Two-dimensional analysis of the interface state effect on current gain for a 4H–SiC bipolar junction transistor

Radiation-induced current gain and $1/f$ noise degradations in NPN bipolar junction transistors are due to accumulation of oxide-trapped charges and interface states at the surface of the device. Based on an available model of base surface current of NPN bipolar junction transistors, a simplified model is presented with some approximations at low total dose level, which can explain the degradation mechanisms of the current gain. Based on the theory of carrier number fluctuation and the simplified model of base surface current, a $1/f$ noise model is developed, which can be used to explain the $1/f$ noise degradation induced by the radiation at low total dose level. The model suggests that the gain and $1/f$ noise degradations can be attributed to the same physical origin, and these two kinds of degradations are the result of accumulation of oxide-trapped charges and interface states. The radiations were performed in a ${\rm Co}^{60}$ source at a dose rate of 10 rad(Si)/s up to a total dose of 70 krad(Si). The degradations of the current gain and $1/f$ noise are compared, and the relationship between $1/f$ noise and the current gain is given, which accords well with the experimental results. Compared to the current gain, the $1/f$ noise parameter is more sensitive, so it may be used to evaluate the radiation resistance capability of bipolar junction transistors.

Bipolar junction transistors (BJTs) of 4H-SiC, with a low collector-emitter forward voltage drop VCE have been fabricated without base contact implantation. A comparison of BJTs on the same wafer with and without base contact implantation shows less than 10% higher VCE for the BJTs without base contact implantation. Omitting the base contact implantation eliminates high concentrations of implantation-induced defects that act as recombination centers. This is advantageous because it allows a shorter distance Wp+ between the emitter edge and the base contact, without affecting the current gain when no base contact implantation is used. The BJTs without contact implantation show a constant current gain as Wp+ was reduced from 3 to 1 mum, whereas the gain decreased by 45% for the BJTs with base contact implantation for the same reduction of Wp+. A key to the successful fabrication of low-forward-voltage-drop SiC BJTs without base contact implantation is the formation of low-resistivity Ni/Ti/Al ohmic contacts to the base. The contact resistivity on the base region (NA ap 4 times1017 cm-3) was measured with linear transmission line method structures to rhoC = 1.9 times 10-3 Omegacm2, whereas the contact resistivity with the base contact implantation was rhoC = 1.3 times 10-4 Omegacm2, both after rapid thermal processing annealing at 800degC.

Epitaxial silicon carbide bipolar junction transistors (BJTs) for power switching applications have been designed and fabricated with a maximum breakdown voltage of 1100 V. The BJTs have high common emitter current gains with maximum values exceeding 60, a result that is attributed to design optimization of the base and emitter layers and to a high material quality obtained by a continuous epitaxial growth. Device simulations of the current gain as function of collector current have been compared with measurements. The measurements show a clear emitter-size effect that is in good agreement with simulations including surface recombination in interface states at the etched termination of the base-emitter junction. Simulations indicate an optimum emitter doping around 1middot10 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">19</sup> in agreement with typical state-of-the-art BJTs

Ionizing-radiation-induced current gain degradation in NPN bipolar junction transistors is due to an increase in base current as a result of recombination at the surface of the device. A model is presented which identifies the physical mechanism responsible for current gain degradation. The increase in surface recombination velocity due to interface states results in an increase in base current. Besides, changing the surface potential along the base surface induced by the oxide-trapped charges can also lead to an increased base current. By combining the production mechanisms of oxide-trapped charges and interface states, this model can explain the fact that the current gain degradation is more severe at a low dose rate than at a high dose rate. The radiations were performed in a Co60 source up to a total dose of 70 krad(Si). The low dose rate was 0.1 rad(Si)/s and the high dose rate was 10 rad(Si)/s. The model accords well with the experimental results.

We investigate the electrical characteristics of 1-kV pnp SiC bipolar junction transistors (BJTs) and compare them with those of npn SiC BJTs. The base resistance, current gain, and blocking capability are characterized. It is found that the base resistance of pnp SiC BJTs is two orders of magnitude lower than that of npn SiC BJTs. However, the obtained current gains are low below unity in pnp SiC BJTs, whereas npn SiC BJTs exhibit a current gain of 14 without surface passivation. The reason for the poor current gain of pnp SiC BJTs is discussed.

This paper examines individual radiation effects caused by 110 keV electrons, 70 keV protons and 170 keV protons, and combined radiation effects induced by 110 keV electrons together with 70 keV protons and 110 keV electrons with 170 keV protons on the forward current gain of bipolar junction transistors (3DG112D, NPN). Key parameters were measured in-situ and the change in current gain of the NPN transistors is obtained at a fixed collector current. Experimental results show that the current gain degradation of the NPN transistors is sensitive to both of ionization and displacement damage. The ionization damage is primarily caused by 110 keV electrons, while the displacement damage is mainly induced by 170 keV protons in this study. Under the combined exposure of 170 keV protons and 110 keV electrons, the current gain degradation lies between those given by the individual irradiations. The current gain degradation is mainly caused by the 170 keV protons, and the 110 keV electrons mitigate the degradation to some extent. In the case of combined irradiation of 110 keV electrons and 70 keV protons, no synergistic effect occurs, the current gain degradation is basically controlled by the 110 keV electrons.

Electronics components such as bipolar junction transistors, diodes, etc. which are used in deep space mission are required to be tolerant to extensive exposure to energetic neutrons and ionizing radiation. This paper examines neutron radiation with pneumatic transfer system of TRIGA Mark-II reactor at the Malaysian Nuclear Agency. The effects of the gamma radiation from Co-60 on silicon NPN bipolar junction transistors is also be examined. Analyses on irradiated transistors were performed in terms of the electrical characteristics such as current gain, collector current and base current. Experimental results showed that the current gain on the devices degraded significantly after neutron and gamma radiations. Neutron radiation can cause displacement damage in the bulk layer of the transistor structure and gamma radiation can induce ionizing damage in the oxide layer of emitter-base depletion layer. The current gain degradation is believed to be governed by the increasing recombination current in the base-emitter depletion region.

This paper addresses the performance of SiC NPN Bipolar Junction Transistors (BJTs) at high and low temperature. A current gain of 50 at room temperature was obtained which decreases to 25 at 275 oC. A maximum current gain (β) of 111 has been reported at -86 oC. At low temperature (below -86 oC), the current gain drops rapidly because of carrier freezout effect. At room temperature, a minimum on-resistance of 7 mΩ-cm2 was obtained. This increases to 28 mΩ-cm2 at 275 oC. The on-resistance of BJTs is approximately unaffected by lowering the temperature down to -86 oC from room temperature. Below -86 oC, the on-resistance jumps up rapidly because of carrier freezeout. Electrical performance of BJTs have been fairly stable during stress measurement at high temperature (120 hours at 100 oC ) at a collector bias of 1000V (with open base) for devices with a breakdown voltage of 1200V.The devices have been stressed further at low (i.e., 6) and high gain (i.e., 15) at room temperature. Initial degradation within first hour of stress test has been reported and then degradation stabilizes out. Packaged devices were tested up to 550 oC and performed admirably well up to that temperature.

Characterizing and modeling current gain degradation in bipolar transistor exposed to heavy ion radiation

The stability of the electrical characteristics of SiC n-p-n bipolar junction transistors (BJTs) is investigated under long-term avalanche-mode, dc, and pulsed-current operation. There is absolutely no change in the blocking I-V characteristics after a 934-h repetitive avalanche stress test. Long-term operation of the base-emitter diode (open-collector mode) alone does not result in any degradation of the ON-state voltage drop V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">F</sub> or current gain β. Long-term operation in common-emitter mode results in negligible V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">F</sub> or β degradation, if the base plate is maintained at 25°C. A greater degradation of β results upon increasing the base-plate temperature. The same total electrical charge, if passed through the BJT as a pulsed current, instead of a dc current, results in smaller β reduction. It is also shown that the β degradation can be reversed by annealing at ≥200°C, suggesting the possibility of degradation-free operation of SiC BJTs, when operating in pulsed current mode at ≥200°C temperatures.

On the base region in very small geometry, bipolar transistors

In this work, total dose effects on 4H-SiC bipolar junction transistors (BJT) are investigated. Three 4H-SiC NPN BJT chips are irradiated with 3MeV protons with a dose of 1×1011, 1×1012 and 1×1013 cm-2, respectively. From the measured reciprocal current gain it is observed that 4H-SiC NPN BJT exposed to protons suffer both displacement damage and ionization, whereas, a traditional Si BJT suffers mainly from displacement damage. Furthermore, bulk damage introduction rates for SiC BJT were extracted to be 3.3×10-15 cm2, which is an order of magnitude lower compared to reported Si values. Finally, from detailed analysis of the base current at low injection levels, it is possible to distinguish when surface recombination leakage is dominant over bulk recombination.

Low-voltage InAsP/InAs HBT and metamorphic InAs BJT devices grown by molecular beam epitaxy

In this paper, a novel bipolar junction transistor (BJT) structure is proposed for high matching characteristics and its performance is compared with a conventional BJT structure. Although the proposed BJT matching structure indicates a decrease of collector current density <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$J_{C}$</tex></formula> and current gain <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\beta$</tex></formula> of about 5.36% and 1.02% compared with those of the conventional BJT structure, the matching characteristics of the collector current <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex Notation="TeX">$(A_{\rm IC})$</tex></formula> and the current gain <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$(A_{\beta})$</tex> </formula> for the proposed structure are improved by about 31% and 24%. The improved matching characteristic of the proposed structure is believed to be due to the reduced effect of the deep n-well or the reduced current path from emitter to collector.

As one of the most attractive postsilicon power semiconductor devices, SiC bipolar junction transistor (BJT) has been studied extensively and commercialized in the past few years. However, SiC BJT has not been widely accepted in the market partially because of high driver consumption in the on-state, which is induced by a relatively large constant base current in order to ensure it is fully turned on . In this paper, a new proportional base driver with a simple circuitry is proposed in order to reduce the steady-state base driver consumption of SiC BJT. It utilizes a silicon small-signal mosfet as a variable resistor placed between the driver and the base of the SiC BJT. The collector current of the SiC BJT is measured via a Hall current sensor whose output is connected to the gate of the mosfet . When the collector current decreases, the gate voltage of the mosfet decreases and its on-state resistance increases, resulting in the decrease of the actual base driver current of the SiC BJT. The operation principle of the proposed proportional base drive technique has been verified by theoretical analysis and experimental demonstration of a 1200 V 10 A-SiC BJT-based dc–dc boost converter. Theoretical calculation, PSpice simulation, and experimental results have shown that the steady-state power consumption of the SiC BJT's proportional base driver is reduced by approximately 70% under the light load condition compared to conventional base driver with a constant base current.