Transcript viju
MOSFET V-I Characteristics Vijaylakshmi.B Lecturer, Dept of Instrumentation Tech Basaveswar Engg. College Bagalkot, Karnataka IUCEE-VLSI Design, Infosys, Mysore Types of Transistors MOSFET (Types) Four types: n-channel enhancement mode • Most common since it is cheapest to manufacture p-channel enhancement mode n-channel depletion mode p-channel depletion mode n-channel p-channel Depletion type n-channel p-channel Enhancement type MOSFET FET = Field-Effect Transistor A four terminal device (gate, source, drain, bulk) Symbols of MOSFET MOSFET characteristics Basically low voltage device. High voltage device are available up to 600V but with limited current. Can be paralleled quite easily for higher current capability. Internal (dynamic) resistance between drain and source during on state, RDS(ON), , limits the power handling capability of MOSFET. High losses especially for high voltage device due to RDS(ON) . Dominant in high frequency application (>100kHz). Biggest application is in switchedmode power supplies. The transistor consists of three regions, labeled the ``source'', the ``gate'' and the ``drain''. The area labeled as the gate region is actually a ``sandwich'' consisting of the underlying substrate material, which is a single crystal of semiconductor material (usually silicon); a thin insulating layer (usually silicon dioxide); and an upper metal layer. Electrical charge, or current, can flow from the source to the drain depending on the charge applied to the gate region. The semiconductor material in the source and drain region are ``doped'' with a different type of material than in the region under the gate, so an NPN or PNP type structure exists between the source and drain region of a MOSFET. •Most important device in digital design •Very good as a switch •Relatively few parasitics •Rather low power consumption •High integration density •Simple manufacturing •Economical for large complex circuits n-Channel MOSFET NMOS Structure MOS (Metal-Oxide-Semiconductor) Nowadays gate is made of poly-silicon Channel length L and width W In most digital design, L is set at the minimum feature size W is selectable by the designer Bulk is connected to the Gnd in NMOS to prevent forward-biased PN junction On state Off state n-MOSFET Characteristics Plots V-I characteristics of the device for various Gate voltages (VGS) At a constant value of VDS , we can also see that IDS is a function of the Gate voltage, VGS The transistor begins to conduct when the Gate voltage, VGS , reaches the Threshold voltage: VT PMOS Structure PMOS transistor has a negative threshold voltage (Vtp) -0.3v~-1.2v A pMOS turns on when Vgs<Vtp P-MOSFET Characteristics The terminal characteristics of the device are given by drain-to-source current Ids against drain-to-source voltage Vds for different values of gate-to-source voltage Vgs. All voltages are referenced with respect to the source voltage, which is assumed to be at ground potential. Switch models of MOSFETs g=0 d nMOS d OFF g s d pMOS d g=1 s s d d g ON OFF ON s s s Thank You