Physics of Semiconductor Devices. Student Project

Transcription

1 Institut für Festkörperphysik Physics of Semiconductor Devices WS 2014/15 Student Project Zener Diode Student: Knoll Alexander Graz, on May 1, 2015

2 Contents 1 Problem 2 2 How does a Zener diode work? Draw a band diagram (conduction band, valence band, fermi energy) Zenertunneling How do you make a Zener diode? Explain how the two sides of the diode are doped and why Doping What is a Zener diode used for? 4 5 What is avalanche breakdown? 5 1

3 1 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

4 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

5 3 How do you make a Zener diode? Explain how the two sides of the diode are doped and why. 3.1 Doping Figure 4: Diode fabrication. 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

6 5 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,