---
title: "Diode current at 100°C (eta 1.8)"
subject: "Electronics Devices and Circuit"
chapter: 1
key: "semester_3/electronics_devices_and_circuit:diode-current-temperature-eta1p8"
source: https://www.bctengineer.com/q/semester_3/electronics_devices_and_circuit/diode-current-temperature-eta1p8
site: BCT Engineering
---

# Diode current at 100°C (eta 1.8)

Electronics Devices and Circuit · Chapter 1: Diodes

- Asked in: 2068 Chaitra, 4 marks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/electronics_devices_and_circuit/diode-current-temperature-eta1p8
- Whole chapter: https://www.bctengineer.com/md/semester_3/electronics_devices_and_circuit/chapter-1

## Question

A diode conducts 1mA at 20°C. If it is operated at 100°C, what will be its current? Given data are: η = 1.8 and negative temperature coefficient value = -1.8 mV/°C.

## Short answer

The forward voltage drops 144 mV from 20°C to 100°C. With VT(100°C)=32.15 mV, ηVT=57.9 mV, current rises to I≈1 mA×exp(144/57.9)≈12 mA.

## Answer

For a fixed diode current the forward voltage falls with temperature at about -1.8 mV/°C. If the applied voltage stays the same, the current rises exponentially.

#### Given
I1 = 1 mA at 20 °C, T2 = 100 °C, η = 1.8, temperature coefficient = -1.8 mV/°C.

#### Steps
```text
ΔV  = -1.8 mV/°C × (100 - 20) = -144 mV
VT (100 °C) = 8.617×10⁻⁵ × 373.15 = 32.15 mV
η VT = 1.8 × 32.15 = 57.9 mV

I2 = I1 × exp(|ΔV|/(η VT))
   = 1 mA × exp(144/57.9) = 1 mA × exp(2.49)
   ≈ 12.0 mA
```

**Answer: I ≈ 12 mA** at 100 °C (about 12 times the 20 °C value).

*Assumption:* VT is taken at 100 °C; using VT = 25 mV gives ≈ 24.5 mA instead.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What is DeltaV for -1.8 mV/degC from 20 to 100 degC?
   A. -18 mV
   B. -1.8 mV
   C. -180 mV
   D. -144 mV
   Answer: D. DeltaV = -1.8 x 80 = -144 mV. (Key point: Voltage shift)

2. What is eta VT at 100 degC for eta = 1.8?
   A. 32 mV
   B. 57.9 mV
   C. 45 mV
   D. 25 mV
   Answer: B. VT = 32.15 mV; 1.8 x 32.15 = 57.9 mV. (Key point: eta VT)

3. How is the new current at constant voltage found?
   A. I1 x |DeltaV|
   B. I1 x exp(|DeltaV|/(eta VT))
   C. I1 / exp(|DeltaV|/(eta VT))
   D. I1 + |DeltaV|
   Answer: B. The diode becomes more forward biased by |DeltaV|. (Key point: Current formula)

4. What is the resulting current?
   A. About 120 mA
   B. About 1.2 mA
   C. About 0.12 mA
   D. About 12 mA
   Answer: D. exp(144/57.9) = exp(2.49), about 12 times 1 mA. (Key point: Result)

## Close variants set in other exams

- [Diode current at 100°C](https://www.bctengineer.com/question.md?key=semester_3%2Felectronics_devices_and_circuit%3Adiode-current-temperature-100c) — Electronics Devices and Circuit, chapter 1, asked 4×

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