330 Lecture 18 haracteristics of Finer Feature Size Processes ipolar Process
How does the inverter delay compare between a 0.5u process and a 0.13u process? DD IN OUT IN OUT SS
How does the inverter delay compare between a 0.5u process and a 0.13u process? Assume n-channel and p-channel devices are minimum sized IN OUT
asic Devices and Device Models Resistor Diode apacitor MOSFT JT
ipolar Junction Transistors Operation Modeling
arriers in Doped Semiconductors n-type p-type
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
arriers in Doped Semiconductors Majority arriers Minority arriers n-type electrons holes p-type holes electrons
arriers in MOS Transistors onsider n-channel MOSFT Saturation Region hannel Triode Region
arriers in MOS Transistors onsider n-channel MOSFT Saturation Region Triode Region arriers in electrically induced n-channel are electrons
arriers in MOS Transistors onsider p-channel MOSFT Saturation Region hannel Triode Region
arriers in MOS Transistors onsider p-channel MOSFT Saturation Region Triode Region arriers in electrically induced p-channel are holes
arriers in MOS Transistors arriers in channel of MOS transistors are Majority carriers
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
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
ipolar Operation onsider npn transistor 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)
ipolar Operation onsider npn transistor 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
ipolar Operation onsider npn transistor npn stack F 1 If no force on electron is applied by collector, electron will contribute to base current
ipolar Operation onsider npn transistor 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
ipolar Operation onsider npn transistor npn stack F 1 F2 When minority carriers are present in the base they can be attracted to collector with reverse-bias of junction and can move across junction
ipolar Operation onsider npn transistor npn stack F 1 F2 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
ipolar Operation onsider npn transistor npn stack F 1 So, what will happen? F2
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 npn stack F 1 So, what will happen? F2 Some will recombine with holes and contribute to base current and some will be attracted across junction and contribute to collector
ipolar Operation onsider npn transistor 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
ipolar Operation onsider npn transistor 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
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
ipolar Operation onsider npn transistor 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
ipolar Operation I I I I I β is typically very large I α I I I I 1 defn 1 I I often 50<β<999
I I ipolar 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
I I ipolar Operation I I I I I β is typically very large ollector current thus varies exponentially with I I ~ S e t This exponential relationship (in contrast to the square-law relationship for the MOSFT) provides a very large gain for the JT and this property is very useful for many applications!!
ipolar Models Simple dc Model I I following convention, pick I and I as dependent variables and and as independent variables
Simple dc model 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 Define I ~ 1 S and substitute this into the above equations J S A
Simple dc model 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
Transfer haracteristics J S =.25fA/u 2 A =400u 2 0.70 0.60 0.50 0.40 0.30 0.20 0.10 0.00 0.1 1 10 I (ma) close to 0.6 for a two decade change in I around 1mA
Transfer haracteristics J S =.25fA/u 2 A =400u 2 0.75 0.70 0.65 0.60 0.55 0.50 0.45 0.001 0.01 0.1 1 10 100 1000 I (ma) close to 0.6 for a four decade change in I around 1mA
Simple dc model Output haracteristics 300 I 250 200 Id 150 100 50 or I 0 0 1 2 3 4 5 ds I J S A e t
Simple dc model etter Model of Output haracteristics 300 I 250 200 Id 150 100 50 or I 0 0 1 2 3 4 5 ds
Simple dc model Typical Output haracteristics 300 I 250 200 Saturation Forward Active Id 150 or I 100 50 0 0 1 2 3 4 5 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
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