1.  Field of the Invention
The present invention relates to an ultra-speed opto-electronic device using a resonant tunneling phenomenon, and more particularly to a resonant tunneling diode in which a peak value in a current-voltage characteristic thereof is changed to a transverse direction with respect to voltage when a light is illuminated into the diode. 
2.  Description of the Prior Art
In semiconductor industries, high integration of a semiconductor device has achieved dependently upon development of a lithographic technique and the like.  However, in case that line width in such a semiconductor device is reduced not exceeding mean free path thereof, unexplainable quantum phenomena in the well-known circuit theory will be found.  Therefore, to achieve ultra-speed operation and high integration of such a semiconductor device, development of new devices are required which voluntarily use quantum phenomena acting as obstacles of the conventional semiconductor technique.  One of the quantum phenomena is resonant tunneling that electrons in a double-barrier quantum well structure are transported at ultra-speed of 10.sup.-12 sec.  and less.  Studies for applying such a resonant tunneling phenomenon to a field of semiconductor devices of ultra-speed operation have been made. 
FIG.  1 shows general current-voltage characteristic of a conventional resonant tunneling diode.  As shown in FIG.  1, such a resonant tunneling diode has a negative differential resistance that only a current signal flowing in the diode is reduced even if an externally applied voltage is increased.  For example, current is rapidly increased in the range of 0.1 to 0.25 volt, but the current is rapidly reduced at 0.25 volt or more.  Also, when the externally applied voltage becomes 0.4 volt or more, the current is more rapidly increased again.  In this current-voltage characteristic curve, a peak signal is generated at approximately 0.25 volt.  The negative differential resistance exists in the range of 0.25 to 0.4 volt. 
Such a resonant tunneling semiconductor device has a bistability with respect to an arbitrary load line because of the above-described negative differential resistance.  This bistability has been applied widely to a memory device, a logic circuit, a high-frequency oscillating device and the like. 
In recent years, a p-i-n diode or a p-n junction diode is more frequently used as a photo-detector, a photo-conductor or the like.  FIG.  2A shows the construction of a p-i-n diode.  In FIG.  2A, a p-region of the diode is electrically connected with a source terminal V.sub. R and an n-region thereof is grounded through a resistor R.sub. L. Then, if the p-i-n diode formed thus is biased in reverse direction and a light is illuminated to the p-i-n diode, electron-hole pairs are produced therein as shown in FIG.  2B. That is, electron-hole pairs are produced because the light is absorbed into the p-i-n diode as a semiconductor device.  Subsequently, Each of the electron-hole pairs is separated into electron and hole, and the carriers separated thus are moved in the diode.  As shown in FIG.  2B, the electron carrier is indicated as "-" symbol, and the hole carrier is indicated as "+" symbol.  If a light is absorbed into the diode under bias of reverse direction, electrons separated thus are drifted from the p-region to the n-region therein, but holes are drifted from the n-region to the p-region, as shown in FIG.  2B. As a result, current flows in the diode, and thereby the diode starts to operate. 
FIG.  2C shows two characteristic curves of the p-i-n diode.  As shown in FIG.  2C, one curve indicated by a dotted line is a current-voltage characteristic curve of the p-i-n device when no light is illuminated to the device, hv=0 (where, h is Planck's constant and .nu.  is frequency of a light), and the other curve indicated by a solid line is a current-voltage characteristic curve of the p-i-n device which when a light is illuminated to the device, h.nu.&gt;0. 
Since such a p-i-n diode can be operated under bias of reverse direction, a dark current is set in the range of about several pA to nA, and a photo-current is set within several tens .mu. A, as shown in FIG.  3C. Therefore, to drive peripheral circuits, the above-mentioned current signal from the diode has to be amplified by an amplifier. 
In addition, as well-known in the art, quantum efficiency or gain, response time and sensitivity or detectivity act as factors determining characteristics of an opto-electronic device such as photo-detector, photo-conductor, or the like.  These factors are determined by generating rate of carriers, generating rate of electron-hole pairs which are produced upon a light being absorbed in the photo-device. 
As will be appreciated, the opto-electronic device, such as a photo-detector or a photo-conductor using a p-i-n diode, has the problem of lowering operation speed, as compared to that using quantum phenomenon.  Additionally, opto-electronic devices using a carrier transporting phenomenon in multi-quantum well structure are used in the art, but it is difficult to manufacture and reproduce the device because of complicated structure thereof.  This causes the problem that yield of opto-electronic devices is relatively lowered.