Volt-Ampere ( i-v ) characteristics of a p-n junction diode
Reverse saturation current, Is:
Fig: Volt-Ampere (i-v) characteristics of a p-n junction diode
- When an external voltage is applied with p side move negative then the n side the junction is said to be under reverse bias condition. This reverse bias adds to the height of the potential barrier. The electric field strength at the junction and the width of the space change region (also called “the depletion region” because of the absence of free carriers) also increases.
- On the other hand, free minority carrier densities (np in the p side and pn in the n side) will be zero at the edge of the depletion region on either side.
- This gradient in minority carrier density causes a small flux of minority carriers to defuse towards the deletion layer where they are swept immediately by the large electric field into the electrical neutral region of the opposite side.
- This will constitute a small leakage current across the junction from the n side to the p side. There will also be a contribution to the leakage current by the electron hole pairs generated in the space change layer by the thermal ionization process.
- These two components of current together is called the “reverse saturation current Is”of the diode. Value of Is is independent of the reverse voltage magnitude (up to a certain level) but extremely sensitive to temperature variation.
Reverse break down:
- When the applied reverse voltage exceeds some threshold value (for a given diode) the reverse current increases rapidly. The diode is said to have undergone “reverse break down”.
- Reverse break down is caused by "impact ionization" as explained below. Electrons accelerated by the large depletion layer electric field due to the applied reverse voltage may attain sufficient knick energy to liberate another electron from the covalent bonds when it strikes a silicon atom.
- The liberated electron in turn may repeat the process. This cascading effect (avalanche) may produce a large number of free electrons very quickly resulting in a large reverse current.
- The power dissipated in the device increases manifold and may cause its destruction. Therefore, operation of a diode in the reverse breakdown region must be avoided.
Carrier density gradients:
Carrier density gradients on either side of the junction are supported by a forward current IF (flowing from p side to n side) which can be expressed as
IF=IS (exp (qv/kT))-1
Where, Is = Reverse saturation current (Amps)
v = Applied forward voltage across the device (volts)
q = Change of an electron
k = Boltzmann’s constant
T = Temperature in Kelvin