Institut für Festkörperphysik
Physics of Semiconductor Devices
WS 2014/15
Student Project
Zener Diode
Student:
Knoll Alexander 1130404
Graz, on May 1, 2015
Student Project
Knoll Alexander (1130404)
Contents
1 Problem
2
2 How does a Zener diode work? Draw a band diagram (conduction
band, valence band, fermi energy).
2
2.1 Zenertunneling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2
3 How do you make a Zener diode? Explain how the two sides of the
diode are doped and why.
4
3.1 Doping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4
4 What is a Zener diode used for?
4
5 What is avalanche breakdown?
5
1
Student Project
1
Knoll Alexander (1130404)
Problem
1. How does a Zener diode work? Draw a band diagram (conduction band, valence
band, fermi energy).
2. How do you make a Zener diode? Explain how the two sides of the diode are doped
and why.
3. What is a Zener diode used for?
4. What is avalanche breakdown?
2
How does a Zener diode work? Draw a band diagram (conduction band, valence band, fermi energy).
Zener diodes are semiconductor diodes which have been manufactured to have their
reverse breakdown occur at a specific, well-defined voltage (its Zener voltage), and that
are designed such that they can be operated continuously in that breakdown mode.
Commonly available Zener diodes are available with breakdown voltages (Zener voltages)
anywhere from 1.8 to 200 V.
Figure 1: Symbol of a Zener diode.
2.1
Zenertunneling
The diode works through Zener tunneling. That is a band to band tunneling. You tunnel
from the Valence band to the conduction band. And this can only happen when you put
enough reverse bias voltage that you lift the top of the valence band above the bottom
of the conduction band. But there is another condition for Zener tunneling and that is
that the distance from the p-side to the n-side must not be to far, because tunneling can
only go a short distance. So you must dope the p and n side very heavily so that you
get a narrow depletion width. Such the way that when you apply a reverse bias voltage,
the electrons can tunnel through. If you have low doping, the depletion width get to
wide so the electrons can’t tunnel through.
2
Student Project
Knoll Alexander (1130404)
Figure 2: Characteristic of a Zener diode.
The characteristic looks like figure 2. In forward bias it turns on exponetially. In
reversed bias you see the reverse saturation current. And at a specific voltage you reach
the condition described in 2.1. Then suddenly because of tunneling you get a very strong
turnon current. You can design the doping so that this onset occurse on a voltage you are
choosing. So you can make Zener diodes with specific reverse breakdown voltage. The
breakdown voltage is typically much lower than the breakdown voltage of an avalanche
diode and can be tuned by adjusting the width of the deletion layer. Lower doping higher breakdown voltage.
Figure 3: Band diagramm.
3
Student Project
3
Knoll Alexander (1130404)
How do you make a Zener diode? Explain how
the two sides of the diode are doped and why.
Figure 4: Diode fabrication.
3.1
Doping
The distance from the p-side to the n-side must not be to far, because tunneling can
only go a short distance. So you must dope the p and n side very heavily so that you get
a narrow depletion width. Such that way that when you apply a reverse bias voltage,
the electrons can tunnel through. If you have low doping, the depletion width get to
wide so the electrons can’t tunnel through.
4
What is a Zener diode used for?
They are mostly used as reference voltage, but can also be used for overvoltage protection
or voltage stabilisation.
4
Student Project
5
Knoll Alexander (1130404)
What is avalanche breakdown?
Avalanche breakdown happens through something called impact ionization. You have
the diode reversed biased and for a large reverse bias there is a large electric field in
depletion area. There are some carriers present and if they get accelerated to an energie
that is larger than the Bandgap they can knock an electron out of the valance band and
put it up into the conduction band. And you get another electron that’s in this large
electric field and this get also accelerated and knocks another electron out of the valance
band and puts it up into the conduction band. You generate some heat and so the effect
gets faster, and you get a lots of electrons moving and this is avalanche breakdown.
Graz, on May 1, 2015
5