a single p-n junction. A P-N junction is the sudden change from p-type conductivity (excess acceptor impurity) to n- type conduction (excess donor impurity) in the same piece of single semiconductor crystal. It is not just an interface between n- type and p- type pieces of semiconductors pressed together. The P-N junction usually has a narrow depletion region(A layer in a semiconductor that has few charge carriers transporting electric charge between zones of different conductivity).

 

    The diode terminals are labeled anode and cathode.

     The diode’s p-n junction is said to be forward biased when the anode voltage is positive with respect to the cathode and the diode carries a forward current. On the other hand, it is said to be reverse biased when the cathode voltage is more positive than anode voltage and it carries a small reverse saturated current.

 

OPERATION CHARACTERISTICS

    

    The operation of the semiconductor diode is described under three conditions, viz:-

•        Unbiased p-n junction: at the boundary where a p- n junction is formed, there is a difference in energy levels and, hence a localized movement of charges occurs within the junction. Electrons and hole move across the boundary to the p- type and n-type materials respectively. The n-type and p- type materials within the junction region.

 

   These space charges( positive and negative) exert repulsive forces on further charges crossing the junction, thereby making the junction region to be depleted of charges(formation of charge depletion region)Because of the positive and negative charge separation within the junction, an electric Field

   With potential known as barrier potential, develop across the junction under this equilibrium condition. At this equilibrium condition, this internal potential energy barrier balances the tendency of electrons to flow by diffusion,

 

and so the net junction current becomes zero. This is because the diffusion current and drift current are equal. Diffusion current is as a result of majority holes moving to n-type and majority electrons moving to p- type due to the concentration gradient. Drift current is as a result of minority holes in n -type near the junction region falling down the potential barrier to the p - type and vice versa with minority electron in the p-type.

 

•       Reverse biased p-n junction:  A p-n junction is said to be reverse biased if the positive terminal of the applied voltage is connected to the n- type material, and the negative terminal to the p- type material. An applied reverse bias voltage to the p-n junction tends to reduce diffusion by effectively increasing the prevented barrier. If it is increase sufficiently, diffusion is prevented altogether and the only current flowing is a small amount due to the minority carriers.

   From the graph above reverse bias voltage may be increases from Vp  to Vq   without materially increasing this leakage current. With this region of the characteristic curve of the p-n junction diode, the resistance of the p-n junction is high.

   If the reverse bias voltage is increase beyond Vq, breakdown of the p-n diode junction occurs (breakdown of the crystal lattice structures occur).

 

 

 

REVERSE BIASED

 

 

 

•       Forward bias p-n junction: this is the situation where the positive terminal of the applied voltage is connected to the p-type and negative terminal is connected to the n- type material.

   Forward bias voltage tends to increase the diffusion current by effectively reducing the potential barrier.

 

   

    If it is increased sufficiently, the effect of the potential barrier may be completely eliminated.

    The variation of the diode current with the forward bias voltage is given from O to R in the graph above. Within the region of OR the resistance of p-n junction diode is low.

  

 

FORWARD BIASED

METHOD OF FORMING P-N JUNCTION

 

    Alloying: this is the oldest method whereby a small amount of impurity atom of opposite type, for example aluminum is deposited on a surface of a semiconductor wafer (N- type silicon). The wafer is then heated in a furnace to allow the deposited impurity to melt.

 

 

    On cooling, melt solidifies into a single crystal which maintains the lattice structure of the underlying semiconductor wafer, but with excess new impurity.

•       Diffusion:  this method is very common nowadays. A semiconductor wafer, called a substrate of p- type silicon wafer, is put into a diffusion furnace kept at a high temperature.

 

   

     A carrier gas (nitrogen or argon) containing some vapour of compound of opposite impurity, say phosphorus in POCl3 is introduced into the furnace. At this high temperature, the compound breaks down and phosphorus atoms, which are deposited on the substrate, slowly diffuse into the semiconductor substrate.

 

 

   

    By strict control of time and temperature, very accurate depth  of diffusion can be achieved by limiting the diffusion to specific regions on the substrate surface through the use of SiO2 diffusion blocking mask( this must be done before hand on the substrate by photographic technique), one can make many junction of small areas on the same wafer.

 

    Ion implantation: Is a new method that is widely used these days. Ions of the desired impurity are first accelerated in a vacuum to high energies and then shot at the semiconductor substrate on which they penetrate the lattice of the substrate thereby disturbing its crystal lattice structure. The disturbed crystal lattice structure is then annealed at elevated temperature which then makes the implanted ions to move to substitutional lattice position where they are electrically active as impurities.

 

P-N JUNCTON BREAKDOWN

    A p-n junction is said to breakdown when the current across the junction begins to increase in applied reversed voltage.

   Some of the types of mechanism of p-n junction breakdown include:

•       Zener breakdown: This is caused by “zener effect”.  zener effect is simply the breaking of the covalent inter-atomic bonds and the consequent dislodgment of many electrons from the valence band of atoms in the depletion region by the application of a very high reverse voltage.

 

  

     It is this very high reverse voltage that produces high electric field that actually knocks out the many electrons and the resulting holes which increases the current across the junction.

    In summary, “zener effect” is the spontaneous generation of hole-electron pairs in the depletion region due to intense electric field produced by the reverse voltage.

 

 

  

   Avalanche breakdown: When the reverse bias voltage is increased beyond values for normal drift current, minority carriers pick up a sufficient energy in crossing the boundary region to knock out valence electrons from their atoms in the p- and n- regions.  Those electrons knocked out are in turn accelerated by the field and knock out more electron and holes. This action continues in a chain reaction.

 

   

 

    These avalanche breakdowns take place at lower electric field intensities than the zener breakdown.

   In summary, avalanche breakdown is as a result of minority charge carriers undergoing ionizing collisions with the atoms of the semiconductor material.

 

 

•       Thermal breakdown: When no or inadequate provision is made to remove heat from a p-n junction operating at a high current, thermal breakdown of the junction will occur.  If the heat sink is not enough, a p-n junction may be heated to a high temperature at which covalent bonds may be broken due to the thermal agitation of atoms in the crystal lattice of the semiconductor material.

   Also, the appearance of excess free carriers causes a still greater increase of the current across the p-n junction and consequently,

 

 

 

    This in turn, leads to greater overheating and a further increase in the current. As a result of this, the p-n is damaged and the diode becomes inoperative. In diodes, this thermal breakdown occurs at much lower field intensities than the avalanche breakdowns.

 

 

 

APPLICATIONS OF P-N JUNCTION

    

    The P-N junction  has many properties that can be used to provide components and equipment used in various engineering applications. Such component and equipment include:

üVoltage-limiting property:  A diode can be used to hold a voltage constant .This is called voltage regulation, a special type called a zener diode is used as a voltage regulator.

 

 

ü Rectifying property: Since a diode will conduct easily in one direction only, just half of the ac cycle will pass through the diode. Devices that make use of this property are known as rectifiers. A rectifier Is a device that changes alternating current to direct current.

 

  

ü      Variable capacitance: The p-n junction exhibits variable capacitance property which is employed In parametric diode used in sensitive microwave amplifier circuits.  The name parametric diode comes from the ability of the p-n junction device to adjust one of its important electrical parameters (capacitor) in response to changes in   parametric diodes are also used as electrically (automatic) controlled tuning capacitors in radio receivers to replace conventional mechanically variable capacitors, particularly in automatically or remotely tuned receivers.

CONCLUSION

          The forward- bias and the reverse- bias properties of the p-n junction imply that it can be used as a diode. A p-n junction diode allows electric charges to flow in one direction, but not in the opposite direction, With this a junction diode can be used as an electronic switch, being open when reverse biased and close when forward biased. negative charges (electron) can easily flow through the junction from n to p but not from p to n, and the reverse is true for holes. 

REFERENCE

• PN Junction Lab free to use on nanoHUB.org allows simulation and study of a p–n junction diode with different doping and materials. Users can calculate current-voltage (I-V) & capacitance-voltage (C-V) outputs, as well.