The main contents of the chapter consist of the following: Introduction, the basic structure of MOSFET, qualitative operation of MOSFET, operation with gate-voltage VG=0, channel for current flow, applying a small VDS, operation as VDS is Increased.
COMSATS Institute of Information Technology Virtual campus Islamabad Dr. Nasim Zafar Electronics 1 EEE 231 Fall Semester – 2012 MOS FieldEffect Transistors MOSFETs Lecture No. 27 Ø Contents: Nasim Zafar Lecture No. 27 MOS FieldEffect Transistors MOSFETs Reference: Chapter4.1 Microelectronic Circuits Adel S. Sedra and Kenneth C. Smith. Nasim Zafar Different Types of FETs Ø Junction FET (JFET) Ø MetalOxideSemiconductor FET (MOSFET) Ø MetalSemiconductor FET (MESFET) Nasim Zafar Different Types of FETs v Junction FET (JFET) The gatechannel insulator is the DEPLETION REGION, and is the same material as the channel. Nasim Zafar Different Types of FETs v MetalOxideSemiconductor FET (MOSFET) The gatechannel insulator is made out of dielectric;SiO2 Nasim Zafar Different Types of FETs v MetalSemiconductor FET (MESFET) The gate is formed by Schottky barrier to the semiconductor layer. The gatechannel insulator consists of the DEPLETION REGION, i.e. the same material as the channel. Very similar to the JFET Nasim Zafar MOS (MetalOxideSemiconductor) Assume work function of metal and semiconductor are same Nasim Zafar Metal Oxide Semiconductor Field Effect Transistors MOSFET Nasim Zafar Circuit Symbol (NMOS) EnhancementType D ID= IS G B (IB=0, should be reverse biased) IG= 0 IS S Nasim Zafar GGate DDrain SSource BSubstrate or Body 10 NMOS with VGS > Vt and a small VDS applied. Figure 4.3: The device acts as a resistance whose value is determined by VGS. Specifically, the channel conductance is proportional to V GS – Vt’ and thus iD is proportional to (V GS – Vt) V DS. Nasim Zafar 32 Modes of MOSFET Operation MOSFET can be categorized into three separate modes when in operation, depending on VGS: v VGS Vt and VDS Vt and VDS > VGS − Vt: The Saturation Mode Nasim Zafar 33 Modes of MOSFET Operation v 1. VGS Vt Nasim Zafar 39 Operation of NMOS as VDS is Increased Ø When VDSis increased to the value that reduces the voltage between gate and channel at the drain end to, Ø VGD= Vt or Ø VGS–VDS= Vt or Ø VDS= VGS–Vt Ø The channel depth at the drain end decreases to almost zero, and the channel is said to be pinched off. Nasim Zafar 40 Operation of NMOS as VDS is Increased Ø Ø Ø At the value reached for VDS= VGS–Vt. The drain current thus saturates at this value, and the MOSFET is said to have entered the saturation region of operation VDSsat = VGS–Vt The region of the iD–VDS characteristic obtained for vDS Vt Nasim Zafar 42 Effects of VDS on Channel Shape Figure4.7: Increasing VDS causes the channel to acquire a tapered shape. Eventually, as vDS reaches vGS – Vt the channel is pinched off at the drain end. Increasing VDS above VGS – Vt has little effect (theoretically, no effect) on the channel’s shape Nasim Zafar 43 Summary: NMOS Operation The MOSFET can be categorized into three separate modes when in operation: v VGS VGS − Vt: The Saturation Mode The saturation mode occurs when VGS > Vt and VDS > VGS − Vt. In this mode the switch is on and conducting, however since drain voltage is higher than the gate voltage, part of the channel is turned off. This mode corresponds to the region to the right of the dotted line, which is called the pinchoff voltage. Pinchoff occurs when the MOSFET stops operating in the linear region and saturation occurs v In digital circuits MOSFETS are only operated in the linear mode, while the saturation region is reserved for analogue circuits Nasim Zafar 45 Summary: NMOS Operation VG > VT ; VDS 0 ID increases with VDS VG > VT; VDS small, > 0 ID increases with VDS , but rate of increase decreases VG > VT; VDS pinchoff ID reaches a saturation value, ID,sat The VDS value is called VDS,sat VG > VT; VDS > VDS,sat ID does not increase further, saturation region Nasim Zafar 46 ... MOSFETs Lecture No. 27 Ø Contents: Nasim Zafar Lecture No. 27 MOS FieldEffect Transistors MOSFETs Reference: Chapter4.1 Microelectronic Circuits Adel S. Sedra and Kenneth C. Smith. Nasim. .. JFET Nasim Zafar MOS (MetalOxideSemiconductor) Assume work function of metal and semiconductor are same Nasim Zafar Metal Oxide Semiconductor Field Effect Transistors MOSFET Nasim Zafar Circuit Symbol (NMOS)... and is the same material as the channel. Nasim Zafar Different Types of FETs v MetalOxideSemiconductor FET (MOSFET) The gatechannel insulator is made out of dielectric;SiO2 Nasim Zafar Different Types of FETs