Location of 1/f noise sources in BJT's and HBJT's—I. theory

The most general case of1/fnoise in transistors can be described by three independent noise current generators: ibebetween base and emitter, ibcbetween base and collector, and iecbetween emitter and collector. By short-circuiting the base and the collector to ground and comparing the base and collector noise spectraS_{IB}(f)andS_{IC}(f)for the case of zero feedback from the emitter with the base and collector noise spectraS'_{IB}(f)andS'_{IC}(f)for the case of strong feedback from the emitter, one can evaluate the relative strength of the three noise sources. By measuring the current dependence ofS_{IB}(f),S_{IC}(f),S'_{IB}(f), andS'_{IC}(f), one can assign physical processes to the current generators ibc, ibe, and iec. It is the aim of this paper to demonstrate theoretically a simple method for locating1/fnoise sources in BJT's and HBJT's by comparing the base and collector1/fnoise for the cases without and with strong emitter feedback. In later papers we shall demonstrate experimentally how this method is applied to practical situations.     

Optimum low-level injection efficiency of silicon transistors with shallow arsenic emitters

The influence of As surface concentration CSEon the emitter efficiency βγand the temperature dependence of βγare reported. The theoretical model that is used to explain the variation of βγwith CSEis based upon the difference in the effective energy bandgaps in the emitter and base regionsDeltaE_{g}. Experimental measurements ofDeltaE_{g}versus CSEare presented. Measurements of βγversus CSEshow that the effective emitter doping densityQ_{E}/x_{eb}reaches a maximum value atC_{SE} cong 1.5 times 10^{20}atoms/cm³, corresponding to the threshold above whichDeltaE_{g} > 0. For the case of a constant active base doping/cm²QB, this also corresponds to an optimum in the emitter efficiency βγ. However, it is shown that in typical sequential diffusion processing of transistors, βγincreases monotonically with CSEbecauseQ_{B} = Q_{B}(C_{SE})decreases. In addition, for devices fabricated in this study,Deltabeta_{gamma}/DeltaC_{SE}atC_{SE}=2 times 10^{20}atoms/ cm³for As-diffused emitters (doped oxide) was ≈ 5 times greater than for ion-implanted-diffused As emitters, showing the superiority of implantation in controlling gain. Finally, transistors that were made withC_{SE} siml 1.4 times 10^{20}atoms/cm³(DeltaE_{g} = 0) showed βgamma(85°C)/ βγ(-15°C) ≤ 1.05.     

Second breakdown of double epitaxial transistors

This paper deals with the second breakdown of transistors with epitaxial collector, epitaxial base, and diffused emitter. Transistors were fabricated with base width WBin the range of 2 to 18 µ and resistivity in the range of 0.1 to 10 ohm . cm. The optimum values of the resistivity and the thickness of these regions were calculated by computer techniques. The devices were mounted onto a TO-63 header and the base and the emitter leads were bonded onto the device ultrasonically. The electrical characteristics, including the frequency response ftand secondary breakdownS/Bcapability, were tested. For the measurement of second breakdown current IM, forward bias condition was used. It was found that for fixed collector and emitter parameters, IMwas controlled by the product of base resistivity ρBand base width WB. The value of IMwas found to increase withrho_{B}W_{B}. However, for a specified device characteristic, an optimum value ofrho_{B}W_{B}was found to exist. For transistors withV_{CEO} =150volts,f_{t}=20mHz andh_{FE}=20, the optimum value ofrho_{B}W_{B}was found to be 6 × 10^-4ohm . cm².     

Noise figure of UHF transistors as a function of frequency and bias conditions

It is shown that the observed values of the minimum noise figureF_{min}of UHF transistors in common base connection can be explained in terms of the device parameter(1-alpha_{dc}) r_{b'b}/R_{e0}and fαfor frequencies up to 1000 MHz. An interesting collector saturation effect is observed that gives a strong increase in UHF noise figure at high currents. Many features of the dependance ofF_{min}on operating conditions can be explained by this effect. The current dependence ofF_{min}for large values of |VCB| and high currents suggests a distribution in diffusion times through the base region. At intermediate frequencies, the noise figure increases with increasing collector bias |VCB| due to an increase inr_{b'b}, which in turn is caused by the dependence of the base width on |VCB|.     

Characteristics, structure, and performance of a diffused-base germanium oscillator transistor

The diffused-base transistor structure affords a degree of design flexibility not found in previous structures. This is true because it has a larger number of independently adjustable design parameters than the previous structures. Its flexibility has been exploited in an oscillator transistor for 200-mc service. Design analysis shows that low ohmic base resistance, low collector body resistance, and operation at about 0.3 of the collector breakdown voltage are desirable in the present application. The methods of Lee have been used in making this germaniump-n-pdiffused-base unit. Alloyed emitter and base electrodes are parallel stripes approximately 0.5-mil apart, each measuring 1 × 6 mils. The collector is about 4.5 × 8 mils. Typical parameters at Vc= -10 volts and 1E= 10 ma are: fα= 600 mc,r'_{b}= 35 ohms, and Cc= 1.0 mmf. Median 200-mc oscillator efficiency of 50 per cent is obtained at the design bias point of -20 volts, 10 ma; this exceeds the performance objective. The unit withstands 20,000-gaccelerations in any direction, an additional demand imposed by the specific application for which it was developed.     

Design considerations of hyperabrupt varactor diodes

This paper describes the design calculations and fabrication techniques used to produce hyperabrupt varactor diodes. The computation consists of determining, the doping profile, the capacitance as a function of applied voltage, the breakdown voltage, and the series resistance; allowance is made for the nonlinear variation of mobility with doping density. Fabrication of the retrograded region was accomplished by diffusion from a doped-oxide source. Two groups of diodes were fabricated. The first group had a breakdown voltage VBof 95 V at 1 µA, a capacitance ratio of 12 over a voltage range of 1 to 40 V (C_{P1}/C_{P40}=12), and a quality factorQof 60 at an operating level of 3 V and 100 MHz. The corresponding values for the second group were VB=45 V (C_{P1}/C_{P40}=4) andQ=210. The capacitance tracking capability of these diodes was found to be within 2 percent. These measurements showed excellent correlation with calculated results.     

Transistor noise at high injection levels

Measurements of transistor noise at high injection levels are compared with the predictions made by the low-level injection theory, The noise is represented by an emf eein series with the emitter and a current generator i in parallel with the collector: eeis split into a part ee' fully correlated withiand a part ee" uncorrelated withi. The measured values ofbar{i^{2}}usually agree very well with theory, even at high currents; this indicates that the low-level injection theory ofbar{i^{2}}remains correct at high injection levels. The measurements ofbar{e_{e}^{"2}, though inaccurate, seem to indicate that the predictions made by the low-level injection theory are approximately correct at high injection levels. The measurements of the cross correlationbar{e_{e}i^{*}}indicate a large discrepancy with the low-level injection theory. The theory predicts thatbar{e_{e}i^{*}}is quite small at low frequency and is mainly imaginary at high frequencies. The measurements indicate thatbar{e_{e}i^{*}}is quite large at low frequencies and that its real part is larger than the imaginary part at high frequencies. The fact thatbar{e_{e}i^{2}}varies assqrt{I_{E}}, so that it approaches zero at low currents, indicates that it is a high-level injection effect.     

A lateral metal—insulator—p-Si tunnel transistor

We report the fabrication of a lateral MIS tunnel transistor whose emitter and collector are Al/SiO2/p-Si tunnel junctions. All processing is carried out at room temperature except for the growth of the passivating field oxide. The small signal common emitter current gain is 20. Two coupled gain mechanisms exist for such a lateral MIS tunnel transistor. The first mechanism relies on a high minority-carrier injection ratio of the emitter junction. Second, the minority carriers injected into the reverse-biased collector junction may produce additional gain through multiplication of majority-carrier current. Lateral MIS tunnel transistors on n-Si make use of the second mechanism. Our device takes advantage of the high minority-carrier injection ratio achievable with Al/SiO2/p-Si tunnel junctions.     

Direct measurement of the potential spike energy in AlGaAs/GaAs single-heterojunction bipolar transistors

A balanced two-step current transport theory, i.e., thermionic emission followed by Shockley diffusion, is applied to study the emitter-base (EB) potential spike energy in the AlGaAs/GaAs single-heterojunction bipolar transistor. It is found, surprisingly, that when the transistor is operated in the active region theI-Vcharacteristics of the collector current (IC) versus base-emitter applied voltage (VBE) exhibits an ideality factor of 1.237. This non-1kT transfer characteristics is due to the bias-dependent potential spike energy at the emitter-base heterojunction. The reverse I-V characteristics of emitter current (IE) versus base-collector bias (VBC), however, shows the traditional 1kT behavior. The difference between ICand IEat the same applied voltage (V_{BE} = V_{BC}) determines the potential spike energy (ΔE). It turns out that Δ E/q = 0.19(V_{BE} - 0.48)whereqis the unit charge. This indicates that the potential spike appears only when the applied voltageV_{BE} > 0.48V.     

Ultralinear UHF power transistors for CATV applications

An investigation of the cross-modulation performance of UHF power transistors and its correlation with the gain parameter curves S21(IC, VCB) shows that such transistors today exhibit an inherent linearity limitation and are not optimized for ultralinear applications. This means that using higher power transistors does not result in a larger output signal with low cross-modulation distortion. Presented measurements clearly demonstrate that the nonlinear products increase, not only with increasing emitter dc current, but also with collector voltage above certain limiting values ICLand VCL, respectively; ICLand VCLare relatively small compared with the corresponding maximum permissible values of ICand VCB. Their existence can be explained by current crowding effects, by the space-charge limitation of the collector current, by the nonuniform thermal conductance, by the temperature gradients, and by the unequal current density distribution across the (large) active areas of power transistors. A correlation between the VCgain parameter falloff effect and the "thermal distortions," has been established. Design and selection criteria for ultralinear transistors needed in CATV amplifers are given. A simple linearity test method is described.     

Epitaxial π-ν n-p-n high-voltage power transistors

A new silicon power-transistor structure has been developed which expands the frontiers of power-switching performance and high-voltage power-handling capability. This new structure employs a unique π-ν (nearly intrinsic p- and n-type) epitaxial-layer construction which utilizes "overlay" emitter concepts to achieve improved volt-ampere densities and expanded second-breakdown performance. An n⁺-p-π-ν-n⁺transistor structure is constructed using alternately grown π and ν, epitaxial high-resistivity layers, p-type base and n⁺emitter diffusions are used in the conventional manner to assure punch-through protection. The depletion region for the n⁺-p-π-ν-n⁺transistor is in the π base and the ν collector, and the maximum electrical field (E_{max}) is at the π-ν interface. The avalanche breakdown VBof the device can be controlled by the thickness (X_{n}, X_{p}) and the concentration (N_{A}, N_{D}) of the π-ν layers. Limiting the thickness of the ν collector region and adjusting the thickness of the π base layer provides a transistor with optimum volt-ampere capability. Various π-ν structures have been fabricated and are evaluated electrically for power switching and for linear applications.     

Measurement of high-frequency equivalent circuit parameters of junction and surface barrier transistors

Methods are described for the determination of the high-frequency parameters of junction and surface barrier transistors and involve, in addition to the knowledge of the usual low-frequency parameters, measurement of the product of Ccand the extrinsic base resistancer_{b0},r_{b0}itself, and the alpha cutoff frequency fα. A previously described method is used to determine the productr_{b0}C_{c}, but new methods are described for the measurement ofr_{b0}and fα. The measurements are of "bridge" type, involving simple circuit adjustments for a response null at a single frequency. Typical experimental results are given for transistors having fαvalues as high as 85 mc, Ccvalues down to 2.3 pF andr_{b0}ranging from 45 to 400 ohms. The limits quoted for fαand Ccrefer to surface barrier transistors. Comparison with results derived by alternative methods of measurement show good agreement.     

Origin of 1/f noise in bipolar transistors

In planar n-p-n transistors fabricated in IC technology, the dependence of 1/fnoise on the base current density jB, the base width WB, and the emitter area FEwas measured. The power spectrumS_{iB}(f)of the base current fluctuations iBcan be represented by the empirical relationS_{iB} = const. jmin{B}max{gamma} cdot Fmin{E}max{beta} cdot wmin{B}max{-1} cdot f^{-1}where1 leq gamma leq 2and β has been found to be 1.3 or 2. The results of measurements on gate-controlled devices indicate that 1/fnoise cannot be explained by McWhorter's surface model. Therefore, a new model is proposed which assumes resistance fluctuations in the base region as the cause of 1/fnoise in bipolar transistors. The model establishes the relation forS_{iB}(f)as well as the magnitude of the coefficients β and γ.     

The temperature dependence of the DC base and collector currents in silicon bipolar transistors

Detailed measurements have been made of the base and collector-current characteristics of both n-p-n and p-n-p silicon transistors as a function of temperature. The collector current shows ideal behavior over the temperature range -60 to 150°C in thatI_{C} infin exp (eV_{BE}/kT). On the other hand, the base current is nonideal:I_{B} infin exp (eV_{BE}/nkT), wheren > 1.0. The nonideality of IBis the main source of the temperature dependence ofh_{FE} = I_{C}/I_{B}. There is no evidence for bandgap narrowing in the devices we have investigated. The temperature dependence of the collector current is given byI_{C} infin T_{m} exp (-e E_{g0}/kT) exp (eV_{BE}/kT), wherem = 1.4or 1.7 for n-p-n or p-n-p devices, respectively.E_{g0} = 1.19 pm 0.01eV. This result is consistent with the findings of others. The base current is a complicated function of temperature due to the presence of nonideal components.     

A new higher ambient transistor

Interest in high speed transistor switching circuits whose operation is unaffected by large changes in ambient temperature led to an investigation of silicon-germanium alloy point-contact transistors because of the larger forbidden energy gap of silicon-germanium alloys. In germanium transistors, as far as temperature stability is concerned, Ic0is particularly poor. Ic0is the value of collector current, at a given collector voltage, with no emitter current. The Ic0of germanium units tested rose rather linearly from 20°C. to about 65°C., with a gradient of 25 µa/°C. but then entered a region of run away. A number of point-contact transistors have been manufactured using 3 per cent silion-germanium (10 ohm-cm, n-type), and the parameters r11, r12, r22, α, fc0and Ic0at room temperature, and values of Ic0as a function of temperature have been measured. Results show that 3 per cent silicon-germanium transistors are as good as germanium transistors in all respects and better in temperature stability. The values of Ic0for silicon-germanium transistors rose linearly from 18° to about 95°C., with a gradient comparable to that of the germanium units below 65°C.     

The influence of post-emitter processing on the current gain of bipolar transistors

The effects of post-emitter processing on the current gain of n-p-n and p-n-p transistors used in linear integrated circuits, is the subject of this paper. It is seen that during the emitter diffusion deep level impurities segregate into the bulk and space-charge regions of the emitter. The aluminum alloy process at 500°C provides a powerful scavenging action that getters the impurities. The post-alloy current gain increases by a factor of ten. This gettering is effective in phosphorus diffused emitters if the dielectric is either a CVD oxide, a plasma deposited oxide, or phosphorus glass under Si3N4. With thermally oxidized emitters there is no improvement in current gain. Boron diffused emitters, however, respond to the aluminum alloy process regardless of the dielectric. This study also identifies two processes that contribute to bandgap narrowing (DeltaE_{g}) these are the emitter diffusion and the post-emitter thermal oxidation. A value ofDeltaE_{g}= 0.04 eV measured after the emitter diffusion was found to increase to 0.09 eV after thermal oxidation at 975°C. This additional component ofDeltaE_{g}is a result of the anomalous accumulation of phosphorus at the silicon surface during oxidation.     

Low-frequency noise in permeable base transistors

The predominant noise is1/fnoise and consists of two parts: a) Noise varying asImin{C}max{2}, generated mostly with conducting channel and predominating for normal values of the collector voltage VCE. b) Noise at low VCEand practically independent of VCE; it is generated chiefly in the space charge region around the base grating and gives collector1/fnoise atV_{CE} = 0. The turnover frequency of the first noise source lies at about 20 MHz forV_{CE} = 0.30V,V_{BE} = 0.20V. At sufficiently high frequencies the PBT shows thermal noise of the output conductanceg_{c0}at zero bias. Generation-recombination noise is observed at large VBEand low VCEand comes mostly from the space charge region around the base grating.     

A coaxially packaged MADT for microwave applications

A coaxially packaged transistor capable of delivering greater than 11 db of power gain at 1000 Mc, with a resultant maximum frequency of oscillation of 3500 Mc, has been developed. This device is a p-n-p micro-alloy diffused-base transistor (MADT). The principal difference between this device and a standard high-frequency MADT amplifier is the reduction of electrode size and use of a coaxial construction. The parasitic elements, rb', and emitter and collector transition capacities, have very striking effects. Also, the excess phase of alpha at alpha cutoff, as described by Thomas and Moll, can be very large (150° on this device); for this reason, fTrather than fcαshould be used as the figure of merit for graded-base transistors. Because of this excess phase, the value of K(0.85 for homogeneous-base transistors), which is used to relate fTto fcα, can be as low as 0.43 in graded-base transistors of this type.     

Influence of base thickness on collector breakdown in abruptAlInAs/InGaAs heterostructure bipolar transistors

The avalanche process in the collector of abrupt Al_0.48In_0.52As-In_0.53Ga_0.47 As heterostructure bipolar transistors (HBTs) is reported. It is reported that the collector multiplication constant decreases monotonically with increasing base thickness. When the base thickness is less than the mean-free path for energy relaxation in the base, the avalanche process in the collector is enhanced by high-energy injection from the emitter. On the other hand, no such dependence is observed for long-base transistors with equilibrium base transport. These effects are expected as the emitter injection energy of 0.48 eV is appreciable compared to the impact ionization threshold of 0.83 eV in the InGaAs collector     

A three mask bipolar integrated circuit structure

A new bipolar integrated circuit structure has been fabricated that compares favorably to the MOS structure in terms of fabrication simplicity and performance. The new structure is basically a modified isolated lateral transistor and requires only three photolithographic masking operations up to and including first level of metalization. The fabrication of the structure is as follows: a shallow nonselective p-type base region is diffused into a lightly doped p-type substrate; n⁺emitter and collector regions are then simultaneously and selectively diffused into and through the p-type base region thus forming a lateral n-p-n transistor. The second and third masks define the contact holes and the metalization pattern, respectively. Lateral isolation of the structure is obtained by encircling the emitter and base regions with the collector region. Vertical isolation is achieved by the large collector-depletion region that extends beneath the emitter and base regions. Since the substrate is lightly doped a low collector voltage will adequately isolate the emitter and base regions from adjacent devices. The new technology permits the fabrication of transistors, resistors, and crossunders. Transistors with 2-to 3-µm spacings occupy 500 µm²of silicon area and have the following characteristics:beta = 35, peakf_{t} = 0.1GHz at 0.5 mA, BV(SUSTAIN) = 3 to 5 V,t_{r} = 20ns,t_{f} = 120ns,t_{s} = 20ns. Resistors with values as high as 40 kΩ have been fabricated within 600 µm². Active nonlinear loads with effective resistance up to 200 kΩ have been fabricated. TTL gates have been made with power-delay products of 3.6 pJ, and propagation delays of 34 ns.