Mos-Gated Thyristors Power Semiconductor Switches

¶ … MOS-Gated Thyristors

Power semiconductor switches have been widely applied in power electronics. However their low usage in pulse power circuitry is due the apprehension that small scale pulse operations might lead to early breakdown. Thyristors have proven to be advantageous in this regard due to their high blocking voltage coupled with an elevated potential to carry steady state current. This paper presents a survey of 7 thyristor structures (Emitter Switched, Base Resistance controlled, Dual Channel EST/BRT, Emitter controlled, Dual gate, Insulated base ECT and floating Base). [1]

The benefits of high input impedance have caused the fabrication of new MOS gated bipolar devices. IGBT (Insulated Gate Bipolar Transistor) is one such device which have their on-state features superior to MOSFET. Thyristor-based devices such as EST (Emitter Switched Thyristor) and BRT (Base Resistance Thyristor) demonstrate how the conduction traits can be enhanced even further. They have turn-off times in the range of 10 ?sec.[2]

Emitter Switched Thyristor (EST)

EST is composed of a vertically placed thyristor within a succession of n-channel enhancement mode MOSFET's. The operational mode is usually forward blocking with the gate voltage at cathode potential. A positive gate bias employed on the device turns it on. This has to be over the threshold voltage of the NMOSFET as the anode potential is maintained positive with regards to the cathode. Bringing the gate voltage down to zero switches the device off. The EST structure shows the development of a parasitic thyristor due to the integration of a lateral NMOSFET with the thyristor P-Base. [2]

Base Resistance Controlled Thyristor (BRT)

A BRT is composed of a vertically placed thyristor with an adjoining MOSFET. An channel MOSFET is created on the P-base surface in amid the N+ emitter and N- drift regions and is utilized to turn the device on. AP channel MOSFET with the P-base and P+ diverter regions acting as its source and drain is used to turn the device off. The on-resistance of the P. Channel MOSFET restricts the maximum controllable anode current density. [2][3]

Dual Channel EST/BRT

The dual channel EST-BST is a capable alternative to the IGBT (Insulated gate bipolar transistor) for applications such as Uninterrupted Power Supplies and Lighting ballasts. It incorporates properties such as high voltage current saturation, lower on-state voltage drop associated with the thyristor structure and BRT like enhanced turnoff potential. an-Channel MOSFET turns it on while a P-Channel MOSFET aids in augmenting the turn-off process using a negative bias. The device functions like an IGBT when anode bias is low. It has demonstrated an improvement factor of 2.5 in manageable current density against a related DC-EST device.[4]

Emitter Controlled Thyristor (ECT)

The Emiter controlled thyristor has a better Forward Biased Safe Operating Area (FBSOA) and Reverse Biased Safe Operating Area (RBSOA) than most MOS gated thyristors and the IGBT. It has a 4 layer PNPN thyristor arrangement along with a P. channel MOSFET (PMOS1) connected to the top N. later. A Floating Ohmic Contact (FOC) shorts the P+ region while the N. emitter operates as the source. The FOC serves as a link which allows the transfer of emitter electron currents. The second P. channel MOSFET (PMOS2) exists at the other side of the cathode contact and lacks an additional control gate. [5]

Dual Gate MOS Thyristor (DGMOT)

Dual Gate MOS Thyristors manage to accomplish IGBT mode by integrating their lateral N. channel MOSFET with a thyristor structure. The device is constituted of a lateral N. channel MOSFET along with a MOS gated thyristor in a way such that a positive gate voltage can short circuit the emitter base intersection at the upper NPN transistor. The device is triggered on when the anode voltage becomes positive compared to the cathode and a positive gate bias is applied to establish a surface channel. This makes electrons flow into the N-drift region. Breaking the thyristor latch-up turns it off. [6]

Insulated Base MOS -- Controlled Thyristor (IBMCT)

The IBMCT structure is attuned to the Insulated Base MOS-Controlled Thyristor production process. This permits rapid hole removal from the p-body region. It is composed of a vertically placed thyristor with separate gates to control the device. It is activated by biasing the on-gate positively and grounding the off-gate. This generates an inversion channel. Its turn off process depends on the presence of a Floating Ohmic Contact (FOC). This is attained by redirecting the holes from the p-body region to FOC resulting in a union with electrons coming from the Cathode. The current capability of IBMCT is relatable to BRT. [7]