3/25/02

Heisenberg UncertaintyGroup

Diodes:Standard Bipolar

Diodes:CMOS andBiCMOS Processes

Summary

Matching Diodes

References

3/25/02 Fig 1: Forward Bias of PN Junction

+ _

+ + +p type

+n type

_ _ _ _------

++++++

Large majoritycurrent flow

Small minoritycurrent flow

Ohmiccontact

Ohmiccontact

pn junction

Vf

EConvention currentor hole motion

1 k�

If

3/25/02 Fig 2: Reverse Bias of PN Junction

+_

+ + +p type

+n type

_ _ _ _------

++++++

small majoritycurrent flow

minority currentflow

Ohmiccontact

Ohmiccontact

Larger space-chargeregion due to VR

VR

EConvention currentor hole motion

1 k�+_IS

3/25/02 Fig 3: Diode Volt Amp Characteristic

iD

vD

Reverse-biasregion

Forward-biasregion

Reverse-breakdown

region

3/25/02 Fig 4: Diode Connected Transistor

Anode

Cathode

Diode Connected TransistorSchematic

Anode

Cathode

Diode Symbol

3/25/02Fig 5: Layout of Diode Connected Transistor

Anode contactEmitter

NBL

Base

Tank

Emitter

NBL

P-subs

BOIA C

Base

EmitterEmitter

Iso

3/25/02

Fig 6: Breakdown Due to Zener vs Avalanche

3/25/02 Fig 7: Base-Emitter Zener Layout

Cathode

Anode

Tank contact

Cathode(emitter)

Anode(base)

3/25/02 Fig 8: Buffered Zener Clamp

Pad

Q1

D1

R1

3/25/02Fig 9: Non-isolated Base-Emitter Zener Diode

3/25/02

Fig 10: Layout of Emitter-In-Isolation Zener Diode

3/25/02Fig 11: Buried Zener Diode Using Deep P+ Diffusion

3/25/02Fig 12: Buried Zener Diode Using Deep P+ Diffusion

3/25/02

Fig 13: Schottky Diode Forward Voltages ForVarious Materials

Material N-type P-typeSilicon Silicon

Aluminum 0.54V 0.40V

Gold 0.62V 0.16V

Molybdenum 0.50V 0.24V

Palladium silicide (Pd2Si) 0.57V

Platinum silicide (PtSi) 0.66V

Titanium silicide (TiSi2) 0.42V

3/25/02 Fig 14: Layout of Field-Plated Schottky

3/25/02Fig 15: Layout of Base Guard-Ring Schottky

3/25/02

Fig 16: Schottky Clamped NPN Transistors

3/25/02

Fig 17: Simplified Schottky Diode Model

3/25/02

Diodes In CMOS andBICMOS PROCESSES

3/25/02

�

– N-well CMOS• (i) NSD/P-epi diode• (ii) N-well/P-epi diode

3/25/02

� PSD/N-well junction or NSD/P-epi junction inCMOS process gives the ZENER DIODE

� CMOS process between noble silicide and N-well gives SCHOTTKY diode

� CMOS process also fabricate the P-type andN-type gate polysilicon

3/25/02

� In this process the NSD/P-epi diode and N-well/P-epi diodes are fabricated

� Both NSD/P-epi and N-well/P-epi diodesconnect to the substrate

� These diode can be forward biased by pellingtheir cathode below the substrate

� These diodes need a negative power supply.– This leads to latch up hazard due to minority

carrier injection into the substrate

3/25/02

� PSD/N-well junction does not conduct as longas the junction is reverse biased.

� Similarly NSD/P-epi junction does not injectminority carrier into substrate in the reversedbiased state

� The Zener diode breakdown voltages alwaysexceed the operating voltage

3/25/02

3/25/02

� This diode is based on the metal-semiconductor contact.– In the P-N junction, the potential barrier appears

due to the difference of the work functions of p-type and N-type semiconductor

– The most important difference is that the Schottkydiodes operate with s single type of carrier(majority carrier)

– The absence of minority carriers means that thestore-charge effect does not exist.

3/25/02

3/25/02

3/25/02

� Forced in a CMOS process between a noblesilicide and the N-well

� The reversed breakdown voltage is limited bythe avalanche voltage of the PSD/N-welljunction

� Schottky diodes in CMOS process haverelatively high series resistance– This is due to the absence of NBL and deep N+

3/25/02

� The resistance in a Schottky diode can beminimized by– (i) elongating the Schottky contact– (ii) Also by surrounding it on all side with cathode

contactsThis diodes are useful in low-currentapplications.It is also very good for fast switchingapplications.

3/25/02

� (i) The moat geometrycoded across the contactopening layer that thecontact resides over thinoxide

� A ring of PSD diffusionplaced around the edge ofthe contact opening servesas a field relief guard ring

3/25/02

� These diodes fabricated in the CMOS processexhibit the following qualities– (i) Have high leakage current than ordinary diodes.– (ii) Gain structure of polysilicon charge with

temperature• The above factors have prevented widespread use of the

poly diodes

3/25/02

� Analog BiCMOS process can fabricate most ofthe diode available in standard bipolar.

� BiCMOS Zener diodes exhibits a higherbreakdown voltage (7V to 10V) than standardbipolar diodes.– This is due to the use of a more lightly dope base

region.

3/25/02

� BiCMOS Zener diodes are more fragile thanBiCMOS standard bipolar diodes– This due to the shallow region of the diffusions in

the modern BiCMOS process which reduces theconduction volume in the zeners

3/25/02

� BiCMOS PROCESS WHICH USE Platinumor Palladium.

� Structure is similar to its bipolar counterparts� A moat geometry coded around the anode

contact take the place of Schottky contactgeometry

3/25/02

3/25/02

� BiCMOS diodes are used as replacements fordiode-connected transistors.

3/25/02

Matching Diodes

3/25/02

� Matching Diodes have three categories :-- PN Junction Diode- Zener Diode- Schottky Diode

� Different diodes obey different principlesof operation.

� Same types of Diodes can match ifproperly constructed.

3/25/02

� Most Junction diodes used in bipolar &BICMOS processes are actually diodeconnected transistors.

� Unique feature found in diode connected isthe merged collector-base contact.

3/25/02

CMOS Process- Produces PN Junction Diodes.- N-well CMOS process can construct a PSD/N-welldiode.- P-well CMOS process can construct a NSD/P-well

diode.� The beta of these bipolar transistors can range from

0.1 to more than 10. Typical value is 2.

3/25/02

� The matching of junction diodes thereforedepends on the matching of parasitic bipolarbetas and well resistances as well asforward voltages

� Layout of CMOS PN junction diode isidentical to CMOS substrate transistor. Thisdevice can be viewed either as a PNjunction diode with parasitic bipolartransistor or as a bipolar transistor with lowgain which resembles a diode.

� By maximizing beta and minimizing wellresistance we obtain matching.

3/25/02

� A device with a small junction-area-to-periphery ratio has better matching than theone having large are-to-periphery ratio.

� Matched devices should be interdigitatedwith one another to produce a common-centroid array.

3/25/02

� Matching improves when high gainparasitic bipolar transistors are incorporatedbecause high gain reduces the currents thatflow through the well resistances.

� Matching can also improve at low currentdensities because of the voltage drop acrossthe well resistances decreases

3/25/02

� Zener diodes are difficult to match becauseof their breakdown voltages depend sostrongly on electric field intensity.

� Any curvature in the junction geometryintensifies the electric field and produce alocalized reduction in breakdown voltage.

� Localized breakdown degrades matchingbecause it reduces the effective area of thejunction.

3/25/02

�Matched Zeners should employcircular junction geometries to avoidcorner curvature.

� Matched Zeners usually employlarge circular geometries to minimizerandom variations and to eliminateedge effects.

� Both Anode and Cathode contactsshould be circularly symmetric.

3/25/02

3/25/02

� Zener Diodes are constructed by thefollowing guidelines:-- Shape of anode contacts resembles a four-leaf clover- Four individual Zeners occupy a commontank to minimize the separation betweenthem.- The tank isolates the Zeners from thesubstrate.

3/25/02

� Above process may not provide precisematching between surface Zeners becausethey are susceptible to Zener walkout- Buried Zener provide much bettermatching because they do not exhibit Zenerwalkout

3/25/02

� Schottky diode are inherently difficult tomatch.

� Factors that Schottky’s barrier depends on:-- metal composition- silicon doping- age effects- annealing conditions- presence or absence of surfacecontaminants.

3/25/02

� Most of the above factors are difficult tocontrol therefore this diode exhibits largermismatches than diode-connectedtransistors.

� Matched Schottky should alwaysincorporate the following:-- diffused guard rings- contact opening should have large area-to-periphery ratio to minimize mismatches dueto linewidth variations.

3/25/02

� Should incorporate NBL and deep N+. Ifthis is used then several matched Schottkydiodes can reside in the same tank if notthen each should have its own tank and allof which should have the same dimensions.

� Schottky diodes are very sensitive toThermal Gradients. They should always beinter-digitated.

� Schottky will not reliably match PNjunction diodes or bipolar transistors.

3/25/02

Semiconductor Fundamentals by RobertCoughlin and Frederick Driscoll,

Prentice Hall 1976 The Art of Analog Layout by Alan

Hastings,Prentice Hall 2001

Electronics by Allan Hambley, PrenticeHall 2000 2nd Edition

Devices For Integrated Circuits byCasey, Craig H. John Wiley and Sons,Inc.