It would be worth starting the story with Edison. This inquisitive man of science experimented with his incandescent light bulb, trying to reach new heights in electric lighting, and accidentally invented a diode lamp. In a vacuum, the electrons left the cathode and were carried away towards the second electrode, separated by space. Little was known about current rectification at that time, but the patented invention eventually found its application. It was then that the current-voltage characteristic was needed. But first things first.
Volt-ampere characteristic of any electronic device - vacuum, as well as semiconductor - helps to understand how the device will behave when included in an electrical circuit. In fact, this is the dependence of the output current on the voltage applied to the device. The diode precursor invented by Edison is designed to cut off negative voltage values, although, strictly speaking, everything will depend on the direction the device is connected to the circuit, but more on that some other time, so as not to bore the reader with unnecessary details.
So, the current-voltage characteristic of an ideal diode is a positive branch of the mathematical parabola, known to most from school lessons. Current through such a device can only flow in one direction. Naturally, the ideal is different from real life, and in practice, with negative voltage values, there is still a parasitic current called reverse (leakage). It is significantly less than the useful current, called direct, but, nevertheless, one should not forget about the imperfection of real devices.
The vacuum triode differs from its younger counterpart with two electrodes by the presence of a control grid that blocks the average cross section of the vacuum flask across. The cathode with a special coating, which facilitates the separation of electrons from its surface, served as a source of elementary particles, which were received by the anode. The flow was controlled by the voltage applied to the grid. The current-voltage characteristic of a vacuum triode lamp is very similar to a diode one, but with one big clarification. Depending on the voltage at the base, the coefficient of the parabola undergoes a change, and a family of lines of similar shape is obtained.
Unlike a diode, triodes operate with positive voltages between cathode and anode. The required functionality is achieved by manipulating the grid voltage. And finally, one last clarification needs to be made. Since the cathode has a finite ability to emit electrons, each characteristic has a saturation region, where a further increase in voltage no longer leads to an increase inoutput current.
Despite the different nature and principles of operation, the current-voltage characteristic of the transistor is not too different from the triode, only the steepness of the parabola is relatively large. That is why tube circuits, upon mature reflection, were often transferred to a semiconductor basis. The order of physical quantities is different, transistors use incomparably lower supply voltages. In addition, semiconductor devices can be driven by both positive and negative voltages, giving designers more freedom when designing circuits.
To fully satisfy the requests for the transfer of ready-made solutions, devices with a photoelectric effect were also invented. True, if the lamps used its external variety, then the improved elemental base, for obvious reasons, functions on the basis of the internal photoelectric effect. The current-voltage characteristic of the photoelectric effect is different in that the value of the output current shifts, depending on the illumination. The higher the intensity of the light flux, the greater the output current. This is how phototransistors work, and photodiodes use a reverse current branch. This helps create devices that capture photons and are controlled by external light sources.
