simulation assignment bjt
Post on 22-Dec-2015
20 Views
Preview:
DESCRIPTION
TRANSCRIPT
BJT Simulation ECE 417 George Vartanov
Introduction
BJT’s consist of two PN junctions constructed in a special way in series, back to back. In this case we are looking at an NPN transistor. We have looked at all of the material parameters of the device and used those parameters to calculate the important values in the device. We are using three different types of software to visualize the different possible interpretations and presentations of the BJT.
Part 1: Matlab Material Parameters:
The values to the left are the parameters that are
unique to this BJT device. They were calculated by using the provided equations in the assignment as well as the equations provided in lecture. The mobility values in all my regions seem very reasonable, as I have seen similar values in the past. The minority carrier lifetimes seem like they are on the correct order of magnitude because I would expect them to be quite small. My minority carrier concentrations are somewhat concerning to me, but the amount of doping that is present makes the values seem a little more reasonable. The diffusivity values I have are reasonable as well because the amount of diffusion occurring in the collector region should be higher due to the amount of doping in that region.
The graph at the left is showing the characteristics of the current as the junction voltages increase in both the collector and base region. In the first portion of the graph while Vce is below .1v the transistor is in saturation mode. After the collector-‐base voltage increases past .1v, the transistor is in saturation mode. At no point on this graph does the transistor reach a breakdown voltage.
uE 88.299 uC 1281.7 uB 287 tE 4.1663e-‐11 tB 1e-‐7 tC 4.386e-‐7 pE0 71.108 nC0 500 pB0 14286 De 2.2826 Db 7.4191 Dc 33.132 Le 9.752e-‐6 Lb 4.77e-‐4 Lc 1e-‐3 Table 1
Figure 1
Results:
The values at the left are vital for
understanding the operation of the BJT. Is is the saturation current of the transistor. This shows the small amount of current that is always present in the transistor. This is a characteristic that determines BetaF and BetaR and is unique to each device. BetaF represents the forward current gain and should be as close to 100 as possible. BetaR is the reverse current gain and should be smaller than BetaF. The forward Early voltage is larger than the reverse Early voltage which makes sense. When comparing the corner current values to the graph the graph from lecture, they made sense regarding the orders of magnitude. Go is greater than Gmu, Gpi is greater than Gmu and Gm is greater than Gm so these values check out with the expectations. The transit times are relatively small which makes for good switching times.
Part 2: HSPICE Complex plot using all variables:
The graph at the left shows the HSPICE representation of the I-‐V curves for the BJT. This is very similar to what I acquired through the Matlab simulation, but it has some visible differences. Although we were able to use a large number of variables in the Matlab plot, we were not able to account for the early voltage that we can see in the plot at the left. In addition, it is visible
Variable Name Values Is 1.1973e-‐18 BetaF 44.785 BetaR 1.5798 Re .1149 Rb 5.1957e3 Rc 6.9617e4 Vjc .784 Vje .9998 Cje0 1.2937e-‐15 Cjc0 2.6869e-‐16 TF 8.4038e-‐12 RF 3.1273e-‐11 VAF 5.9186 VAR 1.2293 IKF 1.1214e-‐4 IKR 9.612 Gm 6.6857e-‐5 Gpi 1.4925e-‐6 Go 2.9257e-‐7 Gmu 6.5313e-‐9 Table 2
Figure 2
that at the minimum value of Vbe, the transistor falls into a cutoff mode that produces a current that is virtually zero. Plot Using only Is, Bf, Br:
The plot to the left
is much more relatable to the Matlab representation. This depicts the current as Vbe increases. This is the same magnitude as the Matlab graph, which is encouraging. The spacing between the current curves is almost identical to the Matlab graph of the same values.
Gummel Plot: The plot at the right depicts the collector current and the base current at various values of biasing in the base-‐emitter region. The collector current is barely impacted until about .3 volts where it increases rapidly. The ratio between the two currents will determine the gain value. The dip in the base current is likely due to some incorrect values so I am slightly disappointed that I was not able to get the correct depiction for this plot. Also the high current roll off is not visible in this range of voltages. My estimate for the maximum CE current gain would be around 78, but due to the lack of precision in the graph, it is hard for me to estimate the exact value. This is the ratio between the collector current and the base current.
Figure 3
Figure 4
Transconductance Values: The small table at the left depicts the various
transconductance values in the BJT. Once again, Gmu is smaller than the other values. In comparison to the values that were calculated in Matlab: Gm is the same order of magnitude, Gpi is slightly smaller, Gmu is almost identical, and Go is slightly smaller.
Part 3: ATLAS
Gummel Plot:
The above plot is a secondary representation of the Gummel plot. This plot
begins at the active region and has a clear representation of which base voltage will produce the largest beta value. Although the variables are still being carried over from the previous calculations, this Gummel plot seems more reliable than the previous Hspice generated plot. There is no unexpected behavior from what I understand from the Gummel plot that I have seen in class.
Gm 1.458e-‐5 Gpi 7.06e-‐7 Gmu 6.2e-‐9 Go 1.36e-‐7 Table 3
Figure 5
Net doping:
The above image is a depiction of the levels of doping within a semiconductor exactly as it would appear in a real life version of the BJT. This matches my expectations seeing as the base is very heavily p-‐type away from the emitter and the emitter being very n-‐type and being almost unaffected by the base region. The n-‐type collector region makes up most of the BJT and definitely is affected by the very p-‐type base due to diffusion across the boundary region. Mobile charge distribution:
The image at the left is a representation of the mobile charges within the BJT. There seems to be very minimum mobile charges within the transistor other than within the base region. This is likely due to the fact that there is transfer of charge through the base region. The reason the majority of the mobile charges are close to the emitter region is due to the proportional amount of n-‐types on the right side of that region.
Figure 6
Figure 7
Donor and acceptor concentration/Net doping concentration The plots at the right are likely the most interesting to me out of all of the ATLAS components. There is a clear connection between the donor, acceptor and net doping concentrations. The first thing worth analyzing is the visible donor doping along the green line that dips down as the emitter region turns into the base region and increases again in the collector region. The acceptor doping is only visible between the emitter and the base region.
The most exciting part that I was able to notice was that the intersection points between the acceptors and the donors match up with the dips in the overall net doping concentration drops. This makes perfect sense because there is recombination occurring at those two points due to the equal numbers of acceptors and donors. Also the net doping looks similar to the donor doping because of how much more impact the electrons have on the doping.
Figure 8
Figure 9
Energy bands/Electric field/Electric potential: The plots at the left can be viewed as a progression. Starting with the energy band diagram, it is clear that there is a large amount of energy for the mobile carriers to overcome when moving from the emitter to the base region. This point where the energy increases dramatically is the same point on the E-‐field graph that has a spike in the electric field.
The electric field also jumps up when the slope of the energy band changes from positive to negative. This induces a negative electrical field and then levels out as the energy band moves into the collector region. At the same spike of electrical field, there is a huge voltage induced in order to compensate for the amount of energy at that point in the band diagram.
This is an interesting visualization as the voltage is similar to a reflection of the energy band diagram across the x-‐axis. The electric field is only induced when there is a change in energy so the graph is accurate in depicting the locations where the E-‐field increases.
Figure 10
Figure 11
Figure 12
Equilibrium/Active/Saturation Modes: The plots at the right are the depictions of the energy band diagrams three different BJT states. The initial picture is the equilibrium plot with 0 bias. This is identical to the plot that is visible on the previous page.
The second graph is more interesting in displaying the energy bands while in active mode. This means that the B-‐E region is forward biased while the B-‐C region is reverse biased. This means that the majority of the mobile carriers will be found in the base region due to the increase in energy in the collector region. It is clear that the energy between the emitter and the base region is greatly reduced which is a proper characteristic of a forward biased region.
The third and final graph is the depiction of the energy bands in a state of saturation. In this case, there is almost no energy difference between the base and the emitter allowing even more diffusion to occur. The collector will be ‘collecting’ a large amount of mobile carriers due to the energy drop that is experienced from the base to the collector region.
Figure 13
Figure 14
Figure 15
top related