How the electrons flow in reverse bias and forward bias in P-N junction diode?

Can anybody explain to me how the electron flow in reverse bias and forward bias in P-N junction diode?

HOw a depletion region gets wide when we apply reverse bias?
I hope somebody can explain to me...

want to know about pn junction

Hi...

In reverse bias, the electrons flow from anode to cathode (P -> N), while the holes (positive charges) flow from cathode to anode (N -> P). This happens because in reverse bias, a greater voltage is wired to N, attracting electrons to outside, while the least voltage does the same with holes. As a result, it has made a 'vaccum' of carriers, in the middle zone, called 'depletion region'.

In other hand, if a forward bias is wired, the greater voltage over the anode attracts electrons from cathode, while the least voltage in cathode attracs holes. There is a stable current anode -> cathode (P -> N)

pn junction carrier principle of dynamic balance

The negative supply repels the free electrons in the conduction band across the junction leaving the N layer more positive. This means that the potential actually opposes the bias voltage. But when you measure the polarity it corresponds the the polarity of the applied voltage. If you could measure the junction, you would find that it actually opposes the battery.

carrier concentration in the pn jun

You already know these things but I guess it doesnt hurt to review.

The PN junction diode has three regions of importance. The P doped side, the N doped side and the PN junction. Since the mass of the proton is 1000X than that of the electron, the electron will in almost all situations be the carrier of charge or transmitter of charge. This isnt actually the case because protons dont move, but rather holes or free spaces in the outer most covalent bond arrange themselves to shuffle along a charge or the opposite which is the absence of one( or a collection of charge)

Anyways if a positive charge is applied to the P terminal, ionization or polarization will occur. Meaning electrons within the P type will want to diffuse to the positive charge and positive charges or holes will want to diffuse the other way(that is toward the PN junction). With a collection of holes or carriers as we shall call them along the PN junction on the P type side, we now have a greater concentration of carriers present in this region. than the originally doped concentration. Similarily, the same type of mechanism is occuring on the N type side. That is electrons will accumulate along the PN junction on the N type side. This is called the accumulation region.


If you havent already noticed + one side, - other side = capacitance, or specifically the PN junction capacitance in this example.

Well as charges accumulate in this region, the capacitance increases. And if the voltage is increased along the P type terminal an inversion layer is created along the PN junction. When the voltage reaches a high enough potential to give the electrons the energy to cross the PN junction. This energy or voltage is related to alot of things like bandgap energy, junction capacitance, doping concentrations, temperature, etc. But usually it is around 0.6 - 0.7 volts. At this voltage the diode is considered on, forward biased, or conducting.

When reversed biased a normal diode does not conduct, unless it is a breakdown diode which operates in the breakdown region, which is a high enough negative voltage or bias that will allow reverse current to flow.
As the voltage is increased, carrier concentrations and will allow conduction in a similar manner as forward bias for conduction to occur. Usually you dont see the same i-v (current-voltage) characteristics in reverse bias as you do forward bias. Normally there is a small reverse leakage current called Irs reverse saturation current.

Reverse bias diodes are known as Zener diodes.

Most of what I just typed seems right but I havent done this stuff for years so theres a good chance something is wrong here. I may have even flip-flopped the P and N regions. But in general this is a good ideal how diodes work, and since transistors work on the same principles of the PN junction diode, this should give you an idea of how transistors work, which are somewhat back to back PN junctions.

Good luck and if any corrections need to be made someone please do so.

junction capacitance of pn junction

Can some body help me in understanding the effect of concentration on the P-N junction break down. Basically I want to know the effect of conc. on the value of the break down voltage.
Like I have P/N diode. So I make it P/N+ then how my break down voltage is going to behave. Moreover what happens to the reverse saturation current value. will it increase or decrease.

pn junction and doping

I'm not 100% sure on this, more like 50-50.

An increase in carrier concentration on the N side (N+) will increase the occurence of breakdown. Breakdown is the operation of the diode in reverse polarity, that is current flow. For reverse current, or current that flows from N to P or in this case N+ to P.

We are describing a zener diode, a specailized diode that operates in reverse bias. A diode that has the characteristics of a constant voltage drop and an extremely high current gain. So I would believe an increase in N (N+) or possibly P- would affect both the breakdown voltage and reverse current. A higher N+ or P-, would decrease breakdown voltage (decrease as in less of a negative bias) and increase reverse saturation current. I don't think carrier concentration has much affect on reverse breakdown current.

**broken link removed**

Like I said previously, I'm not sure. Your best bet is google, text books, and a helpful professor. Whatever you find out keep us posted.

breakdown voltage of a pn junction

Even if you dont apply any bias on PN junction, the depletion region will still exist, due to the dynamic balance of P-si and N-si.

pn junction reverse bias

I have found that with increase in the concentration the breakdown voltage of the P/N juntion decreases (I refered B. streetman for this) but not sure how the leakage current will vary.

capacitance of pn junction

yeah, iwanna know its answer as well

back-to-back pn junction

i think that it will also increase, coz of increase in the concentration of doping

i want to know about pn juction

Munib said:

i think that it will also increase, coz of increase in the concentration of doping

Click to expand...

But if the conc. increases then the population of monority carrier will decrease. And leakage current depends is proportional to minority carrier conc.

more negative bias to p-n junction

Cited from "Microelectronic Circuits" by Sedra/Smith:

Zener breakdown occurs when the electric field in the depletion region increases to a point where it can break covalent bonds and generate electron-hole pairs. The electrons will be swept to the N side and holes to the P side. These electrons and holes constitue a reverse current across the junction that support the reverse breakdown current.

From this citation, try imagine if the concentration fo doping is higher, what's going to happen then?

mechanism of current flow in p-n junction diode

Hi.

The critical (or threshold) electric field to generate the avalanche breakdown can be modelled as constant. If the doping concentration near tjunction is increased, as described in the Gauss Law, the depletion layer width will be narrower for a given reverse bias voltage. Thus, the electric field flowing through the junction will be larger, which caused the decreasement of the avalanche breakdown voltage. But, It can be noted that, the defects, i.e., the traps will be increased as the doping level is increased, which will cause the increasement of the reverse saturation current measured in the experiment.

Bye~~