Chapter 6 Lecture - Part 1

As a result the undoped gaas becomes n type without

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Unformatted text preview: < 10 nm) – Close proximity to the gate (a) Simplified view of modulation doping, showing only the conduction band. Electrons in the donor-doped AIGaAs fall into the GaAs potential well and become trapped. As a result, the undoped GaAs becomes n-type, without the scattering by ionized donors which is typical of bulk n-type material. (b) Use of a single AIGaAs/GaAs heterojunction to trap electrons in the undoped GaAs. The thin sheet of charge due to free electrons at the interface forms a two-dimensional electron gas (2-DEG), which can be exploited in HEMT devices. MISFETs MISFETs • MISFETs: Metal-Insulator-Semiconductor Field MISFETs: Effect Transistors Effect • Working Definition: Working – A FET where the channel current is controlled by a voltage FET applied at a gate electrode that is isolated from the channel by an insulator by • Basic Structure for an enhancement type n-channel Basic MOSFET: – See Figure 6-10 – Common materials of construction: • Si for the semiconductor • SiO2 for the insulator • Al, doped polysilicon for the gate Al, An enhancementtype n-channel type MOSFET MOSFET (a) isometric view of device and equilibrium band diagram along channel; (b) drain current— voltage output characteristics as a function of gate voltage. MISFETs MISFETs • Basic Operation: (Figure 6-10) – Gate voltage = 0 • No current flow due to dual potential barriers of the backto-back p-n junctions – Gate voltage = positive • Positive charges on metal induce negative charges in Positive channel by the formation of a depletion region and a thin electron layer at the surface • Negative charges make p-type region more n-type • The energy barrier becomes lowered because the The conduction and valence bands are lowered and as a result, a conducting n-channel is formed. conducting • The n-type conductivity of the channel is gate voltage The dependent, with a threshold voltage, VT dependent, n-channel MOSFET under different operating conditions conditions (a) linear region for VG > VT and VD < (VG - VT);...
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