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BJT current issue help

  1. Aug 10, 2008 #1
    BJT current issue help urgent

    hello i have a problem with current and the convention and so on. bottom line for a bjt my professor said that Ie = Ic + Ib but the next slide he remixes it telling us that and i quote " the relatively large current flowing from the emitter to the collector is directly controlled by the much smaller base current. so since Ie = Ic + Ib how can the large current flow from Ic to Ie ok the passive sign convention? it should be Ie + Ib = Ic? i am lost and i am tired of being lost on such basic principles can somebody help me out please once and for with this current madness, for me its like voodoo magic thanks for the help in advance much appreciated .
     
  2. jcsd
  3. Aug 10, 2008 #2
    Re: BJT current issue help urgent

    It comes down to positive current being oriented opposite to electron flow. When the bjt b-e junction is forward biased, the electrons emitted from the emitter move towards the base, for an *npn* bjt device. Then the electric field from the reverse biased b-c junction attracts these electrons. Thus the electrons emitted from the emitter, transit through the base region and into the collector. But electrons are negative charge carriers so that a positive current is said to flow from collector to emitter.

    The b-e jcn electric field which moves the electrons from emitter towards base also moves holes from the base towards the emitter. The base current consists chiefly of this hole motion, plus a few electrons that were emitted from the emitter get intercepted in the base region via recombination, so a few more holes flow into the base to preserve charge neutrality in the base region. So base current is hole motion from base to emitter. But, holes are positive charges so that the base-emitter current using positive convention is in the same direction as hole flow.

    The electrons emitted by the emitter are negative charges so that the emitter-collector electron flow is collector-emitter (positive) current.

    It's basically about convention. In a pnp device, the emitter to collector charge motion consists of holes which are positively charged. Hence the positive current convention is in the same direction as the hole flow. The base current would consist of electron flow from emitter to base. This is opposite of positive convention because pnp base current consists of electrons which are negatively charged.

    Have I helped or made matters worse?
     
    Last edited: Aug 10, 2008
  4. Aug 11, 2008 #3
    Re: BJT current issue help urgent

    Don't know if this is better or worse.

    As the last response said, the forward bias on the base causes current to flow from the emittter. (whether this is electron or hole flow is more detail than is needed. )

    The 'current' mainly continues through the thin base region. A small portion of it goes out the base lead.

    Once the current gets into the collector region ( past the junction), it is attracted by the relatively high voltahe field in this region.

    In essence, the forward bias applied to the base starts the process. However, due to the construction of the device, most of the 'current' elements continue on to the collector. The base VOLTAGE actually controls the current flow in a transistor. ( transistors are voltage controlled devices, but current does flow in the base circuits. The design equations work
    better focusing on the current).

    So, in an analog circuit, small variances in base voltage (and current) control large variances in the collector - emitter current.

    In a digital circuit, it's simpler. The base causes current to alternate between saturation and cutoff.

    Hope that helps,

    ford2go
     
  5. Aug 11, 2008 #4
    Re: BJT current issue help urgent

    Good heavens. Are we back to the chicken-egg viscious circle? The bjt, according to Fairchild, Tex Instr, Natl Semi, On Semi, etc. is classified as a current controlled device. Do you know more semiconductor physics than them? Honestly, if the bjt was "really" voltage controlled, every semiconductor maker in the world would say the same. The fact that they don't should say something.

    When I said "forward bias" on the base, I meant Ib and Vbe in unison, not one or the other. Neither Ib nor Vbe alone can "cause" collector current. Also, Ib and Vbe, as well as Ie, cannot exist independently. Either all 3 are zero, or all 3 are non-zero. The FET is similar. In order to change the value of electric field in the gate to source region, a change in Ig, gate current, is needed as well as a change in Vgs. Both are equally important.

    People who insist that Vbe is the "cause" don't accept the idea that current and voltage, i.e. Ib and Vbe, are mutually inclusive. Their relation is not cause-effect. It takes work to change the electric field in the base emitter region because the E field has energy. In order to do work, time is needed. So thw work divided by the time equals the power. But the power is non-zero when the E field is changing. Since power is the product of current and voltage, i.e. Ib and Vbe, non-zero power requires BOTH non-zero Ib and Vbe. It's impossible to modulate the field in a bjt or FET without Ib and Vbe together in unison (or Ig/Vgs for a FET).

    Please don't argue, oppose, or turn this into a flame war. Every semi maker in the world can't be wrong. At the microscopic level, FETs and bjts are universally classified as charge controlled. At the macro level, where we don't concern ourselves with internal physics, only approximate equivalent models, the FET is voltage controlled, and the bjt is current controlled.
     
  6. Aug 11, 2008 #5
    Re: BJT current issue help urgent

    calbraham,

    Not trying to rain on your parade, it was just a casual comment. If it caused any consternation, it was definitely not intentional.

    Have a good day,

    ford2go
     
  7. Aug 11, 2008 #6

    Defennder

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    Re: BJT current issue help urgent

    Having read your entire post I must say I am confused as to your actual stand on this. For one thing the gate current Ig is always zero for a MOSFET. Secondly, it isn't incorrect to think of a BJT as a voltage-controlled current source and in fact this is done in 2nd year electronics classes. The convention for BJTs as current controlled device and MOSFETs as voltage controlled sources stems from the fact that the gate for a FET is electrically insulated while the BJT has no electrically insulated components. I would appreciate if you could provide a source which quotes the semi-conductor manuacturers as characterising BJTs and FETs as current-controlled devices.

    Furthermore I quote the following from my textbook on semiconductor physics:
    Based on what you have written above, you may wish to contact the author to address this seemingly egregious error.
     
  8. Aug 11, 2008 #7
    Re: BJT current issue help urgent

    Ok, let's start here: "one thing the gate current Ig is always zero for a MOSFET".

    I don't know where to start. Have you ever used FETs at frequencies in the 100's or even 10's of kHz? Switched mode power supplies and motor drivers come to mind as examples. To toggle a MOSFET from on to off rapidly requires substantial gate current if losses are to remain low so that the device won't overheat. The Ig in a MOSFET is NEVER ZERO! Who says otherwise! Without Ig, the Vgs cannot change. In a capacitor, which is what a MOSFET gate-source terminals present, the familiar relation is ever present:

    i = C*dv/dt. So, Vgs will not change unless Ig is non-zero. Case closed. You have no position at all. In fact, the unity gain frequency of a MOSFET, aka transition frequency "ft", is defined as that frequency where id = ig. The small signal drain current id equals the small signal gate current ig. The "current gain" is unity at said freq.

    I didn't say FETs were current controlled. At the macro level I said FETs are voltage controlled.

    As far as your text is concerned, the hybrid-pi bjt model uses a resistor r_pi to model the b-e junction. In the small signal mode this is permissable. The gross non-linear b-e junction relationship can be linearized for *small* changes. The value of r_pi is simply hfe/gm, where hfe is the ac beta, and gm = Ic/Vt, where Ic is the dc value of collector current, and Vt is the thermal voltage kT/q. But by definition r_pi = v_pi/ib, and hfe = ic/ib. Thus ic = gm*vbe and ic = hfe*ib are equivalent. The gm*vbe is simply (ic/vbe)*(vbe/ib) = ic/ib which equals hfe.

    Thus for *small signal* operation, we may compute the signal portion of ic as either "gm*vbe* (voltage controlled or referenced), or "hfe*ib" (current controlled/referenced). This duality is valid only in small signal mode. In large signal mode, "gm" changes as Ic undergoes large swings. The "gm" concept is valid only for very small vbe swings, 100's of microvolts.

    The voltage referenced approach is generally used in small signal mode for good reason. The input source to a bjt stage is usually a constant voltage type. The output is generally constant voltage (low Z). The feedback is generally "voltage feedback". Hence when we set up the hybrid pi circuit equivalent, computing parameters based on voltage rather thasn current is usually the norm as common input devices are voltage sources and the amp is usually a constant voltage source itself with voltage feedback. Computing in terms of current should give the same numeric result. Since r_pi is defined as hfe/gm, the relation is valid for constant gm. As ic deviates from Ic, gm changes. If the deviation is small, gm is almost constant and gm*vbe = ic = hfe*ib. Thus voltage control and current control are perfectly equivalent in this narow case as a pure resistor has a linear i-v relation.

    Your text is valid under narrow conditions, i.e. small signal mode. When using a bjt in large signal mode, you never control Ic by connecting a constant low-Z voltage source directly across the b-e terminals. The bjt would be incinerated.

    It's lunch time. Later I'll send quoted from semi mfr's who affirm what I've said. Actually, my position is based on their teachings. I agree with them , not the other way around. BR.

    Claude
     
  9. Aug 11, 2008 #8

    NoTime

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    Re: BJT current issue help urgent

    It's Ie = Ic + Ib.
    As a practical matter, you can think of bjt as two separate circuits.
    B -> E and C -> E.
    Over the linear range of operation Ic = Ib times hfe (or the transistor gain)
    Both currents flow thru the emitter connection.
     
  10. Aug 11, 2008 #9
    Re: BJT current issue help urgent

    References from semiconductor OEMs describing FET as voltage controlled and bjt as current controlled.

    On Semiconductor AN-913: under the heading "Comparing and Contrasting Bipolars and Power MOSFETs", top of page 2 - "The most marked difference is that the gate of the MOSFET is voltage driven whereas the base of the bipolar is current driven."

    National Semiconductor AN 558: introduction on page 1 - "The high voltage power MOSFETs that are available today are N-channel, enhancement-mode, double diffused, Metal- Oxide-Silicon, Field Effect Transistors. They perform the same function as NPN, bipolar junction transistors except the former are voltage controlled in contrast to the current controlled bi-polar devices. Today MOSFETs owe their ever-increasing popularity to their high input impedance and to the fact that being a majority carrier device, they do not suffer from minority carrier storage time effects, thermal runaway, or second breakdown."

    Fairchild Semiconductor AN-7500: under "general characteristics" on page 1 - "A conventional n-p-n bipolar power transistor is a current-driven device whose three terminals (base, emitter, and collector) are connected to the silicon by alloyed metal contacts. Bipolar transistors are described as minority-carrier devices in which injected minority carriers recombine with majority carriers. A drawback of recombination is that it limits the device's operating speed. And because of its current-driven base-emitter input, a bipolar transistor presents a low-impedance load to its driving circuit. In most power circuits, this low-impedance input requires somewhat complex drive circuitry.

    By contrast, a power MOSFET is a voltage-driven device whose gate terminal, Figure 1(a), is electrically isolated from its silicon body by a thin layer of silicon dioxide (SiO2). As a majority-carrier semiconductor, the MOSFET operates at much higher speed than its bipolar counterpart because there is no charge-storage mechanism. A positive voltage applied to the gate of an n-type MOSFET creates an electric field in the channel region beneath the gate; that is, the electric charge on the gate causes the p-region beneath the gate to convert to an n-type region, as shown in Figure 1(b). This conversion, called the surface-inversion phenomenon, allows current to flow between the drain and source through an n-type material. In effect, the MOSFET ceases to be an n-p-n device when in this state. The region between the drain and source can be represented as a resistor, although it does not behave linearly, as a conventional resistor would. Because of this surface-inversion phenomenon, then, the operation of a MOSFET is entirely different from that of a bipolar transistor, which always retain its n-p-n characteristic."

    Fairchild Semiconductor AN-9010: under "advantages of a MOSFET" on page 7 - "1. High input impedance - voltage controlled device - easy to drive. To maintain the on-state, a base drive current which is 1/5th or 1/10th of collector current is required for the current controlled device (BJT). And also a larger reverse base drive current is needed for the high speed turn-off of the current controlled device (BJT). Due to these characteristics base drive circuit design becomes complicated and expensive. On the other hand, a voltage controlled MOSFET is a switching device which is driven by a channel at the semiconductor’s surface due to the field effect produced by the voltage applied to the gate electrode, which is isolated from the semiconductor surface. As the required gate current during switching transient as well as the on and off states is small, the drive circuit design is simple and less expensive."

    These are just a few off the top of my head. You can download any of the above app notes and confirm the above. I've been an EE for 30 years, and every OEM, FAE, app note, and data sheet describes FETs as voltage controlled and bjt's as current controlled. Of course, this is a model based on viewing the device as a 3-terminal black box. This macro view point does not consider internal fields, quantum mechanics, energy bands, doping, traps, crystal bonds and defects, stored charges, etc. It is a "big picture / external" view of the device. At the micro level, both are classified as "charge controlled". At the micro level the fundamental difference between the two is that FET's are "majority carrier" devices, whereas bjt's are "minority carrier devices". The terms "majority" and "minority" refer to *charge* carriers, not currents or voltages. At the micro level, for all of my 30 years as an EE, they are both considered "charge controlled".

    The most common misconception in electrical science in general is that currents are "caused" by voltages. It's not true. The light bulb that illuminates your room can be used for example. Does the voltage across the filament cause the current? Or does the current through the filament cause the voltage? The only rational answer is that neither can exist without the other. It is a circular relation. For the bulb to output heat and light, which is power, power must be inputted. The input power is the product of current and voltage. Both must be non-zero to light up the bulb. The light requires both current and voltage. Because the power source is constant voltage, people can rush to judgement that voltage comes first, then current. But the constant voltage source is that way because the power company forces it to be that way. They could provide constant current, but the losses in the lines would increase.

    BTW, I made a typo in a previous post, but the edit button was locked out. Anyway I said that r_pi = v_pi/ib, and hfe = ic/ib, which should read r_pi = vbe/ib, and hfe = ic/ib. The "v_pi" and "vbe" are not usually equivalent due to re. Enough for now. BR.

    Claude
     
    Last edited: Aug 11, 2008
  11. Aug 12, 2008 #10

    Defennder

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    Re: BJT current issue help urgent

    Hi cabraham,

    Thanks for posting such a long insightful post on this. It got me thinking again on the physics of semiconductor device operations. I'll have to look through my textbook and notes first before I can reply to you on this. Thanks!
     
  12. Aug 13, 2008 #11
    Re: BJT current issue help urgent

    No problem. I've taken 1 undergrad and 3 grad level semiconductor physics courses, and my head still spins with this subject. It isn't intuitive or obvious.

    Regarding one of the OP questions "how does a small current control a large one?", here is an abridged answer.

    In general we don't control collector current with base current, but instead we control it with emitter current. If we input a specific value of Ib, then Ic = beta*Ib. "The "beta" factor is listed in data sheets as "hFE" (dc and low freq) or "hfe" (high freq). The problem with using Ib to control Ic is a phenomenon called "beta dependency". But if we set the emitter current Ie to a specific value, then Ic = alpha*Ie. The chief difference is that beta varies with specimen (part to part values differ by a factor of 2 to 3), temperature (at very low temp, -40 or -55 deg C, the beta value can be 60% or even 50% of that at room temp and at +125 deg C, it can br 150-200% of that at room), and current level (at very low and very high current levels beta droops). Thus a skilled practitioner relies on alpha to control Ic and not beta. Alphs is generally 0.99 give or take 0.01.

    But a good bjt should have a high value of beta to be effective as an amplifying device. This is done by controlling the doping in the 3 regions of a bjt. For an "npn", the collector "n" region is doped with a much lower density of donor ataoms than the emitter "n" region. Also, the "p" base is lightly doped with acceptor atoms. When forward biased, the sparse population of acceptor carriers in the p base region combined with the dense population of donors in the "n" emitter region results in Ie >> Ib. This is desirable. Holes flowing from base to emitter is a quantity to be minimized. The light base and heavy emitter doping achieves that. If an npn was reversed, i.e. collector and emitter swapped, the beta value would plummet, typically much less than 1.

    Does this help? BR.
     
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