✏️POWER TRANSISTORS AND THEIR TYPES
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👆
