What is the Difference between NMOS and PMOS?

What is NMOS

NMOS English is called N-Metal-Oxide-Semiconductor. It means an N-type metal-oxide-semiconductor, and a transistor having such a structure is called an NMOS transistor. The MOS transistor has a P-type MOS transistor and an N-type MOS transistor. An integrated circuit composed of a MOS transistor is called a MOS integrated circuit, and a circuit composed of an NMOS is an NMOS integrated circuit. A circuit composed of a PMOS transistor is a PMOS integrated circuit, and a complementary MOS circuit composed of two types of NMOS and PMOS transistors, that is, a CMOS circuit .

What is PMOS

The PMOS refers to an n-type substrate, a p-channel, and a MOS transistor that carries a current by the flow of holes.

The hole mobility of the P-channel MOS transistor is low, and thus the transconductance of the PMOS transistor is smaller than that of the N-channel MOS transistor in the case where the geometry of the MOS transistor and the absolute value of the operating voltage are equal. In addition, the absolute value of the threshold voltage of the P-channel MOS transistor is generally high, requiring a higher operating voltage. The voltage magnitude and polarity of its power supply are not compatible with bipolar transistors, transistor logic. Due to the large logic swing, the PMOS has a long charging and discharging process, and the device has a small transconductance, so the operating speed is lower. After the NMOS circuit (see N-channel metal-oxide-semiconductor integrated circuit) appears, most of the NMOS circuits are used. replace. However, due to the simplicity of the PMOS circuit process and the low price, some medium- and small-scale digital control circuits still use PMOS circuit technology.

The difference between NMOS and PMOS

In the actual project, we basically used enhanced.

The mos tube is divided into N-channel and P-channel. We use NMOS because it has small on-resistance and is easy to manufacture. As you can see on the MOSFET schematic, there is a parasitic diode between the drain and the source. This is called a body diode, which is important for driving inductive loads such as motors. Incidentally, the body diodes are only present in a single MOS transistor and are usually not available inside the integrated circuit chip.

1, conduction characteristics

The characteristics of the NMOS, Vgs greater than a certain value will be turned on, suitable for the case when the source is grounded (low-side drive), as long as the gate voltage reaches 4V or 10V. The characteristics of the PMOS, Vgs is less than a certain value will be turned on, suitable for the case when the source is connected to VCC (high-end drive). However, although PMOS can be conveniently used as a high-side driver, NMOS is usually used in high-end driving because of its high on-resistance, high price, and low replacement.

2. MOS switch tube loss

Whether it is NMOS or PMOS, there is an on-resistance after conduction, so the current will consume energy on this resistor, and this part of the energy consumed is called conduction loss. Selecting a MOS transistor with a small on-resistance reduces the conduction loss. The current low-power MOS tube on-resistance is generally in the range of several tens of milliohms, and several milliohms are also available. When MOS is turned on and off, it must not be completed in an instant. The voltage across the MOS has a falling process, and the current flowing through has a rising process. During this time, the loss of the MOS tube is the product of voltage and current, called switching loss. Usually, the switching loss is much larger than the conduction loss, and the higher the switching frequency, the greater the loss. The product of the voltage and current at the turn-on moment is large, and the loss is large. By shortening the switching time, the loss per turn-on can be reduced; reducing the switching frequency can reduce the number of switches per unit time. Both of these methods can reduce switching losses.

3. MOS tube driver

Compared with bipolar transistors, it is generally considered that no current is required to turn on the MOS transistor, as long as the GS voltage is higher than a certain value. This is easy to do, but we still need speed. It can be seen in the structure of the MOS transistor that there is a parasitic capacitance between GS and GD, and the driving of the MOS transistor is actually charging and discharging the capacitor. The charging of the capacitor requires a current, because the capacitor can be regarded as a short circuit when charging the capacitor, so the instantaneous current will be relatively large. The first thing to note when selecting/designing a MOS tube driver is the amount of transient short-circuit current available.

The second note is that NMOS, which is commonly used for high-side driving, needs to have a gate voltage greater than the source voltage when turned on. When the high-side driving MOS transistor is turned on, the source voltage is the same as the drain voltage (VCC), so the gate voltage is 4V or 10V larger than VCC. If you want to get a voltage larger than VCC in the same system, you need a special boost circuit. Many motor drivers have integrated charge pumps. It is important to note that a suitable external capacitor should be selected to get enough short-circuit current to drive the MOSFET.