THIN FILM TRANSISTORS AND THIN FILM TRANSISTOR CIRCUITS

In this paper the possible circuit applications of thin film transistors, made by shadow masks, are discussed. Special attention is paid to the basic parameters determining the circuit properties.


INTRODUCTION
The thin film transistor (TFT) is an insulated field-effect transistor made by a simple vacuum deposition process.Although a TFT may not be considered as a competitor of the Si MOSFET, the TFT technology shows some attractive qualities for special purpose circuits: the possibility of making large.area(macro) circuits with a built-in dielectric isolation, and the possibility of handling high voltages.A special feature of TFT tech- nology is the possibility of obtaining a higher level of system integration.This is for instance the case in solid-state displays, where electronic and display functions are merged. 1,2st of the developed TFT circuits are digital circuits.Less efforts are spent on analog circuits, probably due to the proven reliability of the Si technology in microelectronics.
Nevertheless a higher integration level could also be possible in hybrids, using TFT technology.In order to check the feasibility of this goal, the characteristics of TFT test vehicles were investigated.

TFT TECHNOLOGY
The TFT structure used in our laboratory is shown in Figure 1.All depositions are made in one pumpdown of the vacuum deposition system, to minimize the device contamination during fabrication.By means of a mask changer Mo shadow masks are placed one by one in front of the substrate to delineate the device patterns.Devices with a channel length to width ratio of 30/am/100/am have been made previously by metal masks.Prior to the source-drain deposition, the Corning 7059 substrate is coated with a 1000 A thick A12Oa layer to prevent diffusion of impurities from the substrate.
The source and drain contacts are made by the successive deposition of Au and In by E gun.The 300 A thick semiconductor CdSe is evaporated from a heated Mo boat.The gate insulator Al:Oa is obtained by E gun evaporation, and the gate is an 800 A thick evaporated A1 film.Afterwards the TFT is annealed in vacuum at 300C for 3 hours.
Finally the substrate is cut and gold wires are bonded to the TFT.In Figure 2 a TFT is shown mounted in a TO-5 package.
The stability of the device is mainly related to the insulator-semiconductor interface, a AIOa-CdSe seems to be one of the more stable combinations, with a sufficiently small number of slow surface states. 4The thin semiconductor film forms an n-channel, with a conductance determined by the applied gate voltage.The donor concentration is 5 1017 cm -, and the electron mobility #e equals 55 emZ/Vsec.

FIGURE
The thin film transistor (TFT) structure FIGURE 2 A TFT in a TO-5 package

TFT CHARACTERISTICS
In Figure 3 the small signal equivalent circuit is presented of a TFT biased at a gate voltage V and a drain voltage Vo.Only for V > 0 and Vo > 5v, the transconductance gm can be approximated by a linear relation as is predicted by simple theory, From Figure 3 it also follows that the turn-on voltage VT is negative, which means that the n-channel CdSe TFT behaves more like a depletion field-effect transistor.This means that in circuits one can use enhancement loads as well as depletion loads.When the TFT is biased at V 0v, gm it can be fairly well approximated by a parabolic relation and the drain current Io by a third degree parabola.The data presented in Figure 3 are recorded for a TFT with a channel width to length ratio S of 50.In saturation (VD > VG VT) the output conductance gD becomes 0.5/aS.The input capacitance Ci per unit area equals 5.210 -a F/cm2, with a loss tan i 510 -3 (o 104).
With a 1700 A. thick A1203 film the maximum VD is approximately 28v.The frequency behaviour is determined by the unavoidable gate-drain electrode overlap.This causes the Miller capacitance CM.The gain-landwidth G.Bw of the TFT presented in Figure 3 equals: where L is the channel length,/x the electrode overlap and Tr the transit time of the TFT.
Tr is given by the expression: L Tr /ae(VG VT ), and becomes 0.5/asec for Vo v and L 60/am.
An overlap of 10/am results in a G.B w 2 MHz.The equivalent circuit of Figure 3 is valid for frequencies smaller than 1/2rTr.
In digital circuits the switching time of the device is of interest.In the absence of CM, the switching time is composed of a delay and risetime, both determined by Tr.The "on" and "off" state can be characterised by go.In the "on" state Vo VT > VD (-0)go has a similar behaviour as gm at large VD, go increases linearly with V6.

TFT INVERTORS AND CIRCUITS
The invertor stage may be considered as an elementary analog amplifier stage.Two types of load devices are possible: enhancement load (Figure 4a) and depletion load (Figure 4b).The enhancement load gave only an amplification of 2.5, for a geometry ratio S1/S as large as 83.With the depletion load is the fact that the maximum gain occurs at Vc 0, independent of VT.Only the maximum attainable gain depends on VT.

FIGURE 4a enhancement load FIGURE 4b depletion load
This is an interesting feature for designing digital circuits, where an invertor also may be considered as a basic circuit.For analog circuits, the circuit design should be process or VT insensitive, because VT differs from one batch to another.This means that the same design rules should be used as for nMOS.Different small circuits were tried out, such as multivibrators, Schmitt-triggers and analog followers.As an example a single stage voltage follower is shown in Figure 5, with measured input and output impendances respectively larger than 10 m2 and smaller than 10 K2.We recently tackled the problem of the development of more complicated analog circuits, such as an operational amplifier.

CONCLUSIONS
From the above discussion it follows that TFT's can be made by a simple vacuum deposi- tion process.By the use of the A12 O3-CdSe combination stable devices can be made.The use of mechanical masks still allows a suitable miniaturisation, but limits the frequency response due to the unavoidable electrode overlap.Nevertheless acceptable device charac- teristics can be obtained.TFT's with depletion loads show interesting circuit features o 12V 0. S V//d /v.especially.This means that for special purpose circuits, digital as well as analog, where a higher integration level can be obtained, a TFT circuit solution should at least be considered.

FIGURE 3
FIGURE 3 gm and D versus V G FIGURE 5