![]() Metal-Oxide Semiconductor Field effect Transistor (MOSFET) JUNCTION FIELD EFFECT TRANSISTORS (JFETs) JFET is a type of field effect transistor that uses a reverse biased p-n junction (as gate) to control current in channel JUNCTION FIELD EFFECT TRANSISTORS (JFETs) JFETs can be of two types: n-channel and p-channel N-CHANNEL JFET OPERATION Case I: VD = VS = 0V We vary the reverse bias gate voltage as VG = 0 V, VG = -1 V VG = -2 V When reverse bias is large enough so that entire n-channel becomes depleted, the corresponding voltage is called ‘pinch-off’ voltage N-CHANNEL JFET OPERATION Case II: VG = constant and VDS > 0 V We vary the reverse bias gate voltage as VG = 0 V, VG = -0.05 V VG = -1 V N-CHANNEL JFET OPERATION Now, voltage in the channel varies as a function of distance from source (x), depletion width becomes 0.5 2 ∊ □□□□□□ − □□(□□) □□ = □□□□□□ Hence, depletion region is wider at the drain PINCH-OFF VOLTAGE VP = VG at h=0 or W=a For p+-n junction, 2∊ □□□□□□ −□□ □□□□□□ □□ = potential 0.5, where Vbi is the built-in For pinch-off, W = a => □□□□ = □□□□□□ − □□□□□□ □□2 2∊ PINCH-OFF VOLTAGE Animation for explaining pinch off. Junction Field Effect Transistor (JFET) 2. ![]() ![]() FIELD EFFECT TRANSISTORS JFET AND MOSFET FIELD EFFECT TRANSISTORS Unipolar devices current is carried only by majority carriers Three terminal device: Gate, Source, Drain Voltage controlled devices gate (G) controls current flow between source (S) and drain (D) FET can be modelled as a resistance between source and drain, controlled by gate terminal Why “field effect”? Difference between BJT and FET Bipolar Junction Transistor Field Effect transistor Bipolar Unipolar Current controlled Voltage Controlled Lower input impedance High input impedance (implies no gate current) TYPES OF FETS 1.
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