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Technical notes and FAQ on bipolar junction transistors (BJT) :

- What is the difference between bipolar and unipolar devices ?Bipolar transistors can have both minority and majority carriers flowing, whereas FET's only have majority carriers flowing. The fact that bipolar transistors have two types of carriers flowing simultaneously results in the name 'bi-polar'. The monirity carrier flow is responsible for collector conduction modulation and transistor storage time.

-Why is a bipolar transistor current driven and a FET voltage driven ?A FET has a gate that is either insultaed physically (MOSFET) or by a reverse biased junction (JFET). Hence no current is flowing through the gate. A bipolar transistor has the base electrically connected and needs base current to turn on.

- How does a bipolar transistor (BJT) work ?A BJT (NPN) will turn on by base current. For a positive base current, electrons will be drawn from the emitter into the base area.. Most of the electrons that are drawn from emitter into base area will be cought by the collector. For every electron that is taken from the base terminal, several electrons will reach the collector terminal. This is the multiplication effect, or current gain. The BJT is a current amplifier. The BJT has 4 modes of operation :1) Cutoff mode : the base voltage is below threshold (0.4-0.7V for Si) so no collector current is flowing. The base-collector juction is reverse biased, and the collector voltage is blocked with no collector current flowing.2) Linear mode : the collector-base junction is reverse biased, and the base-emitter junction is forward biased. Electrons are injected into the reverse-biased base-collector junction, constituting a collector current which is a multiple of the base current. The collector current is more or less independent from the collector voltage.3) Quasi Saturation : the base-collector junction becomes forward biased. This can appear even if the collector voltage is still higher than the base voltage because of conduction losses in the collector area. In this mode minority carriers will be injected into the collector area though the forward biased base-collector junction. These minority carriers will generate an electron-hole plasma effectively lowering the collector resistance. It is this lowered collector resistance that will offer BJT's a benefit over unipolar devices in switching applications.4) Saturation : the base-collector junction is forward biased and increasing the base current will no longer lower the collector resistance.

- What does the SOA define ?The SOA (Safe Operating Area) defines the permissible region of operation for linear applications. The circuit designer must make sure the transistor is never used outside the SOA boundaries. The SOA defines 4 important boundaries :1) Maximum collector current : this is the maximum continuous current the bonding wires and the transistor metallization can take without damage. Higher peak current is possible. Check the datasheet for peak current values.2) Maximum power : this is the maximum power the transistor can dissipate, usually at case temperature of 25°C. To hold the case temperature to 25°C at maximum power the transistor must be perfectly mounted on an almost infinite cooling fin. Since cooling fins have a limited size and thus have their own thermal resistance, te maximum power must be derated according to the cooling capabilities. Higher power levels can be tolerated for short periods (<100ms) and sometimes the SOA defines peak power for short intervals.3) Second breakdown : this is the maximum power the transistor can dissipate at higher voltage levels. The second breakdown limit is independent from junction temperature. Exceeding the second breakdown boundary is immediately destructive. Sometimes the SOA defines higher power levels for short periods.4) Maximum collector voltage : this is the maximum voltage the collector will sustain. Exceeding this level may result is first breakdown or avalanche breakdown.

- What is the FBSOA ?FBSOA = forward biased safe operating area. This is the same as the normal SOA for linear transistors, and this is the SOA to be used for swichting transistors when the base is turned on.

- What is RBSOA ?RBSOA = reverse biased safe operating area. This does not apply to linear and general purpose transistors. This SOA is to be used for switching transistors when the base is turned off.

- What is the difference between a general purpose, linear, HF and a switching transistor ?General purpose and linear transistors can be used for amplifier and slow switch applications.HF transistors are constructed differently and are suitable only for HF/Video amplifier or oscillator apllications.Switching transistors are designed to switch as fast as possible and are sometimes constructed to withstand switching inductive loads.Important : do NOT attempt to switch large inductive loads (even if these are clamped) with any other device than a switching transistor.Make sure that all SOA conditions are satisfied while the inductive load is switched off !

- How critical is the maximum current limit ?The maximum current limit defines the highest safe current level for the tranistor metallization and the bonding wires, wither for continuous and peak current.Theoretically the transistor can be operated safe near the maximum current limit, but practically it is not advisable to do so, because in that case no margin is left for error and because at the highest current levels the current gain will be poor. If the transistor is to be operated near maximum Ic, then it is wise to select a device with a higher current rating wich will show more linear operation at the same current level.

- How critical is the maximum power limit ?The maximum power limit will define operation at maximum junction temperature and 25°C junction temperature. Maximum junction temperature is usually 150°C, and the maximum power = (150-25)/package termal resistance. Please note that in reality the thermal resistance of the cooling fin has to be taken into account, as well as the thermal resistance of the mounting compound, and that the maximum ambient temperature that has to be taken into account is normally higher than 25°C. The resulting maximum power level will thus be lower than the ideal value specified in the datasheet and will depend on the actual transistor mounting.. If maximum power is exceeded, the junction temperature may exceed its maximum value.

- How critcal is the maximum junction temperature ?Most BJT's are specified at Tj max = 150°C. The reason for this is that above this temperature the collector leakage current will become significant and the transistor will not be usable. Maximum junction temperature can be momentarily exceeded without destroying the transistor. A BJT will shortly withstand temperatures of 200-250°C. However, for reliable operation is not advised to allow the maximum junction temperature to be exceeded. As high temperatures promote device aging, it is discouraged to operate the transistor continuously near maximum temperature if reliable long life is to be archieved.

- How large is the real leakage current ?Most transistors show a very small leakage current at low temperatures, in the order of nA. As the leakage current rises exponentially with rising temperature, collector leakage current may be substantial at Tmax. Therefore the highest possible leakage current should be taken into account when designing a circuit.

- What is the tolerance on the breakdown voltage ?All transistors are designed to go into avalance breakdown at a voltage that is above spec. Usually there is a margin of 10-20%. Breakdown voltage is not temperature dependent. True breakdown voltage may vary slightlty from one transistor to another.

- What happens when a transistor goes into avalanche breakdown ?When a transistor goes into avalanche breakdown with open base (Vceo) and the collector current is limited, the transistor will survive. Check a datasheet, and spot the testing conditions for Vceo. Usually this is done by injecting a small current (usually 30mA) into the collector and allowing the transistor to clamp the collector voltage. Although this mode is not allowed for safe operation, a transistor will normally sustain the stated collector current at Vceo breakdown.When a transistor goes into avalanche breakdown with open emitter (Vcbo) OS with shorted base (Vces) then this will happen at a much higher value than Vceo, whatever the rating in the datasheet. Usually avalanche breakdown at Vcbo will happen at approximately twice the collector voltage of Vceo. Only a very small collector current is needed to destroy the transistor at Vcbo breakdown condition. A current of several mA may destruct the biggest power transistor when Vcbo is exceeded.

- What is the difference between VCEO, VCER and VCBO ?VCEO = breakdown voltage with open baseVCER = breakdown voltage with base-emitter resistor (value to be specifiied in ohm)VCBO = breakdown voltage with open emitter or shorted base.Basically VCBO will have the highest value, because it is the avalanche breakdown voltage of the reverse biased base-collector junction.With VCEO, the base terminal is open and the current that is leaked through the base-collector reverse biased junction is amplified in the transistor itself, thereby increasing the leakage current to high levels at a voltage much lower than the actual avalanche breakdown voltage of the base-collector junction itself. Generally when the current gain is higher then the VCEO breakdown level will be lower.With VCER, a resistor is placed between base and emitter, evacuating collector leakage current from the base. All collector leakage that is take from the base cannnot be amplified in the transistor and cannot contribute in lowering the breakdown voltage level. If Rb=0, then VCER = VCBO. If Rb=infinte, then VCER=VCEO.

Practical exanple :VCEO = 120V VCER= 150V (200ohm)VCBO = 275V HFE= 100

- How critical is the 2nd breakdown boundary ?The 2nd breakdown limit is not to be exceeded under any circumstance. If a transistor goes into 2nd breakdown it will be destructed immediately. It is up to the curcuit designer to make sure a 2nd breakdown condition cannot appear. Special care is to be used in switching applications with inductive loads.

- What mechanism initiates forward bias 2nd breakdown ?Second breakdown is a phonomenon that occurs in BJT's and not in unipolar devices. You may consider a BJT chip to be an infinite number of BJT's connected in parallel. The temperature coefficient of the base-emitter voltage is -2.3mV/K. If any hot spot occurs on the transistor die, Vbe will be lowered at that spot and HFE will increase, leading to a larger Ic at that spot and hence further heating. At low collector voltage and high collector current this unstability is corrected by feedback from the built-in emitter resistance. If the transistor draws locally more current then the emitter voltage will increase, drastically reducing the base current and collector current at that spot. At high collector voltage this feedback will not work since for the same amount of power less current is required, and less emitter feedback voltage is developed. Therefore maximum device dissipation must be reduced at higher collector voltages. The mechanism of 2nd breakdown is basically temperature independent. When a hot spot develops on the transistor die, all power will concentrate immediately (within milliseconds) in one point and local temperature will be so high that device destruction follows immediately. A transistor that has been in 2nd breakdown will exhibit very large leakage currents and will no longer sustain maximum collector voltage.

- What mechanism initiates reverse bias 2nd breakdown ?With the base reverse biased the collecor will not draw current in its linear mode, so the source of breakdown is totally different : reverse bias 2nd breakdown occurs only in switching applications where a transistor is switched off while the collector current is still flowing. If a transistor is switched off then the collector will continue to draw current until the storage time and fall time have elapsed. During turn-off excess minority carriers are either recombined or drawn from the base by negative base current. The withdrawal of minority carriers will never be evenly spread leading to local variations in collector conductivity which may lead to hot spots. Charge stored in the base may turn into a mesoplasma forcing the transistor to conduct at that spot.

- Is there a maximum base and emitter current ?Yes. The maximum base current is specified in the maximum ratings. Sometimes a peak value is also specified. The maximum base current is according to the level that the die metal and bonding wires can tolerate. The maximum emitter current is simply the maximum base current plus the maximum emitter current.

- Can the base be driven negatevely ?Yes, but there is a maximum level that should not be exceeded. The maximum negative base-emitter voltage can be found in the maximum ratings table. Exceeding this value will not immediately destroy the transistor if base current is held to a moderate value, but repeated avalanche base-emitter reverse breakdown may lead to shifts of dopants altering the characteristics of the transistor. In case the transistor base can be driven negatively, make sure the reverse base-emitter voltage is not exceeded, and if there is a risk that this may occur, make sure that the negative voltage is clamped by a protection circuit so that it is not the transistor itself that will need to clamp the negative voltage.

- What is the maximum negative base current, and what is it used for ?A transistor with zero base voltage or a negative base voltage will be in cut-off mode. In switching applications the transistor will turn off when it enters cutoff mode. In case the base current is zero during turn off, the transistor will stop conduction by recombination of minority carriers. This may take up to 10µs or so. If the transistor needs to be switched off faster, these carriers can be drawn from the base by applying a negative base voltage. While the transistor is turning off, the base current will be negative. As soon as the transistor is fully off, the negative base current will fall to zero even if the negative drive voltage is sustained.The maximum negative base current is usually equal to the maximum positive base current.

- What is the relationship between base drive and switching characteristics ?For most power devices including BJT's, the drive waveform is of utmost importance. For fast turn on, a rapid rise (high di/dt) of base current is required. Turn off for a BJT is more difficult : to avoid long storage times, it is important not to drive the transistor too deep into saturation, meaning that the on-state base current must be chosen carefully. For fast tun-off, charge must be extracted from the base. The RBSOA will depend highly on the negative base current and the risetime of the negative base drive. Therefore poor base drive may lead to high switching losses and to initiation of 2nd breakdown when switching inductive loads. Many switching transistor breakdowns can be related to poor base drive waveforms.

- What leakage current is to be expected ?The datasheet will only pubish the maximum leakage current at a given collector voltage. Usually the real transistor leakage current bill be many orders of magnitude lower, but for circuit design the value of the datasheet must be taken into account. Real leakage current may vary between prodcution lots so no typical value can be published. The actual circuit must be designed for the worst possible value.

- What is the temperature coëfficiënt of Vbe ?Vbe drops at a rate of 2.3mV/°C

- What is the temperature coëfficiënt of the gain factor ?There is no real temperature coëfficiënt of the current gain, but current gain will be higher at higher temperatures. Current gain may double at maximum temperature.

- What is the spread of the current gain ?The current gain is very difficult to control exactly in the manufacturing process. The current gain will also vary with changing collector current, dropping significantly at high collector currents, and the current gain is temperature dependent. Therefore each design should take the minimum current gain into account at a given collector current only. Also note that the current gain may drop at very low base currents. This is because there is always some base to emitter resistance on the transitor chip. Although this resistance is very high, it will lead to leaking base current and thus a lower current gain.

- What is the typical current gain ?The current gain is a parameter that varies with temperature, collector current and between production lots. Graphically a typical current gain is published for convenience, but for actual circuit design the minimum values that are published in the datasheet must be taken into account.

- What is the early effect ?The early effect is modulation of the base with by the collector voltage. Linear transistors will be designed to minimise this effect, but as a transistor design is always a compomise between various parameters, it cannot be eliminated. When a transistor is driven by a stabel base current and the collector voltage si increased, the increasing electric field over the reverse biased base-collector junction will push the base charge away making the base electrically thinner, thereby increasing current gain. As a result, current gain will always increase a little with increasing collector voltage.

- Are there requirements on the base drive circuit ?Absolutely : if the power transistor is to remain in cutoff state, the base to emitter voltage should be held close to zero voltage, or can be slightly negative (up to maximum Veb) The base of the power transistor should normally not be allowed floating or driven by an extreme resistance only, as this will increase collector leakake current. When the transistor is to be driven into saturation, the base drive circuit should supply enough base current to ensure the transistor is saturated. Otherwise Vcesat may not be met, resulting in improper circuit operation and high power dissipation. When the power transistor is switched fast, beware that the drive circuit needs to withdraw the base charge in order to allow the transistor to turn off in short time. This will require negative base current drive capability. Failure to do this in a consistent way may result to 2nd breakdown failure in smps or timebase circuits. When the transistor is used in a slow switching application, such as lamp drive, the transistor can be turned off by simply stopping to supply base current.

- Can a linear power transistor be used for switching applications ?When switching easy loads at low speed, yes. But for fast switching of inductive loads at high power the transistor needs to have a wide RBSOA, which is found only in specially designed switching transistors. Furthermore, switching transistors have a much better interdigitated gate/emitter structure, and doping profiles are optimised for fast switching.

- What is fT ?The transistion frequency is where the current gain of the BJT is reduced to unity. This means that it is the highest frequency at which there is amplifying action. Practival circuits will use the transistor at frequencies well below fT.

- What parameters are actually tested in production ?Most of the parameters that are listed in the maximum ratings sheet are tested for each transistor both at the wafer level and after packaging.The values that are production tested are :- Collector breakdown voltage.- Collector leakage current.- Current gain at specified values.- Collector saturation voltage at specified values, is usually maximum current test at the same time.- Base-emitter voltage at specified values.- Base-emitter leakage.Parameters that are inherent to device design are tested on a few devices per production run :- Maximum power (depends on die size)- 2nd breakdown (test is destructive so production test is not possible, and this phenomenon is dependent on device structure)- Maximum frequency (depends on device design)- AC gain (depends on DC gain which is prodcution tested)- Linearity (depends on technology)

- What is Aluminium spiking ?Aluminium spiking is a production error that leads to shorting the base to the emitter, by aluminum reaching though the emitter-base junction. This can also happen to transistors which have been overheated to temperatures more than 400°C.

- Why is the real maximum power dissipation lower than the figure in the datasheet ?Because every manufactures publishes the maximum power dissipation in an ideal situation : with an infinite cooling fin at T(mb) = 25°C. In reality the cooling fin will have a finite thermal resistance, an elevated ambient temperature must be taken into account, there will be some thermal resistance between transistor and mounting base and some margin needs to be maintained to maximum junction temperature. By example, a transistor capable of doing 62.5W at Tj=150°C and Tmb=25°C will have a thermal resistance of 2K/W. If the maximum ambient temperature inside the case of the application is expected to be 70°C, and Tj is designed to be 130°C in that case, then T(j-a) = 60°C only. If the cooling fin has a thermal resistance of 1.5K/W and the transistor mounting leaves a thermal resistance of 0.5K/W, then the total thermal resistance will be 2+1.5+0.5=4K/W. Now the maximum power dissipation will be 60K/4K/W=15 Watt only.

-Is temperature important toward reliability ?Yes ! Junction temperature and transistor failure are directly related. Although most bipolar transistors are allowed to work up to 150°C, continuous operation at high temperature is not recommended if the circuit needs to be reliable for a long time. Keeping the junction temperature low will also keep the failure rate of the transistors low. Another issue is temperature cyrcling. Fast temperature cycling will lead to stress and damage of the transistor package and will also influence device reliability.

-Is reliability dependent on moisture ?Yes ! Water atoms are small (only one oxygen atom surrounded with two very small hydrogen atoms) and diffuse a hundred times as fast though most materials as oxygen or nitrogen (each time two linked atoms). Worse, water vapour can corrode the metal on the die and react with dopants. Therefore operation in high humidity area's should be avoided for all electronic devices. Transistors can be protected using a passivation layer. Normal commercial general purpose transistors do not have such a passivation layer. For ultra reliable operation, transistors can be made with a nitride passivation layer.

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