PIN Diodes

8/11/2019 PIN Diodes

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Jim Stiles The Univ. of Kansas Dept. of EECS

PIN Diodes

Q: Just how do we makeswitches and voltage controlled

attenuators?

A: Typically, they are constructed withPIN diodes.

A PIN diode is simply ap-njunction diode that is designed to

have a very small junction capacitance (0.01 to 0.1 pf).

Sort of theoppositeof the varactordiode!

To see why this is important, recall diode small signal analysis

from your first electronics course.

In small signal analysis, the totaldiode voltage consists of a D.C.

bias voltage(VD) and a small,time-varying signal ( dv ):

) )( (D dD vV tv t = +

For radio engineering applications, the small signal is a

microwavesignal!!! I.E.,:

) )( (D RD F tv vVt = +

Thus, we know that the diode currentiD is:

( )exp 1RFDsD

T

V v ti I

nV

+=


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Since vRFis very small, we can approximatethis diode current

iD(vD) using a Taylor Series expansion around vD=VD:

( )

( ) ( ) ( )

1 ( )

v VD D

v VD D

D

TD

T

D D

RFD D D D D

VnVV

nV SS RF

T

i v

i v i v v t v

I eI e v t

nV

=

=

+

= +

We recognize that:

D1 D.C. Bias Current ID

T

VnV

SI e

=

and thus we can write our small-signal approximationas:

( )( )

( )

sDRFD D

T

RFD

d

I Ii I v t

nVv t

Ir

+= +

= +

where we have defined the diode small-signal resistancerd as:

T

dsD

nV

r I I= +

The diode small-signal resistance is also oftenreferred to as

thejunction resistance Rjor the seriesresistanceRs.


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We can further conclude that the total diode current iDis the

sum of the D.C. bias current ID, and the small-signal current

iRF(t), where:( )

( )RF

RFd

v t

i t r=

Just like Ohms Law!

To a small (i.e., low power) microwave signal, a diode looks like

a resistor.

Moreover, we can controland modifythe resistance of thediode bychangingthe D.C. bias.

Sort of a voltage-controlled resistor!

For example, if we put the diode into forwardbias ( D TV nV>> ),

the bias current IDwill be positive and big, thus the junction

resistance will be very small(e.g., rd = a few ohms).

A forwardbiased diode is very nearly a microwave

short circuit!

Iget it! If wereverse

bias our diode, such thatVD


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Not so fast! The small-signal resistanceof a reverse

biased diode is in fact very large. BUT, we must

also consider the junction capacitance Cj!

Recall that in reversebias, the junction capacitance of a diodecan be significant, and in fact generally increasesas the bias

voltage becomes more negative!

As a result, a good microwave circuit model of a diode includes

both the series resistance and junction capacitance:

1d

Djd

rZ

j r C=

+

For forwardbias, where rd is very small, we find that diode

impedance ZDis approximately equal to this smallseries

resistance( D dZ r )a shortcircuit (approximately):

rdCj

0fb

D dZ r Z


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For reverse bias, where rd is very large, we find that diode

impedance ZDis approximately equal to that of the junction

capacitanceCj:

For low-frequencies (e.g., kHz), this impedance will be typically

beverylarge and thus the diode can be approximate as an open

circuit.

However, at microwave frequencies (where is very large) the

reverse bias impedance rbDZ may notbe particularly large, and

thus the reverse biased diode cannotbe considered an open

circuit.

In order for the impedance 1rb

jDZ j C=

to be very largeatmicrowavefrequencies, the junction capacitance Cjmust be

very, verysmall.

0

1rbD

j

Z Zj C

=

PINdiodes! Ibetthats

why we use PINdiodes!

Thatsexactly why! A PIN diode is approximately

a (bias) voltage controlled resistor at microwave

frequencies. We can select any value ofrdfrom a

shortto an open.

As a result, we can make many interesting devices!

PIN Diodes

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