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Submitted on 1 Jan 1978
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A InSb THIN FILM TRANSISTOR OPERATING AT
CRYOGENIC TEMPERATURES
A. van Calster
To cite this version:
A. van Calster.
A InSb THIN FILM TRANSISTOR OPERATING AT CRYOGENIC
A InSb THIN FILM TRANSISTOR OPERATING AT CRYOGENIC TEMPERATURES
A. Van Calster
Ghent State University, Belgium.
Abstract.- The InSb thin film transistor (T.F.T.) is a field effect transistor, which relies on an induced conduction. Consequently such a device is expected to operate at cryogenic temperatures /l/ This was verified by monitoring the transistor characteristics on a curve-tracer, while the T.F.T. was refrigerated in a He cryostat. We found that the InSb T.F.T. showed satisfactory saturating
characteristics, which are relatively temperature-independent between 150 K and 28 K.
1.INTRODUCTION.- Hall experiments carried out on evaporated n-type InSb films HI indicated that the films are degenerated and that the electron concen-tration is almost temperature-independent in the ex-trinsic region. This temperature-independent beha-viour of the electron concentration is explained by
a very simple model of an impurity band which over-laps the conduction band. This model also predicts that a freeze-out of the carriers does not occur, which eliminates the negative influence of
incomple-tely ionized donor states /3/ on the transistor cha-racteristics at low temperatures. Therefore a InSb is an attractive field effect transistor for opera-tion at low temperatures. Luo and Epstein /4/already reported the operation of the InSb T.F.T. at 77 K. In this paper the operation of the InSb T.F.T. is investigated at a liquid He environment.
2.EXPERIMENTS.- The two-sided structure of the T.F.T. shown in figure 1, was evaporated in a conventional oil-diffusion vacuum system. The vacuum pressure was
5
10" torr. The InSb was flashed on a heated Corning glass substrate of 250°C. The obtained InSb film was 200°A thick. The gate insulator is a SiO film
of 1500 A, which is resistively evaporated at a rate of 10 X/s. For the electrodes we used resistively evaporated Sb. After the evaporation, the T.F.T, nee-ded to be annealed in an oven for 30 minutes at 350°C for obtaining a stoichiometric InSb film with an acceptable electron mobility. The connection of the T.F.T. with the measurement equipment was made by gold wires, which were bonded on the Sb electro-des by a silver epoxy .
The InSb T.F.T. was placed in a He cryostat,
and the transistor characteristics were monitored on a curve-tracer while the T.F.T. was refrigerated
In figure 2 the characteristics for three different temperatures (T = 300 K, 50 K, 28 K) are shown. It follows from figure 2 that the characte-ristics improve when the temperature is lowered. Furthermore no noticeable difference can be seen between the characteristics at 50 K and 28 K. It was even found that below 150 K the characteristics became temperature-independent.
Fig.l : The two-sided thin film transistor JOURNAL D E PHYSIQUE Colloque C6, supplément au n° 8, Tome 39, août 1978, page C6-1186
Résumé.- Le transistor en couches minces (T.F.T.) de Sbln est un transistor à effet de champ, dont le fonctionnement est basé sur la conduction induite. Par conséquent on prévoit que le transistor pourra fonctionner à de très basses températures /l/. Ceci a été vérifié en mesurant les caracté-ristiques pendant que le transistor était réfrigéré. On a constaté que le transistor à Sbln avait des caractéristiques acceptables, qui dépendent peu de la température entre 150 K et 28 K.
Fig. 2 : The transistor characteristics.
3. DISCUSSION AND CONCLUSIONS.- An important opera- tion mode of the T.F.T. is the partial depletion mode
151,
where the semiconductor channel becomes more and more depleted if one moves from source to drain. At room temperature the creation of an inver-sion layer at the drain region prevents a punch-off of the InSb channel and thus makes well-saturating characteristics impossible. The inversion layer is temperature-dependent and its influence weakens when the T.F.T. is cooled.In this case a pinch-off of the InSb channel becomes possible, and the characteris- tics become more saturating, as isseenfrom figure 2. The temperature dependence of the inversion layer can be roughly
estimated
by the temperature depen- dence of the minority carriers pb from the relation.where ni is the intrins.ic carrier concentration and nb the electron concentration which is nearly temperature-independent. Consequently pb has the same temperature dependence as n2
,
and thus thei
influence of pb (inversion layer) is weakened at lower temperatures.
In the partial depletion mode incompletely ionized donors cause a severe reduction of the modu- lation of the characteristics, by trapping of the induced charge. As no freeze-out of the carriers
low temperatures, which agrees with our experiments. We may thus conclude that the InSb T.F.T. shows acceptable temperature-independent transistor characteristics at low temperatures.
ACKNOWLEDGEMENTS
.-
The auth~r wishes to acknowled- ge the assistance of Pr.G.Jacobs for providing the He cryostat equipment.References
/ I / Rogers,C.G., Solid State E1ectron.n (1968)1079
/ 2 / Van Calster A. and Reusens,P., Thin Solid Films
44 (1977) 21.
-
/3/ Van Calster,A. and Pauwels,H.J., Solid State Electron.
18
(1975) 691./4/ Lu0,F.C. and Epstein,M., Proc. I.E.E.E.
60
(1972) 997151 Tickle,A.C., Thin Film Transistors,(Wiley, New York)
