DC Biasing of semiconductor Diodes and analysis in graphical way

avalanche breakdown

avalanche breakdown

Forward biasing with graphical representation

The PN junction diode in the simplest sense is a device that conducts current in one direction and block current in the opposite direction.

Donor impurities (pentavalent) are introduced into one-side and acceptor impurities into the other side of a single crystal of an intrinsic semiconductor to form a p-n diode with a junction called depletion region (this region is depleted off the charge carriers).

 This region gives rise to a potential barrier Vγ called Cut- in Voltage. This is the voltage across the diode at which it starts conducting.The P-N junction can conduct beyond this Potential.

graph of forward bias
forward bias graph

as you can observe from above graph, till potential difference across diode reaches the “barrier potential difference”, current flowing through diode remains zero but, when it crosses barrier it rises in a pretty good rate or pretty quickly.

 And after that you can observe that potential difference becomes constant(just like a straight line) after crossing barrier potential.

Now, if we draw a tangent at any point of that curve and extend it towards the x-axis, the point where tangent intersects the axis is called cut-in voltage. 

cut-in voltage

Now, there are two types of diodes, germanium and silicon. The cut-in voltage for germanium is 0.3/0.4. But, silicon have cut-in voltage value 0.7 (according to industrial standards), you can get 0.6 or 0.5 also.Generally it varies about 0.1 or 0.2 volts, not very much. 

Reverse biasing with graphical analysis

If –ve terminal of the input supply is connected to anode (p-side) and +ve terminal of the input supply isconnected to cathode (n-side) then the diode is said to be reverse biased. 

In this condition an amount equal to reverse biasing voltage increases the height of the potential barrier at the junction.

Both the holes on p-side and electrons on n-side tend to move away from the junction thereby increasing the depleted region. However, the process cannot continue indefinitely, thus a small current called “reverse saturation current” continues to flow in the diode. 

This small current is due to thermally generated carriers. Assuming current flowing through the diode to be negligible, the diode can be approximated as an open circuited switch.

Avalanche VS Zener breakdown

avalanche breakdown
avalanche breakdown

As the voltage across the diode increases in the reverse-bias region, the velocity of the minority carriers responsible for the reverse saturation current Is will also increase.

Eventually, their velocity and associated kinetic energy will be sufficient to release additional carriers through collisions with otherwise stable atomic structures. That is, an ionization process will result whereby valence electrons absorb sufficient energy to leave the parent atom. 

These additional carriers can then aid the ionization process to the point where a high avalanche current is established and the avalanche breakdown region determined.

Avalanche breakdown only happens in p-n junction diodes.

“The value of breakdown voltage for avalanche breakdown is upto 6 volts.”

zener breakdown
zener breakdown

Another mechanism, called Zener breakdown, will contribute to the sharp change in the characteristic. It occurs because there is a strong electric field in the region of the junction that can disrupt the bonding forces within the atom and“generate” carriers.  

Although the Zener breakdown mechanism is a significant contributor only at lower levels of Vz, this sharp change in the characteristic at any level is called the Zener region 

And diodes employing this unique portion of the characteristic of a p-n junction are called Zener diodes.

“The value of breakdown voltage in case of zener breakdown is upto 4 volts.”

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