The MOSFET Metal Oxide Semiconductor Field Effect Transistor • A practical reality in 1960’s • Simpler manufacturing process • For digital circuits the benefits are – – – – MOSFET (also known as IGFET) Enhancement mode N-channel MOSFET (NMOSFET) L Low power High density Easier fabrication Low cost W G • Digital and analog circuits can be easily implemented • Preferred technology for mixed analog/digital systems is CMOS (Complementary MOS) MOSFET Symbols N-channel FETs P-channel FETs M1 S D N+ N+ reverse biased Cox P-type Substrate (Body) B MOSFET Operation Simplified (Fluid Dynamics Analog) VT = turn-on Voltage (threshold voltage) 1) Cut off: VGS < VT, VDS = 0 G S D N+ N+ Source Tank VGS Gate VT M1 2) Strong Inversion: VGS > VT, VDS = 0 G M1 S D N+ N+ M1 If bulk is connected to proper power supply: M1 B at lowest chip potential M1 B at highest chip potential inverted polarity (substrate p-type, surface n-type => inversion VGS VT Drain Tank MOSFET Current Voltage Relationship 3) Linear or Triode: VGS > VT , 0 < VDS < VGS - VT IDS VGS VT 0 Cutoff K'=uCox IDS K' 2 º VDS Wª V V V T DS « GS » VGS t VT , VDS VGS - VT 2 ¼ L ¬ Linear, triode, non-saturation 4) Saturation: VGS > VT , VDS > VGS - VT IDS K' W VGS VT 2 2 L VGS t VT , VDS t VGS - VT Saturation W, L are device geometry parameters Flow becomes independent of VDS N-channel device I-V Characteristics Model Parameter Names in SPICE VGS3 VGS-VT Parameter SPICE Param Units K’ KP A/V2 VTO VTO V ID M1 + VGS - + VDS ID Nonsaturation Saturation - VGS2 VGS1 VDS = -1/O VDS MOSFET Operation (VGS < VT) P-channel device I-V Characteristics IDS VDS VDS = 0, VGS > 0 but < VT VGS=-1.5V VGS > 0 + charge on gate - charge in substrate S VGS + G VGS=-2.5 Saturation VGS=-3.5 Nonsaturation |VGS|- |VT| M1 D Electric field repels holes leaving ionized acceptor atoms depletion region IDS Drain Current for VDS > 0 but Small MOSFET Operation (VGS | VT) The gate voltage required to produce the inversion layer is called the threshold voltage VT VGS !! VT VDS = 0, VGS increasing and | VT 9'6 !9*6 !!97 Depletion region becomes deeper; almost as deep as that under the N+ regions Surface is as attractive for electrons as N+ regions Electrons can enter from the N+ regions The induced n-region under the gate is of n-type Surface is inverted and the electron layer is called the inversion layer The inversion layer forms a channel for current flow when VDS > 0 VDS ! 0 | 0 7KHFKDUJHLQGXFHGLQWKHFKDQQHOLV N+ ID Cox WLVGS VT Qi R channel ID N+ 1 L nCox (VGS VT ) W VDS R channel nCox W (VGS VT )VDS L k' W (VGS VT )VDS L 7KHLQYHUVLRQOD\HUIRUPVDFKDQQHO IRUFXUUHQWIORZZKHQ9'6 !