What are MOSFETs? – MOSFET Threshold Values, ID-VGS Characteristics, and Temperature Characteristics

In succession to the preceding discussion of MOSFET switching characteristics, here we explain the gate threshold voltage, which is a crucial characteristic of MOSFETs, as well as the I_D-V_GS characteristics, and the temperature characteristics of each of these.

MOSFET V_GS(th)： Gate Threshold Voltage

The MOSFET V_GS(th) or gate threshold voltage is the voltage between the gate and source that is needed to turn on the MOSFET. In other words, if V_GS is at least as high as the threshold voltage, the MOSFET turns on.

Some persons may be wondering just how much of a current I_D can be passed on this “MOSFET on” state. And it is true that I_D changes depending on V_GS. Speaking from the standpoint of the V_GS(th) specification, if the conditions are not determined, a value for V_GS(th) cannot be guaranteed; the datasheet for the MOSFET stipulates these conditions. This table is excerpted from the datasheet for the N-ch, 600 V, 4 A power MOSFET R6004KNX.

The blue line surrounds information on V_GS(th), and the conditions column indicates that conditions are V_DS = 10 V and I_D = 1 mA. Under these conditions, and at Ta = 25℃, V_GS(th) is guaranteed to have minimum and maximum values of 3 V and 5 V.

In other words, as V_GS is raised, the MOSFET begins to turn on (I_D begins to flow), and when I_D is 1 mA, the value of V_GS is between 3 V and 5 V inclusive; this value is V_GS(th). Various methods of expression are possible, but we can say that the MOSFET on state is defined as being when I_D = 1 mA at V_DS = 10 V, and the V_GS at this time is taken to be V_GS(th), the value of which is between 3 V and 5 V.

It should also be noted that voltages and currents that change according to some state, such as an input or output or a turning on or off of some function, are commonly called threshold values, and are not limited to MOSFETs.

Temperature Characteristics of V_GS(th) and I_D-V_GS

From the initial graph showing the I_D-V_GS characteristic, the V_GS(th) for the MOSFET can be read off. The condition V_DS = 10 V matches the stipulated condition. When I_D is 1 mA, V_GS is equal to V_GS(th), and so the V_GS when the curve for Ta = 25℃ intersects the 1 mA (0.001 A) line is approx. 3.8 V. The datasheet does not indicate a representative or typical value (indicated by “Typ”), and we see from the graph that the Typ value for V_GS(th) is about 3.8 V. The graph value can in essence be regarded as the Typ value.

Next, we consider the I_D-V_GS characteristic. As the specification value for V_GS(th), the value when I_D = 1 mA is used, but in actual use, a 4 A MOSFET is unlikely to be used with I_D at only 1 mA. For example, when an I_D of 1 A at Ta = 25℃ is required, we see from the graph that the V_GS is about 5.3 V.

From the graph of the I_D-V_GS temperature characteristic, at high temperatures there is a tendency for I_D to increase if V_GS is constant. Taking as an example the previous condition that I_D = 1 A at Ta = 25℃, at Ta = 75℃, an I_D of about 1.5 A can be passed, and so the conditions of use must be considered carefully.

Returning to the gate threshold voltage, the temperature characteristic of V_GS(th) is shown in a graph. As was seen from the I_D-V_GS graph, we see that at 25℃, V_GS(th) is approx. 3.8 V. The temperature in this graph is Tj, but as indicated by the term “pulsed”, the data was obtained in pulsed tests, and it is permissible to assume that Tj ? Ta ? 25℃.

It is seen that the temperature characteristic is such that at high temperatures, V_GS(th) tends to decline. This indicates that as the temperature rises, because V_GS(th) declines, a larger I_D can flow at a lower V_GS. In other words, and as would be expected, this matches the I_D-V_GS temperature characteristic.

V_GS(th) can be used to estimate Tj. The V_GS(th) temperature characteristic is linear, and so a proportionality coefficient can be calculated, and the increase in temperature can be calculated from the amount of change in V_GS(th).

• MOSFET