CO2005: Electronics I (FET) Electronics I, Neamen 3th Ed. 1

Transcription

1 CO2005: Electronics I The Field-Effect Transistor (FET) Electronics I, Neamen 3th Ed. 1

2 MOSFET The metal-oxide-semiconductor field-effect transistor (MOSFET) becomes a practical reality in the 1970s. The MOSFET, compared to BJTs, can be made very small, that is, it occupies a very small area in IC chip. In the MOSFET, the current is controlled by an electric field applied perpendicular to both the semiconductor surface and to the direction of current. The phenomenon applying an electric field perpendicular to the surface is called the field effect. Basic MOS capacitor structure Electronics I, Neamen 3th Ed. 2

3 The Physics of the MOS Capacitor Gate Electronics I, Neamen 3th Ed. 3

4 The Physics of the MOS Capacitor for N-type Semiconductor Substrate Enhancement mode: a voltage must be applied to the gate to create an inversion layer. P-type: a positive gate voltage must be applied to create the electron inversion layer N-type: a negative gate voltage must be applied to create the hole inversion layer Electronics I, Neamen 3th Ed. 4

5 Transistor Structure NMOS Transistor Operation Large enough positive voltage induces an electron inversion layer. Connection between D and S is created so that a current can be generated. Electronics I, Neamen 3th Ed. 5

6 MOSFET Current-Voltage Characteristics The threshold voltage of the n-channel MOSFET is denoted as and is defined as the applied gate voltage needed to create an inversion charge. We can think of the threshold voltage as the gate voltage required to turn on the transistor. V TN Electronics I, Neamen 3th Ed. 6

7 i MOSFET Current-Voltage Characteristics The versus characteristics for small values of D v DS v DS Electronics I, Neamen 3th Ed. 7

8 MOSFET Current-Voltage Characteristics Electronics I, Neamen 3th Ed. 8

9 Ideal MOSFET Current-Voltage Characteristics Nonsaturation (triode) Region v DS vds ( sat) v GS V TN i D K n 2 [2( vgs VTN ) vds vds ] Kn (2 vds ( sat ) v v 2 ( DS DS ) Saturation Region v i DS D v ( also v V ) K DS ( sat) GS TN n ( v VTN ) GS Note: In the saturation region, 1 r o i D / v DS 2 Electronics I, Neamen 3th Ed. 9

10 Conduction Parameter Conduction Parameter K n C ox n W L W L nc 2 ox (conduction parameter) : oxide capacitance per unit area : electron mobility : channel width : channel length C ox 1 t ox, t ox : oxide thickness The conduction parameter is a function of both electrical and geometric parameters. Electrical Parameters: The oxide capacitance and carrier mobility are essentially constants for a given technology. K n W L k n 2 k n : constant Geometrical Parameters: The width-to-length ratio (W/L) is a variable in the design of MOSFETs that is used to produce specific current-voltage characteritics in MOSFET circuits. Electronics I, Neamen 3th Ed. 10

11 Electronics I, Neamen 3th Ed. 11

12 PMOS In the p-channel enhancement-mode device, a negative gate-to-source voltage must be applied to create the inversion layer of holes that connects the source and drain regions. V TP The threshold voltage, denoted an for the PMOS is negative for an enhancementmode devices. The threshold voltage is positive for a depletion-mode device. Holes flow from the source to the drain, the conventional current enters the source and leaves the drain. Electronics I, Neamen 3th Ed. 12

13 Ideal PMOS Current-Voltage Relationship Nonsaturation (triode) Region when v v ) SD v V SD( sat SG TP : i D K p 2 [2( vsg VTP ) vsd vsd ] K p (2 vsd( sat ) v v 2 ( SD SD ) Saturation Region 2 when v v ( ) ( also v V 0) : i K ( v VTP) ) SD SD sat SG TP D p SG Electronics I, Neamen 3th Ed. 13

14 Circuit Symbols N-channel enhancementmode MOSFET P-channel enhancement- mode MOSFET Electronics I, Neamen 3th Ed. 14

15 CMOS Complement MOS (CMOS) technology uses both NMOS and PMOS in the same circuit. To design electrically equivalent NMOS and PMOS devices, adjusting the W/L ratios of the transistors is required. Electronics I, Neamen 3th Ed. 15

16 Summary of MOS Transistor Operation Electronics I, Neamen 3th Ed. 16

17 NMOS Common-Source Circuit Electronics I, Neamen 3th Ed. 17

18 PMOS Common-Source Circuit Electronics I, Neamen 3th Ed. 18

19 Electronics I, Neamen 3th Ed. 19

20 Electronics I, Neamen 3th Ed. 20

21 Electronics I, Neamen 3th Ed. 21

22 Electronics I, Neamen 3th Ed. 22

23 Load Line Load Line V I DS D V 5 20 DD I 5 DS D V 20 R D (ma) 5 I (20) D Electronics I, Neamen 3th Ed. 23

24 Electronics I, Neamen 3th Ed. 24

25 Electronics I, Neamen 3th Ed. 25

26 Electronics I, Neamen 3th Ed. 26

27 中 央 大 學 通 訊 系 張 大 中 Electronics I, Neamen 3th Ed. 27

28 Nonlinear Resistor An enhancement-mode MOSFET is used as a nonlinear resistor. The transistor is always biased in the saturation region and called a load device. v DS v GS v, DS ( Sat ) vgs VTN VTN 0 i D K 2 n( vgs VTN ) Kn( vds VTN ) 2 Electronics I, Neamen 3th Ed. 28

29 Electronics I, Neamen 3th Ed. 29

30 中 央 大 學 通 訊 系 張 大 中 Electronics I, Neamen 3th Ed. 30

31 Electronics I, Neamen 3th Ed. 31

32 CMOS Inverter Electronics I, Neamen 3th Ed. 32

33 If v, the transistor is in cut-off. I V TN NMOS Inverter i v D O 0 V DD If v (and make v V v ), the transistor is biased in the non-saturation region. I V TN I TN DS i v D O K [2( v V ) v v ] v n DD i I D R D TN O 2 O Electronics I, Neamen 3th Ed. 33

34 Electronics I, Neamen 3th Ed. 34

35 Digital Logic Gate Electronics I, Neamen 3th Ed. 35

36 MOS Small-Signal Amplifier We can establish a particular Q-point on the load line by designing the ratio of the bias resistors and. R1 R2 Electronics I, Neamen 3th Ed. 36

37 Constant-Current Biasing Electronics I, Neamen 3th Ed. 37

38 Electronics I, Neamen 3th Ed. 38

39 中 央 大 學 通 訊 系 張 大 中 Electronics I, Neamen 3th Ed. 39

40 Electronics I, Neamen 3th Ed. 40

41 Constant-Current Biasing K V V 2 2 n3 ( VGS 3 VTN 3) Kn4( VGS 4 VTN 4) V GS 4 GS 3 GS 3 V V V V K TN 4 TN 3 1 Kn3 / Kn4 n3 / Kn4 I Q K 2 n2 ( VGS 3 VTN 2) Electronics I, Neamen 3th Ed. 41

42 中 央 大 學 通 訊 系 張 大 中 Electronics I, Neamen 3th Ed. 42

43 Multitransistor Circuit: Cascade Configuration 中 央 大 學 通 訊 系 張 大 中 Electronics I, Neamen 3th Ed. 43

44 Electronics I, Neamen 3th Ed. 44

45 Multitransistor Circuit: Cascode Configuration 中 央 大 學 通 訊 系 張 大 中 Electronics I, Neamen 3th Ed. 45

46 Electronics I, Neamen 3th Ed. 46