Mirza M. Jahan

An analytical expression for base transit time in an exponentially doped base bipolar transistor

Abstract An analytical expression of base transit time for exponentially doped base, which is applicable to both homojunction and heterojunction bipolar transistors, is developed. The present treatment includes dopant dependent mobility variation, bandgap narrowing and finite velocity saturation effects in the calculation of base transit time and thus is a generalization of the results reported recently by Rosenfield and Alterovitz[6]. These effects degrade base transit time significantly and must be incorporated in the calculation. the finite velocity saturation effect will progressively play a pivotal role as the base width is scaled down and for double heterostructure bipolar transistor. The transition of base transit time from the ballistic and the drift-diffusion limit will occur at a longer base width if the base is exponentially doped with a sufficiently high doping index.

An exact current partitioning and its effect on base transit time in double heterojunction bipolar transistors

Abstract The current transport across the abrupt base-collector heterojunction of double heterojunction bipolar transistors (DHBTs) has been studied by partitioning the total collector current into thermionic and tunneling components within a unified formulation. An exact quantum mechanical calculation, including image force lowering of the potential energy barrier, is performed in calculating the total current. A modified junction velocity is calculated and compared with previously reported results. It is shown that the estimation of junction velocity using the previously reported simplified calculation severely underestimated tunneling, especially at low reverse bias. The modified junction velocity is used to study the base transit time of DHBTs.

Junction temperature dependence of high-frequency noise in heterojunction bipolar transistors

The use of the Hawkins model, under isothermal condition, to calculate the bias dependent high frequency noise in Heterojunction Bipolar Transistors (HBTs) is questioned. The inclusion of thermal effects into the noise model of HBTs is necessary as the temperature of the device becomes progressively different from the ambient temperature with increasing bias current. Calculation of noise figure by including the thermal effects explains the experimental measurement whereas the isothermal calculation underestimates the noise figure at high bias current.

Traversal time in an asymmetric double-barrier quantum-well structure

A study of traversal time for asymmetric double-barrier quantum-well structure (DBQWS) is presented. The calculations are self consistent in nature. The analysis shows that traversal time is affected more strongly by the width of the emitter barrier than that of the collector barrier of the structure in the presence of space charge. Bistability in traversal time is observed. Bistability becomes greater if the emitter barrier is made wider than the collector barrier and is explained in terms of space charge developed in the quantum well. However, a wider collector will dominate traversal time at zero bias, especially at eigen energy. >

Shot noise in double barrier quantum structures

Shot noise is calculated in a Double Barrier Resonant Tunneling Structure (DBRTS) by taking the space charge accumulated inside the quantum well into account. The calculation is self-consistent in nature and is obtained by simultaneously solving the Schrodinger and Poissons equations. The calculation shows the suppression of shot noise in the positive differential resistance (PDR) region and an enhancement in the negative differential resistance region (NDR) of the DBRTS. This behavior is explained in terms of the auto-and cross-power density spectrum of current in the presence of space charge in the quantum well. Calculated shot noise factor shows a suppression in the PDR region (γ = 0.31) and an enhancement in the NDR region (γ = 9) as observed experimentally.

Self‐consistent calculation of shot noise in a double‐barrier resonant tunneling structure in the presence of magnetic field

Shot noise is calculated in a Double Barrier Resonant Tunneling Structure (DBRTS) by taking the space charge accumulated inside the quantum well into account. The calculation is self‐consistent in nature and obtained by simultaneously solving Schrodinger and Poisson’s equations. The calculation manifests the suppression of the shot noise in the positive differential resistance region and an enhancement in the negative differential resistance region of the DBRTS. The behavior is explained in terms of the fluctuation of the eigen energy of the structure due to the stored charge in the quantum well.

DC, small‐signal parameters and noise performance for SiGe/Si FETs

An analytical model to evaluate d.c. small signal parameters and noise performance for the SiGe/Si based FETs is presented, that is based on a self‐consistent solution of Schrodinger and Poisson’s equations and an improved velocity‐electric field (vd−e) characteristics. The presence of a self‐consistent calculation provides a better insight in the dependence of the device parameters on the QW properties. Moreover, the inclusion of a modified velocity‐electric field characteristic enables us to calculate small‐signal parameters that are in excellent agreement with experimental data both at 300 K and 77 K, respectively. The theoretical calculation of noise properties for SiGe/Si based FETs are presented.

Far infrared Ga/sub 1-x/In/sub x/Sb/InAs-based strained-layer superlattice detectors grown by OMVPE

Strained-layer, type-II Ga/sub 1-x/In/sub x/Sb/InAs superlattice detectors operating at 9 /spl mu/m have been grown for the first time by OMVPE. In all reported literature thus far, MBE has been exclusively used to grow this particular material system. In the present research, the material was grown by an atmospheric pressure OMVPE reactor using high purity metallorganic precursors (TM Ga, TMSb, TMIn and TBAs). A 300 /spl Aring/ of Ga/sub 1-x/In/sub x/Sb buffer layer was grown on n-GaAs(100) substrate followed by 20 periods (60 /spl Aring/ Ga/sub 0.6/In/sub 0.4/Sb/60 /spl Aring/ InAs) superlattice. A 300 /spl Aring/ InAs was grown as the capping layer. The growth took place at 600/spl deg/C with a precracker temperature of 140/spl deg/C. The absorbance characteristics measured by FTIR showed a shift in the cutoff wavelength /spl lambda/ from 1.8 /spl mu/m to 2.5 /spl mu/m when the In mole fraction was varied from 0.1 to 0.4 in Ga/sub 1-x/In/sub x/Sb layer. The use of InAs in Ga/sub 0.6/In/sub 0.4/Sb/InAs structures introduces strain and /spl lambda/ shifts from 2.5 /spl mu/m to 9 /spl mu/m as confirmed by FTIR. An increase in the number of superlattice periods (from 5 to 20) resulted in an enhanced absorbance edge. p-n photodiodes were fabricated by establishing AuGe/Ni/AuGe contacts on InAs and n-GaAs. The samples were sintered at 430/spl deg/C for 10 minutes in N/sub 2/ ambient. The samples were measured to show good I-V and R-V characteristics. The detector was edge-excited using white light to show change in conductance. The measured leakage current (1 mA<at a bias of -0.2 V) and was attributed to the lattice mismatch between GaAs and Ga/sub 0.6/In/sub 0.4/Sb (/spl sim/8%). A reduction in leakage current (or, improved electrical characteristics) is currently under investigation using a GaSb substrate.

Study of shot noise in a double barrier resonant tunneling structure

Shot noise is calculated in a double barrier resonant tunneling structure (DBRTS) by taking the space charge accumulated inside the quantum well into account. The calculation is self-consistent in nature and is obtained by simultaneously solving the Schrodinger and Poissons equations. The calculation manifests the suppression of shot noise in the positive differential resistance (PDR) region and an enhancement in the negative differential resistance region (NDR) of the DBRTS. The behavior is explained in terms of the fluctuation of the eigen energy of the structure due to the stored charge in the quantum well.<<ETX>>

Noise in Si / Si1-xGex n-channel HEMTs and p-channel FETs

A theoretical model to evaluate noise in SiGe/Si based n-channel MODFETs and p-channel MOSFETs is presented. The analysis is based on a self-consistent solution of Schrodinger and Poisson’s equations. In this study, n-channel FETs exhibit a better noise performance than that of p-channel FETs. The influence of device parameters on noise properties for this class of devices are presented.