- 1. MOSFET I-V Characteristics Contents •Types •Cut off region •Linear region •Saturation region •Comparison •Conclusion and References 1Prepared By S ARUN M.Tech MOSFET VI Characteristics
- 2. Characteristics comes under three category • Cut-off region • Linear region • Saturation region Prepared By S ARUN M.Tech MOSFET VI Characteristics
- 3. MOSFET I-V Characteristics : Cutoff Region •When VGS < VT, there is no channel formed between the Drain and Source and hence IDS=0 A •This region is called the Cutoff Region •This region of operation is when the Transistor is OFF Prepared By S ARUN M.Tech MOSFET VI Characteristics
- 4. MOSFET I-V Characteristics : Linear Region • When VGS> VT, a channel is formed. IDS is dependant on the VDS voltage • When VDS= 0v, no current flows Prepared By S ARUN M.Tech MOSFET VI Characteristics
- 5. • If VGS> VT and VDS> 0, then a current will flow from the Drain to Source (IDS).The MOSFET operates like a voltage controlled resistor which yields a linear relationship between the applied voltage (VDS) and the resulting current (IDS) • For this reason, this mode of operation is called the Linear Region.This region is also sometimes called the triode region (we'll use the term "linear") • VDS can increase up to a point where the current ceases to increase linearly (saturation) • We denote the highest voltage that VDS can reach and still yield a linear increase in current as the saturation voltage or VDSAT Prepared By S ARUN M.Tech MOSFET VI Characteristics
- 6. • when a voltage is applied at VD,its positive charge pushes the majority charge carriers (holes) that exist at the edge of the depletion region further from the Drain. • As the depletion region increases, it becomes more difficult for the Gate voltage to induce an inversion layer. This results in the inversion layer depth decreasing near the drain. • As VD increases further, it eventually causes the inversion layer to be pinched-off and prevents the current flow to increase any further. • This point is defined as the saturation voltage (VDSAT) • From this, we can define the linear region as: VGS>VT 0 < VDS < VDSATPrepared By S ARUN M.Tech MOSFET VI Characteristics
- 7. Prepared By S ARUN M.Tech MOSFET VI Characteristics
- 8. • The Drain to Source current (IDS) is given by the expression: • Where: un = electron surface mobility (units in cm2 /V·s) Cox = Unit Oxide Capacitance (units in F/cm2 ) W = width of the gate L = length of the gate • Remember this expression is only valid when : VGS>VT 0 < VDS < VDSAT ( )[ ]2 02 2 DSDSTGS oxn DS VVVV L WCu I linear −⋅−⋅⋅⋅ ⋅ = Prepared By S ARUN M.Tech MOSFET VI Characteristics
- 9. • what is linear about this equation? • Most of the parameters are constants during evaluation. They are sometimes lumped into single parameters • Notice that W and L are parameters that the designers have control over. Most of the other parameters are defined by the fabrication process and are out of the control of the IC designer. ( )[ ]2 02 2 DSDSTGS oxn DS VVVV L WCu I linear −⋅−⋅⋅⋅ ⋅ = oxn Cuk ⋅=' ( )[ ]2 02 2 ' DSDSTGSDS VVVV L Wk I linear −⋅−⋅⋅⋅= L W Cuk oxn ⋅⋅= ( )[ ]2 02 2 DSDSTGSDS VVVV k I linear −⋅−⋅⋅= Prepared By S ARUN M.Tech MOSFET VI Characteristics
- 10. • What is linear about this equation? • The -VDS 2 term alters the function shape in the linear region. As it becomes large enough to significantly decrease IDSin this function, the transistor enters saturation and this expression is no longer valid. ( )[ ]2 02 2 DSDSTGSDS VVVV k I linear −⋅−⋅⋅= For a fixed VGS, then IDS depends on VDS VDS 2 has a smaller effect on IDS at low values of VDS since it is not multiplied by anything Prepared By S ARUN M.Tech MOSFET VI Characteristics
- 11. • Since we know what the current will not decrease as VDS increases past VDSAT, we can usethis expression to define VDSAT • when VDS>(VGS-VT), then IDS in this expression begins to decrease • we can then define VDSAT = (VGS-VT) • So now we have the formal limits on the linear region and the validity of this expression: • Linear Region : VGS>VT 0 < VDS < (VGS-VT) ( )[ ]2 02 2 DSDSTGSDS VVVV k I linear −⋅−⋅⋅= Prepared By S ARUN M.Tech MOSFET VI Characteristics
- 12. MOSFET I-V Characteristics : Saturation Region • MOSFET is defined as being in saturation when: Saturation Region : VGS> VT and VDS > (VGS-VT) • An increase in VDS does not increase IDS because the channel is pinched-off • However, an increase in VGS DOES increase IDSby increasing the channel depth and hence the amount of current that can be conducted. • Measurements on MOSFETS have shown that the dependence of IDS on VGS tends to remain approximately constant around the peak value reached for VDS=VDSAT • A substitution of VDS=(VGS-VT0) yields: ( ) ( ) ( )[ ] ( )2 0 2 000 2 2 2 TGSDS TGSTGSTGSDS VV k I VVVVVV k I sat sat −⋅= −−−⋅−⋅⋅= Prepared By S ARUN M.Tech MOSFET VI Characteristics
- 13. 1st order expressions for all three regions of operation for the MOSFET Now we have Region Conditions IDS Cutoff VGS< VT Linear VGS> VT VDS < (VGS-VT) Saturation VGS> VT VDS > (VGS-VT) 0=cutoffDSI ( )2 0 2 TGSDS VV k I sat −⋅= ( )[ ]2 02 2 DSDSTGSDS VVVV k I linear −⋅−⋅⋅= Prepared By S ARUN M.Tech MOSFET VI Characteristics
- 14. • Conclusion In this presentation conclude that came to know regards VI characteristics of MOSFET. • References Sedra and smith, “Microelectronic circuits”,7th Ed., Oxford University Press. Thomas L.Floyd, “Electronic devices” Conventional current version, Pearson prentice hall. Robert L.Boylestad, “Electronic devices and circuit theory”,