What's the difference between forward and reverse based P-N junctions?

Question

Also, what is the difference between Zener and Avalanche breakdowns in a P-N junction?

Answer 1

Current flows through a forward-biased PN junction. Current is blocked in a reverse-biased PN junction, but only to the extent that the reverse bias voltage is less than the peak inverse voltage (PIV) rating of the diode.

A reverse biased diode always has a very small current flow due to to what are called minority carrier holes and electrons. They make their way through the space-charge (depletion) region because of the reverse voltage. As the reverse voltage is increased so does the electric field in the diode, and these minority carriers gain more energy, more speed, and actually collide with the lattice. When enough reverse voltage is applied the minority carriers start actually knocking off electrons when they collide with the atoms in the lattice. More electrons means more current and more collisions, and this creates an avalanche effect which breaks down the whole lattice and you get full conduction (avalanche breakdown).

There is a difference between avalanche breakdown and true zener breakdown. Avalanche breakdown is what is described, above. True zener breakdown only occurs in heavily doped junctions where the reverse voltage becomes stong enough to move electrons from their valance bands. The valance electrons move across the depletion region by what's called "tunneling" and there is no avalanche (multiplication) effect, because all the valence electrons are affected at the same time. This process only applies to zener diodes up to about 6 volts, although diodes are called 'zener' diodes whether they use avalanche breakdown or tunneling.

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Answer 2

The n-type region has a high electron concentration and the p-type a high hole concentration, electrons diffuse from the n-type side to the p-type side. Similarly, holes flow by diffusion from the p-type side to the n-type side. If the electrons and holes were not charged, this diffusion process would continue until the concentration of electrons and holes on the two sides were the same, as happens if two gasses come into contact with each other. However, in a p-n junction, when the electrons and holes move to the other side of the junction, they leave behind exposed charges on dopant atom sites, which are fixed in the crystal lattice and are unable to move. On the n-type side, positive ion cores are exposed. On the p-type side, negative ion cores are exposed. An electric field Ê forms between the positive ion cores in the n-type material and negative ion cores in the p-type material. This region is called the "depletion region" since the electric field quickly sweeps free carriers out, hence the region is depleted of free carriers. A "built in" potential Vbi due to Ê is formed at the junction

Answer 3

forward bias is to keep the diode in conduction mode. for example a simple RF switch can be made by using forward bias to allow passing the signal and reverse bias to prevent any conduction. In this case the RF signal is small compared to the bias.

a diode with a zener characteristic has its reverse breakdown voltage occur at a precise voltage which stays constant over a range of pass current. a diode with avalanche characteristic lowers its reverse breakdown voltage once reverse current starts.

Answer 4

A transistor being used as an amplifier is being used in the linear region of its characteristic curve. The same transistor being used as a switch is not operated in its linear region, but rather between its fully saturated region and the cutoff region of its characteristic curve. If it is saturated, then current is flowing from its collector to emitter (on). If it is cutoff, then no current is flowing in that direction (off). A perfect switch indeed. The base current used by the designer determines on/off state.