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The development of a thin film transistor amplifier
Thin Solid Films-Elsevier Sequoia S.A., Lausanne-Printed in Switzerland
21
THE DEVELOPMENT OF A THIN FILM TRANSISTOR AMPLIFIER*
A. E. HILL AND P. A. RIGBY~"
University of Salford, Salford, Lancashire (Gt. Britain)
An integrated amplifier has been designed to incorporate vacuum-deposited thin film field-effect transistors. An array of separate transistors was first produced to gain experience in manufacturing techniques and also to optimize the choice of electrode geometry and materials. A registration accuracy between layers of better than 5 Brn was achieved by the use of a specially designed evaporator. The effects on the characteristics of water vapour and ion migration were largely eliminated by the complementary use of two transistors: The circuit had a midband gain of 20 db and a gain bandwidth product o f approximately 500 kHz.
BASIC THIN FILM TRANSISTOR DESIGN
The thin film field-effect transistors to be used in the integrated amplifier were first tested on a 3 x 3 array. They were prepared on a glass substrate by vacuum deposition at a pressure of l0 -7 torr in a specially designed vacuum evaporator 1 which enabled any combination of nine evaporation sources and six masks to be selected without admitting the atmosphere. The registration accuracy between components on different masks was approximately 5 ~m. The thin film transistors were prepared with an inverted co-planar structure, i.e. the gate electrode was the first to be deposited on the substrate, followed by the gate insulator, the source and drain electrodes and finally the semiconducting channel. The channel was prepared from tellurium, mainly because, as an element, tellurium avoids the problem of dissociation of compound semiconductors during evaporation with all the attendant difficulties of fine adjustment of the source and substrate temperature necessary to achieve a stoichiometric film. It was found necessary, using tellurium, to limit the channel thickness to 50 /~ in order to produce suitable characteristics but it was felt that this was acceptable for evaluation of the devices. The gate insulator was made from silicon monoxide because it was easy to deposit and was able to passivate the tellurium channel to give good * Paper presented at the International Conference on Thin Films, "Application of Thin Films "', Venice, Italy, May 15-19, 1972; Paper 8.5. ~"Now with Messrs. Hewlett Packard Ltd., South Queensferry, Edinburgh, Gt. Britain.
22
A.E. HILL, P. A. RIGBY
characteristics. The thickness of the film was approximately 500 A. The source and drain electrodes best suited for use with a silicon monoxide/tellurium system have been reported 2-4 to be either .gold or aluminium. Gold was finally chosen because it was found that aluminium tended to scatter into the channel region during deposition. To provide good mechanical contact to the gold electrode a layer of copper/manganese alloy (Manganin) was first laid down and the gold was deposited on top. This effectively separated the duties of contact and electrode and gave very satisfactory results. It was not possible to dispense with the gold layer because it was found that, in the absence of the gold, the manganese component of the contact alloy tended to interact with the tellurium film resulting in an increase in channel resistance. STABILITYOF THE TRANSISTOR CHARACTERISTICS The electrical characteristics of the devices proved to be satisfactory and reproducible. However, it was found that exposure of the transistor to atmospheric water vapour caused the characteristics to drift from their initial value, the drift being completely reversed on replacing the device in a desiccator. Silicon monoxide is affected by water vapour but this is not reversible s, 6. A test film of tellurium with gold/Manganin contacts was prepared and tested by first drying and then exposing to water vapour (R. H. 100 ~). The results can be seen in Fig. l(a) which
80 75 7C a ~"
65 time
t~
-~
I ~00 (hours)
I 4000
4a
36
3~ I 0.4
I ~ time (minutes)
[ 40
Fi~. l. The variation of resistance with time of a film of tellurium.
THIN FILM INTEGRATED AMPLIFIER
23
shows a drop in resistance with time. Attempts to avoid this effect by encapsulating the device in silicone rubber* appeared satisfactory but a similar test film now indicated a resistance rise with time (Fig. l(b)). As a result all subsequent tests and measurements were performed with the transistors sealed in a dry inert gas. A more serious cause of instability in the characteristics was due to the migration of ions through the gate insulator from traps at the insulator/gate or insulator/channel interfaces. The effect of this has been shown theoretically7 to give an exponential drift in the pinch-off voltage, which results in an exponential decay of the output response to a step voltage applied to the gate; it has been observed by other workers also 8. The time constant of the decay may differ for positive- and negative-going input steps because the activation energies for the release of ions from the traps at the two interfaces may not be the same. Time constants varying from 27 sec to 1 h have been reported 7' 8 but in our case the two time constants were virtually the same at approximately 250 sec. This ionic process is affected by water vapour and again necessitates keeping the device in a dry environment. It is clear that ionic drift is extremely important when considering the low frequency or static performance of the transistor amplifiers and it is worth noting that the majority of published thin film transistor characteristics have been produced by fast scanning techniques using an oscilloscope. This method neglects ionic drift and usually produces improved characteristics but it can be misleading when designing the d.c. conditions of a transistor amplifier. INTEGRATED AMPLIFIER DESIGN AND PERFORMANCE
The circuit of the amplifier is shown in Fig. 2. Transistor T 2 is connected as a common source amplifier whose drain resistor is replaced by transistor T 1 which is connected as a constant current source. This simulates a very high value of load resistor and permits a much lower supply voltage than would have been possible with the equivalent pure resistive load. The capacitor is necessary to avoid signal degeneration in the current source at low frequencies. A second feature of the circuit is that the values of the resistor bias chains are such that the source-gate and source-drain voltages are the same for both transistors so that the effect of any ionic drift is cancelled 9. The circuit layout is shown in Fig. 3. The bias resistors are made from tellurium and the capacitor dielectric from silicon monoxide. The channel length is 32 lam, the gate width is 36 lam and the maximum misalignment between the two was found to be less than 3 ~tm. The d.c. standing levels around the circuit were found to be within 5 ~ of the designed values. The gain/frequency response had a fiat mid-band gain of 20 db which had fallen by 3 db at 65 Hz and 55 kHz, the lower frequency limit being determined entirely by the value of the capacitor. The gain bandwidth product was 490 kHz which compared well with the theoretical value of 440 kHz. The distortion level was very low for small input signals, rising to 2 ~ T H D at full output (2.25 V peak-to-peak) at which point symmetrical clipping occurred. The average noise level was normally less than 1 mV at the output but this rose as the input was * Silicoset 151.
Thin Solid Films, 13 (1972) 21-25
24
A . E . HILL, P. A. RIGBY (-8V}
R4
600~
'1=i]
(-4V) output
input R2
.1. Fig. 2. The integrated amplifier circuit diagram.
/
output
C
scare (mm
Fig. 3. The integrated amplifier layout.
increased because of minute breakdowns in the silicon monoxide layers. The freedom from ionic drift could be observed by connecting the supply in the absence of an applied signal. It was found that although the supply current fell by 30 ~o over the first 30 rain the output voltage fell by only 3 ~. FURTHER DEVELOPMENT
The above circuit was produced primarily to evaluate the deposition apparatus and techniques. Work is now progressing both to develop better thin film transistors using different channel, insulator and encapsulating materials and also to extend the circuit design. In particular, a differential version of the amplifier is envisaged for use as an attitude or displacement transducer which could be deposited directly onto a variety of rigid or flexible substrates. Thin Solid Films, 13 (1972) 21 25
THIN FILM INTEGRATED AMPLIFIER REFERENCES 1 2 3 4 5 6 7 8 9
A . E . Hill and P. A. Rigby, J. Sci. Instr., 2 (1969) 1084-1086. P . K . Weimer, Phys. Thin Films, 2 (1964) 147-192. T.P. Brody and H. E. Kunig, Appl. Phys. Letters, 9 (1966) 259-260. J.C. Anderson, Electronics and Power (lEE), 15 (1969) 90-93. A . E . Hill and G. R. Hoffman, Brit. J. Appl. Phys., 18 (1967) 13-22. A . E . Hill, A. M. Phahle and J. H. Calderwood, Thin Solid Films, 5 (1970) 287-295. E.J. Swystun and A. C. Tickle, IEEE Trans. Electron. Devices, ED-14 (1967) 760-764. A. Waxman and G. Mark, Solid State Electron., 12 (1969) 751-764. P.A. Rigby, M.Sc. Thesis, University of Salford, 1971.
Thin Solid Films, 13 (1972) 21-25
25
21
THE DEVELOPMENT OF A THIN FILM TRANSISTOR AMPLIFIER*
A. E. HILL AND P. A. RIGBY~"
University of Salford, Salford, Lancashire (Gt. Britain)
An integrated amplifier has been designed to incorporate vacuum-deposited thin film field-effect transistors. An array of separate transistors was first produced to gain experience in manufacturing techniques and also to optimize the choice of electrode geometry and materials. A registration accuracy between layers of better than 5 Brn was achieved by the use of a specially designed evaporator. The effects on the characteristics of water vapour and ion migration were largely eliminated by the complementary use of two transistors: The circuit had a midband gain of 20 db and a gain bandwidth product o f approximately 500 kHz.
BASIC THIN FILM TRANSISTOR DESIGN
The thin film field-effect transistors to be used in the integrated amplifier were first tested on a 3 x 3 array. They were prepared on a glass substrate by vacuum deposition at a pressure of l0 -7 torr in a specially designed vacuum evaporator 1 which enabled any combination of nine evaporation sources and six masks to be selected without admitting the atmosphere. The registration accuracy between components on different masks was approximately 5 ~m. The thin film transistors were prepared with an inverted co-planar structure, i.e. the gate electrode was the first to be deposited on the substrate, followed by the gate insulator, the source and drain electrodes and finally the semiconducting channel. The channel was prepared from tellurium, mainly because, as an element, tellurium avoids the problem of dissociation of compound semiconductors during evaporation with all the attendant difficulties of fine adjustment of the source and substrate temperature necessary to achieve a stoichiometric film. It was found necessary, using tellurium, to limit the channel thickness to 50 /~ in order to produce suitable characteristics but it was felt that this was acceptable for evaluation of the devices. The gate insulator was made from silicon monoxide because it was easy to deposit and was able to passivate the tellurium channel to give good * Paper presented at the International Conference on Thin Films, "Application of Thin Films "', Venice, Italy, May 15-19, 1972; Paper 8.5. ~"Now with Messrs. Hewlett Packard Ltd., South Queensferry, Edinburgh, Gt. Britain.
22
A.E. HILL, P. A. RIGBY
characteristics. The thickness of the film was approximately 500 A. The source and drain electrodes best suited for use with a silicon monoxide/tellurium system have been reported 2-4 to be either .gold or aluminium. Gold was finally chosen because it was found that aluminium tended to scatter into the channel region during deposition. To provide good mechanical contact to the gold electrode a layer of copper/manganese alloy (Manganin) was first laid down and the gold was deposited on top. This effectively separated the duties of contact and electrode and gave very satisfactory results. It was not possible to dispense with the gold layer because it was found that, in the absence of the gold, the manganese component of the contact alloy tended to interact with the tellurium film resulting in an increase in channel resistance. STABILITYOF THE TRANSISTOR CHARACTERISTICS The electrical characteristics of the devices proved to be satisfactory and reproducible. However, it was found that exposure of the transistor to atmospheric water vapour caused the characteristics to drift from their initial value, the drift being completely reversed on replacing the device in a desiccator. Silicon monoxide is affected by water vapour but this is not reversible s, 6. A test film of tellurium with gold/Manganin contacts was prepared and tested by first drying and then exposing to water vapour (R. H. 100 ~). The results can be seen in Fig. l(a) which
80 75 7C a ~"
65 time
t~
-~
I ~00 (hours)
I 4000
4a
36
3~ I 0.4
I ~ time (minutes)
[ 40
Fi~. l. The variation of resistance with time of a film of tellurium.
THIN FILM INTEGRATED AMPLIFIER
23
shows a drop in resistance with time. Attempts to avoid this effect by encapsulating the device in silicone rubber* appeared satisfactory but a similar test film now indicated a resistance rise with time (Fig. l(b)). As a result all subsequent tests and measurements were performed with the transistors sealed in a dry inert gas. A more serious cause of instability in the characteristics was due to the migration of ions through the gate insulator from traps at the insulator/gate or insulator/channel interfaces. The effect of this has been shown theoretically7 to give an exponential drift in the pinch-off voltage, which results in an exponential decay of the output response to a step voltage applied to the gate; it has been observed by other workers also 8. The time constant of the decay may differ for positive- and negative-going input steps because the activation energies for the release of ions from the traps at the two interfaces may not be the same. Time constants varying from 27 sec to 1 h have been reported 7' 8 but in our case the two time constants were virtually the same at approximately 250 sec. This ionic process is affected by water vapour and again necessitates keeping the device in a dry environment. It is clear that ionic drift is extremely important when considering the low frequency or static performance of the transistor amplifiers and it is worth noting that the majority of published thin film transistor characteristics have been produced by fast scanning techniques using an oscilloscope. This method neglects ionic drift and usually produces improved characteristics but it can be misleading when designing the d.c. conditions of a transistor amplifier. INTEGRATED AMPLIFIER DESIGN AND PERFORMANCE
The circuit of the amplifier is shown in Fig. 2. Transistor T 2 is connected as a common source amplifier whose drain resistor is replaced by transistor T 1 which is connected as a constant current source. This simulates a very high value of load resistor and permits a much lower supply voltage than would have been possible with the equivalent pure resistive load. The capacitor is necessary to avoid signal degeneration in the current source at low frequencies. A second feature of the circuit is that the values of the resistor bias chains are such that the source-gate and source-drain voltages are the same for both transistors so that the effect of any ionic drift is cancelled 9. The circuit layout is shown in Fig. 3. The bias resistors are made from tellurium and the capacitor dielectric from silicon monoxide. The channel length is 32 lam, the gate width is 36 lam and the maximum misalignment between the two was found to be less than 3 ~tm. The d.c. standing levels around the circuit were found to be within 5 ~ of the designed values. The gain/frequency response had a fiat mid-band gain of 20 db which had fallen by 3 db at 65 Hz and 55 kHz, the lower frequency limit being determined entirely by the value of the capacitor. The gain bandwidth product was 490 kHz which compared well with the theoretical value of 440 kHz. The distortion level was very low for small input signals, rising to 2 ~ T H D at full output (2.25 V peak-to-peak) at which point symmetrical clipping occurred. The average noise level was normally less than 1 mV at the output but this rose as the input was * Silicoset 151.
Thin Solid Films, 13 (1972) 21-25
24
A . E . HILL, P. A. RIGBY (-8V}
R4
600~
'1=i]
(-4V) output
input R2
.1. Fig. 2. The integrated amplifier circuit diagram.
/
output
C
scare (mm
Fig. 3. The integrated amplifier layout.
increased because of minute breakdowns in the silicon monoxide layers. The freedom from ionic drift could be observed by connecting the supply in the absence of an applied signal. It was found that although the supply current fell by 30 ~o over the first 30 rain the output voltage fell by only 3 ~. FURTHER DEVELOPMENT
The above circuit was produced primarily to evaluate the deposition apparatus and techniques. Work is now progressing both to develop better thin film transistors using different channel, insulator and encapsulating materials and also to extend the circuit design. In particular, a differential version of the amplifier is envisaged for use as an attitude or displacement transducer which could be deposited directly onto a variety of rigid or flexible substrates. Thin Solid Films, 13 (1972) 21 25
THIN FILM INTEGRATED AMPLIFIER REFERENCES 1 2 3 4 5 6 7 8 9
A . E . Hill and P. A. Rigby, J. Sci. Instr., 2 (1969) 1084-1086. P . K . Weimer, Phys. Thin Films, 2 (1964) 147-192. T.P. Brody and H. E. Kunig, Appl. Phys. Letters, 9 (1966) 259-260. J.C. Anderson, Electronics and Power (lEE), 15 (1969) 90-93. A . E . Hill and G. R. Hoffman, Brit. J. Appl. Phys., 18 (1967) 13-22. A . E . Hill, A. M. Phahle and J. H. Calderwood, Thin Solid Films, 5 (1970) 287-295. E.J. Swystun and A. C. Tickle, IEEE Trans. Electron. Devices, ED-14 (1967) 760-764. A. Waxman and G. Mark, Solid State Electron., 12 (1969) 751-764. P.A. Rigby, M.Sc. Thesis, University of Salford, 1971.
Thin Solid Films, 13 (1972) 21-25
25