MOSFET switching characteristics

The turn ON and OFF times of MOSFET gets affected by its internal capacitance and the internal impedance of the gate drive circuit but it does not affect during steady-state operation. 

To understand switching characteristics of MOSFET we can take the simple equivalent circuit for an n-type MOSFET is given below :

Turn ON Process :

• The gate voltage is made positive to turn ON the MOSFET. When the gate voltage has applied the gate to source capacitance C_GS starts charging.
• When the voltage reached through C_GS certain voltage level is called the Threshold voltage ( V_GST ) at the same time drain current I_D begins to increase. 
• The time needed to charge C_GS to the threshold voltage level is known as a turn-on delay time.
• The C_GS charges from threshold to the full gate (V_GSP) voltage. The time required for this charging is known as rising time (t_r).
• During this period, the drain current increases to its full value of I_D because of that the MOSFET is fully turned ON.
• MOSFET turn-on time is given by, 

t_ON = t_[d(on)] + t_r

• The turn-on time can be reduced using a low-impedance gate drive source. 

Turn OFF Process:

• The MOSFET can be turned off with a negative or zero gate voltage. Due to this, the gate to source voltage decreases from V_I to V_GSP.
• C_GS discharges from V_I to V_GSP gate voltage. The time required for this discharge is called a turn-off delay time, during which the drain current also begins to decrease.
• The C_GS continues to discharging and its voltage equals a threshold voltage (V_GST).
• The time required to discharge C_GS from V_GSP to V_GST is called fall time (t_f). The drain current will be zero when the voltage V_GS < V_GST then it is said to have turned off. 
• MOSFET turn-off time is given by,  

t_OFF = t_[d(off)] + t_f

MOSFET switching waveform is shown in the figure below : 

MOSFET characteristics

In general, any MOSFET shows three operating regions following below.

1. Cut-off Region 
2. Ohmic or Linear Region 
3. Saturation Region 

Cut-off Region :

It is a region where MOSFET will be OFF because there will be no current flow through it. In this region, MOSFET acts as an open switch and thus when they are required to function as electronic switches. 

Ohmic Region : 

Ohmic or Linear region is a region where the current increases as the value of the voltage increases. If MOSFET is used in this region, it can be used as amplifiers. 

Saturation Region : 

In this region, despite the increase in voltage, the MOSFET has it's current constant and occurs when the voltage exceeds the pinch-off value. Under this condition, the device acts as a closed switch that saturates the value of current flows. This operating region is therefore selected whenever MOSFET is required to perform a switching operation. 

Now that we know this, let us analyze the prejudice in which these regions are experienced for each type of MOSFET. 

The transfer and output characteristics of the power MOSFET following below :

Transfer characteristics :

• This feature shows the variation of the current  I_D of the drain as a function of the V_GS gate-source voltage. 
• V_GS is the minimum positive voltage to induce n-channel between gate and source. Therefore, for threshold voltage below V_GS, the device is in the off state, a magnitude of is of V_GST the order of 2  to 3 V. This is typical characteristics for n-type MOSFET. 

MOSFET output characteristics :

• The output characteristics of MOSFET shown in the figure indicate the variation of the drain current I_D as a function of the V_DS drain-source voltage with the V_GS gate-source voltage as a parameter.
• For a low V_DS value, the graph between I_D and V_DS is almost linear, indicating a constant value of on-resistance R_DS = V_DS/I_D.
• If V_DS is increased for a given V_GS, the output characteristics are relatively flat, indicating drain current is nearly constant.
• The output characteristics of the load line A and B intersect. A indicates fully on condition and B fully off condition. MOSFET works as a switch either at A or B.
• When power MOSFET is driven with large gate-source voltage, MOSFET is turned on, V_DS is small. Here, MOSFET acts as a closed switch is said to be ohmic region.
• When the device turns on, MOSFET traverses characteristics from cut off to the active region and then to the ohmic region.
• When MOSFET turn off, it takes a backward journey from ohmic region to cut off state.

What is MOSFET | History | Operation | Types | Applications

MOSFET Introduction :

The MOSFET, commonly known as Metal Oxide Semiconductor Field Effect Transistor, is one type of semiconductor device that is widely used in electronic devices to switch and amplify electronic signals. 

It is an integrated circuit core and can be designed and manufactured in one chip. MOSFET consists of four-terminal devices such as source (S), gate (G), drain (D), body (B). 

History of MOSFET :

1925 - Julius Edgar Lilienfield first established the basic principle of this type of transistor. 

1959 - MOSFET was invented on the basis of FET design by Dawon Kahng and Martin Atalla at Bell Labs. 

Operation of MOSFET : 

MOSFET's goal is to be able to control the voltage and current flow between source and drain. Its work depends upon the MOS capacitor and works almost like a switch. The surface of the semiconductor at the below oxide layer which can be located between the source and drain terminal. It can be inverted from p-type and n-type by applying positive or negative gate voltages. The holes present under the oxide layer with offensive force and holes are pushed downward with the substrate when we apply positive gate voltage. The depletion region is formed, populated by the bound negative charges which are associated with the acceptor atoms and therefore electrons reach the channel. The positive voltage also attracts electrons from the source of n+ and drain regions into the channel. If a voltage is applied between them, the current flows freely between the source and drain, and the electrons in the channel are controlled by the gate. If a negative voltage is applied, a hole channel is formed under the oxide layer instead of a positive voltage. 

Types of MOSFET : 

1. Depletion Mode MOSFET 
2. Enhancement Mode MOSFET 

The channel shows its minimum conductance when there is zero voltage on the gate terminal. Since the voltage on the gate is negative or positive, the channel conductivity will be reduced. This type of transistor is called MOSFET depletion mode.

The channel does not conduct when there is no voltage on the gate terminal. The device has good conductivity when more voltage applied to the gate terminal. This is called a MOSFET enhancement mode.

P-channel MOSFET : 

The MOSFET P-channel has a region of the P-channel between drain and source. The MOSFET P-channel consists of negative ions and therefore works with a negative voltage. When the negative voltage is applied to the gate, the electrons present under the oxide layer are pushed downward into the substrate with an excessive force. The depletion region is populated by the bound positive charges which are allied with the donor atoms. The negative voltage attracts holes from p+ source and also drain region into the channel region as well. 

N-channel MOSFET : 

The MOSFET N-channel has a region of the N-channel between source and drain. When we apply the gate voltage, the holes present in the oxide layer pushed downward with a repulsive force into the substrate. The depletion region is populated by the bound negative charges linked to the acceptor atoms. The positive voltage also attracts electrons from the n+source and drain region. If a voltage is applied between the drain and source, the current flows freely between the source and drain, and the electrons in the channel are controlled by the gate voltage. If a negative voltage is applied a hole channel will be formed under the oxide layer. 

MOSFET Application : 

• Used as a switch 
• Used in MOS integrated circuits 
• CMOS circuits 
• Switched-mode power supply ( SMPS )
• Inverters
• Used as a constant current source 

For detailed information :

Read more >> Applications of MOSFET

MOSFET Application

MOSFET is one of the important elements in the design of embedded systems used to control the loads as per requirements.

In applications such as switched-mode power supply, variable frequency drives and other power electronics applications where each device can switch thousands of watts, discrete devices are widely used. 

Different types of MOSFET applications are used as per requirement. 

Application of MOSFET : 

• Used as a switch 
• Used in a calculator
• Used in audio frequency power amplifier for the public address system
• Used in high-frequency amplifier for amplifying electronics signals in the electronic devices
• Used in MOS integrated circuits, CMOS circuits, and VLSI circuits
• Used in both analog and digital circuit
• Used in switched-mode power supplies and inverters
• Used as constant current sources
• Used in brushless DC motor drive
• Used in electronic DC relay
• Used in light intensity control
• Used in motor speed control 
• Used in a high-frequency generator
• Used in sound reinforcement 
• Used in automobile sound system

Explore more information:

1. BJT vs MOSFET

Applications of NPN transistor

NPN transistor with their function in amplifying currents they have many applications and between PNP and NPN, the only difference is the direction of voltage flow.

Let we check the applications of NPN transistor :

• Mostly used in switching applications.
• Used in applications where there is a need to sink current.
• Used in amplifying circuit applications, such as push-pull amplifier circuits. 
• It is used to amplify weak signals in the Darlington pair circuits. 
• Used in temperature sensors. 
• Used in very high-frequency applications.
• Used in logarithmic converters.

Characteristic of BJT

Before we check the characteristics you should know the BJT full meaning. It is helpful to view the characteristic curves of the transistor in graphical form is very similar to the graphical approach used with diodes. 

Now we can check it out the characteristics of BJT. 

BJT Input characteristics :

• A graph of base current I_B Vs base-emitter voltage V_BE gives input characteristics. 
• Since a transistor's base-emitter junction is like a diode I_B versus V_BE graph resembles a diode curve. 
• When collector-emitter voltage V_CE2 is more than V_CE1, base current, for the same V_BE, decreases as shown in the figure. 

BJT Output characteristics :

• A graph of collector current IC Vs collector-emitter voltage V_CE gives output characteristics.
• For zero base current, for example, I_B = 0, as  V_CE is increased, a small leakage (collector) current exists as shown in the figure.
• As the base current is increased from I_B = 0 to I_B1, I_B2 etc, collector current also rises as shown in Figure.

What is BJT

BJT full form is a bipolar junction transistor that uses both electron and hole charge carriers. For their operation, BJT uses two junctions between two semiconductor-type such as n-type and p-type. 

BJTs are manufactured in two types, NPN and PNP, and are available as individual components, or fabricated in integrated circuits, in large numbers. The function of a BJT is to amplify current that can be used as amplifiers or switches. These functions offer a wide range of electronic equipment applications, including computers, TVs, mobile phones, audio amplifiers, industrial control, and radio transmitters. 

Meaning of BJT :

• A bipolar junction transistor is a three layer, two junction NPN or PNP semiconductor device with one p-region sandwiched by two n-region and two p-region sandwiched one n-region. It has three terminal named collector (C), Emitter(E), and base(B). 

Figure of  BJT

• The current flow in the device takes place due to movement both holes and electrons. 
• An emitter is indicated by an arrowhead indicating the direction of emitter current. No arrow is associated with base or collector. 

Schematic diagram symbol of BJT :

NPN                                                  PNP  

Types of  BJTs  :

There are two types of junction transistor :

1. NPN transistor 
2. PNP transistor 

This article gave brief details about NPN and PNP transistor like working principle, advantages and application to better understand this topic.

The working principle of NPN transistor : 

• This circuit is NPN types of BJT transistor shown in the figure there are two types of current flow I_C , I_E is receptively known as collector and emitter current and V_CB , V_EB is collector-base voltage and emitter-base voltage.
• Shown in figure current I_C , I_{E  ,} I_B current going into the transistor is and the sign is taken as positive and if current goes out sign is taken as negative.

 NPN transistor Application:

• Use as an amplifier
• Use as a Darling-tone pair
• Use as a switch

The working principle of PNP transistor :

• In P-N-P junction transistor, emitter current enter through the emitter terminal shown in the figure. 
• When using any BJT device, the junction of emitter-base is forward biased and the junction of the collector-base is in reverse biased.

So conclude that BJT can be operated in the different mode of BJT like cut off, saturated and active mode.

PNP transistor application :

• Used in darling-ton pair circuit
• Used in heavy motors to control current flow
• Used as switch
• Used as a robotic workshop
• Used in the amplifying circuit

IGBT construction

Before knowing about the construction of IGBT one question you should be clear about is  What is full form of IGBT?

IGBT Structure :

Illustrates the basic structure of an IGBT.  IGBT is constructed virtually in the same way as a power MOSFET construct. There a major difference in the substrate. The n⁺ layer subtract at the drain in a MOSFET is now substituted in the IGBT by a p⁺ layer substrate called collector C. Like a power  MOSFET, an IGBT has also thousands of structure cells connected appropriately on a single chip of silicon.
                      

In the IGBT p+ substratum injects holes into n-layer so-called as injection layer. The n-layer is called the region of drift. As in other semiconductor devices, the n-layer thickness determines IGBT's capacity to block voltage. Layer p is called the IGBT body region. the n^- layer between p+ and p regions accommodates the pn^- junction of the depletion layer, Example- junction J_2.

IGBT switching characteristics

How IGBT Switching characteristic works?

Switching characteristics of an IGBT during turn-on and turn-off are sketched in fig. Turn-on time is defined as the time between the instant of forward blocking to forward on the state. Turn-on time is composed of delay time t_dn and rise time ton =  t_dn +t_r.

• The delay time is defined as the time for the collector-emitter voltage to fall from V_CE to 0.9 V_CE. Here V_CE is the initial collector emitter voltage.
• Time t_dn may also be defined as the time for the collector current to rise from its initial leakage current I_CE to 0.1 I_c. Here I_c is the final value of collector. The rise time t_r is the time during which collector-emitter voltage falls from  V_CE.
• It is also defined as the time for the collector current to rise from 0.1 I_C to its final value I_C. After time t_on, the collector current  I_C is and the collector-emitter voltage fall to a small value called conduction drop is said to be V_CES where subscript S denotes saturated value.

• The turn-off time is somewhat complex.
• It consists of three intervals : (i) delay time , (ii) initial fall time and (iii) final fall time.
• t_off = t_df +t_f1+t_f2.
• The delay time is the time during which gate voltage fall forms V_GE to threshold V_GET.
• As V_GE falls to V_GET during t_df, the collector current falls from I_c to 0.9 I_c . At the end of t_df, the collector-emitter voltage begins to rise.
• The first fall time t_f1 is defined as the time during which collector current fall from 90 to 20 % of its initial value of current I_C, or the time during which collector-emitter voltage rise from V_CES to 0.1 V_CE.
• The final fall time t_f2 is the time during which collector current fall from 20 to 10% of  I_c or the time during which collector-emitter voltage rise from 0.1  V_CE to final value  V_CE has shown in  figure.

Applications of power electronics

The era of modern power electronics began with the invention of silicon controlled rectifier by cell bell laboratories in 1956. Its prototype was introduced by GEC in 1957 and subsequently, GEC introduced SCR based systems commercially in 1958.

Since then, There has been the development of many new power semiconductor devices. Today power electronics systems incorporate power semiconductor devices as well as microelectronic integrated circuits.

The terms converter system, in general, is used to denote a static device that converts DC to AC or AC to DC. Conventional power controller based on thyratrons and mercury arc controller using power semiconductor devices in almost all applications. 

The development of new power semiconductor device new circuit topology with their improved performance and their fall in price have opened up a wide field for the new application of power electronic converter. It is said to power semiconductor devices can be regarded as the muscle and the microelectronics as the intelligent brain in the modern power electronic systems.

Some of the typical applications are:

• Domestic and theater lighting.
• Generation and transmission control.
• Power supplies can be used laboratories and uninterruptible power for important loads.
• An industrial application like chemical device, paper, and steel industries.

Some other application of power electronics :

• Aerospace supplies: Space shuttle power supplies, satellite power supplies, aircraft power systems.
• Commercial: Advertising, heating, elevators, light dimmers, uninterruptible power supplies, flashers, and industrial lasers.
• Industrial area: Transformer tap changers, blowers and fans, pumps and compressor, industrial laser, cement mills, rolling mills, textiles mills, cement mills, welding, arc, and industrial furnaces. 
• Residential area: Cooking, vacuum cleaner, lighting, air-conditioning, space heating, refrigerators, electric-door openers, dryers, fans, food warmer trays, personal computers, light dimmer, food mixer, electric blanket.
• Telecommunication device: Power supplies (DC and UPS device), battery chargers.
• Transportation: Electric vehicles, electric locomotives, streetcars, trolley buses, Battery chargers, subways, automotive electronics.
• Utility Process: VAR compensation, HVDC, static circuit breakers, fans, and boiler feed pumps, supplementary energy system.