Winterstone
Banned
To all forum members who are convinced that the BJT is a current-controlled device.
I am aware that the problem as mentioned in the title was discussed already earlier in this forum (for my opinion, without a commonly agreed final result). And also in three currently running threads this question plays a major role:
https://www.electro-tech-online.com...rent-of-an-npn-common-emitter-circuit.134752/
https://www.electro-tech-online.com...tor-not-affect-the-amplifiers-working.134687/
https://www.electro-tech-online.com/threads/voltage-biasing-for-bjt.134806/
In some cases, the difference between „to control something“ and „to determine something numerically“ was not understood. Thus, we had to face some unfair and unobjective contributions leading to misunderstandings, accusations and reproaches.
With this post, it is my intention and hope to enable a fair and openhearted discussion about this - surprisingly - still disputed question. Therefore, I have listed my arguments in the following. To simplify the discussion I have neglected some minor effects that do not contribute too much the principle answer.
To make your replies and critics easier I have numbered the following
Chain of claims and statements:
1.) Due to charged carrier diffusion there is a depletion region (insulation layer) at the pn junction of a diode. As a consequence, there is a so called „diffusion voltage“ across the pn junction with an associated electrical field that establishes an equilibrium between drift and diffusion effects.
2.) A DC voltage applied across the pn junction disturbs this balance - the depletion area becomes smaller and the potential barrier is reduced. The mathematical description of the voltage-to-current relation is given with the famous Shockley equation, which contains as the most important part the expression exp(Vd/Vt).
(Vd being the voltage across the pn junction and Vt=kT/q is the temperature voltage).
3.) A bipolar junction transistor (BJT) is a three-terminal device (E,B,C) which consists - in principle - of two pn junctions connected back-to-back. The middle region (base region) intentionally is very thin. The emitter region is heavily (n) and the base region is lightly (p) doped. Thus, in the following an npn type transistor is assumed.
4.) In analogy to the pn diode there exists, of course, an insulation layer across both pn junctions.
5.) Now we assume normal operating conditions: B-C junction reverse biased and B-E junction forward biased with a rising voltage starting at Vbe=0. Then, the B-E junction will start to conduct as for classical diodes.
6.) There is no reason, why the voltage-to-current relationship across the B-E junction should not follow the above mentioned Shockley equation (exponential law). It is the momentary VOLTAGE across the junction that determines the width of the depletion area and, thus, the amount of moving carriers (current).
And - as everybody knows - all measurements confirm this exponential law.
7.) Because (a) the p-doped base region is very thin and (b) a rising voltage Vbe even decreases the width of the depletion barrier of this central region, most of the the n-charged carriers released from the emitter (equivalent to the emitter current Ie) do not enter the base node but move to the collector region: Ic=alpha*Ie (alpha<1, but close to unity).
Explanation: The E-field accelerates the mobile charges; they are attracted by the collector potential and they become majority carriers in the n-doped collector region.
8.) It is obvious that the corresponding current Ic (which is slightly smaller than Ie because of alpha<1) follows the exponential law as mentioned under 6.).
9.) Thus, we have a collector current that depends on the applied voltage Vbe - following Shockley`s equation: That gives the classical transfer characteristic Ic=f(Vbe).
10.) Note that - up to now - it was not necessary to quantify the current through the base node. This will happen now.
Because - as mentioned under 7.) - not all of the emitted n-carriers reach the collector, the rest forms the base current Ib1. For completeness, it is to be mentioned that there is a second part Ib2 that is caused by p-carriers of the slightly doped base region which recombine with the emitted n-carriers. Thus the resulting base current is Ib=Ib1+Ib2.
11.) It turns out that because of Ie=Ib+Ic the ratio Ic/Ib=(1-alpha)/alpha=beta is a factor, that is relatively constant for different operating conditions (in particular for different Ic values). This leads to a relation that has a clear practical significance: Ic=beta*Ib.
However, from this we cannot derive that Ic would be physically controlled by Ib.
As mentioned, the factor beta is relatively constant but its actual VALUE is connected with a very large tolerance.
Thus, Ib is an unwanted byproduct, that - however - should not be made arbritarily small because of other impacts and restrictions (breakdown voltage, leakage, etc).
12.) Result/Summary : According to Shockley`s equation the collector current Ic is determined by three physical quantities (Vbe, Vt, Is) and can be electrically controlled by an external voltage Vbe.
That means: The BJT is to be treated as voltage-controlled current source.The most important parameter that determines the amplification properties of the BJT is the transconductance g (slope of the Ic=f(Vbe) characteristic.
____________________________________________________________________________
To clarify the main question (current vs. voltage control) it would be helpful if the „defenders“ of current-control could tell me which of the above listed points are wrong (of course, with corresponding verification/explanation). I am hopeful this will lead to a „common agreement“.
I am sorry for the length of this contribution.
Thank you. and regards to all.
Winterstone
I am aware that the problem as mentioned in the title was discussed already earlier in this forum (for my opinion, without a commonly agreed final result). And also in three currently running threads this question plays a major role:
https://www.electro-tech-online.com...rent-of-an-npn-common-emitter-circuit.134752/
https://www.electro-tech-online.com...tor-not-affect-the-amplifiers-working.134687/
https://www.electro-tech-online.com/threads/voltage-biasing-for-bjt.134806/
In some cases, the difference between „to control something“ and „to determine something numerically“ was not understood. Thus, we had to face some unfair and unobjective contributions leading to misunderstandings, accusations and reproaches.
With this post, it is my intention and hope to enable a fair and openhearted discussion about this - surprisingly - still disputed question. Therefore, I have listed my arguments in the following. To simplify the discussion I have neglected some minor effects that do not contribute too much the principle answer.
To make your replies and critics easier I have numbered the following
Chain of claims and statements:
1.) Due to charged carrier diffusion there is a depletion region (insulation layer) at the pn junction of a diode. As a consequence, there is a so called „diffusion voltage“ across the pn junction with an associated electrical field that establishes an equilibrium between drift and diffusion effects.
2.) A DC voltage applied across the pn junction disturbs this balance - the depletion area becomes smaller and the potential barrier is reduced. The mathematical description of the voltage-to-current relation is given with the famous Shockley equation, which contains as the most important part the expression exp(Vd/Vt).
(Vd being the voltage across the pn junction and Vt=kT/q is the temperature voltage).
3.) A bipolar junction transistor (BJT) is a three-terminal device (E,B,C) which consists - in principle - of two pn junctions connected back-to-back. The middle region (base region) intentionally is very thin. The emitter region is heavily (n) and the base region is lightly (p) doped. Thus, in the following an npn type transistor is assumed.
4.) In analogy to the pn diode there exists, of course, an insulation layer across both pn junctions.
5.) Now we assume normal operating conditions: B-C junction reverse biased and B-E junction forward biased with a rising voltage starting at Vbe=0. Then, the B-E junction will start to conduct as for classical diodes.
6.) There is no reason, why the voltage-to-current relationship across the B-E junction should not follow the above mentioned Shockley equation (exponential law). It is the momentary VOLTAGE across the junction that determines the width of the depletion area and, thus, the amount of moving carriers (current).
And - as everybody knows - all measurements confirm this exponential law.
7.) Because (a) the p-doped base region is very thin and (b) a rising voltage Vbe even decreases the width of the depletion barrier of this central region, most of the the n-charged carriers released from the emitter (equivalent to the emitter current Ie) do not enter the base node but move to the collector region: Ic=alpha*Ie (alpha<1, but close to unity).
Explanation: The E-field accelerates the mobile charges; they are attracted by the collector potential and they become majority carriers in the n-doped collector region.
8.) It is obvious that the corresponding current Ic (which is slightly smaller than Ie because of alpha<1) follows the exponential law as mentioned under 6.).
9.) Thus, we have a collector current that depends on the applied voltage Vbe - following Shockley`s equation: That gives the classical transfer characteristic Ic=f(Vbe).
10.) Note that - up to now - it was not necessary to quantify the current through the base node. This will happen now.
Because - as mentioned under 7.) - not all of the emitted n-carriers reach the collector, the rest forms the base current Ib1. For completeness, it is to be mentioned that there is a second part Ib2 that is caused by p-carriers of the slightly doped base region which recombine with the emitted n-carriers. Thus the resulting base current is Ib=Ib1+Ib2.
11.) It turns out that because of Ie=Ib+Ic the ratio Ic/Ib=(1-alpha)/alpha=beta is a factor, that is relatively constant for different operating conditions (in particular for different Ic values). This leads to a relation that has a clear practical significance: Ic=beta*Ib.
However, from this we cannot derive that Ic would be physically controlled by Ib.
As mentioned, the factor beta is relatively constant but its actual VALUE is connected with a very large tolerance.
Thus, Ib is an unwanted byproduct, that - however - should not be made arbritarily small because of other impacts and restrictions (breakdown voltage, leakage, etc).
12.) Result/Summary : According to Shockley`s equation the collector current Ic is determined by three physical quantities (Vbe, Vt, Is) and can be electrically controlled by an external voltage Vbe.
That means: The BJT is to be treated as voltage-controlled current source.The most important parameter that determines the amplification properties of the BJT is the transconductance g (slope of the Ic=f(Vbe) characteristic.
____________________________________________________________________________
To clarify the main question (current vs. voltage control) it would be helpful if the „defenders“ of current-control could tell me which of the above listed points are wrong (of course, with corresponding verification/explanation). I am hopeful this will lead to a „common agreement“.
I am sorry for the length of this contribution.
Thank you. and regards to all.
Winterstone
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