Lecture 30

Lecture 30 - EEE 352: Lecture 30 High Electron-Mobility...

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Unformatted text preview: EEE 352: Lecture 30 High Electron-Mobility Field Effect Transistors (HEMT) * Three terminal devices * Depletion Mode Device (Like the MESFET) * Behavior like the MOSFET * Field effect action Threshold voltage Current saturation The HEMT Device When the semiconductor is connected to the metal by a back contact, the Fermi levels align (no charge in the interface). In MESFETs, the interface charge dominated the zero bias condition; Here, the heterojunction interface is GOODvery small interface charge levels. The GaAsAs surface is dominated by these states, but this is a metal-Gate Inversion layer is created at the interface between GaAs and AlGaAs, just as in the MOSFET, but here it occurs due to the donors in the AlGaAs. E c E v p-type GaAs semiconductor n-type AlGaAs metal ionized donors electron inversion layer Acts like oxide The Schottky barrier causes depletion of the donors near the surface. The conduction band offset at the heterojunction causes the donors to be ionizedthe electrons move into the potential well. E c E v p-type GaAs semiconductor n-type AlGaAs metal ionized donors electron inversion layer If this region exists, it is BAD. The gate has no control over the charge in the channel. The two depletion regions must merge in the center. GaAlAs Layer n n GaAs LAYER CONTROL GATE SOURCE DRAIN n-type contacts are created in the p-type substrate. The n-type channel is INDUCED by the DONORS, and connects these two contacts. The number of electrons in the CHANNEL is determined by the GATE, so the CONDUCTANCE is modulated by the GATE. The HEMT Device The AlGaAs layer acts much like the oxide in the MOSFET. The difference, however, is that the HEMT is a DEPLETION device, and V G < 0, as in the MESFET. Field Effect Action metal E c ionized donors m E c E d E c m E c Quantum well vanishes; no carriers are present at the interface. At sufficiently negative bias, the quantum well vanishes, and there is no charge at the interface. This is the threshold or turn- on or pinch-off voltage. E Fs metal E c m E c E Fm The gate bias gives the difference between the two Fermi levels....
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This note was uploaded on 10/28/2009 for the course EEE 352 taught by Professor Ferry during the Fall '08 term at ASU.

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Lecture 30 - EEE 352: Lecture 30 High Electron-Mobility...

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