Chapter 6 Lecture - Part 1

In this example the device is isolated from others on

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Unformatted text preview: esn’t To increase with V above VP increase GaAs MESFET formed on an n-type GaAs layer grown epitaxially on a semi-insulating substrate. In this example the device is isolated from others on the same chip by etching through the n region to the semi-insulating substrate. MESFET MESFET • Definition: Definition: – • Basically it’s a JFET that uses a reversed biased Basically Schottky contact instead of a p-n junction to regulate current flow in the channel. regulate Main uses: Main – High speed digital and microwave devices that High require very tight geometric tolerances. Schottky contacts have very sharp interfaces with semiconductors, unlike some p-n junctions semiconductors, MESFET MESFET • Example: GaAs MESFET (Figure 6.7) – Description: • Substrate: semi-insulating GaAs • Semiconducting layer: n-type GaAs epitaxial thin film – Why GaAs • Higher electron mobility than Si • Allows for tight geometric tolerances by way of epi Allows growth or ion implantation growth • Channel length L of 0.25 microns possible. HEMT HEMT • High Electron Mobility Transistor – Definition: a device that uses heterojunctions to Definition: increase the conductivity of the channel without using doping to do so. doping – Why important: increasing conductivity via doping Why tends to decrease the mobility due to impurity scattering tends • Structure – A thin undoped well is bounded by doped barriers of a thin higher bandgap (AlGaAs-GaAs-AlGaAs) higher – This leads to a potential well • Effect HEMT HEMT – Electrons in the doped boundary regions diffuse into Electrons the GaAs and become trapped in the well the – The effect is to separate the donors from their The electrons physically electrons – The net effect is to get numerous electrons in an The otherwise undoped (or lightly doped) region otherwise – Conductivity is enhances but mobility is not lowered. • Important characterstics – Large electron density (1012/cm2) – Very thin undoped layer (...
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This note was uploaded on 04/02/2014 for the course EECS 321 taught by Professor Zorman during the Spring '10 term at Case Western.

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