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Digital Integrated Digital Integrated CircuitsCircuitsA Design PerspectiveA Design Perspective

The DevicesThe Devices

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The diodes are rarely explicitly used in modern integrated circuitsHowever, a MOS transistor contains at least two reverse biased diodes that impact the behavior of the device. In particular, the voltage dependent capacitances contributed by these parasitic elements play an important role in the switching behavior of theMOS digital gate.Typically, diodes are also often used as Electro-Static Discharge (ESD) Protection devices to protect chip inputs and outputs.

Therefore, a review of diode fundamentals is desirable (especially reverse bias conditions)

The DiodeThe Diode

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The DiodeThe Diode

p

n

B A SiO2Al

Cross-section of pn-junction in an IC process

n

p

A

B

Al

One-dimensionalrepresentation

A

B

diode symbol

The pn-junction is the simplest of the semiconductor devices

• It consists of two regions of p- and n- type materials• Both the regions are separated by a thin transition region – such a device is called a step or abrupt junction device

• p-type material is doped with acceptor impurities (e.g., boron). Thus, holes are the dominant majority carriers.

• n-type material is doped with donor impurities (e.g., phosphorus). Thus, electrons are the dominant majority carriers.

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The DiodeThe DiodeBringing the p- and n-type materials together causes a large concentration gradient at the boundary.Electron (hole) concentration changes from a high value in the n-type (p-type) material to a very small value in the p-type (n-type) materialThis gradient causes:

electrons (holes) to diffuse from n (p) type material to p (n) type materialmovement of mobile electrons (holes) leaving behind immobile donor (acceptor) ions in n (p) type material. The p (n) -type material is negatively (positively) charged in the vicinity of the pn-boundry.The region close to the junction from where all mobile carriers have left is called depletion (space charge) region.The remaining immobile ions create an electric field across the boundary causing drift currentUnder equilibrium condition: drift currents are equal and opposite to diffusion currents (zero net flow)

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Depletion RegionDepletion Regionhole diffusion

electron diffusion

p n

hole driftelectron drift

ChargeDensity

Distancex+

-

ElectricalxField

x

PotentialV

ξ

ρ

W2-W1

ψ0

(a) Current flow.

(b) Charge density.

(c) Electric field.

(d) Electrostaticpotential.

P is more heavily doped than n (NA>ND), where NA and ND are the acceptor and donor concentrations

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Under zero-bias condition, there exists a voltage across the junction, called built-in potentialThis potential (Φ0 ) has the value

The quantity ni is the intrinsic carrier concentration in a pure sample of the semiconductor. It equals approximately 1.5 x 1010 cm-3 at 300K for silicon

Φ=Φ 20 ln

i

DAT n

NN

Depletion RegionDepletion Region

mVqkT

T 26==Φ at 300K

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Diode: Static BehaviorDiode: Static BehaviorAssume forward biasing – potential of p is raised w.r.t. n-region. This applied potential lowers the potential barrier.

Thus, the flow of mobile carriers across the junction increases as the diffusion current dominates the drift current.

These carriers traverse the depletion region and become minority carriers.

Eventually, these minority carriers recombine with the majority carriers.

The net result is a current flowing through the diode from the p-region to the n-region, and the diode is said to be forward-biased.

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Forward BiasForward Bias

Typically avoided in Digital ICs

x

pn0

np0

-W1 W20p n

(W2)

n-regionp-region

Lp

diffusion

Met

al c

onta

ct to

p-r

egio

n

Met

al c

onta

ct to

n-r

egio

n

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Reverse ConductionReverse ConductionPotential of p is lowered w.r.t. n. This results in a reduction in diffusion current, and the drift current becomes dominant.

Thus current flows from the n to p regions. Since the minority carriers is very small, this drift component is very small.

Thus, it is fair to say that the diode operates as a non-conducting or blocking device when reverse-biased (i.e., one way conductor).

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Reverse BiasReverse Bias

x

pn0

np0

-W1 W20n-regionp-region

diffusion

The Dominant Operation Mode

Met

al c

onta

ct to

p-r

egio

n

Met

al c

onta

ct to

n-r

egio

n

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Diode IDiode I--V CharacteristicsV Characteristics

Deviation due to recombination

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Diode: Static BehaviorDiode: Static BehaviorIdeal diode equation

Is is the saturation current (function in diode area and doping levels)

When VD<<0, ID=-IS

)1( / −= ΦTDVSD eII