Electronics-semiconductor device / DIODE /

 

P-N junction diode

In a N-type 

material the electron is called the majority carrier and the hole of the minority career.

In a P-type

 material the hole is the majority career and the electron is the minority career. The N- and P- type materials represent the basic building blocks semiconductor devices.

The semiconductor diode is simply bringing these materials together constructed from the same base-Ge or Si. At instant the two materials are joined the electrons and hole in the region of the junction will combine resulting in a lake of carriers in the region near the junction. This region of uncovered positive and negative ions is called the depletion region due to the depletion of carriers in this region.

Construction and type of P-N junction Diode

The most extensively used elements in the manufacture of junction diodes are germanium and silicon although some other materials are also assuming importance in recent years. 

A p-n junction diode known as a semiconductor and crystal diode consist of a P-N junction, formed either in germanium or silicon crystal. The diode has two terminals namely anode and cathode.The anode refers to the P-type region and cathode refers to the N-type region as shown in figure below.

n and p-type

The arrow head shown in the circuit symbol, points the direction of currents flow, when it is forward biased. It is the same direction in which the movement of holes takes place.The commercially available diodes, usually have some nations to identify the p and n terminals or leads. The standard notation consist of type numbers preceded by IN, such as IN 240 and IN 1250. Here 240 and 1250 correspond to colour bands. In some diodes, the schematic symbol of a diode is painted or colour are marked on the body.

Diode p-n junction

The commercially available diodes, usually have some notations to identify the p and n terminals and lead. The standard notation consists of type numbers preceded by IN, such as IN 240 and IN 1250. Here 240 and 1250 correspond to colour bands. In some diodes, the schematic symbol of a diode is painted or the colour dots are marked on the body

Potential Barrier and Biasing

A P-N junction diode which consist of P- and N- type semiconductors formed together to make a P-N junction is the place dividing the two zones is known as junction.

Potential barrier

As a result of diffusion some electrons and holes migrate across the junction there by forming a depletion layer on either side of the junction by neutralisation of holes in the p-regional and of free electrons in the N- region. This diffusions of holes and electrons across the junction continues till a potential barrier is developed in the depletion layer which then prevents further diffusion. By the application of an external voltage this potential barrier is either increased or decreased.

The barrier voltage of a P-N junction depends upon three factors namely density,electronic charge and temperature. For a given P-N junction, the first two factors are constant, thus making the value of Vb dependent only on temperature. It has been observed that for both germanium and silicon the value of Vb decrease by 2m Vl degrees celsius.Mathematically, the decrease in barrier voltage is -0.002* delta t, where delta t is the increase in temperature in degrees celsius. 

Forward biasing

Forward biasing

The junction is said to be biased in the forward direction when then positive battery terminal is connected to p-type region and the negative battery terminal to the n-type. This arrangement permits the flow of current across the P-N junction. The holes are repelled by the positive battery terminals and electron by the negative battery terminal with the result that both hole and electrons will be driven towards the junction where they will recombine. Hence as long as the battery voltage is applied large current flows. In other words, the forward bias lower the potential barrier across the depletion layer thereby allowing more current to flow across the junction.

Reverse biasing

Reverse biasing

The junction is said to be reversed biased also called Zener diode when battery connections to the battery are reversed holes are attracted by the negative battery terminal and electrons by the positive battery terminal so that both holes and electrons move away from the junction. Since there is no recombination of electron-hole pairs, diode current is negligible and the junction has high resistance. Reverse biasing increases the potential barrier at the junction, thereby allowing very little current to flow through the junction.

V-I Characteristics of a P-N junction Diode

The V-I voltage-ampere characteristic of a typical p-n junction diode with respect to break-down voltage.

              V-I Characteristics of a P-N junction Diode

• For typical junction concentration the current densities at a temperature of 300K, forward voltage ranges between 0.2 and 0.3 in germanium and between 0.5 and 0.75 volt in silicon.
• The reverse current s related to minority carrier concentration,which depends upon temperature and the energy of the material.
• Reverse current increases exponentially with temperature. It is a limiting factor in the high-temperature junction of semiconductor junction device.
• The high-frequency response of a semiconductor diode may be seriously limited by charge stored in the depletion region. This charge gives a capacitive effect since it changes with voltage the value of the stored charge is that of the ionized impurity atoms in the depletion regions on either side of the junction.
• The width of the depletion region increase with higher doping. The result is lower capacitance, as in the case of a parallel-plate capacitor with wider spacing between plates.
• The maximum reverse voltage of a P-N junction is limited by the field in the depletion region. The field accelerates carriers, which may gain enough energy to create new hole-electron pairs by colliding with atoms of the lattice structure. Each of these carriers may also create a hole-electron pair. As reverse voltage is increased, as avalanche breakdown point is reached at which this multiplicative action causes the current to increase abruptly.
• Avalanche breakdown voltage is higher in lightly doped regions, since depletion region is wider, making the terminal electric field smaller for any given voltage.