Shih-Wei Tan

Characterization and modeling of three-terminal heterojunction phototransistors using an InGaP layer for passivation

Fabrication, characterization, and modeling of three-terminal (3T) heterojunction phototransistors (HPTs) using an InGaP layer for passivation (called P-HPTs) compared with similar nonpassivated devices (called NP-HPTs) were reported. Effects of the base passivated by the InGaP layer on devices optical and electrical performance were investigated. In addition to improving the dc current gain in the small current regime, the photocurrent (I/sub ph/) and responsivity from the p-i-n diode formed by the base, collector, and subcollector are also enhanced in a P-HPT. The measured optical gains are 45 and 27 for a P- and an NP-HPT under 8.62-/spl mu/W optical injection operated as a two-terminal (2T) device with a floating base. When the base bias is applied from a voltage source, both 3T P- and NP-HPTs exhibit degraded optical gains. Although a voltage source applied to the base can be used to push the operating point of a heterojunction bipolar transistor to a higher collector current where the current gain is higher, only a small portion of the photocurrent generated within the base-collector region is injected across the base-emitter junction to be amplified. When the base of an HPT is biased using a current source, the I/sub ph/ and enhanced dc current gain mainly determine both collector photocurrent and optical gain. Thus, a P-HPT biased using a current source shows the best optical performance. Furthermore, the conventional Ebers-Moll equivalent-circuit model was extended to provide simulated results in good agreement with experiment.

Optical and electrical characteristics of InGaP/AlGaAs/GaAs composite emitter heterojunction bipolar/phototransistors (CEHBTs/CEHPTs)

Abstract We report on new InGaP/AlGaAs/GaAs composite emitter heterojunction bipolar and phototransistors (CEHBTs/CEHPTs) with a low turn-on voltage. The composite emitter comprised of the digital graded superlattice emitter and the InGaP sub-emitter is used to smooth out potential spike associated with the emitter–base heterojunction and to obtain a low emitter–base turn-on voltage. A fabricated CEHBT exhibits a small offset voltage of 55 mV and a low turn-on voltage of 0.83 V with a dc current gain as high as 150. In case of a CEHPT’s collector–emitter characteristics with base floating, optical gains increase with increasing input optical power. Furthermore, the collector current saturation voltage is small due to a low turn-on voltage. We obtain an optical gain larger than 6.83 with a collector current saturation voltage smaller than 0.5 V. On the other hand, performance results of a CEHPT with two- and three-terminal configuration were investigated and compared.

Effects of wet-oxidation treatment on Al0.45Ga0.55As/GaAs graded-like superlattice-emitter bipolar transistor with low turn-on voltage

We report an AlxGa1−xAs/GaAs heterojunction bipolar transistor (HBT) with a high Al mole fraction of x=0.45. A digital graded superlattice emitter, forming a step-wise graded composition, is used to smooth out the potential spike resulting from a large conduction-band offset. HBT’s with such a digital graded emitter as a passivation layer exhibit a small offset voltage of 55 mV, a low turn-on voltage of 0.87 V and a current gain as high as 400. Effects of wet-oxidation treatment on fabricated HBT’s are also investigated. Experimental results reveal that the studied HBT’s exhibit reduced collector currents at a fixed base current in the early stage of the wet-oxidation treatment. However, better surface passivation and higher current gain are available after an appropriate wet-oxidation treatment is used to reduce base current. We qualitatively modeled these behaviors by introducing a concept of built-in field along the graded emitter.

An Investigation of Direct-Current Characteristics of Composite-Emitter Heterojunction Bipolar Transistors

Direct-current characteristics of the composite-emitter heterojunction bipolar transistor (CEHBTs) having a composite emitter formed of a 0.04 µm In0.5Ga0.5P bulk layer and a 0.06 µm Al0.45Ga0.55As/GaAs digital graded superlattice (DGSL) were investigated. InGaP/DGSL-passivated and DGSL-passivated CEHBTs were fabricated for comparison with a AlGaAs-passivated CEHBT having an InGaP/AlGaAs emitter and a conventional InGaP/GaAs SHBT. In addition to having an extremely small collector–emitter offset voltage, the InGaP/DGSL-passivated and DGSL-passivated CEHBTs exhibit small base–emitter turn-on voltages, which are 130 and 370 mV lower than those of the InGaP/GaAs SHBT and comparison CEHBT, respectively, at a 1 A/cm2 collector current. These results reveal that the DGSL structure forms a gradedlike wide-gap emitter to effectively smooth out the potential spike at the base–emitter heterointerface. The current gain of the InGaP/DGSL-passivated CEHBT is 250 and is even enhanced to 385 by directly removing the InGaP layer. We qualitatively explain these improvements introducing the concept of a built-in electric field within the DGSL structure.

Improvements in direct-current characteristics of Al/sub 0.45/Ga/sub 0.55/As-GaAs digital-graded superlattice-emitter HBTs with reduced turn-on voltage by wet oxidation

Heterojunction bipolar transistors (HBTs) having an Al/sub 0.45/Ga/sub 0.55/As-GaAs digital-graded superlattice (DGSL) emitter along with an InGaP sub-emitter are reported. The band diagram of the DGSL emitter is analyzed by using a transfer matrix method and the theoretical result is consistent with the experimental observation that the DGSL emitter smoothes out the potential spike at the emitter-base junction. Such passivated HBTs with a high Al-fraction passivation layer exhibit a small offset voltage of 50 mV, a turn-on voltage of 0.87 V, and a current gain of 385. The HBTs are examined by wet-oxidizing the exposed passivated region under various conditions. Experimental results reveal that the HBTs with an exposed high Al-fraction emitter are sensitive to the ambient air. However, with InGaP capped upon the high Al-fraction emitter, the HBTs exhibit better oxidation quality. The wet oxidation brings forth the most remarkable improvements for the InGaP-capped HBTs when the passivation layer is totally wet oxidized. Furthermore, some devices from the same chip have undergone nitrogen treatment for comparison.

Influence of Base Passivation on the Optical Performance of Dual-Emitter Heterojunction Phototransistors

N-p-n InGaP/GaAs Dual-Emitter heterojunction phototransistor (DEPT) with/without InGaP-passivation layer have been fabricated to investigate the influence of surface leakage on the device optical performance. The comparison between DEPT with a voltagebiased emitter and HPT with a voltage-biased base is also included. There are four (three) operating regions appearing in the optical characteristics of the DEPT (HPT): negative-saturation, negativetuning, positive-tuning, and positive-saturation (cut-off, tuning, and saturation) regions. The InGaP-passivated DEPT with the extrinsic base surface passivated by InGaP, exhibits the maximum optical gains of 46.57, 46.86 and 47.39 while the non-passivated one shows those of 32.02, 33.55 and 33.57 under the optical powers of 8.62, 13.2 and 17.5 µW, respectively. However, the optical gains are only in the range of 0.93~2.0 (0.83~1.64) for the InGaP-passivated HPT (non-passivated HPT) for all the illuminating conditions.

Three-terminal dual-emitter phototransistor with both voltage- and power-tunable optical gains

InGaP/GaAs dual-emitter heterojunction phototransistors (DEPTs) with an emitter biased using a voltage is reported for comparison to heterojunction phototransistors (HPTs) with a floating base operated in the p–i–n and transistor modes and to the HPTs with a base biased using a voltage. Although the power- and voltage-tunable optical gains are expected when the base of the HPT is floated and biased using a voltage, respectively, a conventional HPTs configuration does not simultaneously exhibit both. On the contrary, the optical gains of the DEPT can be tuned by both the voltage applied to the emitter and the incident optical power. Experimental results show that (1) the power-tunable optical gains are in the range of 11–29 as determined by the incident optical power upon the HPT with a floating base, (2) the small optical gains tuned by a voltage are 0.83–1.6 with a gain-tuning efficiency of 4.4 V-1 for the HPT with a base electrode, and (3) the DEPT with an emitter biased using a voltage exhibits an enhanced optical gain, with a gain-tuning efficiency of 43.4 V-1, when compared with the HPT with a floating base. Finally, a new concept of current-sharing effects in the base region is introduced to explain power- and/or voltage-tunable characteristics with good agreement found.

Observation of multiple negative differential resistance in InP-InGaAs superlattice-emitter resonant tunneling bipolar transistor

Sequential resonant tunneling behavior of resonant tunneling bipolar transistor with 5-period i-InP/n-InGaAs superlattice emitter has been demonstrated. An interesting multiple negative-differential-resistance (NDR) phenomena resulting from the creation and extension of high-field domain in superlattice is observed at room temperature. Furthermore, the employing of a thin n-InGaAs emitter layer between InP/InGaAs superlattice and p/sup +/-InGaAs base layer helps to lower the potential spike at base-emitter junction and reduce neutral-emitter recombination current. Experimentally, the transistor performance, incorporating multiple NDR, with a relatively large current gain of 454 and an offset voltage as low as 80 mV is achieved.

Sub-quarter-micrometer-like V-gate pseudomorphic doped-channel FETs

This paper reported two reliable and economical methods that one is the using of spin on glass together with re-flowing photoresist to implement 0.4/spl sim/1.5-/spl mu/m U-gate HDCFETs, the other is to employ the wet etching rule to obtain the Sub-quarter-micrometer-like V-gate DCFETs. The measured transconductance available are 225, 250, 275, and 350 mS/mm for a V-gate, 1.5-/spl mu/m, 1.0-/spl mu/m and 0.6-/spl mu/m U-gate devices, respectively. The measured f/sub t/ (f/sub max/) at V/sub GS/=0 V and V/sub DS/=4 V are 22.5(33.5), 16(25), 9.7(20.5), and 7(14) GHz for a V-gate, 1.5-/spl mu/m, 1.0-/spl mu/m and 0.6-/spl mu/m U-gate devices.