4.2 How will the characteristics change if the gate width is increased

In short-gate transistors, the dependence of current on the drain voltage is more pronounced, that is, with a decrease in the gate width, the steepness increases, and with an increase in the gate width, respectively, the steepness decreases. Let 's write this formally in terms of the cutoff frequency [38]:

f_c = = (35)

Thus, an increase in the width of the gate entails a decrease in steepness. At the same time, an increase in the gate width leads to a decrease in the cutoff frequency.

5. Substantiation of the family of input and output volt-ampere characteristics and noise coefficient on the same graph. Let's explain why a SBGFET, despite the high electronic temperature of the media at the output, is referred to as low-noise devices

Picture 34 – A graph of the input and output volt-ampere characteristics of SBGFET, as well as a graph of the noise coefficient [39]

To describe these families of characteristics, we use the Van der Zyl formula [40]:

(36)

So, in expression (33), the parameters depending on the mode are, first of all, the drain current I_d and the ratio of the diffusion coefficient to the carrier velocity D(T_e)/v. It is necessary to evaluate, first of all, the change in the indicated values in the input part of the transistor, which is key for determining the noise in general.

For operating modes, when the drain current reaches saturation, the speed in the gate part of the transistor weakly depends on the gate voltage and the drain voltage. It will tend to the saturation rate. The diffusion coefficient does not change much depending on the electron temperature. Under such conditions, it follows from (33) that the lower the current, the less noise.

However, a characteristic feature of the experimental dependence N_F  f(I_d) is the presence of low noise figure at a current  (0,150,2) . This fact contradicts the stated position on noise reduction when the current is reduced. The contradiction can be eliminated by taking into account the influence of the buffer layer. When the transistor is closed, the electrons heat up already in the initial part of the transistor and acquire the ability to drift in the buffer layer. This leads to a decrease in the transient conductivity (steepness), and, accordingly, to an increase in the noise coefficient. From this point of view, a symmetrical transistor should have the least noise, in which there is no current leaving the substrate [40].

It is worth noting that due to the simpler and more advanced manufacturing technology, SBGFET has a smaller spread of electrical parameters. The current in them does not flow through the p-n-junctions, but between the ohmic contacts of the homogeneous medium of the channel. Due to this, SBGFET have a higher linearity of the transfer characteristic, they do not have current distribution noise, and the current density can be large, therefore, their noise level is less, the power output is greater. The electron mobility in the weak field of gallium arsenide (GaAs), from which SBGFET is made, is about 2 times higher than in silicon (Si), and instead of the capacitances of the emitter and collector junctions, the SBGFET has a relatively small capacity of the inversely displaced gate at the Schottky barrier, so they can operate at higher frequencies up to 90 - 120 GHz. Internal feedback through parasitic capacitances in the SBGFET is insignificant, amplifiers work on them more steadily in a wide frequency range. Despite the fact that the thermal conductivity of GaAs is 3 times less than that of Si, bipolar transistors are inferior to SBGFET in terms of noise coefficient already at frequencies above 1 – 1,5 GHz [40].