ece 340 lecture 26 : reverse bias...

ECE 340 Lecture 26 : Reverse Bias

Breakdown

Class Outline: • Avalanche and Zener Processes

M.J. Gilbert ECE 340 – Lecture 26 10/21/11

Things you should know when you leave…

Key Questions

• What is the Zener effect? • What is avalanching? • When do these occur? • What do they depend on?

M.J. Gilbert

Avalanche and Zener Processes

Most of the preceding analysis dealt with forward bias, what about the reverse bias case?

We can use the same equations and analysis to determine the reverse bias behavior…

Set V = -Vr which biases the p-side negatively with respect to the n-side and examine the relationship for the excess hole concentration…

Vr >> kbT/q

• For large reverse bias, the minority carrier concentration goes to zero. • Minority carrier concentration equations still given by previously derived equations. • Depletion of minority carriers extends one diffusion length on either side of the junctions. • Referred to as minority carrier extraction.

ECE 340 – Lecture 26 10/21/11

M.J. Gilbert


What is happening physically to the carriers…

• Carriers are being swept down the barrier at the junction to the other side.

• They are not being replaced by an opposing diffusion of carriers.

• Reverse bias saturation occurs because of drift of carriers down the barrier

• But the rate of drift depends on the rate of minority carriers arrive by diffusion from the neutral material supplied by thermal generation.

ECE 340 – Lecture 26 10/21/11

M.J. Gilbert


And the quasi-Fermi levels move again…

• Fn moves farther away from EC towards EV because in reverse bias we have fewer carriers than in equilibrium. • Quasi-Fermi levels here go inside the bands but we need to remember that Fp is a measure of the hole concentration and is correlated with EV and not EC. • This just tells us we have very few holes (smaller than in equilibrium).

ECE 340 – Lecture 26 10/21/11

M.J. Gilbert


We already know what happens when we apply a small reverse bias…

Forward bias lowers the barrier and results in a forward current.

• Small reverse bias causes a reverse saturation current caused by EHPs being swept across the space charge region.

• At large enough reverse bias, a very large current begins to flow.

ECE 340 – Lecture 26 10/21/11

M.J. Gilbert


Is the reverse bias breakdown destructive?

• The reverse bias regime is not destructive so long as the current is limited.

• Under these circumstances, reverse bias is no more destructive than forward bias.

The resistance is chosen so as to make the current sufficiently small to keep from harming the device. Failure to do so can also cause significant heating to the p-n junction.

Damage is not necessarily due to mechanisms specific to reverse bias. Similar effects can be present with too much forward bias.

ECE 340 – Lecture 26 10/21/11

M.J. Gilbert


The first of two breakdown mechanisms is zener breakdown…

The Zener effect is the breakdown mechanism if the reverse bias required to force breakdown occurs at low voltages.

Consider the heavily doped p-n junction shown to the left and then apply a reverse bias to the junction.

Reverse bias brings the conduction band very close to the valence band. This brings many occupied states on the p-side into energetic alignment with vacant states on the n-side. Electrons tunnel from the valence band to the conduction band giving rise to a reverse current. This is the Zener effect.

ECE 340 – Lecture 26 10/21/11

M.J. Gilbert


So what do we need to cause Zener breakdown?

Zener effect

The basic requirements to drive a tunneling current are:

• A large number of electrons

• A large number of holes

• Separated by a narrow barrier of finite height.

• Tunneling depends heavily on the barrier width so we want to keep the junction sharp and doping high.

• This will ensure that the transition region W extends only a very short distance from each side of the junction.

• Failure to attain high doping or sharp junctions will result in no tunneling current.

Keep in mind…

ECE 340 – Lecture 26 10/21/11

M.J. Gilbert


What else is important about the Zener process? • As the reverse bias is increased the distance between the bands decreases.

• This is due to higher electric fields increasing the slopes of the bands.

• We assume that the transition region, W, does not increase with bias which is valid for high doping and low voltage.

What does the current look like…

Average electric field in junction

When does it happen… • Occurs in Si for fields ~ 106 V/cm

• Must have high impurity concentrations

• Occurs in general for reverse biases of less than 4Eg/q.

ECE 340 – Lecture 26 10/21/11

M.J. Gilbert


For devices with large breakdown voltages or devices that are lightly doped, the major breakdown mechanism is avalanche breakdown.

In a lightly doped junction the tunneling is negligible due to the large distance between bands. Instead, breakdown is caused by impact ionization of the host atoms by energetic carriers. Normal lattice scattering can cause the creation of EHPs if the carrier being scattered has sufficient energy. These single event scattering interactions are results in carrier multiplication.

ECE 340 – Lecture 26 10/21/11

M.J. Gilbert


But we are not dealing with a single collision…

When the electron scatters it creates an EHP. Each of these carriers then has a chance to be accelerated and collide with the lattice creating a new EHP. This process may continue and is referred to as avalanche breakdown.

Let’s make an approximate analysis of the physics…

• A carrier (either electron or hole) has some probability of having an ionizing collision with the lattice while being accelerated through the transition region, W.

• For nin electrons entering from the p-side, there will be Pnin ionizing collisions and an EHP generated for each collision.

ECE 340 – Lecture 26 10/21/11

M.J. Gilbert


The process continues…

After Pnin collisions by the primary electrons, we have the primary plus the secondary electrons, nin (1 + P). After a collision, each EHP moves a distance of W within the transition region. If the pair is created at the center, the electron will drift W/2 to n and the hole will drift –W/2 to p. For ninP secondary electrons there will be (ninP) P collisions creating ninP2 tertiary pairs. After n collisions we have:

Assume no recombination and equal probabilities of ionizing collisions, then the electron multiplication is…

ECE 340 – Lecture 26 10/21/11

M.J. Gilbert


If the ionization probability approaches unity then the carrier multiplication becomes infinite!

So then what limits the current? External circuit

But this was waaaaaay too easy…

• We assumed no recombination and equal ionization probabilities.

• We expect the probability to increase with increasing electric field so the multiplication should depend on the reverse bias… We can determine an empirical relation…

N is between 3-6 depending on the

material. In general: • Critical voltage for breakdown increases with bang gap. • Peak field in W increases with increased doping, thus Vbr decreases as doping increases.

ECE 340 – Lecture 26 10/21/11

M.J. Gilbert


Can we make any quantitative statements about when this occurs? Let’s consider a step-junction and evaluate the maximum electric field in the junction as we did when we looked into pn junction electrostatics…

But we already know something about the fields and potentials…

when

Now square the field dependence:

But this field should have no doping dependence!

ECE 340 – Lecture 26 10/21/11

M.J. Gilbert


In summary…

Single collision

Primary, secondary and tertiary collisions

ECE 340 – Lecture 26 10/21/11

M.J. Gilbert


For reference, let’s look at some actual devices…

ECE 340 – Lecture 26 10/21/11

ece 340 lecture 26 : reverse bias...

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