I have N-MOSFET gate connected to a 4043 logic. Id is about 100mA. Both the 4043 and MOSFET have +5V. I plan to use a 2N7000.

How large a gate resistor do I need between the 4043 and MOSFET. The logic output is sometimes switched rapidly. How fast? A motherboard HDD LED controls it.
Do I need to place pull down resistor from logic to 0V, between the 4043 and the MOSFET?


It is generally a good idea to include a gate resistor to avoid ringing. Ringing (parasitic oscillation) is caused by the gate capacitance in series with the connecting wire's inductance and can cause the transistor to dissipate excessive power because it doesn't turn on quickly enough and hence the current through drain/source in combination with the somewhat high'ish drain-source impedance will heat the device up. A low ohm resistor will solve (dampen) the ringing.

As @PhilFrost mentiones, a high value resistor to ground is a good idea to avoid capacitive coupling driving the transistor when it is otherwise not connected.







simulate this circuit – Schematic created using CircuitLab

At all times keep the wiring between logic output, transistor gate, transistor source and ground as short as possible. This will ensure fast turn on/off.
