Input vcc
The input characteristic is defined by several values. First is the flashover voltage, that is the voltage at the spot where Ids=0. The expression for flashover voltage:
Supposing
all values are similar for each device except for
(ε is only a bit different, A is a geometry parameter, q is a
constant and N we let be the same), we can see that the bigger
,
the bigger Uf.
GaN has the biggest
and the smallest Uf,
Si has the smallest
and the biggest Uf.
The second value is the voltage of the peak (Up), it nearly equals . That is why the voltages of the peaks goes in the same order.
The third value is the slope. The expression for Ids is the following:
The difference of slopes is defined by velocity of carriers. The velocity of saturation is the biggest for GaN and the smallest for Si, so is the slope.
5. Draw series of input and output VCC and noise factor on one plot. Explain good noise characteristics of MESFET
Input and output vcc and noise factor charecteristic
Cross-sectional view of the gate zone
The FET Schottky has following types of noise:
1.
Channel thermal noise. A channel is a resistance. Like all of the
resistance element it has noise. The noise is the result of the
collision of electrons with atoms and ions that oscillate (phonons).
Thermal noise in this case depends on the electron temperature. Since
collisions are frequent, the electron temperature is high (up to
about 10,000 K) and is directly proportional to the noise power
.
2. The thermal noise of "parasitic" resistance. These resistances include source resistance, drain resistance, gate metallization resistance of a semi-insulating substrate. These noises in conjunction with a channel thermal noise is 40% of the total noise.
3. The shot noise and flicker noise. Shot noise occurs due to generation and recombination of charge carriers, that is, the appearance and disappearance of the charge carriers causes irregular voltage and current fluctuations. Shot noise and low-noise FET are associated with structure irregularities. With the development of manufacturing technology these two kinds of noise became insignificantly small.
To calculate the noise the Van der Ziel formula and an expression for noise factor for amplifier stages can be used. Using Nyquist formula is unacceptable, because this formula describes the balance state, and the current flowing in the channel is essentially non-balance process. The transistor noises are small due to the fact that the transistor, channel region in particular, can be represented as a distributed amplifier (ΔZ). The effective temperature of each amplifier is very small. According to the formula for the stage we can see that the contribution of each subsequent stage to noise factor reduces Кamp n times.
The main contribution is the first stage, the others are very small. Considering the effective temperature of each stage is small, it is evident that the overall noise factor is low. On the characteristics depicted above the noise factor has reference points. Minimum noise point: when drain current decreases so decreases the characteristic slope. The gate influence on the channel decreases and the amplification properties get worse, this causes a noise increase.
The noise factor firstly falls and then grows while the output voltage increases. The decline is caused by an increase of the transistor slope, and the growth is due to increase of the conductivity of the drain and source.
Links: http://www.kit-e.ru/assets/files/pdf/2004_08_40.pdf; http://www.icquest.ru/?section=4&id=35;
Lebedev AI "Physics of Semiconductor Devices", 2008, S. Sze "Physics of Semiconductor Devices", 1984 guidelines for laboratory works on microwave electronics; MVEl lectures on transistors