Why is there knee tension

Junction field effect transistor - FET

General FET properties

The output characteristic field shows two usable working areas. If the drain-source voltage remains well below the pinch-off or pinch-off voltage UDS P, then you are in the ohmic range with largely linear characteristics. The JFET has the property of a voltage controlled ohmic resistance. This work area is used in circuits for automatic gain control and voltage division. In the characteristic area above the pinch limit, the JFET works in amplifier circuits.

Field effect transistors are suitable for high frequencies and can work as fast electronic switches. When switching from the conducting to the blocking phase, the control diode does not have to be discharged first and then the blocking zone must be rebuilt, as is the case with the bipolar transistor.

Like bipolar transistors, JFETs can be operated as amplifiers in three basic circuits. The name indicates the electrode with the least resistance to circuit ground.

Source circuit
It corresponds to the emitter circuit, in contrast, due to the powerless control with a few megohms, it has a very high input resistance. The initial resistance is in the kilohm range. The voltage amplification factor is around 20 and, compared to the emitter circuit, is smaller by a factor of 10. At 180 °, the output signal has a phase position that is inverse to the input signal.
Drain circuit
It is comparable to the collector circuit. The very high input resistance is 10 MΩ. The output resistance is very small and is usually less than 1 kΩ. The voltage gain is less than 1, the phase angle is 0 °. The drain circuit is very suitable as an impedance converter.
Gate circuit
It is comparable to the basic circuit. The input resistance is small and in the kilohm range. The output resistance is comparable to that of the source circuit. The voltage gain is similar to that of the source circuit but the phase angle is 0 °.

Noise source

In the semiconductor, as in all other conductors, the electron flow is irregular in the smallest areas. The sum of the resulting potential differences causes a noise voltage. It covers a large frequency range and broadband amplifiers therefore have a higher noise component. The data sheet contains information on the noise input voltage in nV / √Hz. In comparison to bipolar transistors, field effect transistors have a lower noise behavior. Compared to MOS-FET semiconductors, the noise with the FET is lower by a factor of 10 ... 1000.

Temperature behavior

The FET behaves differently than a bipolar transistor, where as the temperature increases with the base current, the output current also increases and the power loss can lead to a further increase in the currents up to self-destruction. With the JFET, the drain current is determined by the diffusion voltage and the mobility of the carriers in the channel.

Area of ​​larger drain currents
The mobility of the carrier as a determining factor decreases with increasing temperature. The channel narrows towards the drain and the drain current becomes smaller. This effect could be illustrated by the decrease in the mean free path of the charge carriers in the crystal as a result of increasing Brownian molecular movements at higher temperatures.
Area of ​​smaller drain currents
The thickness of the barrier layer is the determining factor. It is inversely proportional to the channel width. As the temperature increases, the junction thickness becomes smaller, the channel wider, and the drain current increases. The temperature coefficient is negative at around 2 mV / K, which increases the pinch-off voltage UP. If the temperature increases, the channel cross-section increases and the conductivity, followed by the drain current, increases. This behavior corresponds to the PTC semiconductor properties.

Both effects work in opposite directions so that there is an operating point on the control characteristic where the drain current is independent of temperature.

Power dissipation Ptot

The power loss converted into heat is limited in the FET to the current-conducting channel, since the gate-source path remains de-energized. Either this value or the maximum permissible duct temperature is specified in the data sheet. If the thermal resistance between the duct and the environment is known, the power loss can be calculated from this.

Specimen scatter

The scatter ranges are usually given in the manufacturers' data sheets. They seem bigger than bipolar transistors. The BF 245 examined above shows particularly large type scatter values ​​in the steepness, the amount of the pinch-off voltage and the associated maximum drain current.