POWER TRANSISTORS AND THEIR TYPES

✏️POWER TRANSISTORS AND THEIR TYPES

📌Power transistors have controlled turn-on andturn-off characteristics.The power transistors can be classified broadly into five categories:

1. Metal oxide semiconductor field-effect transistors(MOSFETs)

2. COOLMOS

3. Bipolar junction transistors (BJTs)

4. Insulated-gate bipolar transistors (IGBTs)

📌Power MOSFETs

👉A power MOSFET is a voltage-controlled device and requires only a smallinput current.

👉The switching speed is very high and the switching times are of the order ofnanoseconds.Power MOSFETs find increasing applications in low-power high-frequencyconverters.

👉MOSFETs have the problems of electrostatic discharge and require specialcare in handling.

 👉it is relatively difficult to protect them under short-circuited fault conditions. 

The two types of MOSFETs are (1)depletion MOSFETs and 

(2) enhancement MOSFETs. 

👉An n-channel depletion-type MOSFET is formed on a p-type silicon substrate, with two heavily doped n+ silicon sections for low resistance connections.

 👉The gate is isolated from the channel by a thin oxide layer. 

👉The three terminals are called gate, drain, and source. The substrate is normally connected to the source. 

👉The gate-to-source voltage VGS could be either positive or negative. 

👉If VGS is negative, some of the electrons in the n-channel area are repelled and adepletion region is created below the oxide layer, resulting in a narrower effective channel and a high resistance from the drain to source RDS. 👉If VGS is made negative enough, the channel becomes completely depleted,offering a high value of RDS, and no current flows from the drain to source, IDS= 0. The value of VGS when this happens is called pinch-off voltage VP.

👉 On the other hand, if VGS is made positive, the channel becomes wider,and IDS increases due to reduction in RDS. 

👉With a p-channel depletion-type MOSFET,the polarities of VDS, IDS, and VGS are reversed. 

👉An n-channel enhancement-type MOSFET has no physical channelIf VGS is positive, an induced voltage attracts the electrons from the p-substrate and accumulates them at the surface beneath the oxide layer.

👉If VGS is greater than or equal to a value known as threshold voltage VT, a sufficient number of electrons are accumulated toform a virtual n-channel, as shown by shaded lines in Figure,and the current flows from the drain to source.

👉The polarities of VDS, IDS,and VGS are reversed for a p-channel enhancement-type MOSFETs. 

👉Because a depletion MOSFET remains on at zero gate voltage,where as an enhancement-type MOSFET remains off at zero gate voltage, the enhancement-type MOSFETS are generally used as switching devices in power electronics.

👉In order to reduce the on-state resistance by having a largercurrent conducting area, the V-type structure is commonly used for power MOSFETs.

👉The cross section of a power MOSFET known as a vertical (V) mosfet. 

👉When the gate has a sufficiently positive voltage with respect to thesource, the effect of its electric field pulls electrons from the n+ layer intothe p layer. 👉This opens a channel closest to the gate, which in turn allows the current to flow from the drain to the source.

👉There is a silicon oxide (SiO2) dielectric layer between the gate metal and then+ and p juncon.

 👉MOSFET is heavily doped on the drain side to create an n+ buffer below the n‐ drift layer. 

👉This buffer prevents the depleon layer from reaching the metal, and also reduces the forward voltage drop during conduction.

👉MOSFETs require low gate energy, and have a very fast switching speed and low switching losses. 

👉The input resistance is very high, 109 to 1011 Ω.MOSFETs, however, suffer from the disadvantage of high forward on-state resistance as shown in Figure above , and hence high on-state losses, which makes them less attractive as power devices, but they are excellent as gate amplifying devices for thyristors etc. 

👆CROSS SECTION OF N-CHANNEL MOSFET👆

👆CROSS SECTION OF P-CHANNEL MOSFET👆