However, silicon thyristors are the most common thyristors. Such device can in principle be made using any semiconductor. This device is also referred to as a pnpn structure or Thyristor. The silicon-controlled rectifier is 4-layer device with alternating n-type and p-type layers as shown in Figure 5.9.2. This larger voltage results in larger on-state power dissipation in the device.ĭarlington transistor structure a) equivalent circuit b) device cross-section.ĥ.9.3. Silicon Controlled Rectifier (SRC) or Thyristor Since the forward bias voltage is much larger than the saturation voltage, the saturation voltage of the Darlington pair is also significantly larger.
#DARLINGTON TRANSISTOR SYMBOL PLUS#
Since the two devices share the same collector, the saturation voltage of the Darlington pair equals the forward bias voltage of transistor Q2 plus the saturation voltage of transistor Q1. The disadvantage is the larger saturation voltage. The key advantage of the Darlington configuration is that the total current gain of the circuit equals the product of the current gain of the two devices. This structure can be fabricated with the same technology as a single BJT as shown in Figure 5.9.1. The proliferation of its use will heavily depend on the material cost and quality of the SiC wafers.ĭarlington transistors contain two transistors connected in an emitter-follower configuration, while sharing the same collector contact. The high saturation velocity (3x compared to silicon) also shifts the onset of the Kirk effect to higher current densities. The higher thermal conductivity (3x) and breakdown field (10x) compared to silicon give it a clear performance advantage. Silicon carbide (SiC) has been hailed as the perfect material for high-power BJTs. Silicon BJTs dominate the power device market, in part because of the low cost of large area silicon devices and the high thermal conductivity of silicon compared to GaAs. Large currents – up to 1000 A – are obtained by making a large area device. Power BJTs therefore are operated at rather low current density of 100 A/cm 2 since the lower current density reduces the power dissipation per unit area and eliminates the Kirk effect. However, for devices with a very high blocking voltage, this may not be an option. The Kirk effect is normally avoided by increasing the collector doping density. The power dissipation is managed by minimizing the power dissipation and spreading the resulting heat dissipation onto a large area. As a result, one finds that the structure needs to be redesigned to a) effectively manage the power dissipation and b) avoid the Kirk effect. Extremely low doping densities, down to 10 13 cm -3, are use to obtain blocking voltages as large as 3000 V.
Such collector regions result in a large blocking voltage. Power BJTs also have a thick and low-doped collector region. The main difference is that the active area of the device is distinctly higher, resulting in a much higher current handling capability. High power bipolar transistors are conceptually the same as the bipolar transistors described in chapter 8. Power MOSFETs and power devices that combine MOSFETs and bipolar transistors are presented in chapter 7. The bipolar-based power devices include high-power bipolar transistors, Darlington transistors consisting of two transistors with a common collector, thyristors – also called silicon controlled rectifiers (SRCs) and triacs, a complementary thyristor structure suitable to control AC power. Power devices can be classified into bipolar-based devices, MOSFET-based devices and devices such as the IGBT that combine a bipolar transistor with a MOSFET.īipolar power devices are the traditional power devices because of their capability to provide high currents and high blocking voltages. BJT power devices Chapter 5: Bipolar Junction Transistorsĥ.9. Bipolar Power Devices 5.9.1. Power BJTs 5.9.2. Darlington Transistors 5.9.3. Silicon Controlled Rectifier (SRC) or Thyristor 5.9.4. DIode and TRiode AC Switch (DIAC and TRIAC)
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