Two new epitaxial technologies have emerged in recent years (molecular beam epitaxy (MBE) and metal-organic chemical vapor deposition (MOCVD)), which offer the promise of making highly advanced heterostructures routinely available. While many kinds of devices will benefit, the principal and first beneficiary will be bipolar transistors. The underlying central principle is the use of energy gap variations beside electric fields to control the forces acting on electrons and holes, separately and independently of each other. The resulting greater design freedom permits a re-optimization of doping levels and geometries, leading to higher speed devices. Microwave transistors with maximum oscillation frequencies above 100 GHz and digital switching transistors with switching times below 10 ps should become available. An inverted transistor strucure with a smaller collectors on top and a larger emitter on the bottom becomes possible, with speed advantages over the common "emitter-up" design. Double-heterostructure (DH) transistors with both wide-gap emitters and collectors offer additional advantages. They exhibit better performance under saturated operation. Their emitters and collectors may be interchanged by simply changing biasing conditions, greatly simplifying the architecture of bipolar IC's. Examples of heterostructure implementations of I2L and ECL are discussed. The present overwhelming dominance of the compound semiconductor device field by FET's is likely to come to an end, with bipolar devices assuming an at least equal role, and very likely a leading one.

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Heterostructure bipolar transistors and integrated circuits

Heterojunction In0.53Ga0.47As/InP avalanche photodiodes have recently been observed to have a response to long‐wavelength optical pulses which contains both fast and slow components. We present evidence that the slow response is limited by charge pile‐up at the semiconductor heterointerface. We find that the speed of response depends on the degree of compositional grading in the heterointerface region. The response time can be significantly reduced for compositional grading lengths of greater than 600 A depending on the doping and depletion region width in the In0.53Ga0.47As layer.

Optical response time of In0.53Ga0.47As/InP avalanche photodiodes

High-speed avalanche photodiodes and high-sensitivity receivers are vital components for future multigigabit-per-second lightwave transmission systems. We review theoretical and experimental performance of high-speed III-V avalanche photodiodes, and also that of multigigabit-per-second lightwave receivers using FET and bi-polar amplifiers. Particular attention is given to APD gain-bandwidth product, and to its effect on high-speed receiver sensitivity. Comparisons between measured receiver sensitivities and calculated performance are presented for bit rates up to 8 Gbit/s.

Multigigabit-per-second avalanche photodiode lightwave receivers

Long-wavelength GaInAs/InP graded double heterostructure p-i-n photodiodes are demonstrated with 3-dB bandwidths over 100 GHz. The heterojunction hole trapping problem is significantly improved and the device contact resistivity is greatly reduced by using superlattice graded bandgap layers at the hetero-interfaces to reduce the barrier height. Self-aligned processes are used in the device fabrication to reduce device parasitics. Pulsewidths as short as 3.0 ps full-width-at-its-half-maximum (FWHM) for 2 /spl mu/m/spl times/2 /spl mu/m device are measured by pump-probe electrooptic sampling. 3-dB bandwidths over 100 GHz are found for 2 /spl mu/m/spl times/2 /spl mu/m and 3 /spl mu/m/spl times/3 /spl mu/m devices. The device with the integrated bias tee can be biased without using the external bias tee. The electrical resonance between the photodiode and external circuits was reduced by integrating an impedance matched resistor in parallel with the photodiode. The 7 /spl mu/m/spl times/7 /spl mu/m device with bias tee and matched resistor has a measured pulsewidth of 3.8 ps and a 3-dB bandwidth over 100 GHz. The calculated pulse shape based on the saturation velocity model fits well with the measured response. A model for different components of the series resistance agrees with the measured area dependence of the series resistance. >

110-GHz GaInAs/InP double heterostructure p-i-n photodetectors

An unusual new line has been observed in the low‐temperature photoluminescence spectrum of GaAs/AlGaAs epilayers grown by liquid‐phase epitaxy, which represents a unique probe of the heterointerface. A very strong, broad and asymmetric line is seen, with peak energy ranging from that of the bound exciton to that of the shallowest acceptor, increasing roughly linearly with the logarithm of the excitation power. The intensity is seen to exhibit a very strong temperature dependence as the temperature is varied from 1.4 K up to about 20 K where the line falls below the background luminescence intensity. Intensity and energy dependence of the luminescence as a function of the Al mole fraction on one side of the junction is also investigated. By employing a novel step‐etching technique, this transition is shown to originate from the GaAs/AlGaAs heterojunction. A qualitative model is proposed to explain the observed data.

New photoluminescence effects of carrier confinement at an AlGaAs/GaAs heterojunction interface

A simplified method for calculating the energy band profiles of graded-gap heterojunctions, based on the generalized model of Oldham and Milnes, is presented. The profiles are derived by superposing an energy band grading function and the electrostatic potential in the heterojunction. The latter is obtained by using the depletion layer approximation as for conventional p-n homojunctions. The energy band profiles of hypothetical p(GaAs)-n(Al0·4Ga0·6As) heterojunctions are calculated using the simplified method. For small grading layer widths, the results are in good agreement with the generalized model. The barrier lowering factor η as a function of the graded layer width l is calculated for such heterojunctions. It is found, for acceptor and donor densities of 1018 and 1016 cm−3 respectively, that the barrier height is reduced from 0·47 eV to zero as l increases from zero (abrupt case) to ≈300 A. The applications of these analyses to practical heterojunctions are discussed.

S.Y. Chiang

Papers

A simplified model for graded-gap heterojunctions