Pnp Heterojunction Bipolar Transistors Research Articles

A Comparative Study of InGaP/GaAs Collector-Up HBTs for High-Reliability Small-Scale PA Applications

In this paper, the characteristics of InGaP/GaAs collector-up (C-up) pnp and npn heterojunction bipolar transistors (HBTs) with the graded InGaAs base and the nonuniform doped collector are demonstrated, and the performances as well as the reliability of the novel HBTs have been compared with the conventional InGaP/GaAs HBTs with various collector structures and thermal schemes. Compared to conventional HBTs, the studied C-up HBTs exhibited better current-driving capability, higher RF efficiency, and improved long-term reliability. Note that the pnp device displayed greater thermal stability enhancements, which are distinct and reproducible, than the npn device. The favorable data of the pnp C-up HBT could be attributed to higher electron mobility, resulting in low base resistance and higher emitter resistance improving the thermal stability. The comparison results, based on pragmatic observations from the C-up HBTs without relatively large heat-dissipation configurations, should be very useful for the reliable and the cost-effective design as small-scale power amplifiers in the wideband CDMA system.

Base doping and dopant profile control of SiGe npn and pnp HBTs

Incorporation of high doping concentrations and the creation and maintaining of steep doping profiles during processing are key enabler for high level RF performance of heterojunction bipolar transistors (HBTs). In this paper, we discuss results of base doping and dopant profile control for npn and pnp SiGe HBTs fabricated within 0.25 μm BiCMOS technologies. High level of electrically active B and P doping concentrations (up to 10 20 cm −3) have been incorporated into SiGe. By adding C to SiGe steep doping profiles have been maintained due to the prevention of dopant diffusion during device processing. It is shown that broadening of P doping profiles caused by segregation could be reduced by lowering the deposition temperature for the SiGe cap. B and P atomic layer doping is shown to be suitable for the creation of steep and narrow doping profiles. This result is demonstrating the capability of the atomic layer processing approach for future devices with critical requirements of dopant dose and location control.

Improved Manufacturability of AlGaAs/GaAs Pnp Heterojunction Bipolar Transistors

Specially designed Pnp heterojunction bipolar transistors (HBT's) in the AlGaAs/GaAs material system can offer improved radiation response over commercially-available silicon bipolar junction transistors (BJT's). To be a viable alternative to the silicon Pnp BJT, improvements to the manufacturability of the HBT were required. Utilization of a Pd/Ge/Au non-spiking ohmic contact to the base and implementation of a PECVD silicon nitride hard mask for wet etch control were the primary developments that led to a more reliable fabrication process. The implementation of the silicon nitride hard mask and the subsequent process improvements increased the average electrical yield from 43% to 90%.

Improved Manufacturability of AlGaAs/GaAs Pnp Heterojunction Bipolar Transistors

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Atomic layer processing for doping of SiGe

Atomic layer processing has been demonstrated for doping of SiGe during Reduced Pressure Chemical Vapour Deposition (RPCVD) in a commercial single wafer reactor. Atomic level control of dose and location has been obtained for B doping using B 2H 6 and for P doping using PH 3. The main idea of atomic layer processing is the separation of adsorption of the reactant gases from the deposition process. By this way, self-limitation has been shown for P doping. By lowering the temperature for B 2H 6 exposure (100 °C), the non-self-limiting character of the B doping process can be changed to self-limitation. By this manner, very shallow doping profiles with low sheet resistance have been obtained, capable for future ultra-shallow junction applications. P atomic layer doping is shown to be suitable for the creation of steep and narrow doping profiles suitable for high-performance pnp Heterojunction Bipolar Transistors (HBTs). This result, together with the already demonstrated usage of B atomic layer doping for npn HBTs, demonstrates the capability of the atomic layer processing approach for future devices with critical requirements for dopant dose and location control.

A comparison of low-frequency noise characteristics and noise sources in NPN and PNP InP-based heterojunction bipolar transistors

Low-frequency noise characteristics of NPN and PNP InP-based heterojunction bipolar transistors (HBTs) were investigated. NPN HBTs showed a lower base noise current level (3.85 /spl times/ 10/sup -17/ A/sup 2//Hz) than PNP HBTs (3.10 /spl times/ 10/sup -16/ A/sup 2//Hz), but higher collector noise current level (7.16 /spl times/ 10/sup -16/ A/sup 2//Hz) than PNP HBTs (1.48 /spl times/ 10/sup -16/ A/sup 2//Hz) at 10 Hz under I/sub C/=1 mA, V/sub C/=1 V. The NPN devices showed a weak dependence I/sub C//sup 0.77/ of the collector noise current, and a dependence I/sub B//sup 1.18/ of the base noise current, while the PNP devices showed dependences I/sub C//sup 1.92/ and I/sub B//sup 1.54/, respectively. The dominant noise sources and relative intrinsic noise strength were found in both NPN and PNP InP-based HBTs by comparing the noise spectral density with and without the emitter feedback resistor. Equivalent circuit models were employed and intrinsic noise sources were extracted. The high base noise current of PNP HBTs could be attributed to the exposed emitter periphery and higher electron surface recombination velocity in P-type InP materials, while the relatively high collector noise current of NPN HBTs may be due to the noise source originating from generation-recombination process in the bulk material between the emitter and the collector.

Tunnel diode collector contact in InP based PNP heterojunction bipolar transistors

A tunnel diode collector contact to InP based PNP heterojunction bipolar transistors (HBTs) is suggested and demonstrated. The additional heavily doped n-type contact layer replaces the thick p-type contact layer required in conventional structures. The thermal and electrical properties of the collector contact layer thus become similar to those of NPN HBTs. A secondary ion mass spectroscopy study explores the maximum tin doping level that can be obtained in the base. Finally, the temperature dependence of the current gain is presented and interpreted.

High-Power GaN Electronic Devices

Gallium Nitride (GaN) and related materials (especially AlGaN) recently have attracted a lot of interest for applications in high-power electronics capable of operation at elevated temperatures and high frequencies. The AlGaInN system offers numerous advantages. These include wide bandgaps, good transport properties, the availability of heterostructures (particularly AlGaN/GaN), the experience base gained by the commercialization of GaN-based laser and light-emitting diodes and the existence of a high growth rate epitaxial method (hydride vapor phase epitaxy, HVPE) for producing very thick layers or even quasisubstrates. These attributes have led to rapid progress in the realization of a broad range of GaN electronic devices.AlxGa1-xN (x=0 ∼.25) Schottky rectifiers were fabricated in a lateral geometry employing p+-implanted guard rings and rectifying contact overlap onto an SiO2 passivation layer. The reverse breakdown voltage (VB) increased with the spacing between Schottky and ohmic metal contacts, reaching 9700 V for Al0.25Ga0.75N and 6350 V for GaN, respectively, for 100-µm gap spacing. Assuming lateral depletion, these values correspond to breakdown field strengths of <9.67×105 Vcm−2, which is roughly a factor of 5 lower than the theoretical maximum in bulk GaN. The figure of merit (VB)2/RON, where RON is the on-state resistance, was in the range 94 to 268 MWcm−2 for all the devices. Edge-terminated Schottky rectifiers were also fabricated on quasibulk GaN substrates grown by HVPE. For small-diameter (75 µm) Schottky contacts, Vs measured in the vertical geometry was ∼700 V, with an on-state resistance (RON) of 3 mΩcm2, producing a figure-of-merit VB 2/RON of 162.8 MW-cm−2.GaN p-i-n diodes were also fabricated. A direct comparison of GaN p-i-n and Schottky rectifiers fabricated on the same GaN wafer showed higher reverse breakdown voltage for the former (490 V vs. 347 V for the Schottky diodes), but lower forward turn-on voltages for the latter (∼3.5 V vs. ∼5 V for the p-i-n diodes). The forward I-V characteristics of the p-i-n rectifiers show behavior consistent with a multiple recombination center model. The reverse current in both types of rectifiers was dominated by surface perimeter leakage at moderate bias. Finally, all of the devices we fabricated showed negative temperature coefficients for reverse breakdown voltage, which is a clear disadvantage for elevated temperature operation.Bipolar devices are particularly interesting for high current applications such as microwave power amplifiers for radar, satellite, and communication in the l∼5 GHz range, powers >l00 W, and operating temperatures >425°C. pnp Bipolar Junction Transistors and pnp Heterojunction Bipolar Transistors were demonstrated for the first time. For power microwave applications, small area self-aligned npn GaN/AlGaN HBTs were attempted. The devices showed very promising direct current characteristics.

Direct-current characteristics of pnp AlGaN/GaN heterojunction bipolar transistors

AlGaN/GaN pnp heterojunction bipolar transistors were fabricated using a low-damage dry-etch process, and the dc characteristics measured up to 250 °C. In the common–base mode, the collector current was approximately equal to the emitter current under all conditions. Although not optimized for power operations, the devices were tested up to power densities of ∼50 kW cm−2. The dc current gain was in the range 20–25 at room temperature. The pnp configuration avoids the problem of high base sheet resistance encountered with npn-AlGaN/GaN devices.

A Gummel–Poon model for pnp heterojunction bipolar transistors with a compositionally graded base

A modified Gummel–Poon model has been developed for the pnp heterojunction bipolar transistor which matches thermionic emission–diffusion of holes across the emitter–base heterojunction with drift–diffusion transport across a graded base. The effects of a high current density on base pushout and the collector junction capacitance have been incorporated to describe the falloff in current gain and high frequency performance. The model's predictions have been compared with reported device measurements and results from a commercial, numerical device simulator for an InP-based device with good agreement found. The model provides an improved description of device performance that will be of use to device designers.

Space-charge recombination currents and their influence on the dc current gain of AlGaAs/GaAs Pnp heterojunction bipolar transistors

The effects of Shockley–Read–Hall, Auger, radiative, and intrinsic surface base recombination processes in the emitter-base space-charge region on the current gain of Pnp AlGaAs/GaAs heterojunction bipolar transistors are analyzed. At low forward emitter-base bias, the current gain of a typical Pnp AlGaAs/GaAs heterojunction bipolar transistor is shown to be reduced substantially below its value calculated while neglecting recombination currents in the emitter-base space-charge region. Excellent agreement between theory and experiment is found for the current gain variation versus collector current density for a Pnp device recently reported by Slater et al. [IEEE Electron Device Lett. 15, 91 (1994)].

Design of a InP/In1−xGaxAsyP1−y/In0.53Ga0.47As emitter-base junction in a Pnp heterojunction bipolar transistor for increased hole injection efficiency

Starting with Burt’s envelope function theory, we calculate the transmission coefficients of holes across an InP/In1−xGaxAsyP1−y/In0.53Ga0.47As heterointerface while varying the width and gallium and arsenic fractions of the InP lattice-matched quaternary compound (x=0.47y). While comparing our results to the case of an abrupt InP/In0.53Ga0.47As interface, we find that the transmission coefficients of both heavy- and light-holes can be enhanced significantly for a 60-Å-wide quaternary layer with an arsenic fraction y=0.4 (x=0.188). This should lead to an enhanced hole injection efficiency of Pnp heterojunction bipolar transistors using the heterointerface analyzed here as an improved design of the emitter-base junction.

Emitter series resistance effect of multiple heterojunction contacts for Pnp heterojunction bipolar transistors

For InP-based Pnp heterojunction bipolar transistors (HBTs), a set of epitaxial layers, frequently incorporating quaternaries, is used to make a low resistance electrical contact to the emitter layer. We describe an analytical approach to investigate the nonlinear effects of the multiple heterojunction interfaces on the emitter series resistance and emitter junction current–voltage characteristics of the device. The simulation results show that heterojunction interfaces can contribute a substantial portion (up to 20%) to the total emitter series resistance, especially at high levels of emitter current.

Hole transport and quasi-Fermi level splitting at the emitter-base junction in Pnp heterojunction bipolar transistors

Hole transport across the emitter-base heterojunction in Pnp heterojunction bipolar transistors is considered and the amount of hole quasi-Fermi level splitting at the interface is analyzed. Hole injection at the emitter is shown to significantly limit the emitter current to a level more than an order of magnitude less than that predicted by the hole-base diffusion model. Due to the low hole mobility, hole drift diffusion across the emitter-base space charge region is shown to be of comparable importance to thermionic-tunneling emission in controlling hole injection into the base.

Implications of hole vs electron transport properties for high speed Pnp heterojunction bipolar transistors

This paper examines the implications of differences in hole and electron transport properties on the microwave performance potential of InP-based Pnp HBTs. In particular, the carrier mobility, diffusion length, saturation velocity and ballistic transport properties for holes vs electrons are compared and their implications for Pnp device performance, such as the current gain and high frequency performance. The effects of the doping level on the mobility and minority carrier lifetime are included using empirical models fit to the available experimental data. The results show that, while the hole's transport properties are inferior to those for the electron, the minority carrier lifetime is larger for holes than electrons so that the hole diffusion length in the base is smaller but comparable to the electron's. As a result, Pnp HBTs with adequate current gain can be achieved with only a modest reduction in the base width compared with the Npn. For the device's microwave performance, the hole's characteristics determine the base transit time and the collector delay time, which limit the Pnp HBT's high frequency performance.

A thermionic-emission-diffusion model for graded base Pnp heterojunction bipolar transistors

An analytical model which matches thermionic-emission-diffusion of holes across the emitter-base heterojunction with drift-diffusion transport across a graded base has been developed and used to examine the performance capabilities of InP-based Pnp heterojunction bipolar transistors (HBTs). Hole drift-diffusion across the emitter-base space charge region is shown to be of comparable importance to thermionic emission in controlling hole injection into the base. The effects of compositional base grading on the recombination currents is also taken into account. Compositional grading of the base is shown to enhance the device’s current gain by as much as a factor of 10 by reducing recombination in the quasi-neutral base. More importantly, compositional base grading significantly reduces the base transit time which improves the device’s peak cutoff frequency by as much as a factor of 1.5. A cutoff frequency as high as 35 GHz is found to be possible. The analysis indicates that composition grading of the base can be useful in developing high performance Pnp InP-based HBTs.

Simulation and design of InAlAs/InGaAs pnp heterojunction bipolar transistors

The performance capabilities of InP-based pnp heterojunction bipolar transistors (HBT's) have been investigated using a drift-diffusion transport model based on a commercial numerical simulator. The low hole mobility in the base is found to limit the current gain and the base transit time, which limits the device's cutoff frequency. The high electron majority carrier mobility in the n/sup +/ InGaAs base allows a reduction in the base doping and width while maintaining an adequately low base resistance. As a result, high current gain (>300) and power gain (>40 dB) are found to be possible at microwave frequencies. A cutoff frequency as high as 23 GHz and a maximum frequency of oscillation as high as 34 GHz are found to be possible without base grading. Comparison is made with the available, reported experimental results and good agreement is found. The analysis indicates that high-performance pnp InP-based HBT's are feasible, but that optimization of the transistor's multilayer structure is different than for the npn device.

Simulation of PNP InAlAs/InGaAs heterojunction bipolar transistors

The performance of single heterojunction InAlAs/InGaAs PNP heterojunction bipolar transistors is modeled numerically and good agreement is found with experimental measurements. Peak experimental values of f/sub T/=14 GHz, f/sub Max/=22 GHz, /spl beta/=170 and U=38 dB are obtained near a collector current density of 10/sup 4/ A/cm/sup 2/ for a nonoptimized device.

Simulation study of InP-based PNP heterojunction bipolar transistors and incorporation of nonclassical effects

This paper describes a numerical approach to the modeling of PNP HBTs in the InP-based materials systems (InP/InGaAs and InAlAs/InGaAs). Initial device analysis was achieved in the drift-diffusion limit by self-consistent numerical solution of the Poisson, carrier continuity and conductor equations subject to the device's geometry and boundary conditions imposed by the device's biasing. Simulation results are compared with the available experimental results and good agreement is found. For the InP/InGaAs and InAlAs/InGaAs heterojunctions, the valence band discontinuities are larger than for the AlGaAs/GaAs system so grading of the emitter-base junction and tunneling effects are important. For completeness, nonclassical effects were also considered. For the emitter-base junctions, hole tunneling was considered, particularly at low forward bias. The inverse dependence of the hole tunneling on effective mass was found to lead to significantly more light hole than heavy hole tunneling in calculating the emitter injection current. In addition, since very narrow base regions (25-35 nm) can be employed while keeping the base spreading resistance low due to the electron's higher mobility, ballistic hole transport should also be considered.

Comparison of pnp and npn InGaAlAs/InGaAs heterojunction bipolar transistors

The molecular-beam epitaxial growth conditions and characteristics of npn and pnp heterojunction bipolar transistors in the InGaAlAs/InGaAs material system are described. Direct-current current gains of more than 15 000 have been obtained for npn heterojunction bipolar transistors (HBTs). pnp HBTs have a larger breakdown voltage compared to equivalent npn HBTs. A novel pnp HBT structure in which a strained hole filter is placed just above the base to inject light holes preferentially is proposed. The high speed performance of pnp HBTs is at present limited by emitter series resistance.