# PN junction band gap - equal across all devices?

With a given material eg silicon, is the band gap constant across devices? For example, at a constant temperature is the band gap of a diode ("diode drop") exactly the same as that of (say) an NPN transistor? If not, what kind of variation would one expect between devices?

• There is no such a thing as "PN junction band gap". Are you asking about the band gap or the Diode/PN junction drop? See my answer below.
– Curd
May 2, 2019 at 9:26
• @Curd Diode drop. Band gap applies to pure material May 2, 2019 at 9:29

For a 100% pure element (no doping) it will be the same.

For a practical diode or transistor, the dopant levels and purity have quite a large effect on both the forward voltage and temperature coefficient.

For pure silicon the tempco is -2.1mV / degree C, but the venerable 2N3904 usually has -2mV / degree C (depends on manufacturer).

If you read the SPICE files for some devices, this can be inferred from ideality factor as seen in the Schockley diode equation.

For a diode, this is N in the model.

For a transistor NF is the forward mode ideality factor and NR is the reverse ideality factor.

A perfect device will have N=1. Here is the model for the MMBD4148 small signal / switching diode

*SRC=MMBD4148;DI_MMBD4148;Diodes;Si; 75.0V 0.250A 4.00ns Diodes Inc. Switching Diode

.MODEL DI_MMBD4148 D ( IS=300n RS=0.422 BV=75.0 IBV=2.50u + CJO=1.99p M=0.333 N=2.77 TT=5.76n )

Here N is 2.77 (the higher this value, the less ideal it is)

Looking at the 2N3904 model

• Model Generated by MODPEX * Copyright(c) Symmetry Design Systems
• Contains Proprietary Information *
• Which is The Property of *
• SYMMETRY OR ITS LICENSORS * *Commercial Use or Resale Restricted *
• by Symmetry License Agreement *

• Model generated on Aug 7, 01

• MODEL FORMAT: PSpice

*\$

.MODEL Q2n3904 npn +IS=1.26532e-10 BF=206.302 NF=1.5 VAF=1000 +IKF=0.0272221 ISE=2.30771e-09 NE=3.31052 BR=20.6302 +NR=2.89609 VAR=9.39809 IKR=0.272221 ISC=2.30771e-09 +NC=1.9876 RB=5.8376 IRB=50.3624 RBM=0.634251 +RE=0.0001 RC=2.65711 XTB=0.1 XTI=1 +EG=1.05 CJE=4.64214e-12 VJE=0.4 MJE=0.256227 +TF=4.19578e-10 XTF=0.906167 VTF=8.75418 ITF=0.0105823 +CJC=3.76961e-12 VJC=0.4 MJC=0.238109 XCJC=0.8 +FC=0.512134 CJS=0 VJS=0.75 MJS=0.5 +TR=6.82023e-08 PTF=0 KF=0 AF=1

Here the forward ideality factor is 1.5 (much closer to an ideal diode). This is one of the reasons that temperature sensors often use a diode connected transistor.

So the forward voltage is process dependent (as is reverse leakage).

• I've read the Ideality Factor is controlled by the abruptness of the junction. Is that true? May 2, 2019 at 12:38
• From memory (a long time ago) the abruptness and ideality factor are inter-related but I don't recall the abruptness controlling it (I could be wrong). :) May 2, 2019 at 13:13

No, it certainly isn't equal for all devices. Semiconductors are not made of pure silicon, but silicon that is carefully mixed ('doped') with other materials. This is done to influence the bandgap voltage, creating areas that are more positive or more negative than other areas on the device. This is how the P and N regions in a PNP transistor, diode etc. are made.

The bandgap is very sensitive to the dopants, and will usually differ slightly even between devices of the same type.

The specifications of the device will usually show what kind of spread can be expected in forward voltage for a given current and temperature.

You said

...is the band gap of a diode ("diode drop")...

suggesting that both things are the same, which is not the case.
Band gap and diode drop are two completely different things. Common for both is only that both are measured in Volts.

Band gap is a constant specific to a semiconductor material (a material constant). It is the difference between the potential of the lower limit of the conduction band and upper limit of the valence band.
For Si it's about 1.1V (with only small dependency on temperature and doping).

Diode drop is a constant specific to a PN junction, i.e. a specific semiconductor device. For Si PN junctions it is about 0.6V; For Si Schottky junctions it is only about 0.3V; at room temperature (with substantial dependency on temperature).