13. Bipolar transistors

Transcription

1 Technische Universität Graz Institute of Solid State Physics 13. Bipolar transistors Jan. 16, 2019

2 Technische Universität Graz Institute of Solid State Physics bipolar transistors npn transistor collector base emitter n+ n p+ p n + lightly doped p substrate Used in front-end high-frequency receivers (mobile telephones), low input impedance amplifier.

3 Ebers-Moll model evbe / kbt / 1 evbc kbt 1 I I e I e E ES R CS evbe / kbt / 1 evbc kbt 1 I I e I e C F ES CS I I I B E C I B I E I RI F F R I C F evbe / kbt evbc / kbt ES 1 IR ICS e 1 I I e

4 Common base configuration evbe / kbt / 1 evbc kbt 1 I I e I e E ES R CS solve for V be I I e I e evbe / kt B / 1 evbc kt B 1 c F ES CS saturation active cutoff I E < 0

5 Common base configuration I c I C ~ I E buffer circuit: the output current is constant over a wide range of output voltages

6 Ebers - Moll Model ea D p ea D n I ES W x W W eabednnb 0 RICS W W be p e0 be n b0 eb e bc be bc be I I F CS ES ea D n W W bc n b0 bc be ea D p ea D n x W W W bc p c0 bc n b0 c c bc be

7 Common emitter configuration evbe / kbt / 1 evbc kbt 1 I I e I e E ES R CS evbe / kbt / 1 evbc kbt 1 I I e I e c F ES CS current amplification ~100 I I I B E C

8 Current amplification factor C fe B I h I B E C I I I 1 1 B C I I 1 1 C e B e I B I B ~ C E I I

9 The Art of Electronics Horowitz and Hill for common emitter configuration

10 Transconductance g m I V C be evbe / kt B / 1 evbc kt B 1 I I e I e c F ES CS The first term depends on V be g m e I ei e I e kt kt kt be B F ES ev / k T C B B B B The transconductance can be very high.

11 Early effect Ebers - Moll: evbe / kbt / 1 evbc kbt 1 I I e I e E ES R CS evbe / kt B / 1 evbc kt B 1 I I e I e c F ES CS I I I B E C I ES ea D p ea D n W x W W be p e0 be n b0 eb e bc be I CS ea D p ea D n x W W W bc p c0 bc n b0 c c bc be I ES and I CS are treated as constants but the depletion widths W bc, W be, W c, and W e depend on the voltages. p ev kt B e0 exp be n ev kt B b0 exp be p ev kt B c0 exp bc n b0 p c0 p exp evbc nb 0 e0 kt B x e W e W eb W x bc W c c

12 Minority carrier concentration n ev kt B b0 exp be p ev kt B e0 exp be n ev kt B b0 exp bc p ev kt B c0 exp bc p c0 p e0 x e x n1 x p1 x p2 x n2 x c

13 Early effect Common emitter configuration Base width modulation: smaller width increases the diffusion current and increases the gain. Punchthrough: The neutral base width goes to zero and all gain is lost. Lightly dope the collector -> voltage drops in collector. Makes circuit slower.

14 I C ~ I B

15 Common emitter configuration I C ~ I B amplifier

16 Small signal response V cc Low input impedance amplifier V transistor V resistor

17 Small signal response i i g v c B EB EB transistor man input conductance: g EB i v B EB transconductance: g m i v c EB

18 Small signal response 1 f 0 1 f ft 0 f

19 Heterojunction bipolar transistors

20 Heterojunction bipolar transistor small emitter-base capacitance to increase speed

21 HBT current gain I C I B 1 n p B0 E0 (npn) Higher doping in the emitter makes the minority carrier concentration lower in the emitter. n B0 p E0 n N 2 i A 2 i n N D N N exp( E / k T) C V gb B NA N N exp( E / k T) C V ge B If the emitter and the base have different band gaps NE NN E c v g exp NB N cn v kbt N D

22 HBT current gain A HBT has an emitter bandgap of 1.62 and a base bandgap of A BJT has an emitter bandgap of 1.42 and a base bandgap of Both have an emitter doping of cm -3 and a base doping of cm -3. How much larger is the gain in the HBT? HBT Eg exp exp 2257 BJT kt B

23 HBT Trade off gain for higher speed Higher base doping lower base resistance reduced Early effect less trouble with punch through base can be made thinner -> faster transistors Because of higher base doping, a higher collector doping is possible without punch through lower collector resistance

24 HBT current gain band discontinuity reduces emitter efficiency Graded layer emitter and base improve performance Si/SiGe AlInAs/InGaAs

25 Heterojunction bipolar transistors Fastest InP/InGaAs HBT's have an f T of 710 GHz. Higher doping in the base allows for a thinner base without punch through and lower base resistance and thus higher frequency operation

26 Microwave engineering Electronics: L<< f <~ 10 GHz Microwave: L 10 GHz < f < 1 THz TeraHertz: L 1 THz < f < 100 THz Optics: L 100 THz

27 Interdigitated contacts in power transistors

28 Thyristors Forward blocking Forward conducting Reverse blocking

29 Thyristors 1 * 2 > 1 Used for switching high currents or voltages

30 Latch-up Both BJT's conduct, creating a low resistance path between V dd and GND. The product of the gains of the two transistors in the feedback loop, is greater than one. The result of latchup is at the minimum a circuit malfunction, and in the worst case, the destruction of the device.

31 Subthreshold current If the p-concentration in the channel is low, electrons emitted into the channel by the forward biased junction diffuse across the channel without recombining.