It would be worth starting the story with Edison. This curious husband of science experimented with his incandescent bulb, trying to reach new heights in electric lighting, and accidentally invented a diode lamp. In vacuum, the electrons left the cathode and were carried away towards the second electrode, separated by space. Little was known about rectification at that time, but the patented invention found its application over time. It was then that the current-voltage characteristic was needed. But first things first.
The current-voltage characteristic of any electronic device - vacuum, as well as semiconductor - helps to understand how the device behaves when connected to an
electric 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 of inclusion of the device in the circuit, but about this 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 of the school lessons. Current through such a device can flow in only one direction. Naturally, the ideal is different from real life, and in practice with negative voltage values there is still a stray current called the reverse current (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 crosses the middle section of the vacuum bulb across. The cathode with a special coating that facilitates the separation of electrons from its surface served as a source of elementary particles that the anode received. The flow was controlled by the voltage supplied to the grid.
The current-voltage characteristic of a vacuum triode lamp is very similar to a diode lamp, but with one big refinement. Depending on the voltage at the base, the parabola coefficient undergoes a change, and a family of lines of a similar shape is obtained.
Unlike a diode, triodes operate at positive voltages between the cathode and anode. The required functionality is achieved by manipulating the grid voltage. And finally, you need to make a final clarification. 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 in the output current.
Despite the different nature and principles of operation, the current-voltage characteristic of the transistor is not too different from the triode one, only the steepness of the parabola is relatively large. That is why tube schemes, as they matured, were often transferred to a semiconductor base. The order of the physical quantities is different; transistors use incomparably lower supply voltages. In addition,
semiconductor devices can be controlled by both positive and negative voltages, which gives a greater degree of freedom to designers when designing circuits.
To fully satisfy the requests for the transfer of ready-made solutions, devices with a photoelectric effect were invented. True, if the lamps used its external variety, then the improved elemental base for obvious reasons functions based on the internal photoelectric effect. The current-voltage characteristic of the photoelectric effect is characterized in that the value of the output current is shifted, depending on the lighting. The higher the luminous flux, the greater the output current. This is how phototransistors work, and photodiodes use the reverse current branch. This helps to create devices that capture photons and controlled by external light sources.