Field effect transistors

Transcription

1 Field effect transistors Junction FETs Metal-Oxide Semiconductor FETs type N type P enhancement depletion type N type P type N type P Slide 1

2 Junction field effect transistor - JFET Construction and circuits symbols of JFETs (Junction Field-Effect Transistors) Slide 2

3 N-channel JFET operation: gate-channel diode slightly reverse-biased (top), and (bottom) gate-channel diode highly reverse-biased (V GS >V p so that channel is pinched off) Slide 3

4 I Junction field effect transistor - JFET For FET operated in saturation region: V 1 = GS I SS Vp 2 where V p is the pinch-off voltage and I SS is the saturation drain current for V GS = 0 The transconductance: g = I V ~ m GS I Bipolar transistor: 1 e C g m bipolar = = ~ re kt I I C Slide 4

5 Idealized common-source characteristic curves for a JFET Slide 5

6 Transient characteristic curves for a n-type JFET Slide 6

7 for R S =0, I =I SS 1.7V, 5.12mA 50V, 5.37mA Self-biasing JFET current source Slide 7

8 Source follower Slide 8

9 Source follower with current-source load ( infinite resistance reduces the nonlinearities) Slide 9

10 Slide 10 Electronics Fundamentals JFET amplifier m GS G GS G GS m R g V R I V V A V V R I V V I g = = = = = =

11 MOS TRANSISTORS Schematic representation of an enhancement-mode N-channel MOSFET Slide 11

12 λ = 0 Characteristic curves of MOSFET showing the regions of operation Slide 12

13 Cuttoff region: I = 0, dla V GS V T 0 Linear region: 0 < VS < VGS VT VS( sat) = VGS VT I ' [( V V ) V / 2] V ( + λv ) = K W / L 1 GS T S S S Saturation region: 0 < V I GS V T V S 2 ( µ C W )/ 2L ( V V ) ( + λv ) = 1 n ox GS T S ' K = C ox µ n λ mobility of electrons MOS channel length modulation factor V T [ ] Φ V Φ = VT 0 + γ BS Slide 13

14 λ 0 λ = 0 Characteristic curves of MOSFET Slide 14

15 V S, L,L Widening of the pinched-off region (an increase of drain current with V in the saturation region Slide 15

16 V T [ ] Φ V Φ = VT 0 + γ BS Transient characteristics of MOSFET Slide 16

17 Types of MOSFETs: N-channel enhancement and N-channel depletion Slide 17

18 Types of MOSFETs: P-channel enhancement and P-channel depletion Slide 18

19 G S I 0 < V GS V T V S PMOS NMOS G S V T V I ( µ C W ) 2L ( V V ) 2 ( 1+ λv ) ( C W )/ 2 L ( V V ) 2 = µ n ox / GS T S n ox GS T I ( V V ) 2 = β V = V = V = V I β GS T GS S T + Active resistors (V GS = V S : saturation mode) Slide 19

20 Voltage division using active resistors Slide 20

21 V out = VS1 = VT 1 + I β1 VS 2 = VT 2 + I β2 I = β ( V ) 2 S V 2 2 T 2 V out ( VS 2 V 2 ) β2 1 = VT 1 + T β ( µ C ox W )/ L = A W L β = 2 n For a given current I the output voltage depends on the ratio W/L Slide 21

22 V Z LOA Current sink Slide 22

23 Current source Slide 23

24 Current mirror (M 1 in saturated mode) Slide 24

25 For identical transistors: µ 0, λ and C ox are the same I out W L W L ( 1+ λvs 2 ) ( + λvs ) Iin = Ai Iin 2 1 = For the same channel dimensions: if V S1 = V S2 if V S1 = V S2 then I in = I out then I in = I out Slide 25

26 MOS inverter with resistive load. Input and output waveforms showing switching delay due to capacitor discharge. Slide 26

27 Resistor-load inverter: current-voltage characteristics of NMOS and load, and composite characteristics of inverter Slide 27

28 MOSFET-load inverter: characteristic of load and composite characteristics of inverter Slide 28

29 CMOS inverter and corresponding PMOS characteristics Slide 29

30 Cross section of the CMOS inverter Slide 30