Trade Resources Industry Knowledge Silicon Carbide Bipolar Opamp Performance at 500ºC

Silicon Carbide Bipolar Opamp Performance at 500ºC

Sweden's Royal Institute of Technology (KTH) has created a monolithic operational amplifier circuit using 4H polytype silicon carbide (SiC) bipolar junction transistors (BJTs) [Raheleh Hedayati et al, IEEE Electron Device Letters, vol35, p693, 2014]. The researchers see applications for such devices that can potentially operate at up to 600°C as being in "Venus exploration, oil and gas drilling, aviation, and automotive".

Although SiC junction field-effect transistors (JFETs) have been reported as working at up to 576°C (Table 1), BJTs have higher speed, better linearity, and higher drive current. Metal-oxide-semiconductor (MOS) transistors are limited by the stability of the oxide layer to temperatures of 400°C and less.

Table 1: Comparison of high-temperature opamps in silicon carbide.

Technology Temperature Open-loop gain Gain bandwidth 4H-SiC MOSFET 300°C 57dB - 6H-SiC MOSFET 300°C 53dB 269kHz 6H-SiC JFET 576°C 69dB 1400kHz 4H-SiC bipolar (KTH) 500°C 64dB 4360kHz

The NPN bipolar transistors were fabricated on a 4-inch 4H-SiC substrate with six epitaxial layers of varying doping concentrations (Figure 1). The mesas for the emitter, collector and base regions were achieved using plasma etch. The ~0.01mm2 devices achieved maximum current gains (β = collector/base currents) of 38 at 25°C and 15 at 500°C.

Silicon Carbide Bipolar Opamp Performance at 500ºC

Figure 1: (a) Cross-sectional schematic and microphotograph of SiC NPN transistor. (b) Measured β versus collector current of NPN transistor from 25°C to 500°C.

The researchers used 17 transistors to create an opamp IC (Figure 2) with two gain stages, buffers, level shifter, and an output stage. The power supply was a dual ±7.5V system. The operating points for the transistors in the circuit varied between 0.4mA (22 gain at room temperature) at the input and 14mA for the output (36 gain).

Silicon Carbide Bipolar Opamp Performance at 500ºC_1

Figure 2: Microphotograph of opamp with integrated feedback resistor (total area ~3.75mm2).

The researchers had integrated a 30pF capacitor to provide 'Miller compensation' for the closed-loop circuit, giving an inverting negative feedback amplifier configuration, but found that a 330pF capacitor off-chip was also needed for stability. The trade-off was smaller bandwidth. External components such as the off-chip capacitor and load resistor/capacitor were kept at room temperature.

Although the closed-loop gain and gain bandwidth degrade between 25°C and 500°C, the 3dB-bandwidth ('half-power point') increases (Table 2). The researchers attribute the wider 3dB-bandwidth to the reduction in bias resistance leading to increased cut-off frequencies of the transistors. Also, the slew rate increases at high temperature.

Table 2: Some measured performance criteria of KTH inverting opamp.

  25°C 500°C DC closed-loop gain with 500Ω load 39.86dB 39.46dB Gain bandwidth 5.92MHz 4.36MHz 3dB-bandwidth 270kHz 410kHz Estimated open-loop gain 76.3dB 64dB Positive/negative slew rates 1.46/1.25V/μsec 1.46/2.16V/μsec Total harmonic distortion -52dB/0.25% -50dB/0.3%

While the researchers estimate that the open-loop gain of the device decreases significantly between 25°C and 500°C, the closed-loop gain is remarkably stable over the wide temperature range.

Contribute Copyright Policy