What is MOSFET and how it works?

MOSFET

MOSFET is acronym of “Metal Oxide Semiconductor Field Effect Transistor”. It is a transistor which uses Silicon Dioxide and an electric field is responsible for its operation when properly powered hence it is named as MOSFET. It is a four terminal device named as Source (S), Gate (G), Drain (D) and Body or Substrate. The body is internally connected to source hence externally we have only three terminals i.e. Source, Gate and Drain. MOSFETs are categorized in two basic types, i.e. N-Channel and P-Channel MOSFET. This categorization is based on conduction channel created during its operations, we will understand in details while explanation of its working.

P-Channel MOSFET

Construction of a N-Channel MOSFET is as shown in the picture above, which consist of a base of P-type material called Substrate also. Holes are the majority current carriers in P-type material and electrons are minority current carriers. There are two blocks of highly doped N-type material which acts as Source and Drain. A very thin layer of Silicon Dioxide is laid on the top followed by a layer of metal connected to gate terminal. This structure of silicon dioxide and metal layer act as a small capacitor giving very high input gate resistance. Similarly Drain and Source metal terminals are taken out for external connection. Similarly we have P-channel MOSFET where the P-type material is replaced by N-type and vice-e-versa.

Sample MOSFET

This structure is housed in a package which finally looks like this sample MOSFET. Again as we saw in transistor, different MOSFETs have different pin notation in terms of gate, drain and source terminals. In this sample MOSFET, left most pin is Gate, middle one is Drain and right most is Source. There is a metal plate extruded out at top with a screw hole, this is used for mounting the MOSFET on a metal heat sink to absorb excess heat produced during its operations. If we do not use heat sink, such MOSFETs will burn out in few seconds of operation. Again we have a large range of MOSFETs which can handle power from some milli watts to hundreds of watts. Based on the power handling capacity the size keep on increasing and at the same time bigger heat sink should be used and even cooling fans are also required along with heat sink. 

MOSFETs are used as signal amplifiers as well as electronic switches as we saw in transistor section. The UPS and home AC inverters mostly uses MOSFETs to drive a transformer from DC supply fed from battery and the output of transformer is used by us to drive load connected to UPS/AC inverter like our computers, lights and fans etc. 

N-Channel MOSFET Operation

Let us understand the working of a N-channel MOSFET. As we see in connection diagram here, Drain is connected to positive supply and Source is connected to negative supply of battery. In this condition the Drain to Substrate junction is reversed biased hence no current flows through drain to source.  As we connect a small positive supply to Gate with respect to Source, the metal plate at Gate act as positive terminal of small capacitor build around thin silicon dioxide layer. Positive charges are accumulated on the metal plate of Gate terminal forcing electrons in substrate to get accumulated near silicon dioxide layer creating a N-channel between two N-type materials, Drain and Source. These electrons are actually the minority charge/current carriers in P-type material.

This channel of electrons provide a path for current to flow from N-type Drain to N-type Source, as they also have electrons as their majority current carriers. As we increase the Gate to Source voltage, more positive charge is accumulated at Gate plate forcing larger amount of electrons to accumulate opposite to it, making broader conducting N-channel hence allowing more current from Drain to Source. This way for a small change in gate voltage, there is a larger current allowed to flow through MOSFET, hence the Gate voltage acts a controller for the Drain current. Remember in Bipolar Junction Transistor, the base current was controlling the Collector current, here Gate voltage is controlling Drain current.

P-channel MOSFET works in exactly the opposite way by reversing all the supply polarities.

MOSFET Graph

Based on above operating criteria if we plot the graph of Drain to Source voltage (V_DS) versus the drain current (I_D) at different gate voltages, we get similar graph as we got in transistor topic for different base current. Sample graph is shown here, where we can see two regions.

On the left of dotted line we have Ohmic region, because current is changing due to change in V_DS for every Gate voltage.

Region on the right hand side of dotted line is flat horizontal that means there is no change in Drain current for any change in Drain to Source voltage for all range of Gate voltage. This region is saturation region, means no more current flow can happen from Drain to Source as the maximum limit has reached. In this condition MOSFET acts as a closed switch.

Below a certain gate voltage, no current flow through MOSFET. In this condition it works as a open switch.

Symbol

Symbol of MOSFET is as shown here, again with arrow in two different direction to distinguish between N-channel and P-channel MOSFET. Gate terminal is shown with a separation as there is layer of Silicon Dioxide in actual construction forming a small capacitor. The diode shown in symbol is actually not a part of original MOSFET and may not be shown in all MOSFET symbols. This is a protection diode added to MOSFET to avoid reverse current when operating any inductive load like relay or motor. If this diode is not shown in any circuit, we need to add separately to protect the MOSFET.

MOSFET Circuit

A sample MOSFET circuit is shown here consisting of a N-channel MOSFET ‘T1’ with a relay ‘RL1’ connected between Drain and positive supply of V_DS. A diode in parallel to relay is connected to bypass the reverse current generated when it switches Off from ON condition. When there is no gate voltage applied, the MOSFET works as a open switch hence the relay is Off. As soon as we apply a suitable gate voltage the MOSFET acts as a closed switch and hence the relay is energized. This is the way the MOSFET is used as a electronic switch.

That's all about MOSFET, keep watching for next upcoming topic.

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