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Looking for books on MOSFET Circuits? Check our section of free e-books and guides on MOSFET Circuits now! MOSFET Equivalent Circuit Models. Transistor Equivalent Book 2sc - Free download as PDF File .pdf), Text File .txt) or read online for free. transistors, diodes, rectifiers and MOSFETs. Good Book on Mosfet - Free download as PDF File .pdf), Text File .txt) or read Figure Vertical Structure Showing Figure Equivalent Circuit Showing.
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This was the initial conception of the Field Effect Transistor. But due to lack of appropriate semiconduc- tor materials and the immature technology, the development was very slow.
Dacey and Ross improved on it in New products were introduced in the s.
And cur- rently, Fairchild produces QFET series using planar technology and low voltage power trench prod- ucts using trench technology. One is an n-channel type and the other is a p-channel type. They both control the drain-to-source current by the voltage supplied to the gate. As shown in the Figure 1 a , if the bias is not supplied at the gate, the current flows from the drain to the source, and when the bias is supplied at the gate, the depletion region begins to grow and reduces the current as shown in Figure 1 b.
Drain Drain Depletion region N N. Gate Gate VGS. The enhancement type is normally off, which means that the drain to source current increases as the voltage at the gate increases.
No current flows when no voltage is supplied at the gate refer to Figure 3. VGS Gate Gate. N N N N Channel. This is suitable for integration but not for obtaining high power ratings since the distance between source and drain must be large to obtain better voltage blocking capability.
Also, the drain-to-source current is inversely proportional to the length. This is suitable for a power device, as more space can be used as source. As the length between the source and drain is reduced, it is possible to increase the drain-to-source current rating, and also increase the voltage blocking capability by growing the epitaxial layer drain drift region.
It is the most commercially successful design. High input impedance - voltage controlled device - easy to drive. And also a larger reverse base drive current is needed for the high speed turn-off of the current controlled device BJT. Due to these charac- teristics base drive circuit design becomes complicated and expensive. As the required gate current during switch- ing transient as well as the on and off states is small, the drive circuit design is simple and less expensive.
Unipolar device - majority carrier device - fast switching speed. As there are no delays due to storage and recombination of the minority carrier, as in the BJT, the switching speed is faster than the BJT by orders of magnitude.
Hence, it has an advantage in a high frequency operation circuit where switching power loss is prevalent. Wide SOA safe operating area. It has a wider SOA than the BJT because high voltage and current can be applied simulta- neously for a short duration. This eliminates destructive device failure due to second break- down. Forward voltage drop with positive temperature coefficient - easy to use in parallel. When the temperature increases, the forward voltage drop also increases.
This causes the current to flow equally through each device when they are in parallel. Parasitic BJT exists between the source and the drain. At this state, if a drain voltage higher than BVCEO is supplied, the device falls into an avalanche breakdown state. If the drain current is not limited externally, it will be destroyed by the second breakdown.
Due to the source region being short, another parasitic component, the diode is formed. This is used in half-bridge and full-bridge converters. Refer to Figure 6. A constant resistance region. If the drain-to-source voltage is zero, the drain current also becomes zero regardless of gate—to-source voltage.
Even if the drain current is very large, in this region the power dissipation is maintained by minimizing VDS on. A constant current region. Here, the drain current differs by the gate—to- source voltage, and not by the drain-to-source voltage. Hence, the drain current is called saturated. It is called the cut-off region, because the gate-to-source voltage is lower than the VGS th threshold voltage.
Transfer characteristics iD characteristics due to VGS in the active region. Refer to Fig. A parabolic transfer curve exists in a logic-level device according to the above equation. This is because the mobility of the carrier is not constant, but decreases due to the.
This is the maximum drain-to-source voltage where the MOSFET can endure without the avalanche breakdown of the body-drain pn junction in off state where the gate and source are shorted. Avalanche, reach-through, punch-through, zener, and dielectric breakdowns are the 5 factors which drive breakdown. Three of these factors are described below:.
It is the main factor among others that drives breakdown.
The drain-to-source leakage current when it is an off state where the gate is being shorted with the source. The formation of the depletion region: When a small positive gate — to — source voltage is supplied to the gate electrode refer to Fig- ure 8 a:. A positive charge induced in the gate electrode, inducts the same amount of negative charge at the oxide — silicon interface P—-body region, which is underneath the gate oxide.
The holes here are pushed into the semiconductor bulk by an electric field, and the depletion region is formed by the acceptors with a negative charge. As the positive gate — to — source voltage increases refer to Figure 8 b , c:. The depletion region becomes wider towards the body, and begins to drag the free electrons to the interface.
These free electrons are created by thermal ionization. The free holes, cre- ated with free electrons, are pushed into the semiconductor bulk. If the supplied voltage keeps increasing, the den- sity of the free holes of the body, and the free electrons of the interface becomes equal. At this point, the free electron layer is called an inversion layer. Threshold voltage: Figure 8: ID starts to flow when the channel has formed and VDD is supplied.
Figure 9: To understand the characteristics, shown in Figure 9, note the voltage drop at VCS x due to ohmic resistance when ID is flowing at the inverse layer. VCS x is the channel-to-source voltage from the source at a distance of x. This voltage is equal to the VGS—Vox x at all x points. Because of this, the resistance increases, and the graph of ID starts to become flat, as opposed to increasing with the increment of VDD.
Silicon starts saturating when the electric field reaches 1. At this point, the device goes into the active region. P-body X 2 C gs. C ds N-drift Source. Figure Vertical Structure Showing Figure The Capacitance between the Gate and Source. The capacitance between the gate and p-body. It is affected by the gate, the drain voltage and the channel length.
Hence, the change of Cgs due to VDS is very small. The Capacitance between the Gate and Drain. This is influenced by the voltage of the gate and the drain. When there are variations in VDS, the area under Cgd n—-drift region meeting with the gate oxide is changed, and the value of the capacitance is affected.
If the Cgs and Cgd which determine the input capacitance become smaller, it is possible to work in high frequency. The Capacitance between the Drain and Source. Figure 12 shows the gate-source voltage, gate-source current, drain-source voltage, and drain- source current during turn-on. They are divided into four sections to show the equivalent circuits at the diode-clamped inductive load circuit. VGS th iD t. So Va varies to IO condition in t2.
This is caused by the voltage drop due to the existing inductance in the line of the circuit. VGS t: VGS is a constant value in accordance with the transfer characteristics as it is in an active region where iD is the full load current IO. So, iG can only flow through Cgd, and is obtained by the following equation.
It is the period where it operates in an ohmic region. RD N-drift. RS N-substrate Drain. To reduce RDS on , the integrity of the chip and trench techniqure are used. This can be stated as shown in the follow- ing equation:. As the gate drive voltage is supplied, charges start to accumulate in N— epi.
The resistance of this accumulation region is RA. The resistance var- ies by the charge in the accumulation layer, and the mobility of the free carriers at the surface.
And if the gate electrode is reduced, its effect is the same as reducing the length of the accumulation layer, so the value of RA is reduced while RJ increases. The N— epi. The resistance of this region is RJ. This is the resistance of the substrate region.
RDS on increases with the temperature. This is an important characteristic of device stability and paralleling. Low VGS th When the VGS th of the n-channel power MOSFET becomes negative due to the existence of charges in the gate oxide, it shows the characteristics of a normally on state, where the con- ductive channel exists even in a zero gate bias voltage.
Even if VGS th is positive, and the value is very small, there could be a turn-on either by the noise signal of the gate terminal, or by the increasing gate voltage during high speed switching. The VGS th can be controlled by the gate oxide thickness.
It increases in proportion to the square root of the background doping.
Temperature characteristic VGS th decreases as the temperature increases, and the rate of decrease can be varied by the gate oxide thickness and background doping level. In other words, the decrease rate increases when the gate oxide becomes thicker and the background doping level increases. It can be expressed as the following equation and represents the amount of change in drain current by the amount of change in the gate-source bias voltage. VDS should be set so that the device can be activated in the saturation region.
As shown in Figure 15, after VGS th is applied, gfs increases dramatically with the increase in the drain current, and it becomes a constant after the drain current reaches a certain point at higher values of drain current. If gfs is high enough, high current handling capability can be gained from the low gate drive voltage. A high frequency response is also possible.
From the following equation which is similar to the RDS on and temperature relationship, it is possible to know the gfs changes by the changes in temperature.
The negative voltage handling capability enables the enhancement of the turn — off speed by providing reverse bias to the gate and the source. Forward or reverse direction is decided by the polarity of the VGS. IGSS is dependent on the quality of the gate oxide and device size. The following figure shows the switching sequence divided into sections. This also means that this period is the charging period to bring up the capacitance to the threshold voltage. It can be divided into 2 regions.
One is the period where the drain current starts from zero increasing with the gate voltage in accordance with the transfer characteristics and reaching up to the load current. The other region is when the drain voltage starts to drop and reaches the on-state voltage drop. As shown in the gate charge characteristics graph, the VGS maintains a constant value as the drain current is con- stant in this region where the voltage decreases. During the rise time, as both the high voltage and the high current exist in the device, high power dissipation occurs.
So the rise time should be reduced by reduc- ing the gate series resistance and the drain-gate capacitance Cgd. After this, the gate voltage continues to increase up to the supplied volt- age level. But, as the drain voltage and the current are already in steady—state, they are not affected during this region. The gate voltage operates in the supplied voltage level during the on state, and when the turn-off transient starts, it starts to decrease.
The td off is the time for the gate voltage to reach the point where it is required to make the drain current become saturated at the value of load current. During this time there are no changes to the drain voltage and the current.
It is the time where the gate voltage reaches the threshold voltage after td off.