Detailed explanation of MOSFET parameters

Thread Content

1. Turn-on voltage VT – The gate voltage required to establish a conductive channel between the source S and the drain D； -A standard N-channel MOSFET has a VT of approximately 3–6 V ； -Through process improvements, it is possible to reduce the VT value of MOSFETs to 2–3V. 2. The DC input resistance RGS – that is, the ratio of the voltage applied between the gate and source to the gate current – is sometimes expressed in terms of the gate current flowing through it. The RGS of a MOSFET can easily exceed 1010 Ω. 3. Gate-source breakdown voltage BVGS – The VGS at which the gate current IG increases sharply from zero as the gate-source voltage is increased is called the gate-source breakdown voltage BVGS. 4. Drain-Source Breakdown Voltage BVDS – Under the condition of VGS=0 (enhanced mode), the VDS at which ID starts to increase sharply as the drain-source voltage is increased is known as the drain-source breakdown voltage BVDS. The reasons for this sharp increase in ID are twofold: (1) avalanche breakdown of the depletion layer near the drain, and (2) tunneling breakdown between the drain and source. In some MOSFETs, the channel length is relatively short; as VDS increases, the depletion layer in the drain region expands toward the source region until the channel length becomes zero, resulting in tunneling between the drain and source. After tunneling occurs, the majority carriers in the source region are directly drawn toward the drain region by the electric field of the depletion layer, leading to a large value of ID.

5. Low-Frequency Transconductance gm – At a fixed value of VDS, the ratio of the small change in drain current to the small change in gate-source voltage that causes this change is called transconductance. gm reflects the ability of the gate-source voltage to control the drain current; it is an important parameter that characterizes the amplification capability of a MOSFET. Typically, its value ranges from a few milliamps per volt to several mA/V.

6. Inter-Electrode Capacitances – There are inter-electrode capacitances among the three electrodes: gate-source capacitance CGS, gate-drain capacitance CGD, and drain-source capacitance CDS. CGS and CGD are approximately 1–3 pF, while CDS is around 0.1–1 pF.

7. On-Resistance RON – The on-resistance RON indicates the effect of VDS on ID; it is the reciprocal of the slope of the tangent line at a certain point on the drain characteristic curve. In the saturation region, ID hardly changes with VDS, so RON has a high value, typically ranging from several tens of kiloohms to several hundred kiloohms. Since MOSFETs in digital circuits often operate at VDS=0 when in the on state, the on-resistance RON at this time can be approximated by the RON at the origin. For ordinary MOSFETs, RON is within a few hundred ohms.

8. Low-Frequency Noise Figure NF – Noise is caused by the irregular movement of carriers inside the device. Due to this noise, an amplifier exhibits irregular voltage or current variations at the output even when no signal is applied. The level of noise performance is usually expressed using the noise figure NF, measured in decibels (dB). The lower this value, the less noise is generated by the device. The low-frequency noise figure refers to the noise figure measured at low frequencies. The noise figure of field-effect transistors is typically a few decibels, which is less than that of bipolar transistors.