Basic Electronics | Bipolar Junction Transistor (BJT) Basics

• The BJT is constructed with three doped semiconductor regions (emitter, base, and collector) separated by two pn junctions.

• One type consists of two n regions separated by a p region (npn), and the other type consists of two p regions separated by an n region (pnp).

• The term bipolar refers to the use of both holes and electrons as current carriers in the transistor structure.

• The pn junction joining the base region and the emitter region is called the base-emitter junction.

• The pn junction joining the base region and the collector region is called the base-collector junction.

• A wire lead connects to each of the three regions.

• The leads are labeled E, B, and C for emitter, base, and collector, respectively.

• The base region is lightly doped and very thin compared to the heavily doped emitter and the moderately doped collector regions.

• Figure 2 shows the schematic symbols for the npn and pnp bipolar junction transistors.

• Figure 3 shows a bias arrangement for both npn and pnp BJTs for operation as an amplifier. In order for a BJT to operate properly, the two pn junctions must be correctly biased with external DC voltages.

• In both cases the base-emitter (BE) junction is forward-biased and the base-collector (BC) junction is reverse-biased. This condition is called forward-reverse bias.

• The heavily doped n-type emitter region has a very high density of conduction-band (free) electrons.

• These free electrons easily diffuse through the forward-based BE junction into the lightly doped and very thin p-type base region.

• The base has a low density of holes, which are the majority carriers.

• A small percentage of the total number of free electrons injected into the base region recombine with holes and move as valence electrons through the base region and into the emitter region as hole current.

• When the electrons that have recombined with holes leave the crystalline structure of the base, they become free electrons in the metallic base lead and produce the external base current.

• As the free electrons move toward the reverse-biased BC junction, they are swept across into the collector region by the attraction of the positive collector supply voltage.

• The free electrons move through the collector region, into the external circuit, then return into the emitter region along with the base current.

• The emitter current is slightly greater than the collector current because of the small base current that splits off from the total current injected into the base region from the emitter.

• The operation of the pnp is the same as for the npn except that the roles of the electrons and holes, the bias voltage polarities, and the current directions are all reversed.

• The directions of the currents in both npn and pnp transistors and the schematic symbols are as shown in Figure 5.

• The arrow on the emitter inside the transistor symbols points in the direction of the conventional current.

• These diagrams show that the emitter current (I_E) is the sum of the collector current (I_C) and the base current (I_B), expressed as:

DC Beta (β_DC) and DC Alpha (α_DC)

• The DC current gain of a transistor is the ratio of the DC I_C to DC I_B and is designated DC beta (β_DC).

• β_DC is usually designated as an equivalent hybrid (h) parameter, h_FE, on transistor datasheets: β_DC= h_FE.

• The ratio of the DC I_C to the DC I_E is the dc alpha (α_DC). The alpha is a less-used parameter than beta in transistor circuits.

• Fig. 6 shows the unsaturated BJT as a device with a current input and a dependent current source in the output circuit for an npn.

• The input circuit is a forward-biased diode through which there is base current.

• The output circuit is a dependent current source (diamond-shaped element) with a value that is dependent on I_B.

Consider the basic transistor bias circuit in Fig. 7.

V_BE: DC voltage at base with respect to emitter

V_CB: DC voltage at collector with respect to base

V_CE: DC voltage at collector with respect to emitter

• V_BB forward-biases the base-emitter junction, and V_CC reverse-biases the base-collector junction. When the base-emitter junction is forward-biased,

• Although V_BE can be as high as 0.9 V in an actual transistor and is dependent on current, 0.7 V is used to simplify the analysis of the basic concepts.

• The characteristic of the base-emitter junction is the same as a normal diode curve.

• Since the emitter is at ground (0 V), by Kirchhoff’s voltage law, the voltage across R_B is

• By Ohm’s law, V_RB=I_BR_B. Substituting for V_RB and solving for I_B,

• The voltage at the collector with respect to the grounded emitter is

• Since the drop across R_C is V_RC=I_CR_C, V_CE can be written as

• The voltage across the reverse-biased collector-base junction is

• Using a circuit like that shown in Fig. 8, a set of collector characteristic curves can be generated that show how I_C varies with V_CE, for specified values of I_B.

• Both V_BB and V_CC are variable sources of voltage. V_BB is assumed to be set to produce a certain value of I_B and V_CC is zero. Thus, both the base-emitter junction and the base-collector junction are forward-biased because the base is at approximately 0.7 V while the emitter and the collector are at 0 V.

• I_B is through the base-emitter junction because of the low impedance path to ground and, therefore, I_C is zero

• When both junctions are forward-biased, the transistor is in the saturation region of operation. Saturation is the state of a BJT in which I_C has reached a maximum and is independent of I_B.

  • As V_CC is increased, V_CE increases as I_C increases. This is the portion between points A and B in Fig. 9. I_C increases as V_CC is increased because V_CE remains less than 0.7 V due to the forward-biased base-collector junction.

• When V_CE exceeds 0.7 V, the base-collector junction becomes reverse-biased and the transistor goes into the active, or linear, region of operation.

  • I_C increases very slightly for a given I_B as V_CE increases due to widening of the base-collector depletion region. This causes a slight increase in β_DC.

  • This is the portion between points B and C in Fig. 9. I_C in this portion is determined only by I_C =β_DCI_B.

• When V_CE reaches a sufficiently high voltage, the base-collector junction goes into breakdown; and I_C increases rapidly, shown by the portion to the right of point C. A transistor should never be operated in this region.

• A family of curves is produced when I_C versus V_CE is plotted for values of I_B. When I_B =0, the transistor is in the cutoff region. Cutoff is the nonconducting state of a transistor.

• When I_B=0, the transistor is in the cutoff region of its operation. This is shown in Fig. 10 with the base lead open, thus I_B=0.

• There is a very small amount of collector leakage current, I_CEO, due mainly to thermally produced carriers.

• I_CEO will usually be neglected in circuit analysis so that V_CE=V_CC.

• Base-emitter and base-collector junctions are reverse-biased. The subscript CEO represents collector-to-emitter with the base open.

• When the base-emitter junction becomes forward-biased and I_B is increased, I_C also increases and V_CE decreases as a result of more drop across R_C.

• When V_CE reaches its saturation value, V_CE(sat), the base-collector junction becomes forward-biased and I_C can increase no further. At the point of saturation, I_C=β_DCI_B is no longer valid.

• V_CE(sat) for a transistor occurs somewhere below the knee of the collector curves, and it is usually only a few tenths of a volt.

• Base-emitter and base-collector junctions are forward-biased.

• Cutoff and saturation can be illustrated in relation to the collector characteristic curves by the use of a load line.

• The bottom of the load line is at ideal cutoff where I_C=0 and V_CE=V_CC. The top of the load line is at saturation where I_C=I_C(sat) and V_CE=V_CE(sat).

• In between cutoff and saturation along the load line is the active region of the transistor’s operation.

More About β_DC

• β_DC varies with both I_C and with temperature.

• Keeping the junction temperature constant and increasing I_C causes β_DC to increase to a maximum.

• A further increase in I_C beyond maximum point causes β_DC to decrease.

• If I_C is held constant and the temperature is varied, β_DC changes directly with the temperature. If the temperature goes up, β_DC goes up and vice versa.

• A transistor datasheet usually specifies β_DC (h_FE) at specific I_C values. Even at fixed I_C and temperature, β_DC varies from one device to another for a given transistor due to unavoidable inconsistencies in manufacturing.

• The β_DC specified at a certain I_C is usually the minimum value, β_DC(min), although the maximum and typical values are sometimes specified.

• The product of V_CE and I_C must not exceed the maximum power dissipation P_D(max). Both V_CE and I_C cannot be maximum at the same time. If V_CE is maximum, I_C can be calculated as

If I_C is maximum, V_CE can be calculated by

• A transistor amplifies current because I_C is equal to I_B multiplied by the current gain, β.

• I_B is very small compared to I_C and I_E. Because of this,

• An AC voltage, V_s, is superimposed on the DC bias voltage V_BB as shown in Fig. 14. The DC bias voltage V_CC is connected to the collector through R_C.

• I_b, I_c and I_e are the AC transistor currents. V_b, V_c and V_e are AC voltages from the transistor terminals to the ground.

• The AC V_in produces an AC I_B, which results in a much larger AC I_C.

• The AC I_C produces an ac voltage across R_C, producing an amplified, but inverted, reproduction of the ac input voltage in the active region of operation.

• The forward-biased base-emitter junction presents a very low resistance to the ac signal. This internal AC emitter resistance (r'_e) appears in series with R_B. The AC base voltage is
  

• The AC collector voltage, V_c, equals the AC voltage drop across R_C: .
  
• Since , the AC collector voltage is .
  
• V_b can be considered the transistor AC input voltage where V_b =V_s-I_bR_B.
• V_c can be considered the transistor AC output voltage.
  
• Since voltage gain is defined as the ratio of the output voltage to the input voltage, the ratio of V_c to V_b is the AC voltage gain, A_v, of the transistor.

• Substituting I_eR_C for V_c and I_er'e for V_b,

• In Fig. 15(a), the transistor is in the cutoff region because the base-emitter junction is not forward-biased. There is, ideally, an open between collector and emitter.

• In Fig. 15(b), the transistor is in the saturation region because the base-emitter junction and the base-collector junction are forward-biased. I_B is made large enough to cause I_C to reach its saturation value.

  • There is, ideally, a short between collector and emitter. A small voltage drop across the transistor of up to a few tenths of a volt normally occurs, which is the saturation voltage, V_CE(sat).

Conditions in Cutoff

• The base-emitter junction is not forward-biased. Neglecting leakage current, all of the currents are zero, and V_CE=V_CC,

Conditions in Saturation

• The base-emitter junction is forward-biased and there is enough I_B to produce a maximum I_C. The collector saturation current is

• Since V_CE(sat) is very small compared to V_CC, it can usually be neglected. The minimum value of I_B needed to produce saturation is

• Normally, I_B should be significantly greater than I_B(min) to ensure that the transistor is saturated.