Pn-Junction diode

A pn-junction diode is a linear, passive and unidirectional device.

Pn-Junction Formation

It is constructed by doping a pure semiconductor (like Si,Ge etc.) base half p-type and half n-type.

Al atom in p-type has 7 electrons in valence shell (3 covalent bonds + 1 unpaired electron). So it needs 1 electron to complete its octet.

P atom in n-type has 9 electrons in its valence shell (4 covalent bonds + 1 loosely bound electron). So, Phosphorous can easily donate this extra electron to gain its octet.

Therefore, P donates and Al accepts an electron, converting P to a +ve charge and Al to a –ve charge.

With each such +ve –ve charge formation electric field is estaiblished from n-type to p-type.

Now, every electron going from P to Al experiences a force in opposite direction, due to the electric field. With every new electron transfer, electric field grows more and more stronger. At some point, the transfer of electron stops when the force due to electric field is strong enough to oppose electron transfer.

The pn junction is said to be formed.

The region so formed by transfer of electrons is called depletion region and it has no free charge present in it.

Biasing of Pn-junction diode

Application of voltage across a diode is called biasing of diode.

Reverse bias condition :-

If potential at terminal K is greater than potential at terminal A or V_KA is positive, the pn-junction diode is said to be reverse biased, as shown below.

Under such condition, the number of uncovered +ve ions in the depletion region of n-type material will increase due to large number of free electrons drawn to the positive terminal of the applied voltage. Similarly, number of –ve ions in the depletion region of p-type material will increase due to the –ve terminal of V_KA .

So, there is widening of depletion region.

Widening of depletion region estaiblishes a too great barrier for majority carriers to overcome, effectively reducing the majority carrier flow to zero.

However, minority carriers (holes in n-type and electrons in p-type) can cross depletion region without any barrier as the force acting on them due to electric field is in direction of their motion.

So, in reverse bias condition, current flow is due to minority carriers only and since minority carriers have very small concentration per unit volume therefore, this reverse bias current called leakage or reverse saturation current is very small.

For si based diodes it is in the range of nanoamperes and is in the range of microamperes for Ge diodes, making si diodes more closer to an ideal diode (zero current in reverse bias condition). This is the reason si diodes are more common to Ge diodes.

With increase in V_KA , the I_S remains almost constant as shown below.

If reverse voltage V_KA is made equal or greater than, so called Peak inverse voltage PIV, suddenly large amount of current flows (avalanche breakdown) and the diode gets permanently damaged.

Therefore, always keep V_KA < PIV .

So, when 0<V_KA<PIV , diode acts like an open switch.

Forward bias condition :-

When the potential at terminal A is greater than potential at terminal K or V_AK is positive, the diode is said to be forward biased.

The voltage V_AK will pressure electrons in the n-type material and hole in the p-type material to recombine with the +ve and –ve ions respectively and reduce the width of the depletion region.

The resulting minority carrier flow of holes from n-type to p-type and electrons from p-type to n-type, has not changed in magnitude but the reduction in depletion region width has resulted in a heavy majority flow across the junction.

As V_AK is increased, the depletion region will continue to decrease in width until a flood of electrons can pass through the junction resulting in exponential rise of current, shown below.

As evident from above characteristics, when V_AK > Knee voltage (V_K) , the diode allows large current I_AK through itself with very small voltage drop across it. Thus, a pn junction diode behaves like a closed switch in forward biased condition.

The complete VI characteristics of a pn junction diode can be obtained by joining the above forward bias and reverse bias characteristics.

Pn-Junction diode current equation