MOSFET
＜Understanding MOSFET Characteristics＞

MOSFET Capacitance and Temperature Characteristics

Parasitic Capacitance

Parasitic capacitance exists in power MOSFETs as shown in Figure 1.

Sometimes known as stray capacitance, parasitic capacitance is unavoidable and typically unwanted that exists between the parts of an electronic component or circuit simply because of how close they are to one another. Capacitance is the ability of a system to store an electric charge.

The Gate terminal in a MOSFET is isolated from the other terminals by an oxide film. The silicon under the gate has the opposite polarity to the drain and source which results in the formation of PN junctions (diode) between the Gate, Drain and Source regions. Cgs and Cgd are the capacitances of the oxide layers, while Cds is determined by the junction capacitance of the internal diode. The gate electrode controls the charge carrier flow through the semiconductor channel, contributing to the parasitic capacitance in the MOSFET.

Generally, all 3 capacitances (C_iss,C_oss,C_rss) listed in Table 1 are included in MOSFET specifications.

As shown in Figure 2 the capacitance characteristics may depend on V_DS (Drain-Source voltage). As V_DS increases the capacitance decreases.

Temperature Characteristics

There are almost no differences in the capacitance characteristics at different temperatures. Temperature measurement examples are shown in Figure 3 (1)-(3).

MOSFET Switching Characteristics and Temperature Characteristics

What is MOSFET Switching Time?

The MOSFET will turn ON or OFF after the Gate voltage turns ON/OFF. The time in between turning ON or OFF is called the switching time. As a voltage controlled device, the application of gate voltage regulates the switching time and overall performance. Various switching times are listed in Table 1 below. Generally,t_d(on), t_F , t_d(off) and t_r are specified.

Temperature Characteristics

The switching time is only slightly affected by temperature rise - on the order of 10% at 100°C. In other words, switching characteristics are largely independent of temperature. Measurement examples are shown in Figure 3 (1)-(4).

V_GS Threshold Voltage: V_GS(th)

V_GS(th) is the voltage required between the Gate and Source to turn ON the MOSFET. In other words, supplying a voltage greater than V_GS(th) will turn ON the MOSFET. The gate voltage controls the formation of the channel by influencing the charge distribution in the semiconductor. To determine the amount of current that flows through the MOSFET when ON it is necessary to refer to the specifications and electrical characteristics for each element.

Table 1 lists the relevant electrical characteristics. In the case of V_DS=10V a threshold voltage of between 1.0V and 2.5V is required for an I_D of 1mA.

Table 1: Electrical Characteristics

I_D-V_GS and Temperature Characteristics

I_D-V_GS and threshold temperature characteristics examples are shown in Figures 1 and 2 below. As seen in Figure 1, a large Gate voltage is required for large current flow. Applying a positive gate voltage enhances the conductivity of the channel by repelling holes under the oxide layer and attracting electrons from the source and drain, which increases the current flow.

Although the models listed in Table 1 feature a threshold value less than 2.5V, 4V drive is recommended. Always make sure there is a sufficient Gate voltage to turn the MOSFET ON.

Referring back to Figure 2 we see that the threshold value decreases in proportion with the temperature. Therefore, the element channel temperature can be calculated by monitoring the change in threshold voltage.