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PHYWE Characteristic Curves of Semiconductors with FG-Module |
Learn about:
- Semiconductor
- P-n Junction
- Energy band Diagrams
- Acceptors
- Donors
- Valence band
- Conduction band
- Transistor
- Operating Point
When measuring the current-voltage characteristic of a semiconducting diode, the collector current in dependency on the emitter-collector voltage is measured for different values of base current strength through a NPN transistor.
A p-doped semiconductor contains impurities called acceptors whose energy level catch an electron from the valence band is that near to the band edge, that at room temperature a considerable part of these levels is occupied thus forming holes in the valence band as mobile charge carriers and immobile “ons” in the crystal lattice. A n-doped semiconductor contains impurities called donors capable of delivering electrons by thermal exitation to the conduction band as mobile carriers (having energy levels near the band edge considerably occupied at room temperature). The Fermi level usually lies in between the band edge and the ionized impurity levels.
When a n-doped and a p-doped semiconductor are brought in contact, in the contact area some electrons from the donors of the n-doped semiconductor recombine with the acceptors of the p-doped semiconductor without creating mobile charge carriers but creating a space charge, a barrier layer, (more of a contact surface charge), until it’s field equalizes the Fermi levels of both parts. So the contact area is depleted of carriers - the depletion zone - is formed.
If a voltage is applied to such a device the polarity makes a big difference:
If the negative terminal is connected to the p-doped part, this is called reverse biassing the diode. The energy level of the electrons is raised in the negatively charged part. The space charge increases creating a stronger field reverse to the applied outer field and the depletion zone gets larger. Electrons from the valence band of the p-doped part could lower their energy by entering the conduction band of the n-doped zone, but they can’t do so, because they may not cross the forbidden region unless it is that narrow (by heavy doping), that they can tunnel through it (tunnel diode). So no current can flow with reverse voltage. Applying high voltage will either result in finally an avalanche breakdown of the device, if the electrons get accelerated in the depletion region in a way, that they can ionize other atoms, or in a tunnel breakdown, depending on the doping circumstances.
Diodes designed as rectifiers usually get destroyed by avalanche breakdown; zener diodes are especially made to break down at a certain reverse voltage and with them tunnel breakdown dominates at low and avalanche breakdown at high breakdown voltages. Since the temperature coefficients of avalanche and tunnel breakdown have opposite signs, tunneling works better in the cold due to sharp band.
Forward biassing the diode means to put the positive terminal to the p-doped part. Then, at low voltages, still no current flows since the carriers would have to get over the diffusion potential to cross the depletion layer. Only if the voltage equals the diffusion potential, the band edges "get straight", the space charge and the depletion layer get dissolved and the current can flow freely. Holes and electrons can enter the oppositely doped region and recombine there (in case of direct semiconductors - not silicon - emitting their energy difference not only thermally but also as photons - useful for LEDs. The band edges are not straight in momentum space but periodical.
With this experiment you will be able to measure the current - voltage curve for 1N4007 and 1N4148 silicon diodes, as well as measure the collector current - emitter-collector voltage curve for different values of base current. All data is easily read of the computer display, each variable is easily adjusted and monitored.
Includes Cobra3 Basic Unit; power supply, 12 V; RS232 data cable; PowerGraph; Cobra3 Function generator module; digital multimeter; potentiometer, 1 k O, Plug-in board, Transistor in plug-in box; resistor in plug-in box, 47 k O; silicon diode 1N4007; silicon diode 1N4148; connecting cords; and manual on CD-ROM.
System requirements:
PC, WindowsR 95 or higher
For your convenience this experiment is available to you as a complete set.
Everything you require to perform the activities are included in the total price.
For assistance in using your new equipment we also offer professional development (WLS1808-24) on all PHYWE line products.
For customization options and pricing please contact your local Cenco representative.
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