MOSFET Basics - Microelectronic Circuits | ECE 3040, Study notes of Electrical and Electronics Engineering

Download MOSFET Basics - Microelectronic Circuits | ECE 3040 and more Study notes Electrical and Electronics Engineering in PDF only on Docsity! ECE 3040 - Dr. Alan DoolittleGeorgia Tech Lecture 24 MOSFET Basics (Understanding with no math) Reading: Pierret 17.1-17.2 and Jaeger 4.1-4.10 and Notes ECE 3040 - Dr. Alan DoolittleGeorgia Tech Flow of current from “Source” to “Drain” is controlled by the “Gate” voltage. Control by the Gate voltage is achieved by modulating the conductivity of the semiconductor region just below the gate. This region is known as the channel MOS Transistor Qualitative Description ECE 3040 - Dr. Alan DoolittleGeorgia Tech MOS Transistor Qualitative Description Consider now the Inversion case: First, VDS = 0: •when VGS > VT , an induced n- type region, an “inversion layer”, forms in the channel and “electrically connects” the source and drain. P-type Inversion layer (n-type) ECE 3040 - Dr. Alan DoolittleGeorgia Tech MOS Transistor Qualitative Description Inversion case, VGS > VT(continued): When VDS >0 , the induced n- type region allows current to flow between the source and drain. The induced channel ast like a simple resistor. Thus, this current, ID, depends linearly on the Drain voltage VD. This mode of operation is called the linear or “triode”* region. P-type Inversion layer (n-type) * “Triode” is a historical term from vacuum tube technology. ECE 3040 - Dr. Alan DoolittleGeorgia Tech Inversion case, VGS > VT(continued): Drain current verses drain voltage when in the linear or “triode”* region. MOS Transistor Qualitative Description ECE 3040 - Dr. Alan DoolittleGeorgia Tech MOS Transistor Qualitative Description Inversion case, VGS > VT(continued): The inversion layer eventually vanishes near the drain end of the channel. This is called “Pinch-Off” and results in a Flat ID-VDS curve ECE 3040 - Dr. Alan DoolittleGeorgia Tech Inversion case, VGS > VT(continued): ID-VDS curve for the “Saturation Region” The drain-source voltage, VDS, at which this occurs is called the saturation voltage, Vsat while the current is called the saturation current, IDsat. MOS Transistor Qualitative Description IDsat ECE 3040 - Dr. Alan DoolittleGeorgia Tech MOS Transistor Qualitative Description Inversion case, VGS > VT(continued): For VDS>Vsat the channel length, L, effectively changes by a value ∆L. The region of the channel, ∆L is depleted and thus, is high resistivity. Accordingly, almost all voltage increases in VDS>Vsat are “dropped across” this portion of the channel. High electric fields in this region act similarly to the collector-base junction in a BJT in active mode, “stripping” or “collecting” carriers from the channel.