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Characteristic Frequencies of Field Effect Transistors and Related Measures for Improving FT

Definition of characteristic frequency

Used for identifying and measuring frequencies in a given transmission, such as carrier frequencies that can be specified as feature frequencies.

For components, characteristic frequency refers to a cutoff frequency when their main function drops to a point where it is difficult to use. For example, for active devices such as bipolar Transistors and field-effect Transistors used as amplifiers, the characteristic frequency refers to the frequency at which the current amplification coefficient drops to 1, which is the cutoff frequency used for amplification in a common emitter configuration.

For passive diodes used as detectors, switches, etc., their characteristic frequency refers to the frequency at which their impedance drops to a small level and cannot absorb signal power. At this point, the cutoff frequency is also their characteristic frequency.

Field effect transistor (MOS transistor)

The characteristic frequency ft of Field-effect transistor (JFET, MOSFET, HEMT) refers to the frequency when the common source, output short circuit, current amplification factor is 1 (that is, input current=output current), also known as the gain bandwidth product of the common source configuration; It is mainly determined by the gate capacitance Cg.

The simplified small signal high-frequency equivalent circuit can provide a formula with ft=gm/2 π Cg=1/2 π τ， The ft is determined by the transit time of the streamer downloaded from the gate τ。

For long channels（ μ Devices with constant: τ = L/ μ Ey ≈ L2/ μ Vds, then ft= μ Vds/2 π L; For devices with short channels (Drift velocity saturation vs): τ = L/vsL, then ft=vs L/2 π L.

If parasitic capacitance CL is included, the cutoff frequency is ft=gm/[2 π (Cg+CL)]=(1/2 π τ） [1+(CL/Cg)] -1.

Measures to improve the ft of field-effect Transistors:

Increase transconductance gm, reduce gate capacitance Cg, reduce channel length L, and increase mobility μ Or saturated Drift velocity vs.

For HEMT (High-electron-mobility transistor), because the thickness of the control layer of HEMT can be made smaller, the Cgs is smaller (that is, the gm is larger), so it has a higher cut-off frequency and faster operating speed.

The characteristic frequency ft of a bipolar transistor is the current amplification coefficient of its common emitter configuration β The frequency at which it drops to 1, also known as the gain bandwidth product of a transistor.

if β O is the current amplification coefficient at low frequencies, f β It's called β Cut-off frequency, then when f>>f β Sometimes there may be β │ f= β O f β = Ft. Therefore, as long as it is above f β Measured at a frequency of | β You can obtain ft by doing so.

The characteristic frequency ft can be used to determine the effective transit time of electrons from the emitter to the collector τ EC is represented as:

(2 π τ Ec) -1, where τ Ec= τ E+ τ B+ τ D+ τ C, τ E=(kT/q Ic) CjE is the charging time of the emission junction, τ B ≈ τ F is the time when electrons cross the neutral base region,

τ F is the time required to remove the stored charge from the base and emission regions (slightly greater than τ B) , τ C=(kT/qIc+rc) CjC is the charging time of the collector junction, τ D=Xdc/vs is the time when the electrons cross the collector Depletion region Xdc at the saturated Drift velocity vs;

The main determining factors for ft are generally τ B. Next is the junction capacitance (especially the collector junction capacitance). FT is related to the working point of Transistors, so it is necessary to choose the working point reasonably when using Transistors and testing FT.

The measures to increase the frequency of BJT features are:

① When ft is not very high, it is often τ If B plays a major role, it is required to reduce the width of the base region (using shallow junction technology to make thin base regions) and increase the electric field factor of the base region η Increasing the doping concentration on the emission junction side of the base region and increasing the steepness of impurity distribution in the emission region can reduce the retardation field, but if the doping concentration is too high, it will actually reduce the diffusion electron coefficient η Generally controlled between 3 and 6);

② When ft is high, the width of the base zone must be very small, τ If B is shorter, it must be considered τ E τ D and τ The impact of C requires reducing the dynamic resistance of the emission junction (using a larger collector current) and the barrier capacitance (reducing the emission junction area), reducing the barrier thickness of the collector junction (which can reduce the resistivity of the collector area, but also considering the breakdown voltage), reducing the series resistance rC of the collector (which reduces the resistivity of the collector area), and the barrier capacitance Cjc (which reduces the collector junction area).