MicroNoteSeries 701

PIN DiodeFundamentalsA PIN diode is a semiconductordevice that operates as a variableresistor at RF and microwavefrequencies. The resistance value ofthe PIN diode is determined only bythe forward biased dc current. Inswitch and attenuator applications,the PIN diode should ideally controlthe RF signal level withoutintroducing distortion which mightchange the shape of the RF signal.An important additional feature of thePIN diode is its ability to control largeRF signals while using much smallerlevels of dc excitation.

A model of a Microsemi PIN diodechip is shown in Figure 1. The chip isprepared by starting with a wafer ofalmost intrinsically pure silicon,having high resistivity and longlifetime. A P-region is then diffusedinto one diode surface and an N-region is diffused into the othersurface. The resulting intrinsic or I-region thickness (W) is a function ofthe thickness of the original siliconwafer, while the area of the chip (A)depends upon how many smallsections are defined from the originalwafer.

The performance of the PIN diodeprimarily depends on chip geometryand the nature of the semiconductormaterial in the finished diode,particularly in the I-region.Characteristics of Microsemi PIN

diodes are controlled thickness I-regions having long carrier lifetimesand very high resistivity. Thesecharacteristics enhance the ability tocontrol RF signals with a minimum ofdistortion while requiring low dcsupply.

Forward Biased PIN Diodes

When a PIN diode is forward biased,holes and electrons are injected fromthe P and N regions into the I-region.These charges do not recombineimmediately. Instead, a finite quantityof charge always remains stored andresults in a lowering of the resistivity ofthe I-region. The quantity of storedcharge, Q, depends on therecombination time, (the carrierlifetime), and the forward bias current,IF, as follows (Equation 1):

Q = IF [Coulombs]

The resistance of the I-region underforward bias, RS is inverselyproportional to Q and may beexpressed as (Equation 2):

RS = [Ohms]

where: W = I-region widthN = electron mobility

p = hole mobility

Combining equations 1 and 2, theexpression for RS as an inversefunction of current is shown as(Equation 3):

RS = [Ohms]

This equation is independent of area.In the real world the RS is slightlydependent upon area because theeffective lifetime varies with area andthickness due to edge recombinationeffects. Typically, PIN diodes display aresistance characteristic consistent

with this model as shown in Figure 2.Resistance of the order of -0.1 at 1Amp forward bias increasing to about10,000 at 1A forward biasrepresents a realistic range for aMicrosemi PIN diode.

Figure 2 RS vs Forward Current (UM4000 PIN Diode)

The maximum forward resistance,RS(max), of a PIN diode is generallyspecified at 100 mA forward biascurrent. For some PIN diodes,Microsemi specifies not only theRS(max) but also the RS(min) at alower forward bias current (10A).These specifications ensure a widerange of diode resistance which isparticularly important in attenuatorapplications. At the lower frequenciesRS is not constant but increases asthe frequency is lowered. The normalPIN diodes which are designed tooperate in RF and microwavefrequencies exhibit this increase in RSin the 1-10 MHz range. A properlydesigned PIN will maintain a constantRS well into the 10 KHz region. Goodexamples for this frequency range arethe UM2100 series devices.

The results obtained from Equation 3are valid over an extremely broadfrequency range when Microsemi PINdiodes are used in a circuit. Thepractical low resistance limitationsresult from package parasiticinductances and junction contactresistances, both of which areminimized in the construction ofMicrosemi diodes. The highresistance range of PIN diodes isusually limited by the effect of thediode capacitance, CT. To realize the

Figure 1 -PIN Diode Chip Outline

by Bill Doherty, Microsemi Watertown

PIN

W

RS

= D

iod

e R

esis

tan

ce (

oh

ms)

IF = Forward Bias Current (mA)

100

10

1

0.1.001 .01 .1 1.0

W2

(N + p) Q

W2

(N + p) IF

maximum dynamic range of the PINdiode at high frequencies, this diodereactance may have to be tuned out.

It should be noted that skin effect ismuch less pronounced in relativelypoor conductors such as silicon, thanwith good metallic conductors. This isdue to the fact that the skin depth isproportional to the square root of theresistivity of the conducting material.Thus, RF signals penetrate deeply intothe semiconductor and skin effect: isnot a significant factor in PIN diodesbelow X-Band frequencies.

At dc and very low frequencies, the PINdiode is similar to a PN diode; thediode resistance is described by thedynamic resistance of the I-Vcharacteristics at any quiescent biaspoint. The dc dynamic resistance pointis not, however, valid in PIN diodes atfrequencies above which the period isshorter than the transit time of the I-region. The frequency at which thisoccurs, fT, is called transit timefrequency and may be considered thelower frequency limit for which Equation3 applies. This lower frequency limit isprimarily a function of W, the I-regionthickness and can be expressed as(Equation 4):

f= [MHz]

where W is the I-region thickness inmicrons. For Microsemi PIN diodes,this low frequency limit ranges fromapproximately 5 KHz for the thickestdiodes (UM2100 and UM2300 series)to approximately 1 MHz for the thinnestdiodes (UM6200, UM7200).

Associated with the diodecapacitance is a parallel resistance,Rp, which represents the netdissipative resistance in the reversebiased diode. At low reversevoltages, the finite resistivity of the I-region results in a lossy I-regioncapacitance. As the reverse voltageis increased, carriers are depletedfrom the I-region resulting in anessentially lossless silicon capacitor.The reverse parallel resistance of thePIN diode, Rp, is also affected by anyseries resistance in thesemiconductor or diode contacts.

Equivalent Circuits

Because of the unique construction ofMicrosemi diodes, the RF equivalentcircuits are generally different andactually more simplified than thoseassociated with PIN diodesconstructed using conventionaltechniques. These equivalent circuitsfor Microsemi diodes are illustrated inFigure 3. Because of the absence ofsmall wires or ribbons, the packagecapacitance is directly in parallel withthe PIN chip, there is virtually nointernal package inductance toconsider as is the case withconventional PIN diodes. The fullfaced bond achieved between thesilicon chip and the metallic pins,combined with the relatively largechip area, result in negligible contactresistance. Hence, the residualseries resistance in conventionaldiodes, is for all practical purposes,nonexistent in Microsemi PIN diodes.Any self-inductance presented by theMicrosemi diode is external to thediodes capacitance and is similar tothat of a conducting cylinder havingthe same mechanical outline as thediode chip and pins. Calculationsusing self-inductance equationsshow the Style A package inductanceto be on the order of 0.10 nH for allMicrosemi PIN diode types.Additional self-inductance isintroduced by any lead attached to theStyle A package. Thus at frequenciesbelow 1 GHz, Microsemi packageparasitic effects are usuallynegligible. At higher frequencies, theoverall dimensions and materials ofthe diode package should beconsidered in both the diodeselection and RF circuit design.

Series 701-PIN Diodes

Reverse Biased PIN Diodes

At high RF frequencies when a PINdiode is at zero or reverse bias, itappears as a parallel plate capacitor,essentially independent of reversevoltage, having a value of(Equation #5):

C = [Farads]

where: = silicon dielectric constantA = junction areaW = I-region thickness

The lowest frequencies at which thiseffect begins to predominate isrelated to the dielectric relaxationfrequency of the I-region, f

, which

may be computed as (Equation #6):

f= [Hz]

where: = I-region resistivity

For Microsemi PIN diodes, thisdielectric relaxation frequency occursbelow 20 MHz and the total packagedcapacitance, CT, is specified for mostMicrosemi diodes when zero biasedat 100 MHz. Additional data issupplied in the form of typical curvesshowing the capacitance variation asa function of reverse bias at lowerfrequencies.

At frequencies much lower than f,

the capacitance characteristic of thePIN diode resembles a varactordiode. Because of the frequencylimitations of common testequipment, capacitancemeasurements are generally madeat 1 MHz. At this frequency the totalcapacitance, CT, is determined byapplying a sufficiently large reversevoltage which fully depletes the I-region of carriers.

1300W2

Figure 3 - PIN Diode Equivalent Circuits

Metal Pin

Glass

IntrinsicLayer

L

RS

(a) Cross Section of Basic Diode (b) Forward Bias (c) Reverse Bias

AW

I2pi

CT Rp