Kyu-Heon Cho

New Power MOSFET Employing Segmented Trench Body Contact for Improving the Avalanche Energy

We have fabricated the 60 V power MOSFET employing the segmented trench body contact which results in low conduction loss and high avalanche energy (EAS) under undamped inductive switching (UIS) condition without sacrificing the device area. The proposed device employs the CMOS compatible deep Si trench process. The segmented trench body contact suppresses the hole current beneath the n+ source region under the avalanche breakdown mode because the impact ionization begins at the bottom of the trench contact, which suppresses the activation of parasitic NPN bipolar transistor and improves the EAS. We have investigated the avalanche characteristics by testing devices under UIS. The measured EAS of the proposed device is 4.5 mJ while that of the conventional one is 1.84 mJ. Although the breakdown voltage decreased from 69.8 V to 60.4 V by 13% due to the trench body contact, EAS improved by 144%. Trench segmentation increases the n+ source contact area which results in reducing the on- resistance and improving the uniformity of the trench body contact and active cells.

Molecular cloning and expression of a novel family A endoglucanase gene from Fibrobacter succinogenes S85 in Escherichia coli.

A Fibrobacter succinogenes S85 gene that encodes endoglucanase hydrolysing CMC and xylan was cloned and expressed in Escherichia coli DH5 by using pUC19 vector. Recombinant plasmid DNA from a positive clone hydrolysing CMC and xylan was designated as pCMX1, harboring 2,043 bp insert. The entire nucleotide sequence was determined, and an open-reading frame (ORF) was deduced. The nucleotide sequence accession number of the cloned gene sequence in Genbank is U94826. The endoglucanase gene cloned in this study does not have amino sequence homology to the other endoglucanase genes from F. succinogenes S85, but does show sequence homology to family 5 (family A) of glycosyl hydrolases from several species. The ORF encodes a polypeptide of 654 amino acids with a measured molecular weight of 81.3 kDa on SDS-PAGE. Putative signal sequences, Shine-Dalgarno-type ribosomal binding site and promoter sequences (-10) related to the consensus promoter sequences were deduced. The recombinant endoglucanase by E. coli harboring pCMX1 was partially purified and characterized. N-terminal sequences of endoglucanase were Ala-Gln-Pro-Ala-Ala, matched with deduced amino sequences. The temperature range and pH for optimal activity of the purified enzyme were 55 approximately 65 degrees C and 5.5, respectively. The enzyme was most stable at pH 6 but unstable under pH 4 with a K(m) value of 0.49% CMC and a V(max) value of 152 U/mg.

Increase of Breakdown Voltage on AlGaN/GaN HEMTs by Employing Proton Implantation

The breakdown voltage of new AlGaN/GaN high electron mobility transistors (HEMTs) was increased considerably without sacrificing any other electrical characteristics by proton implantation. The breakdown voltage of proton-implanted AlGaN/GaN HEMTs with 150 KeV 1 times 1014 -cm-2 fluence after thermal annealing at 400 degC for 5 min under N2 ambient was 719 V, while that of conventional device was 416 V. The increase of the breakdown voltage is attributed to the expansion of the depletion region under the 2-D electron gas (2-DEG) channel. The depletion region expanded downward into the GaN buffer layer because implanted protons acted as positive ions and attracted electrons in the 2-DEG channel.

A New Soft Self-Clamping Scheme for Improving the Self-Clamped Inductive Switching (SCIS) Capability of Automotive Ignition IGBT

The improvement of self-clamped inductive switching (SCIS) capability of ignition IGBT widely used in automotive coil driver ensures the protection of the ignition IGBT from a severe thermal stress under the abnormal switching condition such as the open secondary. We have proposed and verified a new self-clamping scheme for improving the self-clamped inductive switching (SCIS) capability of ignition IGBT by employing the collector coupled turn-on of IGBT. The proposed self-clamped IGBT has an additional power MOSFET connected between the gate and collector of main IGBT with a large gate resistance. The additional power MOSFET is turned-on temporarily at the beginning of the self-clamping mode, which increases the collector voltage of main IGBT gradually so that the proposed self-clamped IGBT reduces the hard switching energy loss, which results in improvement of the SCIS capability of main IGBT.

Investigation of HCI reliability in interdigitated LDMOS

Novel time-dependent kinetic model for interface trap formation is developed resulting in consideration of hot electron/hole injection in Interdigitated LDMOS. Proposed kinetic model replaces Si-H equation with Nit equation. HCI degradation of Interdigitated LDMOS is classified into two mechanisms. First mechanism is attributed to decreased electron densities due to electron trapping in interdigitated active region. Second mechanism occurs in accumulation region around side STI due to hot hole injection. First mechanism leads to an increase in RON upon stress, whereas second mechanism decreases RON.

1.4 kV AlGaN/GaN HEMTs employing As+ ion implantation on SiO 2 passivation layer

We proposed As+ ion implantation on SiO2 passivation layer of AlGaN/GaN HEMTs to improve the forward and reverse electric characteristics of AlGaN/GaN HEMTs. SiO2 passivation layer which was applied in this work suppressed the electron hopping from gate to the surface states of gate-drain region so that the virtual gate formation was suppressed and the electric field concentration was terminated. As+ ions which were implanted on SiO2 passivation layer changed the depletion region curvature so that the electric field distribution became moderate. To verify that As+ ions exist as positively charged after ion implantation, we measured electric force microscopy (EFM) with EFM test sample. After ion implantation, 2DEG concentration was slightly increased due to the induced electrons in channel which is the countercharge of positive ions in SiO2 layer generated by As+ ion implantation. Therefore, forward electric characteristics were slightly improved after ion implantation. Reverse characteristics were improved significantly after As+ ion implantation. The leakage current when VGD is -100 V and the breakdown voltage of the conventional device was 51.36 uA/mm and 523 V. The leakage current and the breakdown voltage under the same condition were improved to 38.82 uA/mm and 1380 V after As+ ion implantation under the condition of 80 keV energy and 1 times 1014/cm2 dose. After annealing (400degC, 10 min and N2 4 SLPM), most of positive charge was removed, which was observed by EFM measurement. The breakdown voltage was decreased and the leakage current was increased. These results showed that the improvement of electric characteristics of AlGaN/GaN HEMT is due to implanted As+ ions.

High breakdown voltage AlGaN/GaN HEMTs employing fluoride plasma treatment

We proposed channel-modulated AlGaN/GaN high electron mobility transistors (HEMTs) employing fluoride plasma treatment and carried out a detailed numerical simulation of device operation using ISE TCAD software. A reduction of peak electric field is required for achieving the high breakdown voltage of AlGaN/GaN HEMTs. The proposed channel-modulated AlGaN/GaN HEMTs created the fluoride plasma-treated region between the gate and the drain. The fluoride plasma-treated region modulated the two-dimensional electron gas (2-DEG) channel concentration, and effectively reduced the peak electric field without any field plates. The reduction of peak electric field in the gate-to-drain region caused the breakdown voltage to increase by 206%. The dc characteristics of channel-modulated AlGaN/GaN HEMTs remained practically the same as those of the conventional AlGaN/GaN HEMTs. The breakdown voltage could be increased without a significant degradation of dc characteristics due to the SiO2 passivation layer. The deposition of the SiO2 passivation layer combined with fluoride plasma treatment is a powerful process for channel-modulated AlGaN/GaN HEMTs.

High Breakdown Voltage AlGaN/GaN HEMTs by Employing Proton Implantation

The breakdown voltage of AlGaN/GaN high electron mobility transistors (HEMTs) was increased considerably without sacrificing any other electrical characteristics by proton implantation. The breakdown voltage increased from 416 V of conventional device to 719 V by proton implantation with 150 KeV, 1x1014cm-2 fluence after a thermal annealing at 400degC for 5 min under N2 ambient. The increase of breakdown voltage is attributed to the expansion of depletion region under the 2-dimensional electron gas (2-DEG) channel. The depletion region expanded downwards to the GaN buffer layer because implanted protons act as positive ions and attract the electrons in the 2-DEG channel. Proton implantation successfully reduced the electric field concentration so that the breakdown voltage increased.

New voltage sensing terminal of the IGBT for the short-circuit protection with suppressed floating effect by employing the internal PMOS

We have proposed a new floating p-well voltage reset scheme for reliable and fast discharging voltage sensing terminal by employing the negative gate bias, which stabilizes the floating p-well voltage spontaneously during the transient state and off-state by turning on the internal PMOS of the floating p-well IGBT. We have investigated the switching stability characteristics of the floating p-well voltage. Experimental results show that the discharging time and voltage spike of the floating p-well voltage is successfully controlled by employing the gate bias polarity and filter capacitor. Simulation results also shows that the hole current diverting effect due to the turn-on of the internal PMOS suppresses the floating p-well voltage during the turn-off transient.

Low leakage current circular AlGaN/GaN Schottky barrier diode

We proposed circular AlGaN/GaN Schottky barrier diode, which has no mesa structure near the current path. Proposed device showed low leakage current of 10 nA/mm at -100 V while that of the rectangular device was 34 nA/mm at the same condition. Proposed circular AlGaN/GaN SBD showed high forward current of 88.61 mA at 3.5 V while that of the conventional device was 14.1 mA at the same condition.