Diodes are classified, based on two criteria.
- Based on the width of the depletion region during Reverse Biased condition
- Based on the Reverse Recovery Time (to be discussed under ‘Reverse Recovery Characteristics’) and manufacturing process.
Only the first category will be discussed here and the next category of classification will be discussed under the Reverse Recovery Characteristics topic.
- Power diodes in Reverse Biased condition and need of the Drift Region
Why do we need Drift Region in Power Diode?
The only need of Drift Region is to increase the Reverse Breakdown Voltage of the Diode. So, we can say that the high voltage that a diode blocks/bears (during Reverse Biased condition), is achieved due to the Drift Region.
We will see how Drift Region does this.
We know from the above figure that Diodes can be classified as Non-punch-through and Punch-through based on the Reverse Biased condition.
- Non-punch-through Diodes: Diodes in which the depletion region width at the breakdown does not penetrate into the n+ layer are called Non-punch-through diodes. In Non-punch-through diodes, the width of Drift Region is more than the maximum width of depletion region (at the breakdown), hence penetration does not take place.From the analogue electronics theory
Where, VBD = Reverse Breakdown Voltage of the diode
Nd = doping concentration of the n-type layer.From the above equation, if doping concentration Nd is decreased then the Breakdown voltage of the Diode increases. e.g. if Nd= 1014 cm – 3 then VBD comes out to be nearly 1000V (from the above equation). Hence, a new layer with less doping concentration i.e. the n- Drift Region is introduced which then increases the Reverse Breakdown Voltage of the power diode.
- Punch-through Diodes: Diodes in which the depletion region width at the breakdown penetrates into the n+ layer are called Punch-through diodes. In Punch-through Diodes, the width of Drift Region is less than the maximum width of depletion region (at the breakdown), hence penetration takes place.