General Study Material

8/12/2019 General Study Material

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http://abyss.uoregon.edu/~js/glossary/quantum_tunneling.html

The phenomenon of tunneling, which has no counterpart in classical physics, is an important

consequence of quantum mechanics. Consider a particle with energy E in the inner region of a one-

dimensional potential well V(x). (A potential well is a potential that has a lower value in a certain region

of space than in the neighbouring regions.) In classical mechanics, if E < V (the maximum height of thepotential barrier), the particle remains in the well forever; if E > V , the particle escapes. In quantum

mechanics, the situation is not so simple. The particle can escape even if its energy E is below the height

of the barrier V , although the probability of escape is small unless E is close to V . In that case, the

particle may tunnel through the potential barrier and emerge with the same energy E.

Impact ionization is the process in a material by which one energetic charge carrier can loseenergy by the creation of other charge carriers. For example, insemiconductors,anelectron(or

hole) with enough kinetic energy can knock a bound electron out of its bound state (in the

valence band)and promote it to a state in theconduction band,creating anelectron-hole pair

AVALANCHE BREAKDOWN

So as we increase the applied reverse voltage, the electric field across junction will keep

increasing.

If applied reverse voltage is Va and the depletion layer width is d;

then the generated electric field can be calculated as Ea =Va/d

This generated electric field exerts a force on the electrons at junction and it frees them from

covalent bonds. These free electrons will gain acceleration and it will start moving across the

junction with high velocity. This results in collision with other neighboring atoms. These

collisions in high velocity will generate further free electrons. These electrons will start driftingand electron-hole pair recombination occurs across the junction. This results in net current that

rapidly increases.

We learned that avalanche breakdown occurs at a voltage (Va) which is higher than zener

breakdown voltage (Vz). The reason behind this is simple. We know, avalanche phenomenaoccurs in a diode which is LIGHTLY doped and junction width (say d) is high. A zener break

down occurs in a diode with heavy doping and thin junction (here d is small). The electric field

that occur due to applied reverse voltage (say V) can be calculated as E = V/d.

So in a Zener breakdown, the electric field necessary to break electrons from covalent bond is

achieved with lesser voltage than in avalanche breakdown. The reason is thin depletion layer

width. In avalanche breakdown, the depletion layer width is higher and hence much more reversevoltage has to be applied to develop the same electric field strength

ZENER BREAKDOWN
http://abyss.uoregon.edu/~js/glossary/quantum_tunneling.htmlhttp://abyss.uoregon.edu/~js/glossary/quantum_tunneling.htmlhttp://en.wikipedia.org/wiki/Charge_carrierhttp://en.wikipedia.org/wiki/Charge_carrierhttp://en.wikipedia.org/wiki/Semiconductorhttp://en.wikipedia.org/wiki/Semiconductorhttp://en.wikipedia.org/wiki/Semiconductorhttp://en.wikipedia.org/wiki/Electronhttp://en.wikipedia.org/wiki/Electronhttp://en.wikipedia.org/wiki/Electronhttp://en.wikipedia.org/wiki/Electron_holehttp://en.wikipedia.org/wiki/Electron_holehttp://en.wikipedia.org/wiki/Kinetic_energyhttp://en.wikipedia.org/wiki/Kinetic_energyhttp://en.wikipedia.org/wiki/Valence_bandhttp://en.wikipedia.org/wiki/Valence_bandhttp://en.wikipedia.org/wiki/Conduction_bandhttp://en.wikipedia.org/wiki/Conduction_bandhttp://en.wikipedia.org/wiki/Conduction_bandhttp://en.wikipedia.org/wiki/Electron-hole_pairhttp://en.wikipedia.org/wiki/Electron-hole_pairhttp://en.wikipedia.org/wiki/Electron-hole_pairhttp://en.wikipedia.org/wiki/Electron-hole_pairhttp://en.wikipedia.org/wiki/Conduction_bandhttp://en.wikipedia.org/wiki/Valence_bandhttp://en.wikipedia.org/wiki/Kinetic_energyhttp://en.wikipedia.org/wiki/Electron_holehttp://en.wikipedia.org/wiki/Electronhttp://en.wikipedia.org/wiki/Semiconductorhttp://en.wikipedia.org/wiki/Charge_carrierhttp://abyss.uoregon.edu/~js/glossary/quantum_tunneling.html


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Zener breakdown occurs in heavily doped pn-junctions.The heavy doping makes thedepletion layer extremely thin. So thin, in fact,carriers cant accelerate enough to causeimpact ionization.With the depletion layer so thin, however, quantum mechanicaltunneling through the layer occurs causing current to flow.

At low doping levels and higher voltages the avalanche mechanism dominates while atheavy doping levels and lower voltages the Zener mechanism dominates

"Neither Zener nor avalanche breakdown are inherently destructive in that the crystallattice is damaged.However, the heat generated by the large current flowing can causedamage, so either the current must be limited and/or adequate heat sinking must besupplied."

The forward voltage at which the current through the junction starts increasing rapidly, is called the

knee voltage or cut-in voltage. Semiconductor diodes begin conducting electricity only if a certain

threshold voltage or cut-in voltage is present in the forward direction


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In many cases, as the forward resistance of diode is small and cut-in voltage is also small, the

diode is assumed to be an ideal diode.

In case of ideal diode, it is assumed that it starts conducting instantaneously when appliedvoltage is just greater than zero and the drop across the conducting diode is zero. So conducting

diode can be ideally replaced by a short circuit, for the analysis of various diode circuits. The

Fig. 2 shows the ideal diode characteristics.

We have seen that in forward biased diode, the total voltage drop across the diode is Vf which

consists of drop due to barrier potential which is almost equal to cut-in voltage Vand the drop

across the internal forward dynamic resistance rf of the diode. While when reverse biased,reverse saturation current is very small and practically neglected. Hence reverse biased diode is

practically assumed to be open circuit.

Thus the practical diode model consists of a battery equal to cut-in voltage and the forward

resistance, in series with an ideal diode, in forward biased conditionAnother way to analyse the diode circuits is to approximate the V-I characteristics of a diode

using only straight line i.e. linear relationships.

In such approximation, the diode forward resistance is neglected and the diode is assumed toconduct instantaneously when applied forward biased voltage VD is equal to cut-in voltage V.

And then it is assumed that current increases instantaneously giving straight line nature of V-I

characteristics. While in reverse biased condition whenVD< 0, the diode does not conduct at all.

Hence when diode forward resistance is assumed zero, the circuit model of diode is asshown in the Fig. 3(a). In reverse biased, the diode is open circuit as shown in the Fig. 3(b). As

the diode conducts at VD= V, the V-I characteristics with straight lines is as shown in the Fig.

3(c).As the method models the diode with the pieces of straight lines, the name given to such

approximation is piecewise-linear method.

DIFFUSION CURRENT

Despite the presence of the electric field, which creates an impediment to the diffusion of carriersacross the electric field, some carriers still cross the junction by diffusion. In the animation below, most

majority carriers which enter the depletion region move back towards the region from which they

originated. However, statistically some carriers will have a high velocity and travel in a sufficient net

direction such that they cross the junction. Once a majority carrier crosses the junction, it becomes a

minority carrier. It will continue to diffuse away from the junction and can travel a distance on average

equal to the diffusion length before it recombines. The current caused by the diffusion of carriers across

the junction is called a diffusion current


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A diode forward voltage refers to thevoltage dropthat occurs when an electrical current passes through

a diode in an electrical circuit. The electrons must be pushed by a voltage with enough force to cross the

p-n junction, and this push is the source of the diode forward voltage, or forward bias voltage drop.
http://www.wisegeek.com/what-is-voltage-drop.htmhttp://www.wisegeek.com/what-is-voltage-drop.htmhttp://www.wisegeek.com/what-is-voltage-drop.htmhttp://www.wisegeek.com/what-is-voltage-drop.htm


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ZENER DIODE

The zener diodeis the simplest types of voltage regulator and the point at which a zener diode breaks

down or conducts is called the Zener Voltage (Vz ).

There are a couple of requirements to consider. First, the input voltage has to be higher than the Zener

voltage. Second, the resistor value must be chosen such that there is always current flowing through the

Zener

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