Infrared optical bulk channel field effect transistor for greater effectiveness

An infrared optical field effect transistor has been developed using a thin film of Lead Titanate (PbTiO.sub.3) deposited on a n/p.sup.+ Si substrate by RF magnetron sputtering. This transistor possesses excellent pyroelectric properties and can, therefore, be operated even at room temperature. The infrared optical field effect transistor has the following features associated with rapid bulk channel structure and higher mobility: PA1 1. Can be operated at room temperature, unlike quantum type IR sensors which can only operate at very low temperature (-100.degree. C..about.-200.degree. C.), which results in higher costs. PA1 2. High speed response with only 2.3 .mu.s of rise time. This is much faster than other types of thermal infrared optical field effect transistors. PA1 3. Easy to fabricate an integrated sensor device.

FIELD OF INDUSTRIAL APPLICATIONS 
This "high speed infrared optical field effect transistor (FET)" uses 
titanic acid lead to form the ferroelectric thin film. Its design 
incorporates the high mobility of high speed bulk channel field effect 
transistors for greater effectiveness. The sensor has a wide range of 
applications including scientific, commercial and military applications, 
including laser detection, missile guidance, spectrum analysis, remote 
control, burglar alarm and thermoimage detection. 
TECHNICAL BACKGROUND OF THE INVENTION 
There are two main types of infrared sensor. 1) Thermoelectric sensors, and 
2) quantum sensors. Thermoelectric sensors are superior to quantum sensors 
because: 
1. They are able to operate at room temperature; Quantum sensors require a 
low temperature environment between -100.degree. C..about.-200.degree. C. 
to operate. 
2. Rapid response; Response time for thermoelectric sensors is faster than 
that for Golay cells (see Computers & Telecommunications, Volume 21, 
p.265, M. Okuyama, 1985). 
Referring to Addison Wesely (John P. Uyemura, p. 21 Chapter 1) reveals that 
traditional thermoelectric FETs which use surface channel designs suffer 
from scattering on the transistor surface and traps created during 
fabrication which reduce mobility. 
According to M. Okuyama (Computers & Telecommunications and Ferroelectrics 
volume 63, p.243, 1985), the rise time of traditional thermoelectric FETs 
is 3.5 .mu.S and electrical current is less than 1 .mu.A. As direct 
switching is not possible, an amplifier is required, which delays 
switching time. 
SUMMARY OF THE INVENTION 
The main purpose of this invention is to provide a high speed and sensitive 
infrared optical FET that operates at room temperature, in this case by 
using rapid bulk channel structure of metal/ferroelectric thin 
film/semiconductor. The second purpose is to accommodate fabrication of 
VLSI for infrared optical FET which can also be developed as integrated IR 
OEIC. 
The use of surface channels in conventional thermoelectric FETs is the 
reason behind their lower mobility and slower switching time. The inventor 
discovered that bulk channels can reduce equivalent resistance; 
furthermore, infrared rays cause compensatory charge variation in the 
depletion layer of components; this compensatory change is equivalent of 
its ferroelectric capacitance series with depletion capacitance. The 
compensatory charge variation of traditional surface channel FET is 
located at the inversion layer, and equivalent capacitance is only 
ferroelectric capacitance. According to the comparative method found in 
Physics of Semiconductor Devices(S. M. Sze, 1981),the developed infrared 
sensors using ferroelectric thin film will have lower equivalent 
capacitance. 
Because the developed infrared optical FETs using Ferroelectric thin film 
have lower resistance and capacitance, response speed is faster than that 
of conventional FETs. Additionally, its electrical current is 
significantly increased to reach above 100 .mu.A, making it more sensitive 
.

DETAILED DESCRIPTION OF THE INVENTION 
The architecture of infrared optical FETs which use ferroelectric thin film 
(5) and p-n junction of Si semiconductor (4) is illustrated in FIG. 1. 
Ferroelectric thin film is put on the top of central Si semiconductor, 
then metal thin film is put on as gate (2). At either sides of the gate 
are the source (1) and the drain (3) of metal thin film. At the bottom of 
Si semiconductor is substrate of metal thin film. 
The p-n junction of the Si semiconductor described above can be either n/p+ 
or p/n.sup.+. The substrate generally adopts semiconductor materials such 
as Si of IV family, CdS of II-VI family and GaAs of III-V family. 
Ferroelectric thin film (5) is used to sense infrared ray, and the gate is 
used as the radiation absorbing electrode. 
Fabrication Procedure: 
1. Clean the Si semiconductor substrate. 
2. Ferroelectric thin film grows on Si semiconductor by using RF magnetron 
sputtering system. 
3. Evaporate a gate of metal thin film on ferroelectric thin film. 
4. Etch ferroelectric thin film by using photolithography to open source 
and drain windows. 
5. Evaporate source and drain on either side of ferroelectric thin film. 
6. Evaporate a substrate electrode on the other side of Si semiconductor. 
An infrared sensor of ferroelectric thin film is now complete. 
The diameter of the target is 5 cm, the distance between target and 
substrate is 5 cm, growing power is 100 W, sputtering gas is mix of 90% Ar 
and 10% oxygen, growing pressure is 6 mtorr, temperature of substrate is 
500.degree. C. to 600.degree. C., growing method of ferroelectric thin 
film can be either RF magnetron sputtering or laser evaporation. 
Because ferroelectric thin film with good thermoelectric characteristics 
can be operated at room temperature, infrared sensors using ferroelectric 
thin film have the following features: 
1. Able to operate at room temperature. It overcomes the necessity of low 
temperature devices for traditional quantum infrared sensors which can 
only operate in a -100.degree. C..about.-200.degree. C. environment. This 
greatly reduces production costs. 
2. Rapid response superior to that of other thermoelectric infrared optical 
FETs; rise time of only 2.3 .mu.S. 
3. It is easier to manufacture components for integrated infrared sensor. 
COMPONENT CHARACTERISTICS TEST 
Current/voltage curve is measured using the HP 4145B semiconductor 
parameter analyzer; response time is measured using the HP 54600A 
oscilloscope with an IR LED (wavelength of 970 nm) light source. FIG. 2 
illustrates typical drain current/voltage curve of ferroelectric thin film 
infrared optical FET with different infrared intensities. FIG. 3 shows 
effect of drain current using different infrared intensities. FIG. 2 and 
FIG. 3 show that drain current increases as infrared intensity increases. 
FIG. 4 shows loading waveform which is measured using the HP 54600A 
oscilloscope. An infrared ray is used to irradiate the sensor to generate 
the current waveform. 
FUNCTIONS 
We can see from the above that the invention has the following functions: 
1. Although the quantum infrared sensor has a faster response time, it must 
operate in a -100.degree. C..about.-200.degree. C. environment to inhibit 
noise. This can increase costs substantially. The optical field effect 
transistor has not only an excellent response time, but can also be 
operated at room temperature. No low temperature devices are needed. 
2. Its high rapid response, with a rise time of only 2.3 .mu.S, exceeds 
other thermoelectric infrared optical FETs. 
3. It is easier to produce components for an integrated infrared sensor. 
The explanation is made by a physical example with illustrations, which are 
intended to aid comprehension. 
EXAMPLE 
The sensor is able to be operated in depletion mode after it is processed 
by polarization, at 200.degree. C., with -8 V on gate about 20 minutes. 
After the polarization process, remanent polarization of the ferroelectric 
thin film generates an electrical field. The electrical field repels 
electrons in the n layer and creates a depletion region which has a 
positive charge. The energy band of semiconductor will bend upward as 
shown in FIG. 5(a). When the sensor is exposed to infrared radiation, the 
ferroelectric thin film absorbs it, causing a rise in temperature, 
weakening the polarization. The depletion region becomes narrow; the n 
type channel and drain conductance increase. Its energy band is shown in 
FIG. 5(b). 
From the above description, an optical field effect transistor (FET) with 
high speed infrared response which uses titanic acid lead for its 
ferroelectric thin film is a real innovation.