General Introduction to Fabrication techniques of a P-N junction diode:
pn-junction-fabrication
In practice, the P-N junction is formed from a single mono crystalline structure by adding carefully controlled amounts of donor and acceptor impurities. Here discussion is limited only to acquaintation with the basic techniques and terminology (not expertise in fabrication).
The first and foremost requirement is to obtain an extremely pure germanium or silicon. Impurity of less than one part in ten billion (1010) is required for most semiconductor device fabrication to-day. For obtaining pure semiconductor material it is first purified chemically. For reducing the impurities further, and to ensure the formation of a mono crystalline structure, a technique known as flatting zone is quite often employed.The mono crystalline structure is formed through the use of a small seed of semiconductor (e.g., silicon or germanium). The seed itself is a monocrystal that has been very carefully cut along the face of its cubic lattice. Support clamps are employed to hold the low-purity polycrystalline rod.
The rod, seed, and support clamps are placed in a quartz cylinder. The process may be carried out in vacuum or by surrounding the semiconductor with an inert gas. Great care is required to ensure that the semiconductor is not further contaminated during the floating zone process.
Induction coils encompass the quartz container, as shown in figure. The coils are excited by a radio-frequency voltage. Circulating currents are induced in the semiconductor due to the magnetic field set up by RF currents flowing through the induction coils. The heat so produced melts the zone which is exposed to it. As a result, a molten region is formed.
Slow rotation of rod makes the semiconductor atoms to align themselves with the atoms in the monocrystalline seed. Thus as the induction coils are moved downward, the molten region follows, and a monocrystalline structure continues to grow from the seed. Purification of the semiconductor rod occurs simultaneously with the formation of the monocrystal. The impurities in the semiconductor rod tend to become ‘more liquid’ than the semiconductor. Thus as the induction coils are moved downward, the impurities tend to follow the molten region. Once the induction coils have traversed the length of the semiconductor rod, the impurities are collected in its lower end. The lower end of the semiconductor rod may then be cut off, and the process is repeated until the desired impurity level is attained.
Once a pure monocrystalline semiconductor has been produced, carefully controlled I amount of donor and acceptor impurities are added to the semiconductor without disturbing the orderly monocrystalline structure. Thus a P-N junction is formed.
Semiconductor diodes are normally one of the following types:
1. Grown junction diode
2. Alloy type or fused junction diode
3. Diffused junction diode
4. Epitaxial grown or planar diffused diode
5. Point contact diode
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