Information AboutHemt |
| CATEGORIES ABOUT HEMT | |
| transistor types | |
| microwave technology | |
| terahertz technology | |
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Ordinarily, the two different materials used for a heterojunction must have the same Lattice Constant (spacing between the atoms). An analogy - imagine pushing together two plastic combs with a slightly different spacing - at regular intervals, you'll see two teeth clump together. In semiconductors, these discontinuities are a kind of "trap", and greatly reduce device performance. A HEMT where this rule is violated is called a PHEMT or ''pseudomorphic'' HEMT. This feat is achieved by using an extremely thin layer of one of the materials - so thin that it simply stretches to fit the other material. This technique allows the construction of transistors with bigger bandgap differences than otherwise possible. This gives them better performance. Another way to use materials of different lattice constants is to place a buffer layer between them. This is done in the mHEMT or ''metamorphic'' HEMT, an advancement of the PHEMT developed in recent years. In the buffer layer made of AlInAs, the Indium concentration is graded, so that it can match the lattice constant of both the GaAs substrate and the GaInAs channel. This brings the advantage that practically any Indium concentration in the channel can be realized, so the devices can be optimized for different applications (low Indium concentration provides low Noise , high Indium concentration gives high Gain ). To the best of the author's knowledge, PHEMTs and related devices are the fastest transistors available. They can be used to make amplifiers which work at over 200 GHz. Applications are similar to MESFET s - Microwave and Millimeter Wave Communications , Radar , and Radio Astronomy . ( Heterojunction Bipolar Transistor s were demonstrated at frequencies over 600 GHz in April 2005.) See also Heterojunction Bipolar Transistor . EXTERNAL LINKS |
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