134 PHILlPS TECHNICAL REVIEW VOLUME 30

The MOS tetrodeT.Okumura

In the various designs of MOS transistor that have so far been produced the feedback capac-itance is relatively high. This means that they are less suitable for amplifying signals'of very high frequency, The article below describes a related circuit element which can beusedforfrequencies up to 200 MHz and higher. This new device has been developed at theTakatsuki-Osaka laboratory of the M atsushita Electronics Corporation (MEC) a com-pany ownedjointly by the Japanese company Matsushita Electronic Industries (MEI) andthe Philips Group of Companies, and active in various fields of common interest.

Introduetion

The most advanced of the various field-effect tran-sistors known at present is the metal-oxide-semicon-ductor (MOS) transistor. MOS transistors are relativelysimple to manufacture and they combine the advantagesof high input impedance and characteristics that givevery little cross-modulation. However, they are notvery suitable for amplification at frequencies aboveabout 100 MHz because of their relatively high feed-back ("Miller-effect") capacitance.The same problem is encountered in thermionic

devices in the triode valve. The usual solution adoptedthere has been to replace the triode either by a pentodeor by a pair of riodes connected in a "cascode" circuit.In solid-state electronics an analogous arrangement tothe cascode has been made up from two MOS transis-tors, and this gives a feedback capacitance much lessthan that of a single MOS transistor. At MatsushitaElectronics Corporation we have been looking intothe possibility of producing a practical circuit elementof even better performance by making a cascode oftwo MOS transistors as a solid circuit [1]. Our invest-igations have been successful and their practical end-result is a new circuit element, the MOS tetrode [2]. Thisnew device does indeed have the desired very smallfeedback capacitance, and it also surpasses the ordi-nary MOS transistor on two other counts: in circuitswith automatic gain control (AGC)-readily producedwith the MOS tetrode, as we shall see below - thereismuch less cross-modulation with the tetrode, and thereis much less variation of the input capacitance withinput voltage. The characteristics are also very stable.

In a valve cascode circuit (jig. 1) the operating conditions oftwo triodes are such that' the grid current is very small, and

-Dr. T. Okumura, is with the Reseárch Laboratory of MatsushitaElectronles Corporation, Takatsuki, Osaka, Japan,

the anode current in both valves is about the same. If the grid G2is decoupled to prevent the appearance of an a.c. voltage, thenthe circuit behaves like a triode of very high internal impedance(anode impedance) Re and very high amplification factor ,."t;

the transconductance gm t is about the same as that of one ofthetriodes. In this case, which corresponds to the situation in theMOS tetrode, we have [3]:

(1)

(2)""t = /-111-'2 + 1-'1 R; ""1""2and

, . . (3)

from which these statements can readily be verified.

Fig. 1. A valve cascode circuitusing triodes. The circuit canbe adjusted in such a way thatthe characteristic curves areanalogous to those of a singletriode. The internal impedanceand the current amplificationfactor are however much higher,and the feedback from anode tocontrol grid is much less. Thetransconductance is about thesame.

Fig. 2 shows a diagram of the cross-section of oneof our designs of a MOS tetrode, next to a section of anordinary MOS (triode) transistor. The diagram showsthat the tetrode has two control electrodes (gates) GIand G2. Between the two gates there is an N-type regionwhich' is surrounded by the P-type silicon of the sub-strate, This N-type region is the "island", which servesas the drain electrode for the triode formed by S, GI

1969, No. 5 MOS TETRODE 135

D~Gd ~~ ."

a

b

Fig. 2. a) Diagrammatic cross-section of a MOS transistor(MOS triode). S source. D drain. C metal gate electrode, insulat-ed from the other parts of the device by a thin oxide layer O.Sand D are of the same semiconductor material as the rest ofthe substrate (shaded) but are of the opposite conduction type.b) Diagram of the cross-section of a MOS tetrode on a P-typesilicon substrate. There is a third N-type region, the island J,between Sand D. There are now two gate electrodes, Cl and G2.The device can be considered to be an integrated cascode circuitof two MOS triodes.

Each of the two circuit elements has a substrate contact whichis usually connected to the source. The standard symbolsfor use in circuit diagrams are shown on the left. The arrowrepresents the source.

on Vg, and the same is therefore true for the current Idwhich flows through it from S to D.If curves of the variation of this current with the

voltage Vd - Vs are plotted for constant Vg, it is foundthat the first part of each curve is parabolic and theremainder is very nearly a horizontal straight line(fig. 3; Vs is assumed to be zero, as usual). The maxi-mum value Id sat of the current is the ordinate value ofthe straight-line region and occurs consequently in alarge interval of Vd values (saturation). The value ofId sat is dependent on Vg (fig. 3b). In the "saturated"region Id and Vg are related at constant Vd by

lel sat = 1- fJ(Vg - Vth)2 (4)

to a very good approximation. Here fJ is equal toCfl-JI2, where C is the capacity between the gateelectrode and the channel, fl- the mobility of the major-ity carriers (the electrons in this case) and I is the lengthof the channel. Vth is the threshold voltage, i.e. thevalue of Vg for which Id becomes zero. The variationof the current with Vg is thus quadratic (fig. 3c). Itcan be shown directly from (4) that gm sat, the valueof the transconductance under saturation conditions,

Q

°o~--~~---------vth -~

bfig. 3. a) The Id- Vd characteristic for a MOS triode for a fixed value of Vg. The curve canbe fairly accurately represented by a combination of a parabolic region and a nearly horizontalstraight line. b) Set of J,,- Vd characteristics for various values of Vg. c) Ie- Vg characteristicfor constant Vd (see the dashed curve in b). At Vg < Vth the current fd is virtually zero.The parabolic and the horizontal part of the Ie- Vd characteristics meet at Vd = Vg - Vlil.

and I and as the source electrode for the triode formedby T, G2 and the drain electrode D. The island has nocontacts. In most applications the signal to be ampli-fied is applied to Gl and a capacitor is connected be-tween the electrode G2 and earth. The substrate has acontact (not shown), which is connected to the source.

Before describing the special electrical and techno-logical features of our new tetrodes, we should perhapsrecapitulate some ofthe more important features oftheMOS triode. The fundamental mechanism in the oper-ation of a MOS triode (fig. 2a) which has a P-typesubstrate, is the formation at the substrate surface of athin N-type layer when a positive bias Vg is applied tothe gate (inversion). The charge density of the mobilemajority carrier in this layer, the "channel", depends

IS given by:

gmsat = ('Md satJàVg) = j3(Vg - Vth). (5)

The transconductance gm sat is thus not independentof Vg.It is the very nearly quadratic relation between Id sat

and Vg which accounts for the very small cross-modu-lation obtained from the MOS transistor [41. If a

ru Characteristics and production methods for solid circuits arediscussed in: A. Schrnitz, Philips tech. Rev. 27, 192-199, 1966.

[2J During the development of our tetrode some details of similarwork were given by N. H. Ditriek and M. M. Mitchell at a meet-ing ofthe IEEE Electron Devices Group in October 1965.

[3J See G. Klein and J. J. Zaal berg van Zeist, Precision electron-ics, Centrex Pub!. Co. Eindhoven 1968, Chap. 18.

[4J This is discussed by P. E. Kolk and I. A. Maloff, Electronics37, Dec. 14, 1964, p. 71.

136 PHILIPS TECHNICAL REVIEW VOLUME 30

•

radio receiver is tuned to a, particular frequency,the modulation of the signal received is affected tosome extent by the modulation' of other transmit-ters, particularly those at neighbouring frequencies.The magnitude of this effect is mainly determined bythe third-power term --:- and to .a lesser extent by thehigher-power odd terms - in the expansion of Id sat asa power series in Vg. For the MOS transistor theseterms are very small and the cross-modulation is there-fore correspondingly small. .

MOS transistors for use in computers usually havea substrate of N-type silicon, arid hence a P-type chan-nel. In these transistors Vth usually has such a valuethat the transistor does not conduct when there isno bias on the gate, corresponding to the usual require-ments in digital applications. MOS transistors foramplifying high-frequency signals, on the other hand,are preferabl~ made on a P-type substrate. The channelthen shows N-type conductivity, which is more suitablehete since the electrons in silicon have a mobility aboutthree times that of the holes. (It is true that the mobil-ity in the layer obtained by inversion is smaller thanit is in the bulk materialof similar type, but the relativedecrease. isabout.the same for both kinds of chargecarrier [51.)

The operation of a MOS tetrode

Let us now calculate how the current in a MOStetrode depends on the various potentials and charac-teristic quantities. We shall proceed as if the tetrodewas a cascode circuit oftwo triodes : in fact this assump-tion is not quit~jüstified since a tetrode has one sub-strate contact, while the substrate of each triode of acascode circuit is connected to its own source elec-trode. We shall also assume that the two triodes areidentical, so that they each have the same (3, Vth, etc.If an equation analogous to (4) is set down for the

two triodes, then since both transistors carry thesame current it can be shown that

. . . . . (6a)

or

and

This equation for the saturation current of thetetrode is identical with the one that holds for thefirst triode (see eq. 4). The diagrams oî fig: 4 show howthe triodes adjust themselves for the different cases(èf. eq. 6b).

When Vgl is greater than Visl + Vih, the currentin the fi~st t~i~cie is no longer' saturated, and is given by:

From this relation and the similar one for Id2 it fol-lows that

ViSI= -!-[Vgl+ Vg2 :- 2 Vth -

- {(Vg! + Vg2 - 2Vth)2 - 2(Vg2 - Vth)2}t], (9)

and(3

!ct = '8 [Vg2-- Vgl + {(Vg! + Vg2 - 2Vth)2-:-

- 2(Vg2 - Vth)2}t]2: (10)

Let u~ now look at a number of special cases. WhenVg! is equal to Vth, the first triode is cut off and thepotential ViSIOf the island becomes equal to Vg2 - Vth;this means that the second triode is just driven intocut-off. If Vg! is allowed to increase indefinitely, then,as (9) shows, VISI goes to zero (i.e, Visi becomes equalto Vs) and Id goes to the limiting value -!-(3(Vg2 - Vth)2.

\ I,I

TDI-..:.J.I.!A--t+--I---lmA

8~~~r4-~-~Idi 6

TDV

Fig. 4. Operating curves for a pair of MOS triades in a cas-cade circuit, those for the first triode on the left and thosefor the second on the right. The drain potentialof the first tri-ode, which is also the source potential for the second, is labelledVisl, since it corresponds to the potentialof the "island" of atetra de. The curves for ViSl and Vd- ViSl apply for constantVg2. At a lower value of Vg2 this curve takes up the positionindicated by the dashed line, and at higher Vg2 it takes up theposition shown by the dotted line. .

(6b)

(7)This limiting of !ct has advantages on the score ofreliability, since a high voltage applied at the gatecannot set up a current large enough to damage thetetrode.

Finally, we should say a word or two about the mostimportant quantity of all in the MOS tetrode - thefeedback capacitance. In the MOS triode the feedbackaction arises because the gate electrode overlaps thedrain. electrode a little (fig. 5a). In the MOS tetrodethe main cause of feedback is that any a.c. voltage

1969, No. 5

.. ~D

DVd.. rl"

Cfb

G C,••o{/ /-- I",' Vist

~I

"1~ 1Q 5 5 12.

MOS TETRODE 137

c"O}~[ r-1 0':61-_

1

, __ ~

o ~--~W~--~70~O~--~7~OO~OkQ

---- Rg2

Fig. 5. a) The small stray capacitances in a MOS tetrode betweenG and D and between G and S arise mainly because G slightlyoverlaps the two other electrodes. The stray capacitance Crbbetween G and D is responsible for the feedback action. b) Dia-gram referring to the calculation of the feedback capacitanceof a MOS tetrode. c) The feedback capacitance Crb as a functionof the resistance in the lead to the second gate electrode. 1f thisresistance is greater than a few tens of kn, the advantages thatcan be gained with the tetrode are partly lost.

at D sets up a corresponding (but much smaller) a.c.voltage at J, and this in turn is coupled back to Gl(fig. Sb). The direct effect of the potentialof Gl onthat of D is very small, since G2 functions as a screen-ing electrode, provided that the resistance in thelead of G2 is not too high (fig. Se).

The feedback via the island can be calculated asfollows (fig. Sb). If the feedback capacitance of thetetrode is represented by Crb and the a.c. current in-duced in Gl by an a.c. voltage VI superimposed onVd is represented by is, then:

or

Crb = CIVI/Vd.

Since the same current flows in both parts of thetetrode, VI/Vd is equal to R; divided by the totalresistance of the cascode circuit, which is equal toRl(l + fl2) + R2. Consequently:

RlCrb= Cl---·----- R,j CI/fl2. (13)

Rl(l + fl2) + R2

This shows that in the MOS tetrode the feedbackcapacitance is about fl2 times smaller than that of thetriode.

If one attempts to verify experimentally that ViS! becomesequal to Vg2 - Vtl! when Vgl is made equal to Vth, as the relation(6) predicts, a fairly large discrepancy is found. With triodes

whose characteristics were like those of fig. 3, and for whichVtl! = - 2.7 V, we found a value of 6.2 V for Vis! at Vg2 = 5 V,as compared with a calculated value of 7.7 V. A much smallerdiscrepancy was found when we used triodes made on a muchless strongly doped silicon substrate (20 ncm). This agrees withthe calculations of Van Nielen and Memelink [6], from which itcan be shown that there will be no discrepancy at all in transis-tors made on an intrinsic subtrate.

Some suitable configurations

Fig. 6 shows a diagram of the cross-section of twoof our experimental MOS tetrodes, which fulfilledexpectations in every respect. The configuration shownin fig. 6a is very simple and does not really needfurther explanation. In the configuration of fig. 6b itcan be seen that the oxide layer does not have thesame thickness everywhere. Because ofthe step in thick-ness the two triodes have different values of C (thecapacitance is of course inversely proportional to thethickness of the oxide layer). In the tetrode of fig. 6b Cis greater for the second triode, and it can be shown

(11)

Fig. 6. Diagram of the cross-section of two types of experimen-tal MOS tetrode. In the lower diagram the oxide layer of thesecond tetrode is thinner than that of the first one, which givesa smaller feedback capacitance and also makes it easier to applyautomatic gain control (AGC) via G~.

that this has the effect of making its current ampli-fication factor fl greater. It follows that for this confi-guration Crb will be even smaller than it is for thatof fig. 6a. This arrangement also has the advantagethat a smaller voltage is needed at G2 when the tetrodeis used for AGe. As in other tetrodes the cross-mod-ulation in reverse AGC with Vg2 (see below) is lessthan in reverse AGe with Vgl, so that this configurationcombines a number of attractive features.

A variation of the tetrode drawn in fig. 6a is now

[5] O. Leistiko, A.S. Grove and C. T. Sah, IEEE Trans. ED-12,248, 1965.

[6] J. A. van Nielen and O. W. Memelink, The influence of thesubstrate upon the DC characteristics of silicon MOS tran-sistors, Philips Res. Repts. 22, 55-71, 1967.

138 PHILJPS TECHNICAL REVIEW VOLUME 30

Q

~ n n 9:~. ëill llilliTIfB:b

Fig. 7. Stages in the production of the Matsushita 3 SK 32MOS tetrode. a) Oxidation of the silicon substrate. b) Oxide isetched away in a particular pattern and phosphorus is diffusedinto the substrate to obtain source, island and drain. c) Removalof the oxide layer. d) Second oxidation; the oxide between S, Iand D is etched away. e) Third oxidation. I) Oxide is removedfrom parts of Sand D and aluminium is deposited.

Fig. 8. Microphotograph of a silicon wafer on which a 3 SK 32MOS tetrode has been produced. Starting at the outside, theyellow lines are the source, the first gate, the second gate and thedrain. The four dots are the connections. The four lead wiresare blurred but can just be made out.

Fig. 9. Experimental MOS tetrode in which the second triodeis the one with the thicker oxide layer.

in production (3 SK 32 MOS tetrode). Its characteristicfeatures will be described below; a diagrammaticcross-section and the method of making it are shownin fig. 7. The cross-section has been made differentfrom the one of fig. 6a to reduce as far as possiblethe effect of the overlap of the two gate electrodeson S, I and D. Fig. 8 is a microphotograph of the sur-face of a silicon wafer on which a tetrode like that offig. 7fhas been produced.

Experiments have also been carried out with two otherconfigurations besides the ones shown in fig. 6. The first is likethe configuration of fig. 6a, but with the island left out. Wefound that tetrodes of this type were not very successful; thepotentialof the oxide between Gl and G2 changes when Vgl

or Vg2 changes, but with a very high time constant. The conduc-tivity of the channel between Gl and G2 is therefore not uniquelydetermined by Vgl and Vg2.

The fourth configuration that we have investigated [7] is drawnin fig. 9. Here the oxide layer of the second triode is thicker thanthat of the first one. This has the result that Vth is higher for thesecond triode than for the first, which means that the islandpotential is fairly high even at Vg2 = 0; consequently atVg2 =0 the current lel is so large that a fairly high transconduc-tance is obtained.

The Matsushita very high frequency MOS tetrode type3 SK 32

The 3 SK 32 MOS tetrode, whose configurationhas just been described (fig. 7 and 8), is intended foruse at VHF (30 to 300 MHz), and is herrneticallysealed in a standard TO-72 package. The Id- Vdcharacteristics of a sample of this type are reproduced infig. ID. The value of Vth is the same for both triades

20mA

Vgr=2V/'

I1J/'

7.5

1/V 71

IV

0.5_

V0

70

70 20V-t{j

Fig. 10. Set of Id- Vd characteristics for the 3 SK 32 MOStetrode for Vg2 = 5 V. At Vgl = 0 the current Id is about 1.5 mA.

1969, No. 5 _MOS TETRODE 139

and is in the region of - 0.6 V.O At Vgl' 0 the satura-tion value of Id is about 1 mA, and this means that abias on Gl is desirable in most applications (enhance-ment mode "operation).When Vd is set equal to zero and Vgl and Vg2 are

made so strongly negative that there is no currentin the tetrode (Vgl = Vg2 = -10 V), the small-signalinput capacitance and output capacitance are 4.5 pFand 3 pF respectively at a frequency of 455 kHz.The variation of the feedback capacitance Crb as afunction of Id is shown in fig. 11. The value of Crbis about 0.02 pF, except at very low Id. In general Crbis never greater than about 0.035pF.

a03pF

a~~ ~ ~

Cfb

j0.G1

°0~--~--~~--~6~--~8~--~ro~m-A~-Id

Fig. 11. The feedback capacitance Crb for the 3 SK 32 tetrodeas a function of Id, at a frequency of 445 kHz, with Vd = 10Vand Vg2 = 5 V.

Fig. 12 shows how the transconductance varieswith Vgl, with Vg2 as a parameter. It can be seen thatall the curves have a maximum, which means that gmcan either increase or decrease with increasing Vgl.

A reduction in Vg2 always gives a reduction of gm.This means that there are in principle three ways ofobtaining automatic gain control: two using Vgl andone using Vg2. The most attractive method of the threeis the one using Vgl - which is chosen to give anoperating point in the downward-going part of the curve- since the magnitude of the variation of gm with Vgl

can be anywhere within certain limits; all that has tobe done here is to set Vg2 to a suitable value. The factthat a fairly high Vgl has to be used in this method ofcontrol so that Id also has a rather high value, is adisadvantage.Fig. 13 shows how the power gain G and the noise

figure F of the 3 SK 32 tetrode depend upon Id atthe frequency 200 MHz and for three values of Vg2.

For still higher values of Vg2 the measured curves al-most coincide with those for Vg2 = 5 V. It can beseen that for normal conditions and optimum matchingthe power gain at 200 MHz is 20 dB or more. _< _

T6r------------------,mA/V Vci=lQV

T4

gm

f 12

108

8

6

6

4

2

Fig. 12. The transconductance gm of the 3 SK 32 tetrode as afunction of Vgl for nine different values of Vg2, with Vd = 10.V!.

(: .

20dB

///II/II

ITV/\\ / »:~ / ..

~-- 3V _--. _~---- _-- - - --=--=--=--=- - ~2=5V -

G---.F-~

5

%~--------~5~--------~T~0----------+'$~mA-Id =,

Fig. 13. The power gain G (solid lines) and the noise figure F(dashed lines) of the 3 SK 32 tetrode, measured at 200 MHz.Both quantities were measured under optimum conditions andwith a bandwidth of 4 MHz.

;."

The cross-modulation characteristics for. circuitsusing two methods for obtaining A9C_ are shown.in fig. 14. The curves were obtained with an untunedcircuit which had two signals applied to it, one witha carrier frequency of 200 MHz and the other with a

. . ~[7) See also Electronics 39, May 16,)~66, p. 212-213._

140 PHILlPS TECHNICAL REVIEW VOLUME 30

1V'r---------------~--------~--~

Vct=10V0.01

10 L-d

20mAo

Fig. 14. The interfering signal Vlnt (frequency = 212 MHz),which gives a cross-modulation of 1% in a signal at a frequencyof 200 MHz in the 3 SK 32 tetrode, shown as a function of Idfor Vd = IOV. The solid line applies for constant Vg2 - i.e. thenormal situation with the tetrode driven at Gl - and the dashedline refers to constant Vgl.

carrier frequency of 212 MHz. It can be seen that fornormal values of Id a cross-modulation of 1% occurswhen the interfering signal is 0.1-1 V, and that the casewith fixed Vg! is the most favourable one.

In MOS triodes and tetrodes the input capacitanceCi is not entirely independent of the applied voltages.Since this capacitance can contribute to the tuning ofa circuit, it is desirable that the variation of Ci shouldbe as small as possible. Fig. 15 shows how Ci varies withId; in one case Id is controlled by Vg! and in the otherit is controlled by Vg2. It can be seen that the mostfavourable situation is the one in which Vg! is heldconstant. In this respect AGC via G2 is thereforepreferable.

Since the mobility of the charge carriers and thepotential Vth both depend on temperature, the currentId and the transconductance gm are also to some extenttemperature-dependent at constant Vd, Vg! and Vg2.Conditions can be found for which the two effectscancel one another out. If rJ.p, is substituted for (J, itcan be readily shown from (4) and (5) that the two tem-perature coefficients are given by:

bh Id bp, V·----bVth-- = --- 2 rJ.p,Id--er P, sr sr 'bgm gm bp, ö Vth- =---rJ.p,--.sr P, bT »r . . . (15)

Fig. 16 shows how Id varies with the channel tempera-

ture at Vd = 10 V and Vg2 = 5 V. At -Vg! = 0 V(or Id = 1.5 mA) the effect ofthe temperature is almostzero, for Vg! < 0 the temperature coefficient is positivebut very small, and for Vg! > 0 it is negative. This neg-ative temperature coefficient is another featurewhich is useful for reliability. (Bipolar transistors donot have this attractive feature, and the current inthese is also more strongly temperature-dependentthan the current in MOS transistors.)

As in other MOS transistors the Vth of our tetrode

6pF

Ci

t----__ 11rconst.(1.4 V)-_-_--_

5

\{{=10V200MHz

Fig. IS. The input capacitance Cl of the 3 SK 32 tetrode at afrequency of200 MHz as a function ofthe current Id at constantVg2 (5 V) and constant Vgl (1.4 V), with Vd = 10 V. The varia-tion of Cl with Id is smallest at constant Vgl.

(14)

25mA

20Id

t T5

TO

5

~20 0 20 40 TOO T200C

2.0

1.5

T.O----------------- 0.5

0--0.5

60 80-T

Fig. 16. Variation of Id with channel temperature at constantVd and Vg2 for a number of values of Vgl. When Vgl is small, Idis almost independent ofthe temperature.

1969, No. 5 MOS TETRODE

is not absolutely constant. Fig. 17 shows values ofVth recorded for four samples over a period of1000 hours. At first there was a relatively rapid varia-

0.5~

tion, although it was not very large (about 0.05.V in150 hours), and after that Vth remained virtuallyconstant for all four tetrodes.

~=+10V_~0.4 __ _ ,,_ _ _f~;---------~---------------------~~:--10V---------------

at°0~--~--~2~OO~---L--~40~0~--~--~6~070--~----8~0~0~--~~t==OOOh

-tFig. 17. A record ofthe variation ofthe values of Vtll measured over a period of 1000 hoursfor four samples ofthe 3 SK 32 tetrode at a temperature of 150°C. During the experiments Swas connected to D and Gl to G2. The voltage between the two pairs of electrodes was +10 Vfor two of the tetrodes, and -10 V for the other two.

Summary. The MOS tetrode, an integrated cascode circuit oftwo MOS triodes, has been developed to obtain a MOS devicewith such a low feedback capacitance that operation in the VHFband would be possible. The slope is about the same as for a MOStriode, and the current amplification factor and the internal im-pedance are very high. In normal use the gate G2 of the secondtriode is kept at a fixed potential (say zero) and the tetrode isdriven at Gl. In the 3 SK 32 VHF tetrode developed by Matsu-shita Electronics Corporation the threshold voltage Vtll is about

-0.6 V; at Vgl = 0 the current Id is about 1.5 mA, so that abias is needed for certain applications. The power gain is 20 dBor more at 200 MHz. The transconductance (about 10 mA/V)has a maximum for a particular value of Vgl and also dependson Vg2, so that there are three possible ways of obtaining AGC.The feedback capacitance is about 0.02 pF. The cross modula-tion is very low. At low Vgl the current Id is very nearly inde-pendent of the temperature. During the first 150 hours of lifeVtll changes a little, but after this changes hardly at all. .

141