EE 330 Lecture 19. Bipolar Devices

Transcription

1 330 Lecture 19 ipolar Devices

2 Review from last lecture n-well n-well n- p-

3 Review from last lecture Metal Mask A-A Section - Section

4 Review from last lecture D A A D

5 Review from last lecture Should now know what you can do in this process!! an metal connect to active? an metal connect to substrate when on top of field oxide? How can metal be connected to substrate? an poly be connected to active under gate? an poly be connected to active any place? an metal be placed under poly to isolate it from bulk? an metal 2 be connected directly to active? an metal 2 be connected to metal 1? an metal 2 pass under metal 1? ould a process be created that will result in an answer of YS to most of above?

6 asic Devices and Device Models Resistor Diode apacitor MOSFT JT

7 ipolar Junction Transistors Operation Modeling

8 arriers in Doped Semiconductors n-type p-type

9 arriers in Doped Semiconductors I urrent carriers are dominantly electrons Small number of holes are short-term carriers I urrent carriers are dominantly holes Small number of electrons are short-term carriers

10 arriers in Doped Semiconductors Majority arriers Minority arriers n-type electrons holes p-type holes electrons

11 arriers in MOS Transistors onsider n-channel MOSFT Saturation Region hannel Triode Region

12 arriers in MOS Transistors onsider n-channel MOSFT Saturation Region Triode Region arriers in electrically induced n-channel are electrons

13 arriers in MOS Transistors onsider p-channel MOSFT Saturation Region hannel Triode Region

14 arriers in MOS Transistors onsider p-channel MOSFT Saturation Region Triode Region arriers in electrically induced p-channel are holes

15 arriers in MOS Transistors arriers in channel of MOS transistors are Majority carriers

16 ipolar Transistors npn stack pnp stack ipolar Devices Show asic Symmetry lectrical Properties not Symmetric Designation of and critical npn transistor pnp transistor With proper doping and device sizing these form ipolar Transistors

17 ipolar Transistors npn transistor pnp transistor n-channel MOSFT p-channel MOSFT In contrast to a MOSFT which has 4 terminals, a JT only has 3 terminals

18 ipolar Operation onsider npn transistor Forward Active Operation npn stack Under forward bias current flow into base and out of emitter urrent flow is governed by the diode equation arriers in emitter are electrons (majority carriers) When electrons pass into the base they become minority carriers Quickly recombine with holes to create holes base region Dominant current flow in base is holes (majority carriers)

19 ipolar Operation onsider npn transistor Forward Active Operation npn stack Under forward bias and reverse bias current flows into base region arriers in emitter are electrons (majority carriers) When electrons pass into the base they become minority carriers When minority carriers are present in the base they can be attracted to collector

20 ipolar Operation onsider npn transistor Forward Active Operation npn stack F 1 If no force on electron is applied by collector, electron will contribute to base current

21 ipolar Operation onsider npn transistor Forward Active Operation npn stack F 1 If no force on electron is applied by collector, electron will contribute to base current lectron will recombine with a hole so dominant current flow in base will be by majority carriers

22 ipolar Operation onsider npn transistor Forward Active Operation npn stack F 1 F 2 When minority carriers are present in the base they can be attracted to collector with reverse-bias of junction and can move across junction

23 ipolar Operation onsider npn transistor Forward Active Operation npn stack F 1 F 2 When minority carriers are present in the base they can be attracted to collector with reverse-bias of junction and can move across junction Will contribute to collector current flow as majority carriers

24 ipolar Operation onsider npn transistor Forward Active Operation npn stack F 1 So, what will happen? F 2

25 Size and thickness of base region and relative doping levels will play key role in percent of minority carriers injected into base contributing to collector current ipolar Operation onsider npn transistor Forward Active Operation npn stack F 1 So, what will happen? F 2 Some will recombine with holes and contribute to base current and some will be attracted across junction and contribute to collector

26 ipolar Operation onsider npn transistor Forward Active operation npn stack Under forward bias and reverse bias current flows into base region arriers in emitter are electrons (majority carriers) When electrons pass into the base they become minority carriers When minority carriers are present in the base they can be attracted to collector Minority carriers either recombine with holes and contribute to base current or are attracted into collector region and contribute to collector current

27 ipolar Operation onsider npn transistor - Forward Active Operation npn stack Under forward bias and reverse bias current flows into base region fficiency at which minority carriers injected into base region and contribute to collector current is termed α α is always less than 1 but for a good transistor, it is very close to 1 For good transistors.99 < α <.999 Making the base region very thin makes α large

28 ipolar Transistors npn stack pnp stack principle of operation of pnp and npn transistors are the same minority carriers in base of pnp are holes npn usually have modestly superior properties because mobility of electrons is larger than mobility of holes

29 ipolar Operation onsider npn transistor Forward Active Operation npn stack In contrast to MOS devices where current flow in channel is by majority carriers, current flow in the critical base region of bipolar transistors is by minority carriers

30 ipolar Operation onsider npn transistor Forward Active Operation I I I I β is typically very large I α I I I I I 1 defn 1 I I often 50<β<999

31 ipolar Operation onsider npn transistor Forward Active Operation I I I I β is typically very large I ipolar transistor can be thought of a current amplifier with a large current gain In contrast, MOS transistor is inherently a tramsconductance amplifier urrent flow in base is governed by the diode equation ollector current thus varies exponentially with I I I ~ S e I ~ S e t t

32 Preliminary omparison of MOSFT and JT (Saturation vs Forward Active) S G D n-channel MOSFT D G I D npn JT I S OXW I 2L 2 D GS TH I I ~ S The JT I/O relationship is exponential in contrast to square-law for MOSFT Provides a very large gain for the JT (assuming input is voltage and output is current) This property is very useful for many applications e t

33 ipolar Models Simple dc Model I I Following convention, pick I and I as dependent variables and and as independent variables

34 Simple dc model onsider npn transistor Forward Active Operation Summary: I I t I ~ S e I ~ S kt q e t t I I This has the properties we are looking for but the variables we used in introducing these relationships are not standard It can be shown that I ~ S is proportional to the emitter area A J S I ~ 1 S J S A Define by and substitute this into the above equations

35 Simple dc model npn transistor Forward Active Operation I I t I ~ S e I ~ S kt q e t t I J I J t kt q S S A β A e e t t J S is termed the saturation current density Process Parameters : J S,β Design Parameters: A nvironmental parameters and physical constants: k,t,q At room temperature, t is around 26m J S very small around.25fa/u 2

36 Transfer haracteristics npn transistor Forward Active Operation J S =.25fA/u 2 A =400u I (ma) close to 0.6 for a two decade change in I around 1mA

37 Transfer haracteristics npn transistor Forward Active Operation J S =.25fA/u 2 A =400u I (ma) close to 0.6 for a four decade change in I around 1mA

38 Simple dc model npn transistor Forward Active Operation Output haracteristics 300 I Id or I ds I J S A e t

39 Simple dc model etter Model of Output haracteristics 300 I Id or I ds

40 Simple dc model Typical Output haracteristics 300 I Saturation Forward Active Id 150 or I ds utoff Forward Active region of JT is analogous to Saturation region of MOSFT Saturation region of JT is analogous to Triode region of MOSFT

41 nd of Lecture 19