2.2 Advantages and disadvantages of transistors with ballistic transport

If l , hat is, the length of the active region is less than or equal to the relaxation length of the pulse, then the movement of charge carriers in the active region of semiconductor elements can occur collisibly or with a small number of scattering acts. In the first situation, we call the movement of charge carriers ballistic. In the second situation, we will talk about a quasi-ballistic movement.

A ballistic transistor, as well as a bipolar transistor, has an emitter - an electron injector, a base and a collector - collector of injected electrons. However, a ballistic transistor differs in principle from a conventional bipolar transistor by the high energy level of the electrons injected into the base, as well as by the fact that it is usually unipolar (although bipolar ballistic transistors are also found). All elements of a unipolar transistor (emitter, base, collector) have the same type of conductivity, and nonequilibrium main carriers are injected into the base. This circumstance contributes to an increase in the speed of the transistor, since it allows you to choose the fastest charge carriers as working carriers and, in addition, reduces the capacitances of the emitter and collector junctions due to the absence of diffusion components associated with non-primary carriers. The speed of the transistor is determined in the same way as in BT (bipolar transistor), by the time of passage of the base electron and the product of the greatest resistance in the transistor circuit (the longitudinal resistance of the base) by the collector and emitter capacitance. The time of flight of the base is determined by the value of the initial velocity of the electrons, and in the case of a ballistic collisionless flight, it is equal to. Due to the high speed of the injected electrons, the flight time of the base in ballistic transistors on hot electrons is fractions of a picosecond. It is precisely in achieving a short time of flight of the base due to the high initial velocity of electron injection that the main idea of increasing the speed of the transistor consists [26].

Thus, diode, transistor and other semiconductor structures in the conditions of ballistic and quasi-ballistic transport can exhibit very non-trivial static and especially dynamic properties. An important feature of the ballistic and quasi-ballistic modes of operation of various semiconductor devices is that the flight time of charge carriers through the active region is small even at low voltages. With characteristic sizes of active regions slightly smaller than 1 µm, semiconductor ultrahigh-frequency devices in ballistic and quasi-ballistic transport conditions can operate at frequencies corresponding to the submillimeter wavelength range, and elements of digital circuits can have high performance. That is, the advantages are smaller dimensions, no shot noise at low temperature, lower power consumption and a higher (THz) switching frequency [26].

At the same time, one of the big disadvantages of these devices is the low operating temperature range necessary for the absence of scattering on phonons, that is, for the correct operation of the transistor.