Diode Biasing

When a single crystal of semiconductor is doped with acceptors on one side and donors on the other side a p-n junction is formed. A schematic diagram of a p-n is shown in fig (a) . In this figure donor ions are denoted by plus signs and the electrons they donate are represented by the small filled circles. The holes are shown by unfilled small circles and acceptor ions by the minus signs.

A p-n junction cannot be formed by simply joining the two pieces together because it would produce a discontinuous crystal structure. Special fabrication techniques are needed to form a p-n junction. An important property of semiconductor diode is that it allows the conduction current in only one direction. For this it is used as a rectifier. A schematic symbol of crystal diode is depleted in fig (b). The arrow in the symbol represent the direction of easier conventional current flow. A semiconductor diode contains two terminals. When it is connected in the circuit, one this is to decided is whether the diode is forward or reverse biased. The p-side of the diode is always the positive terminal for forward bias and designated the anode. The n- side is called the cathode and is the negative terminal when the device is forward biased. The holes and electrons are the mobile charges, while the ions are immoblies charges.

WORKING OF A SEMICONDUCTOR DIODE

Forward Bias :- When an external voltage is applied with the polarity i.e positive terminal of baterry is connected with positive terminal of diode and negative terminal of the battery is connected with negative terminal of the diode, it is known as forward bias. An ideal p-n diode has a zero ohmic voltage drop across the body of the crystal. For such a diode, the height of the potention barrier at the juntion is lowered by the applied forward voltage V.

The equilibrium initially establishes between the forces tending to produce diffusion of majority cariers and the restraining influence of the potential energy barrier at the junction will be disturbed. Therefore, the holes cross the junction from the p-type to the n-type and the electrons cross the junction in the opposite direction. These majority carries can then travell around the closed circuit and a relatively large current will flow. 

Reverse Bias :- The applied voltage in the direction i.e negative terminal of baterry is connected with positive terminal of diode and positive terminal of the battery is connected with negative terminal of the diode, it is known as reverse or blocking bias. Due to this type of connection, holes in the p-type and electrons in the n-type move away from the junction. As a consequene, the region of negative-charge density is spread to the left of the junction and positive charge density region is spread to the right. However, this process cannot go on indefinitely because in order to have a steady flow of holes to the left, these holes must be supplied across the junction from the n-type germanium and there are very few holes in the n-type side. Therefore, nominally, zero current results.

In fact, a small current does flow because a small number of holes - electron pairs are produced throughout the crystal due to thermal energy. The holes formed in the n-type germanium will wander over to the junction. A similar remark applies to the electrons thermally produced in the p-type germanium. This small current is known as the diode revers saturation current. This reverse current will increase with increasing temperature. Therefore, back resistance of the crystal diode decreases on increasing the temperature.

The another way to be understand is :- When no voltage is given to the p-n diode , the potential barrier across this junction exists. If a voltage v is given to the diode in the reverse direction, then height of the potential- energy barrier is increased by the amount eV. Due to this increase in barrier height , the flow of majority carriers is reduced. But the minority carriers are unaffected by the increased height of the barrier.