When building an amplifier, a question that often arises is whether we should use a MOSFET or a regular transistor. As a matter of fact, both MOSFETs and regular transistors are widely used for making amplifiers. And, we should also understand that MOSFETs are actually also a type of transistor. So, what is the difference between MOSFETs and regular transistors? What are the advantages and disadvantages of each of MOSFETs and regular transistors? Below, we will take a more detailed look at both amplifier MOSFETs and amplifier transistors. Continue reading!
Overview of MOSFET
The term “MOSFET” actually stands for “Metal Oxide Semiconductor Field Effect Transistor”. So, it is also a type of transistor – a type of Field Effect Transistor (FET), to be precise. The MOSFET is a type of transistor that is usually used for switching or amplifying electronic signals. It features an insulated gate. The voltage of the insulated gate determines the conductivity of the device. This very feature to change conductivity according to the amount of voltage is the reason why MOSFETs can be used for switching or amplifying electronic signals.
Today, the word “metal” in the name MOSFET is often a misnomer. This is because the gate material is now made from a layer of polycrystalline silicon rather than metal. The word “oxide” is sometimes also a misnomer, as different dielectric materials are utilized in order to obtain strong channels on small voltages.
“Metal Insulator Semiconductor Field Effect Transistors” (MISFETs) and “Insulated Gate Field Effect Transistors” (IGFETs) are usually synonymous to MOSFETs. By far, the MOSFET is the most common transistor used in digital circuits, memory chips, and microprocessors. Due to the fact that MOSFETs can be made using either n-type or p-type semiconductors, complementary pairs of MOSFETs can be utilized to create switching circuits with very low power consumption. MOSFETs are often preferred due to the significantly low resistance, low heat dissipation, and ‘automatic’ conductivity control.
Overview of Transistor
On the other hand, the transistor is quite a broad term. A transistor is a semiconductor device that is utilized to switch or amplify electrical power and electronic signals. The name “transistor” is actually derived from the device’s ability to transfer a current across a resistor (transistor = transfer + resistor). Transistors are often used on complex circuits such as logic gates, memory chips, and microprocessors.
Typically, a transistor features three electrodes in a conducting path so as to alter the voltage or current in the electronic circuit. A transistor may be composed of either p, n, p semiconductors or n, p, n semiconductors. Transistors can be classified into two groups: bipolar junction transistors (BJTs) and field effect transistors (FETs). Bipolar junction transistors are those that use both majority and minority carriers to conduct, and have low input impedance. Field effect transistors are transistors that use either electrons (n-channel FETs) or holes (p-channel FETs) to conduct, and generally have higher input resistance.
There are four terminals on a FET, which are source, gate, drain, and body. However, most FETs have the body connected to the source inside the packaging.
MOSFETs vs. Transistors: Heat Dissipation
Despite being a type of transistor itself, the MOSFET can be quite different if compared to a regular transistor of either the BJT or FET type. The primary difference that often affects people’s preference in building an amplifier is the heat dissipation. MOSFETs tend to dissipate very little heat, thanks to the extremely low resistance, much lower than that of regular transistors.
A transistor has a variable power; it can be turned to any level in between the on and off positions. Such feature is great if you need to control the transistor level. However, the problem is that, if the transistor is not fully turned on, it may dissipate a fair amount of heat. Even a BJT, which has low input impedance, can still dissipate considerable heat if not fully turned on. And we generally want to produce as little heat as possible when building an amplifier or any other circuit. If not properly controlled, the heat dissipation may cause overheating, which in turn may cause various problems. The last thing that you want to happen is your amplifier circuit getting burned due to excessive overheating.
On the other hand, MOSFETs dissipate significantly less heat due to extremely low input resistance when turned on. An amplifier using MOSFETs is much less likely to overheat. By using MOSFETs, you won’t have to worry so much about heat dissipation.
MOSFETs vs. Transistors: Control
As mentioned above, a transistor has a variable power which allows the unit to be not fully turned on or off. It can be at any level in between the on and off positions. In addition, let’s make a note that bipolar junction transistors (BJTs) are current controlled devices. In order to turn them on, you have to flow current into the base for NPN transistors or to flow current out of the base for PNP transistors.
On the other hand, MOSFETs are either on or off. It can only be in one of two possible positions: on or off. Don’t get confused when people say MOSFETs have extremely high or low input resistance. When turned on, the input resistance is incredibly low. When turned off, the input resistance is extremely high. The ultra high input impedance makes a MOSFET very easy to bias. However, MOSFETs are voltage controlled devices. You turn it on by applying a voltage that exceeds the threshold voltage on the gate. You turn it off by removing the voltage from the gate.
|- Voltage controlled||- Current controlled|
|- Is either on or off||- Can be anywhere in between the on and off positions|
|- Dissipates very low heat||- Dissipates quite some heat, especially if not fully turned on|
|- Better for preventing overheating||- May experience overheating|
MOSFETs are a type of transistor, but MOSFETs are different from the regular transistors. A regular transistor is current controlled, and may dissipate a fair amount of heat if not fully turned on. On the other hand, a MOSFET is voltage controlled, is either on or off, and dissipates very little heat. For an amplifier, MOSFETs are generally much more recommended than transistors.