FET Datasheet Specifications:

Maximum Ratings – A portion of a FET Datasheet Specifications is reproduced in Fig. 9-17. As with other device data sheets, a device type number and brief description is usually given at the start. Maximum ratings follow, and then the electrical characteristics are stated for specific bias conditions. From Fig. 9-17, the maximum drain-source voltage (V_DS) for the 2N5457 through 2N5459 devices is 25 V, and the maximum drain-gate voltage (V_DG) is also 25 V. This means, for example, that if a -5 V gate-source bias is used, the drain source voltage should not exceed,

Note on Fig. 9-17 that the maximum reverse gate-source voltage (V_G(r)) is specified as 25 V. This is considerably greater than the (typically 5 V) maximum base-emitter reverse voltage for a BJT.

No maximum drain current is specified in Fig. 9-17, but this can be calculated from the maximum power dissipation and the V_DS level. The specified gate current (I_G) is the maximum gate current if the gate-channel junctions become forward biased.

Saturation Current and Pinch off voltage:

The drain-source saturation current (I_DSS) and the pinch-off voltage (V_P or V_GS(off)) have already been discussed. Values for these are listed in the FET Datasheet Specifications portion showing the Off Characteristics and ON Characteristics in Fig. 9-18. It is seen that V_GS(off) for a 2N5457 (underlined) ranges from a minimum of 0.5 V to a maximum of 6 V. Also, the I_DSS level for a 2N5457 (dashed underline) is a minimum of 1 mA and a maximum of 5 mA.

The FET transfer characteristic approximately follows the equation,

When I_DSS and V_P are known, a table of corresponding values of I_D and V_GS can be determined from Eq. 9-1. These may be used to construct the FET transfer characteristic. Because of the wide range of specified values for I_DSS and V_P, the transfer characteristic can differ substantially from one device to another one having the same type number. This creates a problem in FET bias circuits.

Forward Transfer Admittance:

The forward transfer admittance (Y_fs) [also known as the transconductance (g_m or g_FS)] for a FET defines how the drain current is controlled by the gate-source voltage.

The units used for Y_fs are microSiemens (μS), which can be restated as microamps per volt (μA/V). MilliSiemens (mS), or milliamps/volt (mA/V) might also be used. For a FET with Y_js = 2 mA/V. I_D changes by 2 mA when V_GS is altered by 1 V. In the portion of a FET Datasheet Specifications in Fig. 9-20, the Y_fs units are μmhos. The mho (ohm written backwards) is another (older) name for the Siemen, the unit of conductance. For the 2N5457 (underlined) the value of Y_fS is specified as a minimum of 1000 μmhos, and a maximum of 5000 μmhos. An inverted ohm symbol is also sometimes used instead of the Siemens symbol.

Because Y_fs defines the relationship between I_D and V_GS, it can be determined from the slope of the FET transfer characteristic. This is illustrated in Fig. 9-21.

The transfer characteristic for a FET Datasheet Specifications is defined by Eq. 9-1, and Y_fs is determined from the slope of the transfer characteristic. So, an equation for Y_fs can be derived by differentiating Eq. 9-1.

Equation 9-3 may be used to calculate the value of Y_fs for any V_GS level. As in Eq. 9-1, the negative sign for V_GS is already included, so that only the numerical value should be entered.

Output Admittance:

The drain resistance (r_d) of a FET is the ac resistance between drain and source terminals when the device is operating in the pinch-off region of its drain characteristics. it is also the slope of the drain characteristics in the pinch-off region; (see Fig. 9-22). The drain characteristics are almost flat, so, r_d is not easily determined from the characteristic. Because r_d is usually the output resistance of the FET at the drain terminal, it may also be expressed as an output admittance (Y_os = 1/r_d).

The output admittance is defined as,

Drain-Source ON Resistance:

The drain-source ON resistance (r_DS(on)) (also designated R_D(on)), is a dc quantity, not to be confused with the ac drain-source resistance (r_ds). r_DS(on) is the resistance of the FET channel when the depletion regions are absent; when the device is biased ON in the channel ohmic region of the drain characteristics. (see Fig. 9­-23). In this condition, the voltage drop along the channel from drain to source is I_D x r_DS(on)). This is the drain-source ON voltage (V_DS(on)). which is similar to the V_CE(sat) of a BJT.

The drain-source ON resistance might typically be 60 Ω or lower. It can be an important quantity for FETs used in switching circuits. V_DS(on) can be much smaller than V_CE(sat) making FET gates superior to BJT gates for some applications.

Gate Cutoff Current and Input Resistance:

The gate-channel junctions in a JFET are pn-junctions, and because they are normally reverse biased, a minority charge carrier current flows. This is the gate-source cutoff current (I_GSS), or gate reverse current. [see Fig. 9-24(a)). For a 2N5457 FET. I_GSS is specified as 1 nA at 25°C, and 200 nA at 100°C.

The gate input resistance (R_GS) is the resistance of the reverse-biased gate-channel junctions [Fig. 9-24(b)], and it is inversely proportional to I_GSS. Typical values of R_GS for a JFET are 10⁹ Ω at 25°C and 10⁷ Ω at 100°C.

Breakdown Voltage:

The are several ways used for expressing JFET breakdown voltage. BV_DGO is the drain-gate breakdown voltage with the source terminal open-circuited. BV_GSS is the gate-source breakdown voltage with the drain terminal shorted to the source. Both are a specification of the voltage at which the gate-channel junctions might break down.

Noise Figure:

A FET usually has a much lower level of thermal noise than a BJT. This is because there are very few charge carriers crossing junctions in a FET. As in the case of a BJT, the FET noise figure (NF) is specified as a spot noise figure at a particular frequency and bias conditions, and for a given resistance at the input. The figure varies if any of these conditions are altered. Noise calculations for a FET circuit are performed exactly as for a BJT circuit.

Capacitances:

Terminal capacitances for FETs may be specified as gate-drain capacitance (C_gd). gate-source capacitance (C_gs), and drain-source capacitance (C_ds). The input capacitance is sometimes expressed as the common-source input capacitance (C_iss or C_gss). This is the gate-source capacitance measured with the drain terminal shorted to the source, (Fig. 9-25). In this case, a reverse transfer capacitance (C_rss) is also specified: C_rss being another term for C_gd. These quantities are very important for FET high-frequency and switching circuits.