junction breakdown

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Junction Breakdown

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Kamal Jeet SinghRoll No. 141610ME Modular ECE 2014 BatchNITTTR Chandigarh13-Aug-1411Junction Breakdown13-Aug-14Kamal Jeet Singh, Roll No. 141610, ME-MOD-ECE-20142What is Junction Breakdown ?

In the ideal PN junction device, when a reverse bias voltage is applied, a small reverse bias current flow through the device. This reverse current remains very small until a critical voltage is reached, at which point the current suddenly increases. This sudden increase in current is referred to as the junction breakdown2Junction Breakdown13-Aug-14Kamal Jeet Singh, Roll No. 141610, ME-MOD-ECE-20143

Junction Diode Symbol and Static I-V Characteristics3Junction Breakdown13-Aug-14Kamal Jeet Singh, Roll No. 141610, ME-MOD-ECE-20144General Breakdown Characteristics

The maximum reverse bias voltage that can be applied to a p-n diode is limited by breakdownBreakdown is characterized by the rapid increase of the current under reverse biasThe corresponding applied voltage is referred to as the breakdown voltage

4Junction Breakdown13-Aug-14Kamal Jeet Singh, Roll No. 141610, ME-MOD-ECE-20145Types of Junction Breakdown

There are two physical mechanisms which give rise to the reverse bias breakdown.

Zener Effect (Zener Breakdown)

Avalanche Effect (Avalanche Breakdown)5Junction Breakdown13-Aug-14Kamal Jeet Singh, Roll No. 141610, ME-MOD-ECE-20146

6Junction Breakdown13-Aug-14Kamal Jeet Singh, Roll No. 141610, ME-MOD-ECE-20147Phenomenological Description of Breakdown Mechanism

For qualitative understanding of the breakdown mechanism in p-n junctions, it is helpful to consider the Impurity concentration, the depletion region width and the electric field in the junction region.

Let consider P+-n junction neglecting the depletion width in the P+- region , the following relations are Obtained

Where VR = -Va is the applied reverse voltageW= Space charge width

7Junction Breakdown13-Aug-14Kamal Jeet Singh, Roll No. 141610, ME-MOD-ECE-20148Zener Effect (Zener Breakdown)

The Zener effect or Zener Breakdown is a type of electrical breakdown in a reverse biased p-n diode in which the electric field enables tunneling of electrons from the valence to the conduction band of a semiconductor, leading to a large number of free minority carriers, which suddenly increase the reverse current8Junction Breakdown13-Aug-14Kamal Jeet Singh, Roll No. 141610, ME-MOD-ECE-20149Zener breakdown phenomena occurs in a pn junction diode with heavy doping & thin junction

Zener breakdown occurs when the electric field across the diode junction increases

This results in a force of attraction on the negatively charged electrons at junction

This force frees electrons from its covalent bond and moves those free electrons to conduction band.9Junction Breakdown13-Aug-14Kamal Jeet Singh, Roll No. 141610, ME-MOD-ECE-201410When the electric field increases (with applied voltage), more and more electrons are freed from its covalent bonds

This results in drifting of electrons across the junction and electron hole recombination occurs

So a net current is developed and it increases rapidly with increase in electric field

Electrons generated in this way are swept by the electric field into the n side10Junction Breakdown13-Aug-14Kamal Jeet Singh, Roll No. 141610, ME-MOD-ECE-201411Holes generated in this way are swept by the electric field into the p side

Zener breakdown occurs through tunneling mechanism

Zener breakdown does not result in damage of diode

Since current is only due to drifting of electrons, there is a limit to the increase in current as well

Temperature coefficient of the Zener mechanism is negative11Junction Breakdown13-Aug-14Kamal Jeet Singh, Roll No. 141610, ME-MOD-ECE-201412Tunneling Mechanism in p-n junction

12Junction Breakdown13-Aug-14Kamal Jeet Singh, Roll No. 141610, ME-MOD-ECE-201413

Barrier must be thin: depletion is narrow doping on both sides must be large

Must have empty states to tunnel into Vbi + VBR > EG/qZener Breakdown - Tunneling13Junction Breakdown13-Aug-14Kamal Jeet Singh, Roll No. 141610, ME-MOD-ECE-201414Theoretical Treatment of Internal Field Emission

Tunneling is a Quantum Mechanical Phenomenon Considering the figure A If we consider electron asParticle then it is clear that Electron can pass from regionI to region III when its energy E is higher than voltage Vo Otherwise electron isreflected back after strikingThe barrier

14Junction Breakdown13-Aug-14Kamal Jeet Singh, Roll No. 141610, ME-MOD-ECE-201415 When we consider wave nature of the electron andemploy the result of Quantum mechanics, we find that there exists a finite probability for the Electron to tunnel through the barrier into the region III when energy E of Electron is less than voltage Vo15Junction Breakdown13-Aug-14Kamal Jeet Singh, Roll No. 141610, ME-MOD-ECE-201416When large fields are applied to a semiconductor, the energy band diagram may be represented as

16Junction Breakdown13-Aug-14Kamal Jeet Singh, Roll No. 141610, ME-MOD-ECE-201417The slope of the band edges in the previous figure is a measure of applied electric fieldWidth of potential barrier (Wt) is given by

q = - Eg/Wtor Wt = Vg/||

WhereVg = voltage corresponding to the gap energy Eg = Electric field17Junction Breakdown13-Aug-14Kamal Jeet Singh, Roll No. 141610, ME-MOD-ECE-201418Temperature Dependence of the Zener Breakdown Voltage

For PN junctions in which junction breakdown occurs by the Zener mechanism, the breakdown voltage decreases with temperature Assume T as temperature coefficient of breakdown voltage and it can be defined as

where VBo = Breakdown voltage at a referencetemperatureAlso, T is negative for junctions showing the Zener breakdown


Graph showing Temperature Dependence ofZener Breakdown voltage19Junction Breakdown13-Aug-14Kamal Jeet Singh, Roll No. 141610, ME-MOD-ECE-201420Avalanche Effect (Avalanche Breakdown)

The Avalanche breakdown process occurs when electrons and/or holes, moving across the space charge region, acquire sufficient energy from the electric field to create the electron-hole pairs by colliding with atomic electrons within the depletion region20Junction Breakdown13-Aug-14Kamal Jeet Singh, Roll No. 141610, ME-MOD-ECE-201421

Depletion width larger than mean free path lots of collisionsReverse Bias Avalanche Breakdown21Junction Breakdown13-Aug-14Kamal Jeet Singh, Roll No. 141610, ME-MOD-ECE-201422 Reverse bias = Very little current flow = Open circuit

As Reverse voltage increases, a point is reached where current increases dramatically, therefore dynamic resistance decreases

Avalanche breakdown causes high flow of current under reverse bias condition!

The question is: How does this happen?22Junction Breakdown13-Aug-14Kamal Jeet Singh, Roll No. 141610, ME-MOD-ECE-201423Reverse bias 1. Thick depletion region causes high electric field and tremendous acceleration

2. Very few electrons make it through depletion region with high velocity 3. These electrons collide with atoms in the depletion region and free more electrons ( Process called Multiplication).

4. Results in higher and higher current flow

23Junction Breakdown13-Aug-14Kamal Jeet Singh, Roll No. 141610, ME-MOD-ECE-201424 Avalanche Breakdown occurs in lightly doped pn-junctions (Multiplication effect) where the depletion region is comparatively long.

The Doping density controls the breakdown voltage

Avalanche breakdown occurs when the applied reverse voltage is so large that electrons that are pulled from their covalent bonds are accelerated to great velocities

The accelerated electrons collide with the silicon atoms and knock off more electrons

24Junction Breakdown13-Aug-14Kamal Jeet Singh, Roll No. 141610, ME-MOD-ECE-201425 The collisions in high velocity will generate further free electrons and electron-hole pair recombination occurs across the junction which increases the net current rapidly

The voltage at which the net current is maximum or we can say breakdown occurs is called the breakdown voltage

Temperature coefficient of the Avalanche mechanism is positive25Junction Breakdown13-Aug-14Kamal Jeet Singh, Roll No. 141610, ME-MOD-ECE-201426Avalanche effect can be illustrated from the figure below

This figure is showing that electrons are colliding with the lattice resulting in generation of free electrons and further electrons are accelerating and colliding and generating more free electrons26Junction Breakdown13-Aug-14Kamal Jeet Singh, Roll No. 141610, ME-MOD-ECE-201427

Another Graphical Representation that howelectrons are multiplying27Junction Breakdown13-Aug-14Kamal Jeet Singh, Roll No. 141610, ME-MOD-ECE-201428The multiplication in terms of other parameters can only be found from experiment and is

where n is between 4 to 6 in Si and between 2 to 6 in Ge. V= Reverse VoltageVo= Breakdown Voltage

28Junction Breakdown13-Aug-14Kamal Jeet Singh, Roll No. 141610, ME-MOD-ECE-201429Recalling the VI characteristics ofpn-junction

It is illustrated that Avalanche voltageis greater than the Zener voltage

Why the avalanche voltage is greater than Zener voltage?

29Junction Breakdown13-Aug-14Kamal Jeet Singh, Roll No. 141610, ME-MOD-ECE-20143030Junction Breakdown13-Aug-14Kamal Jeet Singh, Roll No. 141610, ME-MOD-ECE-201431As the depletion layer in Zener breakdown is thin

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

And the depletion layer in Avalanche breakdown is thick or long

So in a Avalanche breakdown, the electric field necessary to break electrons from covalent bond is achieved with higher voltage31Junction Breakdown13-Aug-14Kamal Jeet Singh, Roll No. 141610, ME-MOD-ECE-201432Electron and Hole Current Components through the Space Charge region during Avalanche Multiplication

If we assume that a reverse bias electron current In0 enters the depletion region at x=0 as shown in fig, the electron current In will increase with distance through the depletion region due to avalanche process.32Junction Breakdown13-Aug-14Kamal Jeet Singh, Roll No. 141610, ME-MOD-ECE-201433At x=W, the electron current may be written as

In = Mn In0

Where Mn is a multiplication factor

The hole current is increasing through the depletion region from the n to p region (see previous figure) and reaches a maximum value at x=0

The total current is constant through the pn junction in steady state33Junction Breakdown13-Aug-14Kamal Jeet Singh, Roll No. 141610, ME-MOD-ECE-201434Temperature Dependence of Avalanche Breakdown Voltage

Temperature coefficient of theAvalanche mechanism is positive

From the graph, breakdown voltage is increasing as the temperature is increasing 34Junction Breakdown13-Aug-14Kamal Jeet Singh, Roll No. 141610, ME-MOD-ECE-201435

Critical Electric Field at Breakdown in a one sided junction as a function of impurity doping concentration35Junction Breakdown13-Aug-14Kamal Jeet Singh, Roll No. 141610, ME-MOD-ECE-201436

Breakdown Voltage versus impurity concentration in uniformly doped and linearly graded junctions36Junction Breakdown13-Aug-14Kamal Jeet Singh, Roll No. 141610, ME-MOD-ECE-201437Difference between Zener and Avalanche BreakdownZener BreakdownAvalanche BreakdownVI characteristics in the breakdown region is softBreakdown voltage decreases with temperatureNo instability is observed in Zener breakdownVI characteristics in the breakdown region is hardBreakdown voltage decreases with temperatureAvalanche breakdown is always accompanied by instability in the current at the onset of breakdown37Junction Breakdown13-Aug-14Kamal Jeet Singh, Roll No. 141610, ME-MOD-ECE-201438Applications of Breakdown Diodes (Zener and Avalanche)

Breakdown diodes are used as voltage regulators in Power Supply Circuits

Breakdown diode can also be used as sources of a reference voltage

Zener diode can be used as waveform clipper, as a voltage shifter etc.

Let us take an example of Zener diode as clipper circuit in next slide


In this figure you can illustrate how the Zener diode clipping the input waveform depending on the way in which is being used39Junction Breakdown13-Aug-14Kamal Jeet Singh, Roll No. 141610, ME-MOD-ECE-201440References:

Semiconductor Physics and Devices, Donald Neamen, Tata McGraw Hill Introduction to Semiconductor Materials and Devices, M S Tyagi, Wiley India Physics of Semiconductor Devices, S M Sze http://www.circuitstoday.com/ http://www.science-campus.com/engineering/electronics/semiconductor_theory

40Junction Breakdown13-Aug-14Kamal Jeet Singh, Roll No. 141610, ME-MOD-ECE-201441Thanks

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junction breakdown

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